The Feeding Behaviour of a Sit-And-Wait Predator : Ethological Studies on Ranatra Dispar

WAIT[. INlìTITIJTE P¿,. I r.fsj rr l_j i1{ì R Ï

THE FEEDING BEHAVIOUR OF A SIT-AND-I,JAIT PREDATOR

(IIITER0PTERA ETH0L0GTCAL STUDTES 0N lì4\'l\l4JRA qIÐ\,S

NEPIDAT), TI.IE !\'ATER STICK INSECT

By

PauI Charles Edurard Bailey B.Sc. Hons. (A.N.U.)

A thesis submitted for the Degree of Doctor of Philosophy in the faculty of Agricultural Science at the University of Adelaide.

Department of Ent.omology, litaite AgricuJ.tura]. Research Institute, University of Adelaíde.

April, I9B4

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TABLE OI- CONTTNT:J lcae

SUMt\1ARY 1V

DECLARA I I ON X

ACI(NOI,jLIDGEMENTS Xi

CHAPTIR 1 INTRODUCTION I

CI.IAPTtR 2 TflE PREDATOR AND PREY 2:I Tlre Predal-or - Ranatra {içpgf t1 2 1. I InLroclucl-ion 11 2. 1,.2 0bservatíons on Generaf t3iology 12 2 I"3 Mainl-enance of R.djspar jn ttre laboratoly 25 2-.2 T he Prey - &¡L-ggpg -dejl]-çl 2l 2, z?,I Introduction 21 2 :2.2 Maint.enance of A"deanei irr the laboratory 28 2 22.3 CollecLing Teclräi*flã-Jãnd Init-ial ldenLif i- cation and Size Grading 30 2:2"4 Techrriques used in ltleasurenrenl- of body J-engths and ureL and dty ueights of A.deanei 32 223 The Temporal and Spa t-ial disì:ribuLion of' tì.dispar and A.deane-l in the fi.eld 34 2:3.I Inl-roducti on J4 2:3.2 Ilatelials and l"1el-hods 35 223,3 Resu ll-s 40 223 "4 Di. scussi.on 42 CIjAPTER I Tllt PREY CAPTURING BEHAVIOUR 0r R.dispar 3zI Intloducl-ion 47 3zZ Description of General Prey Captur:e oncJ Feeding Behav-iour 52 3:2,I Materials and Methods 52 322.?- Resufts 5J 3:3 Variation ì.n Prey Capturing Behaviour and other observatior-rs 59 3:J.1 Observal-ions on the Arousal- Component o1" Predal-ory Behaviour 60 3:3,? Observations on the Prey Capture Contponent of Predat.ory Bchaviour 6I 323,3 0bselvations on the Defens-ive and [vasive BeLrav -iour of' !"._çl:=-gqf and l-heir influence on Predatory Behaviour / 65 3;3.3 .l- Inl,roducl-ion 65 3z'3.J "?- Materiafs and Plelhods 66 323.3 .3 Resul-ts and Discussion 66 324 Description of tfie Predator's 'Arousalr Space and the Efl1=ect of Food Deprival-ion 70 324.I Introduction 70 3 ztt.2 l4aLe ::i- a l-s and Methods 70 324,J Resu Its 74 3ztt. 3.r Holling's llodel f'or Lhe Estimation ol' ttle Reactive Fie.l.d of H"clrassa B?. 324.3"2 Model-s l'or tlre fst.imatic¡n of the Arousaf Field of !ì_r_ç!'5pg:. B6 iz5 Description of and l-l'l'ect of Predat-c¡r Fasting on tlre rCapl-ure Spacel 92 J:5 .I Mel-hods 92 _tl

I-{-qs.

325 .2 ResuÌt s 92 3:5 .3 Discussiorl 101 3:6 ffl=ect of, Pr:ey Sj.ze anc.l Food Deprivatjon on Prey LlaptLrri nq Behaviour 106 326.I If f'ect of, Pley Size and l-ooc1 Deprival-íon 106 JtC¡.I"I ftlatelials and Mel-hods 106 326.I.2 Resul-l-s ¿rrrd Di scussion t0B 3;6.2 The tf 1' ect of Distance lretueetr Prey and Predal-or on l-he Prey Capt.uring Betraviour 114 326.2.I Methods 114 3:6.2.2 Results and D-iscuss.icln LI6 327 Discussion 118 3zB Summary L24

CHAPTER 4 THE TFFECT OI' PRIY DENSITY ON THI NUI'IBTR OF PRIY EATEN 4:l Introducl-ion t27 4zZ The Parame'Lers of' l-he Funct.ionaJ- Response Ilu 422.1. Reviev of' Functj.onal- Response Equations; l-10 4:2,2 Random Preclat-.or-' Equat.ion r33 423 Basic Experimental- Procedulre 13l 424 The El'1'ect of, Te-'nperaLure on the FtlnctionaÌ r39 Response o1' Adult. -B-!jru¿a¿ 424 "I Introductiot'l I39 4ztt,2 Methods 140 424.3 ResuLts l4c) 4zlt.4 Discussion llr I 425 The El'fecl- of Aq¡e Sl-rucl-r¡re of Lrot.lr Predator and Prey on the Funct-ional Response r43 4:5.1 Introduction L47 425.2 lYethods r45 425.3 Resul-ts r46 4z5.tt Di.scussion l4B

CHAPTER 5 TI]E TFFTCT OF PREY DENS]TY ON THI FITDING AND PREY CAPTURiNG BEIIAVIOUR 5:l LntroducL-iorr L5B 522 Tlre Feeding Dyir amics of, R.dispar 160 522.1 Mel-l-rods 160 5zZ .2 Results and D-ì-scussion r64 5:2.3 Discussion L69 5zJ Experiment to Demonsl-rate 1-he Passage of tda'uer inl-o l-he P::ey During Feeding by R . di:;par 171 523.I Introduction L7l. 5¿3,2 Experiment I r72 5 z3 .2.I Metlrods 172 523 .2.2 Resull-s L7J 5 z3 .3 Exper-iment 2 174 523,3,I MeLhods I7 Lt 523.3.2 Results L74 523,1+ Discussion l.l t-t 5:4 Mr"rltJ ple Pley Capture and Handlirrq of' .B:9r*:.llrå r71 5:4.1 Introduction "I71 5 24.2 l'lat-erial-s and l4ethods 178 5:4..3 flesults 180 5 ¿4,4 Discussion 184 525 Discussion lBB l_r1

-þ-se

CHAPTTR 6 FACT0RS AFFECTING TtlE ENC0UNTIR RATE 0F A.deanei tnJITll R. dispaT: - THE EF FECT OF PREY DTNSITY AN' I./ATER TEMPERATURE ON S\.JII4I.1ING AND AGGREGATING B[I]AVIOUR 6: I lntroduction I90 622 Descrtptiorr o1' Basic Sruimnring Behavi.oul of &Sre" !gg.l"-1, Adults l_91 6:2.I Material-s and MeLhods r.9l 6¡2.2 ResuLl-s 192 623 Effect o1' lrlater Temperature and Density o¡r the MovemenL Pattelns ol' Adult A.deanei 19J 623.I Material-s and Metlrods 193 6:3 "2 Resul-Ls 194 6'.4 I he El'fect of' Prey Derrsity and tn/ater Tem¡:erature on the Subsequent fncounter Ral-e with an 'Anbush' Predator l9B 624,I [lel-hod l9B 624.2 Resulls and Discuss.ion 200

CHAPTIR 7 THE EFFICT OF PREY DINSITY ON AMBUSI-I SITt SELECTIOI! BY R. dispar 7zI Introduction ?"04 7:2 The Effect. of Prey Density on Movements Betueen and Around an Ambush Site 207 7 22.). Material-s and i4ethorls 207 7 22.2 Resul"Ls n2 7zj The EÍ'fect of Prey fncounber versus f'ood Intal

CHAPTER B GENERAL DISCUSSION 226

BIBLIOGRAPIIY 23r l.v

SUI'ïUARY

The aquatic bugs, lþl1âl*l:1 .*!qef arrd 4'å!,qqUs -(qA¡lgi1' \\¡ei:e uscd to stucly ethological and ecological j-nterrel¡rtiorrshíps i"n the pre

The sequence of behavioural cornponents in tlre predaLory behaviour follovrs the basíc anbush paLtern, i.en initial precapLure posture, arousal, orientation, capture, consolidaLion of gr1p, exploralion, lnjection of venom/enzymes, feeding, discard of prey, although much variation betr,¡een components ex;Lsts " Much of this variation is described, inclurling the predaLorrs defensj-ve and evasive behaviour, along ruiLh their effect on the preclatory behaviour. The preclaLr;rrs

arousal ancl strj-ke space are describe

space and the effecl- of hunger using the model designed by I'Ioll:Lng (1966) proved reasonably satisfactory n aî.Ler s1-íght modifÍcation., for the horizontal plane buL totally unsatisfactory for the vertical p1ane. This is believed to be due to Lhe unusual eye structure of Relq!{a-.

The additional effect on predatory behaviour of prey size was invest-lgated. It was found that l-he hunger 1evel clel-ermines r'rhether

R"dÍspar rr':111 inítia1l y be aroused or noL llut Ehe disî:ance at v¡h-Lch the

arousal t¿rlces p1.ace ls influenccd by the size of the prey. This is l:e1íeuecl to reflect the capaciLy and intei:relat-Lon beLtt'eerl visual and l¡echaltoreceptor, sensory organs. Ttre decision to str:ilte at a prey isn alttrough again inf-l.uencerl by hunger, signÍficanLl-y affected by prey size" The

cornponent,s of pretlatory behavi<¡ur of Ld.-igPgJ- leading up to prey capture, by increas:Lng not only díst¿rnce of response but al*;o the ntilnber of strikes, hits, and capl-ures per unit preserrl-a1-ion of prey. Tt does noL affect capture efficiency rvhich rernaÍns at about 70 Lo BO%. Food' deprivatíon also increases the range of prey sizes that lì"clispar responds t-o ancl âttempts to captul-e. The effecL of food deprivation is considered to reflect a notivational change in responsiveness Lo

.I parLicularpreystimuli.usuall¡'descrj.lredasasellsitizatÍorrof H particular stirnulus-response rel-ations raLhcr than l-he food deprrl-vatj-on I affecting the sensory mechanisms. The predat.ory success in relal-ion to size of nodel prey suggested an toptimumr size Lhat could be capturc:d, irrespecLive of p::eclator motivational leve1, rvlti-ch ís based pri-mari1-y on tlre relatlonship between the shape of the grasping leg and sj-ze of prey.

I i i Functional respons;e experiments ivere pcrformed to examine the rneasure ::ates t effect of prey densíty on feecling behaviour and to attaclc i /, i ancl handling t.imes. In both the temperature ancl age structure of

response seemed tt- pledalor and prey effects, the most generally applicabl-e tì t I to 6e the Type 2" Progress:ively nore prey v¡ercì eaten tretvreeu l5.0oC and I inves;Líga{:ed the at1-ac-k-rat-.e 29 "OoC. Over the ternpet:ature range q i' v_t

inc.re¿rses alruost l:'-nearly rqhi-le the lrandling-t-irrrc increases dramatl.cal1.y

between 15oC ancl 20oC after rvh:Lch i.t renains corrsl-ant up to 3OoC. Generally the handli-ng-tirne increases â{ì prei¡ sizre incrcases ¿rnd

'i. decreases as the predator size fncreasús" 'l-he aLtacl'--::aLe surface is far more complex.The naximurn al-tack rarte for ilts:iars I antl II occurred on the s¡nallest prey size (1 and 2)ur+hj-le as the prerlator sj-ze coilti-nued to increase, so the att-ack-rate on each of the prey sizes increased. Unlilte the prevj.ously published atl-aclc rale surfac.e, with srnall

preclators at-.tacki.ng srnall prey and lar:ge predators attacking large PreYr

t-he maximum attack-rate stops at the intennedia-te prey sÍze 3" Predator Ínstar V has the largesL al-tack rate values over all- prey sizes. Adult R"díspar have noticeabl y lorver altack-rates fcr various prey sizes than j-nstars V, IV and to some degree fII. The results suggest that sma1l preclator instars r+:t11 usual-Ly conpete with large instars for prey, unless there is spatial or teroporal separation between them. Observations conducted in the fÍeld indícate that a clisr:inct d iù age-specific spatial dLstrlbution exj-sts in R.dåWg (and j-n the prey I A deanei wiEh the smallest individuals being found preclorní-nernt.1y in the

shall-ow (1ittora1 zone) water while the larger individuals are fotrnd in the deeper watel:.

RelaLed behaviours of the predator rvere investigated itr order to

identi.fy factors that r¿e::e belíeved to be involved in the observed I i increase in tlte numtrers of prey eaLen aL differelÌt prey densities.

I f /, t--o i. The feeclíng behaviou:: of R"dispar rqas examined wiLh respect

:, (intercat-ch !l Lhe prey utilj.zation, the tirne between successive capLu::es r t ínterval) ancl the fecding time. The feediûg process consisted of three I sLages. (1) Injection of venc¡n, (2) breal

ancl (3) excract-i-on of fooci. 'l'he::al,c of exLracL-Lon from an i.ndjvidual

prey clecreases as the prey -LLem is dep-l.eted" 'Ihe extracLÍon raLe wâs

shown l-o ircrease sign:Lficantly during the first 15 nj.nutes br:forc+ decreasing. Ilven afte-r 30 miriutes i.he e¡ll-r¿rct-.j-ou rate t\'as stj.1l ma::ginaLly higher than the i.nil-ial- exl-raction rate. Tiris phenomenon j-s quite differenl- to what has previously bc'en repo::tecl for sucl

There \^/as a uegaLive relationsh:i.p between in,creasirrg prey densì-ty and prey depletion, r,ríth ttre pretlators t¡eing s;ignifj-canLly ntore rrras'Lefulr r( 1.e" prey were dlscarded l¡efore all exLractable food was rernoved) at the two higher prey dens:i-ties compared witl'r those aL the lot+er densitj.es. As the prey densitlr çls.r.ased fronn 60 to 1pre)' per container, sc¡ the resultant inLercatch inLerval and feeding ttme inc.reasecl" In

¡ conjunction rsith this the average dr'y weight exLracted per prey also

increased. No correlation t,Jas found beLrseen i-ndividual inLercaLc.h, interval and subsequenl feeding time when exarnined through an B prey catch seguence. This is tallen to supporL the Optimal feerling model in

I fl which the predator reacts to tire average profitabiliry of the l environment (i.e" mean intercatch interval) ::ather than to the specific level of foocl j-n the gut h'hiclì has resulted from the length of the intercatch interval. The overall effect of the extraction dynamícs atld duration of feeding on orìe prey ltern at different prey densítj"es is that I at higtrer densities, although spending l-ess than half the tlme on a prey

item compared ruith the lors densÍl-y, the predator stil-l obtains al-most I i 6O% of. the avaíl;rble prey contenls before discar

I I i B)' the use of fluorescent dyes i.t v¡as shot*'n thal dur:ing feeding

'1 . !lt water frorn the external environment pírsses into the preyrs bocly, r through, it 1s bel-1eved, punct.ure holes; frorn previous feerli.ng sites. I Such a phenomenon, resulti.ng in Lhe dí-luti.rg of the prey bocly c-ontetttso 4 I v] 1l-

n¿ty be used as a physiological stlruu-Lus Lc.r Crelect t-he quality of Lhe

prey r+hl.ch rnay j-n Lur¡r be used in deciding r';hethr:r: Lo cease feeding at one sile and nove to anat-her partr of tl:e Preyrs trody or:

Prey.

R"cl:i.spar.' Ís sltorrir 1-o be able to capture and hold a nunber of prey simultaneously. Capture of prey characLerisl--ically occurs ín three distinct paLterus, each character:ized by a dj-f :ferent nu.lnber of prey caught. The tine since last- feeding by the preclator has a signifi-cant effeci: on tyhether Lhe predator will capture llore tb.an one prey" Otrce feeding starLs there is a c.ritical per:iod during vrhich, :Lf an encounter takes place, the predator rçil1 attcmpl- to capture'the prey" Once past that pe::l.od prey are ignored" The critj-cal- periorl is longer Lhe higher the moLivatíona1 level of the preclator" It is sugges'ced thaL this rnulti-prey capture behaviour potenl-ially inr:reases the size of a rrreal as

groups or richools of prey move pass the ambush predaLor.

Particular features of the preyrs beiraviour believed to l¡e involved in deterruining i.ts movement patLerns, and t-hus eucounter-rate

v¡ith an anbush predator, were investigated.

Four basic stvinurri-ng patterns of A.deanei- are clescribed. The effects of water temperature and density on movement pal-terns shorv that þotir havc; a.signj-ficant effect, wíth animals rnoving mo::e a.t higher ternperatures but less as density increases " f t is srtggested tl-rat these íntraspecific Ínt.eractlons in gr:oups of À=4gryçå lead to the format-ion of groups or schools of inriividual-s, and l;hat this observed behaviour is 1 art anLi-predator defence. This aggregating behaviour is; strongly

{ supported by data ol¡taj.necl from Lhe fie1d, AdrliLional resul-ts inclicated l.x l-hat. the effect of this aggr:egal-ing behavj,our signi'ficant-ly reduces the encount-.er-raLe wj-th an ambush predalor compared wi-t"h a predicLed rate basecl on Lhe mov€:rüenL of one .individual , This l-rerrrl rv¿rs repeated over the three experj-nenLal ternperüLures.

Prey densífy was e-lso shorsn Lo iirfluence the choÍce and duraLion of stay at an ambush site by &-4æ:¿U". It r.¡as shor,nr Lhat preclators changed anbush sites significernLly nore frequently when prey clensil-y tr'as low cotnpared rsith t+hen pre¡' clensity was high. fnitÍ.a1ly, p::edat'ors renain at an anbush site for signifi-cantly lotrger thrrn later staYs at different sil-es, irrespecLive of the pr:esence or abse¡rce of prey. 'fhere tùas a tendency, over Lhe three prey densíLies exaniined (0, 3, 15 prey per litre), for the durati-on of stay at a site to become shortcr as the experintent progressed. Exanilratíon of the angular changes by the predator while at an ambush site showed that B-rdlgg*f remainerl longer periods rshen üo stationary for significanlly Prey "u,,u iiu=.rrt, while the incidence of moving through rellatively srnall angula:: changes occur significantly ruore ofLen rtÍren prey rvere present.

By usi-ng model- prey tlìaL conferred no nutritlonal reward it was shor,rn Lhat encountering a prey caused BISM- to remain signj.ficantly longer aL a rpotentiallyr profitable anbush site contpared rvith tshen no encounter was given. It is suggested that n-.4i"¿q. uses not only a mean i¡rtercatch interval. but a prey ercounLer rale nechanism in assessing the availabi,lity of preyo It is predicted that Lhe assesslnent of pr:ey

availability by ambush predators may lnvolvecl more thatr one factor and that, depending on the circumslanc-es, pl'edators nray switch between

different nnechanisrns or uti.lize information from several simultaneously. Dt.Cl-ARAT I0N

The uork presented in this thesis is my ovn unl-ess othelurise acl

April , I9B4 Paul. C.E. Bailey X1

AC KNOI\IL I-.DG EI'1EN T S

Many peopfe have lrel-ped me jn a greaL mi:rn)u ¡r¿ys i-hrouglrout tlre duration of tlris sl-udy. To aIl- of thenl I extend rny thanks and appreciatiorr for all tlrey have done" There are some t-haL I very nuch ruould l-ike to single oul- hovever'.

I urish to sincerely thank my two principal- supervisors, Prof,essor T.0. Brouning and Dr. Roger Laughlin, for t.heir advice, suggesLions, profitable discussions, and critical- reading ol' nry thesis. rProf,r continuousl-y caused me to question and critically exanine nry vorl< and. pointed out nry rapid jumping to concl.usions. Rager vas continuously avail-able f'or discussion ancl advice on every aspect ol'this study, and provjded aclditionat heLp and suggesti.ons in regands to the design (t¡ol-h theoret.ical- and practical) of experiments and the anal-ysj.s; all- vitlr incredible palience. I am very grateful.

, Discussions ruith Dr. Derek l'lael-zer, Dr. Peter Mil.es, Pli:s. Eve Lever, and ottre¡ postgraduate studer-rts helped me in formufating and refirring ideas and tecf-rniques t-hat- I used.

In particular I ruish to thank the fol-lo',uing very good friends for their help: Andy Austin ancj David Dal] for their comments and advice, especially in regarcls Lo some of" tlre resul.Ls j.n Chapter 5 and l-he Brisbane

ì ri¡r. George Hof fman, Duncan McKay arrd Peter Sanlson f or tl-re many hours spent in act.ing as 'sounding boardsr f or my ideas, discussing the resul"l-s and helpinq Lo provide a stinrulating at.rnosphere (not only on Frj-day evenings) 1"or formulating ideas. Duncan pr:ovidecJ much help in stat.istical aclvj-ce ancJ l-tren sharcd -ì-onq hours on the 'l

cornments concernj-rìg tl're layout arrd consl-¡:uction of Cl-rapter J, and along ruith George have been such good l'riends over the last l-0 motrl-hs.

I a-lso tyj.slr to l-hanl< rUncler Dr. Paul [1adge f or nol- only l-r-is friendship but. his gener:ous qil'1, vlrich enabled me 1-o go to Brj srbane in 1983, The discussions I llad uhile attenclì.rrg l-he l.B I.t.C. \r/ere ¡:art-icu- J-ar:Iy lreJ-pful and came at a critical l-j-nre" Tc other memt¡ers of the Fri-clay niqht Choir plactice at the Eclinburgh I extend nry gratitude; Lhe nrany hours spent. in 'l-alking shop', the arlvice and discLtssions (and jugs of beer) uere much appreciated. Marija Tadic kindly converted my lough sl

Drs. Stan Eckett and Debbie Street (Biometry Section, I^J.A.I.T.) provided much st.atistical. advice in addition to ruhictl Stan kindJ y vrote the program for analysing the::esul-ts in Chapl-er:4. Noel Stew¿rrt and

Ken tnJ.ilkerson helped by obtain-ing many pieces of equipmenl- and advising and helping to buíld much ol' the apparatus used; the Library staf,f at the lJaite and S.A. Museum acquired irrnumerabfe (sometimes obscure) inter- library l-oans for me; Mr. Brjen Palk photographed ernd printecl the l-ine j-ll-ustratj-ons and prinl-ed the. half toncs; CoJ.our Quick Pty. Ltd. advised and printed the col-our pJ-ates. I tuish to thank al-l these people, and others t.hat I may have f'orgotten.

The folloving people kindly gave tne pernrission to either conduct. field vork -in, or col.l.ect an-imals f,rom, l-hei-r properties :

Barry Riddle (former ouner) , Lee t{ajor.s (ovner) niO¿ters Dam, Hahndorf ; Tim and Ívloir'¿r Atkinson (Crafers) ; Mr " F red l)onaldson (Hahnclorf ) ; l4r. Hilìy (Lobethal); [1r. llunster (Lubellral); t4r. & l4rs. von Doosa (üraf,ers);

A-lice and Rocl Viel.ls (Uraidla). xI1r

I vould like to especially thank [.le]en Mil]er for the hours spent in typing my thesis ir¡to therApple'mini-computer and producing, as ever, an exceLlen{: typescript.

FinaIIy, and most ol" all, I thank Louise for her untiring support and encouragement through this etudy, especÍally over the last 6 months during the gestation of this thesis. l^lithout her advice and heJ.p in every phase of, this work it tuould have been an unenviable task.

This project ruas supported by a Commonveal-th Postgraduate Research

Atuard and a Special Research Grant, Postgraduate Scholarship, University of Adelaide.

tlV¡rìi i: il':.,ri l' : : : . Ii:rrl":i'i

1

Early forms of life probably obtained their nut-.ri-ents directly from the surroundj-ng water, but- the capacity to engulf other organisrns

developed very early in evolution. These were the first predat-ors and predatj-on has remained one of t-he important ways of obtaining energy

along rvith photosynthesis, deLrj-Lous feedj-ng and herbivory (if one

distinguishes herbivory and prerlation). Predat:'-on is therefore an

ecological factor of almost universal importance for the biologist who aims at rrnderstanding the habits and structures of animals. Despite this, opinions differ as to what predation really is. The Oxford Dictionary defines predation as trthe habit of hunting and killing other animals for foodtr, whereas a cuisory glance at current biological literature reveals that normally predation is defined (by biologists) in negative terns. Thus, it is thought not to tre parasitism, the other

comnron process by which an organism may gai.n benefit at the expense of another, nor filter-feeding, carrion eating, nor browsing.

Accordingly, one could define preclatiorl as a process by which an

animal expends sorne effort to locate a 1j-ve prey and, in addition,

expencls further effort to muLilal-e or kill it (Curio 1976, p.1). From the definition just proposed, consumption of the prey, following its capture, has been intentionally omitLed. Hence, the biological

significance of predation may be more than the maintance of nutritional

homeostasis. fn fact, predation may have something in common with the (i,'; more direct forms of competitÍon (Curio 1976; see also B{Ðgon and Mortimer 19Bl). Therefore predation nay be besl- clistinguished from other forms of foraging by only one of its consequences, i¡: that it 2 concludes wil-h Lhe mutilaLion or toLal destructíon of an animal that generally offers sorne resistance against being discoverecl and/or being harmed. Obviously, one can cite various parasites that may harm and eventually kill their hosts, but whereas it is in the interest. of the parasite to ensure that its host remains alive long enough for ít to complete its life-cycle, such needs are not necessary in the case of the' predator.

As in other disciplines, the significance of predation can be studied broadly from the point of view of the populations of predator and prey (see for example Bergon and Mortimer 1981; Bertram 1978; Hassell L97B; Kruuk I972b; Scha1.1er 1972), to its effect on the individual (for example Bauer L982; Beard 1963; Evans 1976¡ Lubin o 1980; Ì{fl.ernond 1981; Rees and Offord L969; Schlee 1983; Schmidt 1982). Irrespective of the point of view, whether from the populatíon or índividual level, the effect Ís the result of-a predator-prey interaction.

Predation within an ecosystem can be considered to be t-he sun of all interactions between single predators and siäg1e potentíal prey animals. A single interaction can result in success or failure for the predator, corresponding to cleath or escape for the prey. If a successful interaction is defined as one that results in a meal for thä predator, then the rate of successful interactions is the rate of - predation. For a given predator or prey, within a certain time interval, there Ís an associated probability of a successful interaction (Gerritsen and Strickl-er 7977). To examine this, the predatoiy interacLion needs to be dissected furLher 3

A complete predatory inter:action can be broken down into a series of chronological events, culminat.ing in ingestion. The number of events is soner,¡hat arbitrary; Edmunrls (I974) considers tr+o evenLs, encounLer and ingestion, while Gerritsen and Strickler (1977) list four, (1) the animals rnust first encounter each other, (2) the predator must recognise the other animal as a potential prey item and attack it, (3)

Ít musl- capture the prey, and (4) it musL successfully ingest tiie prey. It is to be expected that each of the animals in question (i.e. the predator and the prey) is likely to have a set of adaptaLions to change the probabilities in its favour at each of the four steps. As

Dawkl-ns and Krebs (1979) so elegantly suggest, the development of predator-prey interactions can be viewed as a co-evolutionary ttarms racett; predator specÍes evolve more efficient techni-ques for any of the above chronological st.eps and prey species counter r"¡ith defensive adaptations.

Nevertheless, no matter how many arbitrary divisions that the predator-prey interaction is broken down into, the encognter of predator ' with prey is of pararnount imporLance. Obviously, if there is no encounter between predator and prey, no predation can take p1-ace, irrespective of how efficient particular adaptations may be later in the series of events culminating in Íngestion. Ilowever, prey species differ in their defence tactics (see Cloudsley-Thompson 1980; Edmunds 7974). Therefore, predators preying on more than one type of prey often employ a number of various hunting techniques to bring about the all-important encounter, while others employ one method throughout their lives. This, in addition to the fact that the diets of most predators are different 4 from each other point to the exislence of almost ¿ls lnany encounLering techniques as there are types of prey (for: exarnplc Chinery (1979) describes over 250 strategies and techniques). Yet, within the predato::s as a ruhole, two broad modes of predatiotr can be idenLified: rrthose that go forLh and hunt their prey and those thal- sit anci .r¿aitrt

(Edwards 1963).

fn the former category, Curio (1976) describes two techniqties used by predators vi-2. sustained movenent over, sometimes, cons:Lderable '"ho"rr, distances ." for exarnple, from the seemingly random blunclering ínto prey by predatory mites ancl insect larvae (Sandness and McMurtry 1972; Dixon 1959) to the relentless pursuit by African huntiirg dogs (Lvcaon pÍctusl (Estes and Goddard 1967) ancl wolf (Canis !-U=-) (Burkholder 1959; Mech 1970). The seconcl i-s the sudden rush onto a prey (normally over very short distances) vhich is often preceded by a period of stalking (Curio 1976, p.136), during which the predator manoeuvres itself into such a position that it is close enough for rapid assault. Examples of predator:s that often ut.ilize such a technique are the lion (Panthera 1eo) (Schaller 1972), cheetah Acinon jgÞ.tra-) (Eaton 1970) and cuttlefish (Sepia gffl..ifgliq-) (Neill and Cullen re74).

Stalkers are usually capable of at least a short run or a swift dash. lJhich mode of approach is used depends upon the speed or the manner of movement of the prey, with often some subtle properties of the situation as a t,¡ho1e having their influence (see Curio 1976). With a prey that moves slowly, praying manLises (Hollíng 1966), salt:i.cid spiders (Drees L952), and cats (Co1t 1957, p.11+3; Biben 1'979) stalk as 5

unobtrusj-ve1y as possible, thus; nj-nimizing ttre risk of untimely discovery before the final rush or strike" Furthermore, dragoufl.y larvae, lil

even nore slorvly than v¡hen stallcing noving prey (Iloppenheil I964b, quoted in Curio IcIl(¡), presumabl.y because still prey are more lj-ke1y to

deLect rnovenent.

In conlrast, prey rvith a rapid or jerky movemenL, that often

precede sudden disappearance, is approached quickly until within striking distance by all of the above nentioned predaLors. Obviously, the predator is taking a chance, albeit a sma11 one, instead of losing an opportunity. (In reference to the paper by Krebs and Dar+kins

(op.cit.) it is in all probability no chance coincidence that -escapg. tactics may be sírni1ar1y structured, see for example references in

Cloudsley-Thompson 1980; Murj.e 1944; White and l{eeden 1966).

The other principal mode of predation ís often referred to as either, rsiL-and-waitr or rambusht. The difference Ís that, unlike pursuit predators, these predators rely bn the movement of prey to bring about an encount,er, while they remain motionless and often concealed in L,l the surroundings. The Lerrn rambushr used in this context (as any lerþ]al- ,! . \ of predation literature reveals, ê.8. Curio 1976 pp. 138--140) js, I believe, unfortunate. Firstly, there ís a teleological nuance in Lhe

word. Secondly, most references Lo an ambush situation involve/ the I predator ruaiting until the predator comes to rvithin a set distance and then rush:ing the prey in an atternpt to capture it" Thus, the final

component of the prey-capture sequence of a stalkirrg predator is often

via an ambush situatj.on. Yet, in the rsit-and-waitr technique no 6 pursuit is observerl ín the capl-ur:e sequerce, i.e. the predator relies totally on the movement of the prey to br:ing about the encounLer. Therefore, T believe that care should be taken in ttre use of the terms rambushr and rsit-and-waitr and for those predators that do not utÍlize a rapid rush or pursrrÍt, relying, on the preyts movement' we should' standardize and increase the use of the term, rsit-and-r'¡aitr.

Sit-and-wait predaLors can be divided into two distincl- groups, princípa11y on whether or no¡: the predator uses a trap (or snare), for example a spiderrs web or an ant-lion pitfall, which is normally constructed in the external environment. Examples of the trap constructing predators include all of the num;erous web-buildi-ng spirlers (including the orb-web spiders, Argiopidae; the sheet r,reb builders, Linyphiidae; the scaffold r¡eb builders, Theridiidae; and the r+eb or net casting spiders, Dinopidae); the various trap-door and rtrip-wiret spiders that utilize silk; and ant-lion larvae (Myrmeleonidae). Common examples to be found in the non-trapbuilding group include, frorn the invertebrates, l.¡!le-, tiger beetle larvae (CiÇindeli-jdae), scorpion fly Bittacus sp.), c.ertai-n praying mantises (for example, the florr'er mantis, Hymenopus coronatus) manlis flies (Mantisira spp.), crab spiders Thomisidae , and lrater stick insects Ranat-ra ¡ while vertebrate examples Ínclude many of the vipers (for example the Gaboon viper, Ligæ xanthina), the angler físh (Antennariidae, Lophiifornes) and the snapping turtle (Chelvdra serpentina).

As found in the earlier group, much overlap of techniques exisL within one species of predator vrhere, clepending on the prey sought, the predator adopts a differenL prey-capt-uring method. Furthermore, oLher 7 predators, for exarnple the mantid (Pa-raienqþra -ry9lg-EfigngLÞ.) (Inove and Matsura 1983) and the larvae of the v¡ater beetle, Dytiscus verl-ica1ís as reported by Formanor+icz (J982), stvitch between an ambuslt tactic (r,'hen prey density is high) to an act-ive searching and pursuj.t of prey (when density is 1ow). In addition, as Ctrinery (L979, p.2B) correctly points out, there is often no clear-cut clistinction i-n the technique adopted by a predator in capturing the prey, with many animals falling into the grey area between them.

To enable predators to be classified accordj-ng to their predatory techniques, and thus prevent arnbiguity and confusion creeping into the literaLure, thorough details of the predatory behaviour of the animal nust be collected and classifÍed.

The alleviation of such problerns. aside, the merjor significance of behavioural studies of predation is best seen j-n'regard to two particular points: (1) Predators exert a significant selection pressure on their prey species because, in general, the action in terrns of natural selection is unequivocal; the prey is either kíIled or escapes from the attack. Because of this, studies on the natural selection of prey species by predators provide opportunities to observe, both directly and numeríca11y, the efficacy of the anti-predator mechanísms involved (see for example Glasser L979¡ Schall and Pianka 1980; Vermeij L982). (2) Predation has been widely studied in both ecology and ethology. Both of these fields have macle signifícant advances in the recent past, ntost of which, unfortunately have been in isolation frorn each other (see for exarnple Hinde I9B2; Manning 1979; Thorpe 1979). Klopfer (1962) attempted to integrate ecological thinking into B stuclÍes of behaviour but, as Curio (1976) points out, this pioneering work has been largely outdatecl by the cclnspicuous tlp.surge of model building by present ecologists. From these rnodels Curio (1976) suggests thal- ethologísts could profit through i.nspiration for a fresh and evolution-oriented appr:oac.h. Ecologists, on the other hand, may profit by assimílating finclings on behaviour into theír models. These models (for reviews see MacArthur 1972; Emlen 1973) make assunptions on parameters of predatory behaviour, such as search-time, pursuiL-Lime, sel-ection of diets, etc., fror'l r¿hich it is not clear, fron an ethological point of view, how cornplete this list of behavioural parameters is, how they interrelate and interact in reality, and Ín which ways they are affected by the type ancl density of PreI, and the presence of other predators. Holling (1965, L966) atternpted to study one aspect. of predation, i.e. the factors influencing the number of prey captured, by rexperimental component analysisr. The rationale was that a complex behavioural phenomenon such as predation can be broken down into sirnple components. Bach component could be analysed experirnentall-y and the relationships within the component described ¡raLhematically.

The rnathematical relationships could then be synthesized into a model which would, it r+as hoped, describe what goes on in the real wor1d.

The main criticisms levelled at the experimental component analysis is that this approach has been more concerned wj-th determining the existence and effect of behavioural processes than rvith elucidating

the mechanisms underlying each process. However, it is here that ethological studies of preclal-ory behaviour can contríbute greatly. Once the behavioural properti,es of a predator-prey system are more completely knovn, ecological models that make p::edicEions about the systern wi.ll 9 almost certainly gain realism, precision, and perhaps, as Curio (1976) suggests, generality.

At present the extelìt of our knoruledge on predatory behaviour is heavily biased tor+ards those predators that actively search and/or' pursue their prey. Líttle j-s known about eitlrer arnbush or siL-and-wait predators. This is unfortunale, for, as listed above, the sit-and-wait predators constitute a significant proportion of predators in general.

In addition, many of thern (in particular t-.he insects) are known to be beneficial in relation to biological control programs against various pest species (see examples in De Bach 1974).

lrlith the above in rnind, a study r*as initi.ated to exa¡nine the predatory behaviour of a sit-and-r.¡ait predator with one of its conmon prey.

The corunon aquatíc bug, Bqnatra ÈL"pa¡-, that inhabits farm ponds or dams rsas selected for the study, as it was readily available' appeared to be abundant aL most times of the year, nymphs and adults could be easily distinguished and they are large insects making them suitable for behavioural observatÍons and manipulation. The prey animal, Anisops dÇglg!, another water bug, was chosert because it was available in large nunbers, relatively easy to identify and large enough for observation and experimentation.

Originally the program of study rvas designed to examine various behavioural componenLs in the laboratory and then synthesize these with fÍe1d observatj.on and manipulations. However, due to the tine involved 10 in the laboratory experiments, coupled with the fact that R.dispar proved to be a difficult anÍrnal to r^¡ork with in the fiel-d, due to the turbid nature of farrn dams, the study developed into primarÍly a laboratory examinatj-on.

This thesis follows the following pattern. Initially, aspects of the natural history of R.di-spar and to some extent A.deanei, are examined, in particular the temporal and spatial distribution of both species in the field. The behavioural conponents of prey-capture by R.dÍspar are then clescribed and classified, following which the effect of food deprivation and prey-size are exam:lned to observe their effect on arousal and prey-capture. Further, the effect of prey-density and associated encounter-rate are exarnined to determine the overall form of the functional response (i.e. the effect of prey-density on the observed number of prey eaten); the effect of water temperature and developmental stage of both predator and prey arê also examined.

Next, the effect of prey density is examined in relation to (1) various feeding and prey-capturing techniques exhibj-ted by R.dispar and

(2) the sruimming and aggregatíng behaviour of A.deanei; the subsequent effect on prey encounter-rale rvith a sit-and-wait predator was then examined. Finally, the effect of prey density and encounter-rate on the predatorrs duratíon at a foraging site and subsequent rate of movement between thern was investigated in order to identify possible mechanisms involved in prey assessment.

11

2zI THE IREDATOR - Ranatra disp?r

'i 2zl.I Introduction

Ranatra disp-a: Montandon (Nepidae : I{eteroptera) belongs to a very cosmopolitan family of aquatic insects erroneously referred to 1n

much of the literaLure as rr¡aterscorpions. The genus is found worldwide

as Table 2:1 shorvs.

Table 2:1 Caooranhi ca1 di stri l.inn nf thp oonrrs RanaFra

Location Knov¡n No- 0f Soecies.

Europe 1

Australia 3

Anerica T2

Asia 16

. Our scant knowledge of the bíology of Ranatra is príncipally

based on observations and studies carried out on either the European species, R.linearis or the North American species, &¡!usca,the predominant species in each continent respectively. Additional studies have also been undertaken on a few oriental species. Table 222

summarizes an extensive literature search on Ranatra, and lists (1) the area of investigatlon or observation, (2) the species, and (3) the original source. lAtlLt Z:2 SunrmaLy of ¡;ublislrcd rcscorcl¡ rnateriol otì Riìnatra

ÏOPIC OF STUDY SPIC IE5 AUTI'{OR AND YEAR

A. NATUIìAL IIISTONY AND CININAL BIOLOGY.

l. NoLe on Lhe habits oF Ranotra lincaris Ranatra Iirrearis LAKIR, A.c. tB79 2. lhc history of R.lincaris, |.JILM0ÌT I C. lB89 J, Cencral bioÌogy, note on habitaL. I LUCAS, N.J. 1900 4. Note on large numbers of R.linearis Flying. KtRITSCil[NK0, A.H. t9t1 5. CenelaL biology. BUTLERT E.A. 1923 6. Cene¡al biology. LARSDEN, 0. 7916, 1949 7. Cene¡al bioloqy I ARN0|.JD, Br. 1966 B. AquaLic heteroptera from the CerutLi Collection, Sryitzerland I I DEtftIER, 1't. 7911 9. Nev record From Leonr,Spain. LucAS, M.T. & SALGADO, J.M. 1977 ÍIt 10. Nev records from Norvay vÍth note on biology. HANSEN & JACOBSEN I97S tt. Ceneral bioJogy. Ranatra fusca RADINOVSKY, S. 1964 ¡tÍ 12. Neu state reco¡ds Írom l.JashingLon and Faunistical notes. NIESERT N. 7972 ll. As prey ior the notonectid predator, Notonecta lobaLa Í[ ZALoH, F.G. t97B . General 14, biology and AnatonicaL notes. Va¡ious Nth.Amer spp. TORRE-UUEN0, J.R.- 1905b 15. Ceneral biology and Comparative anatomy. ilI[ HUNGERFORD, H.D. 1919 16. Notes on Geographical Distribution !lÍfi û0NS0ULIN, c.T. 1974 17. General bioJogy. Ranatra elonoata RAGr-tUNATltA RAO, T. 1962 lB' DescripLion and habitats [Í BAQAI, I.U. et al. 1977 t9. General bioÌogy. Ranat¡a vicina ; TAI/FIK & AI'ADALLAI,I 1975 20. General - triology. Sanatra HACUENDER, H. t970 Neu 21. records from Arkansas, USA. . Ranatra australis HARP,C;1.& HARP, P. 1980 a) Nev sLaLe records irom Virginia. Íil cRoss, J.L. 1972 Nev record From Bhagalpur, India. Ranatra filiFormis I1ANDAL, B.K. et al. t974 24. Nev record from llortobagy, Hungary, and contribution (?) ü to knoruledge. Ranqtra_ LASZL0, t4. 1917 25. Nev íecorcJs From Kansas, USA. Ranatra 0LDHAI4, T.!/. I97B T 26. Neu oF record Ranatra in the Philippines. Ranatra diminuta P0LHEMUS, J.T. & REISIN, u.K. 7916 Nev records From Arizona, Florida, USA. Africa R.montezume rR. texana, 8. spaLulata P0LHÚ'|USr J.T. I976 tú. Notes and revieu, oF systematic positiorr vith American Ranatra R, compressicollig LANSBURY, I. I914 29. Aquatic heteroptera of Ahmedabad Sanatra spp. VYAS, A.B. et al. 1972 10. A reviery oF lhe Oriental species oF Ranatra Ranatra spp. LANSBURY, I. I972

B. BEHAVIOUR.

l. Eliecb oF vater temperalure on movement. Ranatra linearis KûSICKI, S. 1966 2. Respiratory U trehaviour and activity. CLOAREC, A. I969a ,b. Rythms 3. in respiratory behaviour. I972a Ir. I PosturaÌ variation in forelegs. 1914a 5. Post-moúlt behaviour. l9B0b 6. lhe size oF the binocu.lar zone oF the visual, field in insects. FRANTSTVICH, L. l. & PICftKAT ,' V.E, r916 7. Notes on stridulation behaviour fìanatra Fusca TORRË-BUENO, J.R. l90l

+ B. The reactions of Ranatra to liqht. H0LI'1ES, S.J. 1905 Death 1 9. feigning in Ranatra. lt0Lt,rES, s.J. 1906 i 10. ObservaLions cn young, HOLl4ES, S. J. I 907 Some l¡ I ¡. Jl. reflex responses of B,flusca. ABB0ÌT, C.E. 1940 12. Salinity and temperaLure Ë" tolerance. Eanatra eloqùta SHIRGUR, C.A. & KntALRAI4ANI, T H. I97J ,t I

{ I' ¡ TOPIC OF STUDY SPECIES /\UTHOR AND YtAR

c. tcOL0GY ì, Association viLh aquatic vegetation. Ranatra linealis BUTLER, [.4. I9]B 2, Reduced flight nruscl.es and niqr¿rtion. BRO|JN, E.S. ì951 l. Abundance and energeLics Ín lield, I lr l./AITZRAUER, tl. 1974.1976a&b 4. tJse of in methods oF bio-geographic research 0KLAND, J. 1971 5, Seasonal. flucluations in population. Va¡ious Asian spp. JULKA, J.H. J977 6. Bio-ccological studies. R. filiFo¡mis, R . elonclaLa RAGTIUNAIHA RAo, T.K.1976 on use B conLrol 7. Preliminary observations oi l4alarisl oil, to DESAIT V.R. & aquatíc insecLs in Fish ponds. Ranatlq SPP. RAo, K.J. r972 B. Di¡;tribution and abundanco in desert stream. 8"!" t"a BRUNS, D.A. & 14INCKLEY, I,J.L l9B0 9. Association uith annual temporary ponds in Canada. ll I,,IGGINS, G.B. l9B0

D. REPRODUCTION t. Note on oviposiLion. Ranatra Linearis ENOCH, F. 1900 St¡ucture and funcl-ion of the egg shell. HINTON, H.E. 1961. t, fgg architecture, embryology'and eclosion. C0BDEN, R.H. l96B tt. 0ogenesis, 0G0RZALEK, A. 1974 I 5. Grouth on uiater lily leaves. SAUER, F. 1916 6. NoLe on egg. Ranatra Fusca PETIT, R.H. 1902 7. llatching. Íil DAVIS, C.C. 1r6l_ 8. Life history. hanatra quedridentata IoRRE-BUEN0, J.R. 1906 9, Notes on lile history. Ranatra chirrensis HOFFr.lANN, tt.E. l9l0 I0. Some observations on the life hisLory. Ranatra unicc¡lor. BAN, Y. t9Bt

E . ANAI OI4Y

t, llorphological and histologicdl sLudy of CNS Ranatra linearis CL0AREC, A. 197la tl ü 2. Developmentol changes of eye. t97tb ,'t t. The sensory nervous structure ol the prothoracic leg. It I97Ja ¡'l Variations tn,,rn" ot,,tn" prothoracic leg I. T97Jb 4. "r"u":u"" 5. " II, during nymphal developnent I llorphogenesis. I 1974b lI. Distribution ol the sense organs. I 6. Anatomy and hisLology ol Neuro hemal organ. BAUDRY-PARTIIIAOGLOU, N. I97B 7. Anatomy and physiology Ranatra Fusca LOCY, t,.4. lBB4b B. Ana tomy }IARSI.ÍALL & SEVERIN 1904 o Anatomy of head and thorax I NEIST/ANDIR, C.R. 1926 10. Ttrc P.l.europoclia ll HUSSIY, P.B. 1926 ll. StrucLure and function of the thoracie/abdominal spiracles. PARSONS, L, I972a,b,I97 3,1974 L2. Tonal apparatus Ranatra ctuadridentata T0RRE-BUENO,J.R. Ì905a r). AnaLomy of FernaLe qenitalia. Ranatra CARL0, J.tl.de 7968 14. l4orpho).ogy and anatomy oF alimentary canal Ranatra chinensis CH0TJDHURY ¿( BEGUM 1974 t5. Ilr-rrphology and secretions of the salivary glands. Ranatra filiformis QUAYUH, I'1. A. !915 ,' t6. Comparative anatomy of the spiracles. I I BHATNAGAR, B.S. I97B I7. Structure ol the compound eye. Ranatra seqreqa PILLERANO' G.N. ¿{ I CARL0, J. 1977 1 lB. l,licroscopic anatomy of maxillary gLands. CARL0' J.14. ¿, ¡ì PILLIRANO, G.N. 1980 19. ïlroracic structu¡aI myology. Ranat!e eþnqeþ DESHPANDI, S,B. 1980 l, ,t- 20. I'lalpighian Lal¡ules. SHUKLA, G.S. & $ GOPAL, K. l980 I 21. Atlaptive FeaLure of leg structure. Ranatra (?) PUCHKOVA, L.V. t97B I

4 r l TOPIC OF STUDY SPIC IES AUIHOR AND YEAR

F. PHYSIOLOGY l. Inqestion rote. S"nCUSl_.-"""r.i!. CL0AREC, A. t975 2. Fíne structure and Function ol the chloride colls. I Kot'lNICK, H. 1916 J. Natural diet - Analysis by Electrophonesis f GILLER, P.S. 19g2 4. PuÌsaLing organs Renai:rn fusca LOCY, l.J.A. lBB4a 5. Proline metablism. RanaLra Filiformis stNGH, J. 1978 6. Histochemical. observations on ths Neuro endocrine conplex. FARUQUI, S.A. 1917 7. Phospho-lipid content in Malpigian tubules and haemoLymph. l4AH01.1tD, U.V. t977

C. PREDATORY BTHAVIOUR l. As voracious predator. Ranatro linearis stJA¡lMERDAf.t, J. t937 2. ALtacking small Fish. Ílr 0tlERoDr E.A. IBTB l. Predator on mosquito larvae (precipitin test). Ranatra fusca BR00KE , t'l. t4. & PROSKE, H.0. 1946 4. Descriptive study. Ranatra linearis CL0AREC, A. 1969b 5. Inberactions betureen diffe¡ent receptors. Ílt L916 6. Estimation of hi.t distance. Itr t979 7. Ontogeny ol hit distance estimation. " l98oa 8. EFfect on pred. performance of prey after moulting. " 198r 9. EFFect on pred. performance of prLfV ofLer deprivaLion. I l982a ',1 " 10. RoIe oF visual and mechanical stimulalion after moulting. lt I " 1982b t' ll. Deprivation of prey during development. ' l98l 12. Density dependent prey oize selection I BLoIS & CLoAREC, A, l9|l lf. Predation on lish populations in S.t'J. Sueden. It ERIKSS0N, 14. EggL. 1977 J4. Nocturnal rythm in predatory behaviour. Ranatra montezuma BLINN, D.l/. e!_ e.!. 1962

il ï

,¡

t

L 'f 1

; $' {: I

,t I {

"{ lr .t J 72

Studies on the Äustralian species, Rana_tJa -ü.spêr_, are restricted to taxonomy :- R.dispar Montandon , 1903; Lundblad, 1933; Hinton, 1961, L962 (Structure of egg and key to eggs); Menke and Stange, L964i Lansbury,

7972. R.di.spar var. longicollis Montandon , 1903; Lundblad, 1933; Menlce and Stange, 1964.

R. australiensi-s Ha1e, 1924 syn.n,; Menke and Stange, l96t+; Lansbury,

1967.

(after Lansbury, 1972)

The paper by Hale (L924) contained some brief notes on natural history, being prirnarily concerned with a taxonomic description. f have been unable to trace any addÍtional published works on any aspects of the biology of R.díspar. Therefore , because of the depauperate state of our

knowledge on even basic biological information concerning R" dispar, whenever possíble observations and notes were collected on various

aspects of the biology of this ambush predator. The following secLions

summarize these observations, obtained from both the laboratory and fie1d. Aspects of the temporal and spatial distribution of all developmental stage s of R.clispar in the field are gÍven in Section 223.

221.2 0bservations on General Biologv A. Reproductive behaviour.

I 1 (i) Mating: Copulation was observed on a number of occasions in the l

i laboratory, but never in the field.It took place usually between I il T 1700 1900 (i"e. torvards dusk) during the Spring .t and hours

J (September to November) and Sununer (December to Fetrruary), t : rnonths. --{

I

I 13

Adult mal-es have been observed to approach the female from belorv as rve1l as from above, normally while Lhe female is clinging to a subnerged object (e.g. water plant stems or twÍgs). 0n several occasiotrs, when the male approaches from above and j-n

front of the female, the male was observed to move sloruly, extending íts prothoracic legs r+hlch in turn grasped the femalers prothoracÍc legs, about the tibio-tarsí/femur, thus locklng the lclawf onto the fennur, The male would then manoeuvre itself to the side of the female, adjusting its grip on the proLhoracic tegs, until Ít was positioned to the side of and slightly belorr the female. The malets meta- and meso-thoracic legs are such that they either grasped the fernale or remained in contact wiLh the submerged object on whích the fernale was clinging. Norrnally the former was observecl however, with the male rhookingr íts meso-metathoracic legs over the body and legs of the female.

Often, the female v¡ou1d swirn away fron thê site r,'ith the male

still clinging to her body. trlhen positioned to the side (either

right or left) of the female the malers prothoracic 1eg was inclined upwards and hooked over the thorax (pronotum) of the fernale. The dorsal aspect of the posterlor portÍon of his

abdomen faced the ventral part of the posterior ."gio., of the

female. The ventrally situated aedeagus is then curved upwards (the two halves of the malets respiratory siphon (filaments) being separated near the base to permit this) to a dorsal position, where it is clasped by the female genitaf sclerites. The female genitalia were observed to move spasmodically

I t before/unti1 copula took p1ace. Transfer of sperm persumably T4

takes place at this time. Adults renained in copula for beti.'ee¡

3 to 37 minut-es (x = 1.7.1, SD = 11.7, Range 3 to 37 mins, n = 10), vrith up t.o Lhree separate copirlaLions being observed to occur within a one houi: period. Follorri.ng nal-ing the anirnals separate with l-he female norrnally rnoving of f , either swimmiig ro ttre bottom of the tanlc or to other plant naterial.

(íi) OvipositÍon Behaviour. Eggs were laíd betrrcen 24 and 48 hours afLer matíng \{as seen to occur at a water temperature of 25.5 .f 1.5oC. The fertílized female rselectst a suiLable pÌece of submerged vegetation, for example the stem of water planLs or subrnerged 1eaf, and grasps it r¡ith her second and thírd pair of legs, her head being lower than the abdomen. The head is raised abouL 2 crn vertically away from the stem with the tip of the

ventral surface of the abdomen pressed against the plant. The

prothoracic legs are held away from the head, normally at an angle of about 45o to the longitudínal body axis. As the

ovipositor is extended the prothoracic legs are lo',"ered to come

to lie Ín the same line as the body being held flat l-ogether (similar to the rleg extensj.onr phase described in Chapter 3).

The sarv toothed oviposit.or is t,hen slowly drlven int.o the plant by downward, backward and forward movements, while the respiratory filaments rest along the p1ant. AfLer piercing the

plant tissue the ovípositor is partially rvithdra\\¡n, opened

laterally and the egg is extruded and forced into the hol-e. The two long respiratory filaments of the egg spring apart in a tvt shaped fashion as the ovipositor is withdrawn. The female then moves vertically down the plant, maintaini-ng contact with the 15

ventral tip of the abdomen, about 4 to 6 mn, and repeats the hole borlng/egg laying process. Up to 20 eggs have been observed to be laid in one period, (see PIaLe Ic).

B. Life Cycle Stages (i) The colour of the egg is pale ye11ow to whj-te rrÍth the micropylar region being dark brown. The trvo respiratory filaments of the

egg are pale yellow to amber becoming darker towards the base.

Size of the egg is shown in Table 2:3.

lAIlT,E 2:3 Size of egg of R.dispar.

Ae Removed from FollowÍng 24 hrs Fenale in water

Length (mrn) 3.325 ro 3.55. 3.50 to 3.755 Greatest diameter (mrn) 0.85 to 0"97 0.95 ro 1.11

Respiratory filament (nn) 2.22 to 2.55 No observable change

During developrnent the fertilized .gg continues to increase in

size before hatching while unfertilízed eggs do not. About 5 days later (at 25 + 1.0oC) two pinlcish spots appear near the extremlty of the anterior end (location of respiratory filament),

these being the developing eyes, while one day later an area of a lÍght. orange tinge appears in this area. As development

continues the area darkens until finally is distributed over Lhe

ventral surface of the ovum. 16

(ii.) Eclosion : During t-he last da¡'s .¡ incubation the egg becomes bulged at the cephalic end unti-l finally rupture of the chorion

takes place. The split al-rvays occurred in about the same place, occurring posterior to Lhe end of the egg on the dorsal surface, spreading round Lo the slde. A membrane (possibly two) was the

first object to appear, pushirrg the partÍally detached toperculumt or cap of the egg, wit-h the attached respi-ratory

fílaments, towards the ventral surface of the egg. The membrane

is atLachetl to a protuberance on the vertex.of the hatching nyrnph

that apparently also presses agaínst the operculum, affordi_ng additional rliftr as the head emerges from the egg. After emer8ence of the anterior part of the head, emergence temporarily

ceases wlrile the head is observed to nove rythmically back and forth rviLh a pumping action. Following about 3 to 5 nínutes of

this movement the merubrane(s) burst rapidly and the head pushes

completely out of the egg, followed by active bending movements of the still- encased body segments. After about 2 to 5 minutes of thís movement the appendages become freed by the continued effort of observed nuscular movements wÍthin the head and the bending/pushing rnovements of the body (Davis 1961, attributes the muscular movements within the head to be associated with the rdrinkingt of water by the nymph Lo increase the body volume. r am unable to substantiate his conclusions wiLh my available observatÍons). Complete eclosion j-s achieved with the aicl of the liberated legs pulling the rest of the body free of the egg case. The complete eclosion process takes between 10 to 15 nínutes. L7

The newly hatched rryrnph remains notionless for up to 1 rilinute

af l-er hatching, suppor:Led by any available plant or subrnerged malerial" I{ithin a fei,' niuutes however, it r-alces up the characterÍstic head-dorm prey capturing posit-ion as descr:Lberl in

detail for adul-ts in chapter 3. osLrocod prey have beer-r obderved

to be caughL within 60 to 80 minr¡tes follor,¡íng eclosion. The

incubation period lasts betrseen 7 days at 2g.5 + 2.5oc to 14 days

at 20 + 1-5oc (n = 10 i'each case). No eggs hatched v¡hen maintained at 15 + 1.0oC.

(iíi) Duration of rnstar stages : Following egg hatch there are five nymphal stages. Hale (1924) has provided a detailed cornparatÍve account of the rnorphology of each of ttre lnstars a¡d therefore

following a routine comparison, the results of which n¡ere almost

identical to Halers, no additional descri-ptions seemed necessary. However, where addi-tional work dicl seem warranted was in ascertaining the duration of each stad:'-run at both constant (laboraLory) and fluctuati.ng (fielcl) temperaLures. For laboratory observations, animals ryere housed individually in

either smal1 (250 m1) or large (1 litre) plasr,ic beakers

containing water to a depth of 3.0 cm ancl l-0.0 cm respecLively.

Each bealcer was provri-ded wíth aquatic plant materj-al and. an

appropriate piece of cocktail stick for support. Each day animals r+ere fed either a nixture of ostrocods and small- (ínstar

1 and 2) notonectids for predator lnstars r and rr, or a mixt.ure

of larger notonectids for instars III to V, in sufficient nurnbers to maintain e4 libum condi.tlons, in additÍon to checking for

moulting actì-vity. Iror field observati_ons ani-mals were 1B

naintaíned in 2 large childrenrs plast-ic paddling pools (2.0 x 2.0 x 0"30 ruetres) the bottom of each be-Lng covererl in a sloping tbedt of pond rnud (wi-th associaled pelophil-e organisrns) to produce a graded vater depth pr:ofile o:f 5 to 20 cm. The pools

had been set up 12 months previously and supported rnotlerate

breeding populations of var:Lous nolorrectid speci-es and ostrocods

in addition Lo various transient visíLors (for exarnpJ.e zygoptera,

coleopi:era, Diptera). va::ious aquatic plants hacl also been established along r,¡ith their various phytorheoptrile populat-ions.

During oÏ¡servatÍon periods Lhe populations of ost.rococls arrfl small

notonectid instars were supplemented handsomely z to 3 times per week. Both pools rrere covered by a tent of shacle cloth (sarlon ,BoT") Lrr¡o meLres above, but uere opened on trço sides (North and. south) which prevented localized heatíng but permittecl normal air circulat.ion and rain fall. The temperature regime of both pools tras very sÍmilar Lo that of a rnoniLored farm pond for c.omparable water depth, although the rat,e of both increase anci decrease of temperature in the morning and evening was moclerat-e1y faster in the pools.

R.dispar individuals h¡ere individually housed in eÍther sma1l (7 x 5 x 5 cm deep) or large (10 x 10 x 15 cm cleep) cages for instars r and rr, and rrr to v respectively. The cages were consLructed from a wire frame and covered with either a coarse nylon netting (small cages) or a rvire gauze (lar:ge cages). This permitted the passage of r+ater and prey animals inLo the cages bul prevenLed the escape of the predators. In addirion, rvhen possible, cages were supplemented wj-th freshly caught prey of the 19

appropriate type. tr{ater and air temperatures were recorded each hour by lray of a Lhermistor wÍrecr 't to a calibrated Rustralc

I recorder. Each predator was checked tr¡ice daily; early each

morning and late afternoon for signs of noulting or an obvious

absence of prey" Table 2:4 shor+s the observed durations foi each insLar, (see P-Late la).

(iv) Moulting. Anlmals r+ere observecl not to feed at least 24 hours before noultì-ng, and in most cases there was a significanL reduction in Lhe number of prey eaten up to /rg hours before

rnoulting. Prior to moulting the indi.vidual becomes cornpletely

ínactive, unresponsive to prey (natural or model) and sonewhat enlarged. As moulting begins, the skin splits first just posterior to the head on the dorsal surface. The split increases

Ín length along the pronotum and rernainder of the thorax as shown in !-ig . 2',r. As the new individual moves'forward out of the

exuvi-a the legs remain pressed agaínst the body. The body moves

in a rhyLhmic, pulsating fashion ruith tperistalticr waves of musculature (?) movenent passing down the body as the anirnal slowly extricates itself. üIhen the thorax and the majority of

the abdomen are out of the exuvia the legs are freed and are used

1n the fi.nal part of the extraction process. when completely free of the exuvia the nyrnph, which is pale cream wÍth a tÍnge of ye11ow in colour, remains stationary for about 3 rninutes, before

swirnmlng and coming to rest on a suítable piece of sub¡nerged

vegetation on which i-t remains stationary while Lhe

scleroLization and melanization of the culicle takes p1ace.

Althorrgh the predator performed various movements rvith the Fiq. ?-zI The sl-ages of the rnoull-inc; p rocess of R. dispar as depicLed in pì.an and lateraf vievs.

(1) to58 h

(2) l1o2 h L l )_ J /' Ll (l) ito4 h

Ga) l1o7 h

(4) lloe h

(4a) lll-r tr

(5) ttt5 h

(6) rr22 t't See text for details. (r)

(2al

(2)

(3a) (3)

(4) (¡l¡)

(c)

(5) TABLE 2:4 The rnean (1 Std.Dev.) duration (days) of cach inetar qf 8:9j-æ.gf rshen maintained eb either (A) Constant: laboratory or (B) iluctuating semi-naLural uater temperatures (see text For deLails).

I.,IATER TEMPERATURT (OC) Il,II.lATURE STAGE 15 1 r.5 20 + I.0 25 + I.g 30 g I.5 (A)

+ I I2.B + 6 .B 6.0 3 4.0 3.0a0 2,7 + l.I

(BL : 8.3 mm) (l to zz )a 5 (2 to 14) 5 (0) 5 (2Lo5)5

II I5.0 a J.0 7.6 a J.I 7,5 ¡ t.3 6.6 a I.3 (BL = l) to tl.2 nun) (l to 3l) 4ob,18c (4 to 14) 40, ,B (6 to 9) 40, J6 (6 to 9) 40, 27

III ALL DIED I0.6 3 3.7 9.2 a L6 I2.3 ¡ 2.5 (BL (l (8 (10 = I7.2 to 20 n¡m) to 14) lB, lI Lo 12) 36, 34 to 15) 27, I '.

IV 16.2 ¡ 5 .9 19.7 ¿ 1.5 2O.4 al.2 \ (BL = 24.5 to l0 mm) O te ?z) Jr, 25 (I9 to 22) 34, 24 (I8 to 24) 8, 4

V lB.0 a 4.7 10.0 + 1.0 ALL DITD (BL (L6 = 19.5 to 41.5 mm (14 Lo 23) 25, 17 to 20) 24, r7

TEHPTRATURE (oC) DIJRATION (daye) .]r (B) RANGE Y SD RANGE SD N

I 17.0 to 2.5 ZO.t 2.4 StoB 5.2 I.7 9

II I7.5 Lo ?5,5 22,I 3.6 9tol2 10. t r.t D

III l7.O Lo 2I.2 2I.? 4.4 5to15 IO.2 l.t 9 IV I7.0 to l0 ?2.7 5.9 L2 lo 16 t3.6 1.5 2I v lB.0 to l0 24 6.t 17 to 20 l0 1.1 79

s. Number ln parenthesis is the range b. Number oF animaLs tested c. Number oF animal.s that successiully moulted and used in calculating the mean. BL. Body length ol instar exeluding rosl-rum and respiratory siphon. + The small sample size (5) used ís due to lack of ferlilized eggs. 20

prothoracic legs for a nunber of hours following rnoulting ro prey

were observed to be caught until. 6 hours lnd e1-apsed since the

moult. Cloarec (1981, I9B2a, 1982b) has shorqn thaL Llìe flrst 4 hours following a moult are crucial Ín the subsequent predatory performance of nJinçarfs., tlr.e European species. Predators that

noulted in the presence of prey performed more prothoracic 1eg

movenents and shoryed a higher subsequent strike efficiency than those indlviduals that moulted in the absence of prey. Time did not perrnit the required observatíons and experiments to see if

R.dispar exhibited the sane de pendence of presence of prey at moulting. Following moultlng, R.díspar were observed to perform

numerous prothoracic leg movemenls however.

C. Feeding Behavlour

(i) Range of Prey Types taken by R.dispar. Ranatra has developed a

reputation from the earlÍest literature for beJ-ng amazingly

voracior"rs. A good example is thaL of Swammerdam (I737)z- ttThere is not perhaps in all the animal creatlon so outrageous or fierce a creature against those weaker than itself than the water

scorplon. It destroys like a wolf among sheep, twenty times as

many as its hunger requires. I have often seen one of these when

put into a basin of water in which were 30 or 40 of the r,¡orms of the niddle Libella, whlch are at least as large as itself, destroy them all in a few minutes.tt Such dramatlc descriptlons 2L

rspectacularr aslde, Sgnqæ. does capture (often in a fashion) and consume a large array of both invertebrate and vertebrate prey. Chapter 3 presents a detailecl account of the significant effect thaL both the internal moLivatÍona1 state (i.e. hunger) of the predator and the sÍze of the prey have in the prey capturl-ng behaviour of R.dispar. hlhat T propose Lo do in thís section however is to present additional records of observatir:ns on the range of prey types taken (in both the field and laboratory) and predatory behavÍour.

The range of prey l-aken by all instars of R.dispar is rsj-de in regards to both prey size and type. First lnstars capture prey ranglng in size frorn less Lhan l rnn (srnall ostrocods) to 1.5 cm ( sma1l tadpoles ) in length, with a body mass of some 3 to 4 tir¡es

(small ta

hlhen capturl-ng larger prey it is not unconmon for the smallest lnstars to be pulled from their anbush support and pulled round by the srvimmlng prey, often for a consíderable period, before being able to locate a suitable intersegmental membrane and inject the venom/enzymes into Lhe prey. All instars are capable of capturing prey that are on the water surface, either terrestial 1n orÍ-gln or semi-aquatic, surface f11m dv¡ellers. To do this the predator positi.ons itself in such a way that on strÍking it can brealc through the wat,er surface and thus capture 22

ttre prey. This ls in t,urn pulled beneath the r¿ater surface ruhere feeding talces place. Table 2:5 presents a list of various prey Lypes that were either observed in the field as they were being

fed on, or presented to the predator in the laboratory. 0n1y those prey that were subsequently fecl on are listed.

(iÍ) Cannibalism : All stages of R.4iÊpar have been observed to cannibalise in both the laboratory and fie1d. Table 2:6

summarizes the stages that either feed on or are fed on as observed in the laboratory. All stages are, suscepLlble to cannlbalísn follorving moulting. This is especially noticeable

for the larger instars that, followÍng a moull-, require up to 4 hours before sclerotlzatlon and melanization of the cuticle

render it hard enough to apparently prevent cannibalisn by

smaller instars. It v¡as not uncommon to observe, in the

laboratory, up to 3 smaller fnstars (e"g. II) all feeding on a newly noulted indivÍdual (e.g. V or adult), although I have not observed thís phenomenum in the field. Radinovsky (196¿+) suggests that the observed tendency in R.fusca for the eggs to hatch in darkness, during the evenlng or early morning, is adaptÍve in that. it offers sone protection to the newly emerged firsL lnstars from older siblings during those flrsL critical hours. !/cre seen to be fed ÍABLE 2:5 Summary oF the variou8 prcy l-ypes (bol-h aquaLic ând terDestiaL) that on by R.clispar in either the laboratory or tlre tield'

LABORATORY F IILD

I II III IV A II III IV V

AQUATIC PREI A. INVERTEBRATI-S ANNIL IDA Branchiura sP. + + + + + + + + lmbr.&gltls. sP. + + + +

ARANEAE + + + !919¡9d"9 sp. + +

ACAR INA Hydracarina

CRUSIACEA + * * + * 0sLracoda + + + + + + + + t * Daphnia spp. + + + + + + + + + * * * + * AusLrochiltonia + + + + Artemia salina + + + + + + * + + + * Cyclopoida + Paratya sPP. + + + 14aerobrachium +

INSECTA

EPIIEI'IEROPTERA + + + * + + + + + + * + !!Eo¡. "p. ODONATA * + + + + Zygop tera + + + + + + + + * + + AnisopLera + + +

HEI,IIPTERA + + Anisops deanei + + + + + + + + + + * { + * + + Anisops dorís + + + + + + + * * * * + EniLhares berqrothi + + + + + + + + + * + + + + + + !1-d"g¡elg "p + * * * + r t + * Esgglg sP' + * + * t + + * + Gerridae + + + + + + + * * * * Laccotrephes t¡istis + + + + + Ranatra dispar + + + + + + + + + + {- + + + + + + + * + + + $ge.r.g =p. * * + t * Micronecta sp' + + + + + + +

DI PTERA * + + * * * + * + !h""!gly-:- * + + + * * + + * ¡!9!¡iÞ sP. + + + + + + * + * * * * CuLex sp. + + + + + + + * * * * '* Aedes sp. + + + + + + + * + + + + Chi ronomidae + + + + + +

TRICIlOPTERA Leptoceridae (+ case) Leptoceridae (- casc)

COLEOPTERA CvÞister tripunctatus (A) + + * * * * sP (L) + + + + + + * * .ÇLÞ,is!gt * * + + + * + * * * !g$!_"n_9ottu-9. sp. + t + * * Hydrophilidae + + * * * Gyrinidae (A) + LABORATORY FIELD

I II III IV V A II III IV v A B, VERTEBRA'IES

ANURR. (Larvae) * t * + LimnocJynasLes tasmaníensis + + + + + + * * * + LiLoria (?) + + + + + +

P ISCES (Mosquito l"ish) # * Gambusia affinis + + + + +

(Guppy ) Poecilie reticulaLa + + + + + +

TERRESTIAL INSECTS

BLAÏTODEA Peripleneta americana + + + + + + Blattella qermania + + + + + + Çalolampra irrorata + + + + + + + l4antidae nymph

ORTHOPTTRA * * t leleoqryllus (N) + + + + + + * feleoorvl lus (A) + + * i + + * Flòcrotona sp. (A) + + + + + * + + + * t + Atracl-omorpha sp. (A) * * * * + + +

HETEROPÏERA * + Lygaeidae (N) * + + + * Lygaeidae (A) + * * + + Nabidae (A) * t + + + + * * l4iridae (N) * + * + Miridae (A) + * * + + +

COLEOPTERA Lycidae ( Met riorrhynchus -@eAT + Elateridae (A) + Tenebrio nrotitor (L) + + + + + +

DIPTERA Tipulidae + + + + + Ð"phu=. "p. Tabanidae + Asilidae + Drosophi la + + + + + + + + * * 4 + cilia cu ln (L) + + + + + + * * * * * + LuciLia cuprina (A) + + + + + + * * + * + + Callíphora (A) + + + + + + t I f + + + .@. spp. (n) + + + + + + * t + * + + 0thers + +

HYI4ENOPTERA Formicidae Myrmecia + + Ichneumonidae + + Pompiliclae + Sphecidae + + + Thynnidae + + + Vespidae + + Apis meìIiFera + + +

0 Lhe rs + + + + +

(*) 0bserved being fed on (+) Lab: not offered

(-) Oflered but NOT fed on (*) Fíeld¡ likely as prey 23

TÀÞLB ZL6_ The íncidence of cannibalísrn by R.Dispar on varlous developmental stages observed in the laboratory or field.

n."¿re p.a_q stage acting as Prey

Acting as Predator I II III ÏV v A 2 ï + t- + ¿ +2 +

II + + +

III + + + + + +

1 IV + + + + + +

1 1 v + + + + + +

1 A + + +I + + +

I Observed 1n field.

2 Mo"" than I predator has been observed feeding on the same individual at same time .

D. Natural Enemies (í) Predators : Apart.from conspecifics the principal invertebrate predators appear to be Odonata and Coleoptera. Observations in

the field revealed on two occasions Anisoptera nymphs feeding on a II and III instar, while a Zygoptera nynph was seen to capture

and consume a I insLar. In the laboratory both AnisopLera and

Zygoptera nyrnphs have been observed feeding on instars I to ÏV.

ïn arlditÍon adults of the dytiscid beetle, Cybister tripunctatus, capture and feed on insLars f to V, and probably adults although 24

I have noL observed this. I conducted no feeding trials using vertebrate preclators. I did observe, h.ovever, a white-faced Heron, Ardea nqVaçlg]lggdaag., talce either a V instar or adult

R.dispar on one occasion during field observations.

(ii) ParasÍEes: R.disle!, like a rrumber of other acluatic fteteroptera, have been observed to ofLen carry conslderable nunbers of epizoic Hydrachnid water mites. I have observed all stages of R.dispar to have these external parasites, carryl-ng populations of 2 to 40 individuals.

Following moulting, R.dispar is susceptible to invasion by

various hydronycete fungi. In the laboratory this can prove Lo

be fatal. Ilolever, whether this is a natural occurrence I am

unsure, although dead R.dispar collected in the field inevitabl v show the presence of fungal hyphae. Follor,'ing overr¿intering ln particular, adult R.dÍspar: often

have a rlch growth of algae over their body. ThÍs may be

particularly thick along the respiraLory filaments and 1egs. Hale (1924) reported that one adult R.dispar he collected had rra mass of SpiroRvra attached to and growing frorn the base of the (respÍratory) filamentsrf ... âûd that iL ttrendered these organs wholly non-functíona1 by forcJ-ng thern apart at the base; notwlthstanding this dlsability, the bugs appeared quite healthy .tt I have also observed R"dispar to have , in some cases, nasses of Spi.rogyra atl-ached to varíous regions of the body. For sma1l instars encounLering and becoming entangled in this filamentous algae can prove fatal. 2.5

2:1"3 Maintenance of _R"digpar !n thq laboratory ALternpLs to maintain an adequate breeclÍng culture of R.diqpql dÍd not prove possible, therefore in the vast najority of cases animals r+ere col-lected from field locations and maj-ntained in the laboralory as outlined below.

Following the final moulE, adults r,rere separated by sex and placed into 4 large plastic tubes (60 x 36 x 52 crn deep ) filled to a depth of 50 cn rvith aged, dechlorlnated tap water. Each tank had an abundance of stems of PlUagrnitçå ¡¿ suspended from the sfde wal1s and across the top onto which the Ranatra settled, Between 30 and 50 adults were housed in each tank and they were fed 4 tines weekly a collection of notonectids, corixids and rnosquito larvae, tadpoles, and young guppies. Usually sêvera1 hundred prey vlere added to each tanlc per feedlng (except. tadpoles and fish). In addition, tv¡o tanks r,rere set up and mainLained containing both ruales and females. Frcn these the feru fertllÍzed eggs used in described sLudíes rqere obtained over a 3 . 5 year period. The instars of R.dispar are easy to identlfy because of the non-overlapping size range. Individual instars collected frorn the field were identified and, depending on the insLar, transferred to a suitable container (see Table 227 for list). PLATE ]

(A) From l-efl- to right : Instars I to V and

adLrlt. female of R,dispar. (Scal-e l¡ar i cm).

(B) Adult A*{ææ:.

Scalc bar 5 rnm. $__ "--__.!

(C) Eggs (one arroryed) of' R.dispar in aquatic vegel-ation.

(D) Leg extension behaviour. ol' R.dis¡rar

(See Chapter l: J f or clescri pl-ion ) . o

(,^ -ã+| u^ l+

./ Jt

lrltì -l*= +o iq

-4 -û

ì 26

TABLE 2:7 Size of contalner used to hold various inslars of R.dispar in the laboratory.

INSTÄR CONTAINER DESCRIPTION

I Plastic trays (34x22x6cm), I,rIater depth 2 cm.

II Plastic trays (34 x 22 x 6 cn), Wat.er depth 4 cn.

III Plastic trays (30 x 20 x 10cn), trüater depth I crn.

IV As IïI above v Plastic tubs (53 x 35 x 20crn), hlater depth l8cm.

Ad Plastic tubs (60 x 36 x 50cm), trrlater depth 48cm.

Each container had adequate plant or cocktail stick supports for the @-qrand animals k¡ere fed mixLures of suitably sized prey as shor¡n 1n Table 2:B every second day.

TABLE 2:8 Types of prey given to each R.dispar during maintenance.

AR

I Ostrocads, srnall notonectid and corixid nynphs, smal1 mosquiLo larvae.

il As ï above. III Notonectids, corJ-xids, mosquito larvae. ïv Notonectids, corixids, moscluito larvae, various aquatÍc insects as available. v As IV above plus tadpoles and young gupples.

Adults As V above.

A1l- contaíners were housed in a cubicle w-ithin a solar central 27

heat,ed insectary bul1ding. Tanks rrere arranged ouL of direct

sunli.glit but bhe adec¡uate glass area of the roon nteant no additional lighting was required. The air tenperature ranged from 17 to 30oC in \ùinter and 23 to 35oC irr Summer. Thís

produced a range of water temperatures fron 17 to 24oC and.20 uo

30oC in hlinter and Surumer respecLively.

222 THE PREY : Aqisgp€_ de_anei 2¿2.L Introduction Anisops deanei Brooks (Notonectidae : Hetenoptera), as do the nepids, belongs to a world wirJe family of aquatic bugs (Brooks 1951).

Members of thls family are easily recognizable because of the active swlnming movements whilst they are upside-down (hence Lhel-r common name

rbackswírnmersr). They have a pronounced convexly keeled dorsal surface, a feature related to their method of swinming while the abdo¡nen has a mid-ventral keel, with a hydrofuge hair- covered grove on each side, for trapping air. Prey consists of srnall submerged animals, including mosquito larvae and Daphnie spp., rvhÌle the larger indivi

tadpoles and smal1 fish (Bllis and Borrlen L97O; Giller 1980; see references 1n McCormick and Polis I9B2). Cannibalisrn has also been reported to occur (for example Fox 7975a).

The cosmopolitan genus Anisops is represented 1n Australia by 23 species (Lansbury 1969). They are very commonly found in ponds, farm dams, sma1l freshwater lakes, creeks and slow lowland rlvers the water bodies not needing to persisl for all the year. Sweeney (1965) has published an account of the geographícal distribul-ion in S.E. Ausbralia, 28

to which Lansbury (1969) has added additional reports.

At present, as far as I can asce-¡:taln frorn the lÍterature, our

knowledge of A.deanei is restricted to taxonomic studies (Brooks 1951; HaÍe L923; LansburyI964, 1969 )and tvo unpublished honours thesis

concerning studies on a po1-ymorphlc backsr'¡-inmer, Aniqgpg thierenanní

Lundbland , later found to have been nisnamed ,,(Lapworl-h L97B) and prey I size selectlon on various morphs of DapLqia (Reynolds 1980 ).

Lansbury (1964) pointed ouL that A.- deanei is very similar to

A.hypelionrand only nales can be separaterl with certaj-nty. Care was

taken during behavioural observations for any signs of sr¡imming or other

abnormality of prey anÍmals. ltparL f ro¡n the nat'ural variatÍon lro m¿rrked behavioural differences were seen. Therefore, for thís thesis the Ânisons species used has been regarded as AJleanei, (see Plate lb).

Because of the time constraints, no additional laboratory observations on general biology vrere carried out on A.deanei. Hale

(L924) briefly reported some general naLural history observatÍons he made on the closely related specie s A"hyperíon. The results of a field censufing program to identify the temporal and spatial dlstribution of ¿rdeege! 1n the field are presentecl in Section 2:3.

2:2.2 Ì,laintenance of A.deane.L in the Laboratogr Because the nature of the experiments often required la::ge

numbers of varlous size classes of A"deanei. to be available aL any one

time, attempts were made to develop and maintain a culturj-ng program.

Unfortunately, apart from some minor success, the program proved 29 unsatisfacLory and l-herefore anirnals r¿ere collect.ed at regular intervals from 1ocal fleld populatlons and maintained through the different instars in the laboratory. Different A.deanei- instars (hereafter referred to as Size Classes) rvere kepü in B large plastic tubs (53 x 35 x 2O cm deep ) that rvere arranged on a large stand by a windort but'out of direct sunlight. The top of the staud supported a I-arge (150 litre) water reservoir tank whlch in turn ttrickler fed each of the holding tanks through a system of pÍpes at a rate of 100 to 120 m1 per hour"

Two overfl.oru pi.pes per tank, connected to the rrvasl-e plumbingr of the stand, maintalned a constant Ieve1 of 18 cn in the tanlcs. As anímals. moult.ed they were neLted and transferred Lo the nexL appropriate tank (there were 2 tanks per size class 1 to 4). As animals of a parti.cular size cl-ass \,¡ere requÍrerl they were removed, acclimatized to the experimental conditions and used. Adult A.deanei were maintaí.ned ín separate 4 1-arge plastic tubs (60 x35 x 52 cn deep) 1n the same room as the stand but not connected to ít. All water us'ed r¿as filtered, dechlorinated tap waLer either dírectly from Lhe maÍns (for reservoir tank) or from 200 lítre plastic drums l-eft to stand for 48 hours.

Ani.nal-s were fed every second day a variety of different aquatic organisms as they v¡ere avallable, as shown in Tabl,e 229. 30

T43Lq ¿:i. Types of prey glven to

s.rzE CLASS ru. MrxtrrRE I Ostracods, sma1l mosquito larvae, DqphnÍa

2 As above plus sma11 corixid n¡'mphs.

3 As above, larger rnosquito larvae.

4 As above, sma1l notonectíds.

5 As above, other notonectld spp.

All tanlcs were housed in the same room as R.dispar described in Section 22L.3. Because of the rapid growth and thus short duration of the juvenile Â.cleanei, especlal-ly Lhe smallest si.ze classes, ln Summer, the running water tanks provided cooler.conditions that resulted in extending the duration of each slze class by up to 8 to 10 days. This provided extra time for experiments during the summer when juveniles were available from the field. In Summer the running water temperature rarel-y got above 20 to 22oC conpared with the stl1l water tanks that would reach 3OoC.

222.3 Collectine Techniques and Initial Identifí cation and Size Gradin&

Animals were hand-netted from llield locatious prevÍ.ously selected because of their large and domlnant populations of A"dea_nei, and returned to the laboraLory for ldentiflcatíon. Anlrnals were poured through two nets, a coarse and fine, to separate the larger fron the smaller individuals. Subsamples were transferred, ln wat.er, to a white 31 plastic tray that in turn was resting on a bed of ice. The cooling process slor¡ed t,he movement of the animals suffl-ciently for sorti.ng, without causlng significant mortality.

The other species of notonectld that were collected at the field sites were A.hyperion, A.thier:enannÍ Lundblad A.qraLus Hale and

Enith¿¡res bererothi l{ontandon. For adults and larger size classes

A.deanei/A.hyperion could be separated quil-e easily from A.thieremanní as the l-atter has a significantly larger pronotum; AdratuÊ has a longer pronotum and long shaggly hairs on the verl;ex, and EnÍthares is a much larger () 10 mn) very robusL and generally much darker in colour (Lansbury 1964, 1969). The srnaller slze classes proved extremely difficult to separate as no adequate keys are available for the juvenile notonectids in Aust'ra1Ía. Therefore, in a1l- likelihood the size classes

1 to 3 contain a number of different species, this was however unavoidable. To compensate to a certain degree, the size grading procedure (descrlbed below) ensured Lhat indivíduals were of very slnilar sizes when used and pilot experlments, using the smaller size classes, showed no signifícant differences in the behaviour or swimning pattern of the anlnals used.

Fo1lor+ing the inltial sorting and approximate size gradíng described above animals r+ere washed through a set of Edicott Sieves and then, following a visual inspectlon, ernptied into the appropriate holding tanlc. Table 2:10 lisLs the 5 size cl-asses of AnÍsops used along with various physical rneasurements, and the correspondi-ng instar stages. TABLE 2:10 Summary of certain physical characteristics of A.deanei used in categorizing Prey Size Class.

SIZE INSTAR B0DY LENGTH (mm) Ì,'JET !'JEIGHT (mg) DRY WEIGHT (ms) CLASS STAGE

X SD RANGE X SD RANGE X SD RANGE

+ 1(50) I and II 2.39 0.09 ?.2 to ?.5 0.439 0.110 0.20 to 9.7 0.195 0.047 0.105 Lo 0.256

2(50) IiI 3.Jz 0.163 3.I Lo 3.7 I.zAi 0.110 O.75 to 2.0 O.t+76 0.129 0.257 to 0.71 l(50) IV 4.36 A.243 3 .6 Lo 4.7 3.292 O.636 2.3o to 4.75 O.959 O.364 0.51 to 1.75

4(50) V 5.74 0.247 5.3 Lo 6.1 8.028 I.693 5.70 to 12.55 2.I5O 0.600 I.16 to 3.40

5(120) Adult 6.81 0.433 5.7 Lo 7 .9 12.674 2.702 8.10 to 20.00 3 .626 O .770 2.39 to 5 .99

tÊ " Number in parenthesis equals number of animals measured. 32

2:2.4 Techníques used in measuremenl- gL lgltgths_ and wet. _qqg. gI_L ¡ve_rght_e" of Anísop.s-

222.4.7 BodL L""gth.

Anl.mals were placed on their lateral surface utrder a Zeiss binocular microscope fttEed wlth a calibrated ocular eye piece. The measurement was tal

2t2.4.2 l{e! Weighíru7. Technique To allow for the varj-ation in wet weights caused by water cllnglng to the external surface of the animal after blottingr a standard drying curve was constructed using a range of different size classes of A.deanei.

Aninals r¡rere removed frorn l-he water, carefully blotted on absorptlve tissue and placed onto the weighing tray of an electríc nicrobalance (Beckman Ltd. Model LM600) which is accurate to 10-3 mg. The weight of the animal was read directly from the dial every 30 seconds over a ten minute period. This was repeated for 35 anlmal-s.

Follorving the construction of a dryJ.ng curve the rate of water loss \,¡as found to be consLant aL 0.05 mg rnin'l .ftrr 5 minutes drying. Thls was taken to demonstrate that the variabLe amounts of external water r,¡hich 33

lras st.ill adhe::ing to the insect hacl evaporated and thaL the r*ater loss from the insect was constant.

From thís pilot experiment a standardi-'zed wel weighing method was

used Lhroughout this thesis as follows :

Animals ü¡ere carefully removed from Lhe water, blotted on absorptive tissue (in the same v¡ay each time), placed into a smal1 petri.., dish, covered rql-th a gauze lld and left for 5 mins. It was then placed onto the balance and the weight read inanediaLely.

Dr¡ l{eÍehinå Met-hod

Individual animals vrere placed into'numbered glass durham tubes (15 x 5 ttn) ancl dried at 105oC for 48 hours, transferred ín dessicator jars over silica ge1 and weighed wiLhin 15 to 30 minutes.

2:2.4.3 Prev Leneth/l^leisht Resress l_ons rn order to predict the initial weight of a prey animal (1 error) prior to experiment,s, separate regression equatÍous of dry weight- (ng) on body length (mm) were consLructed for each of the sl-ze classes of

A.deanei. 50 contro1 animals frorn each size class 1 to 4 and 120 anirnals from size class 5 were measured and welghed as outlined above.

Table 2:11 shorus the resuLtant regression equations for each of the si-ze classes. By substitution of the appropriate terms into a polynornical ec¡uati-on the lnltlal weight (+ S.E.) of the animal frorn its body lengrh could be calculated. These weights were then used j-n turn to predíct the body weight 1oss, due to feeding by the predator, by simple subLraction. TABLE 2:11 Summary of the regression equations of dry rueight (mg) on body length (mm) for five size classes of A.deanei used to predict initial dry veights of individual prey.

S IZT ÏNTTRCEPT SLOPE R t-stati-stic sig. CLASS

WTT -1. BB7 o.97t .69 4.5I8 p <0.001 1 DRY -o.654 o.355 70 4.652 p <0.001

!IET -4.29? r.656 .87 9.126 p <0.001 2 DRY -r.528 0.604 .76 6 .01 p <0.00J.

l¡\,ET -6.424 2.228 .85 B.58 p <0.00I 3 DRY -3.450 1.011 .67 4.84 p <0.001

ll.JET -26.92? 6.080 .BB 10.04 p <0.001 4 DRY -9.O52 r.949 BO 6.98 p <0.00J.

l',ET -28.151 5.997 .96 25.3r p <0.001 5 DRY -7 .893 t.692 .95 22.t+6 p <0.001 34

rrlE AND- Il'l 2¿3 TISL0RAL 4"{q. SPArrjL -DJËTSTBULæ]!.9L &-4'.¿q. -{¡lea!ej= THq }'IJU. 2:3.1 Introiluction FieJ-ci studies on any species of }glgg5q are conspicuous in the literature because of their absence" Reference has already been made to a numher of short papers (see for example Blinn L9B2i HaLe L924¡ Hungerford 191"9; Raghunatha Rao 1.962i Radinovslcy 1964; Tawfík and

Arvadallah l975) thaL are primarily concerned with reporting some aspecLs

of the nal-ur:al history. Truo exceptions however are those by l{aitzbauer (I976arb) who provirles a detailed account of tire energy turnover of a

number of water bugs (including Re,natra and the problems encounLered in sanpling and determining Lhe abundance of tr¡ater bugs in lakes.

ü lI Fíeld studies on various notonectÍd species are far more nurnerous I with aspects of their field biology, for exarlple comparative life historles and general ecology (Bane 7926i Essenberg 1915; GÍttleman L973), preciaLory behaviour (Ellis and Borden I97O" Fox and Murdoch

L976; Toth and Clnev 1972), cannibal.ism (Fox 1975arb,c; ZaIotn' 1978)'

age-dependent inti:aspecífic interference (Murdoch and Sih 1978; Sih 1979, 1981; Strearns and ller,¡fie1'd 1972; Streans and Shubeck I9B2i

Taylor 1968), foraging strategies (Giller ancl I'lcNeill 1981; Sih

LgBZarb) and development aud energetics (Toth and Che¡v L972; trrlaitzbauer

1976b) being r+el1 docrmented. Ttris is by no means an exhaustive lÍsto but a selection of what I consÍclereld to be extensi-ve sLudi.es rather than short reporLs or natural history observaLions.

In reference to Lhe two species conce:rned j.n this study, R"dispar E i' i t' 35

and A-dgggi, the aim c'f ttris section rua"s to invesl--îgate the spa't-.i-al and temporal struclu-re of a fiel-d populaLíon of both species t-o rietermine LntraspecifÍc and interspecific simíl-arilies and differences" Rather than being an exleusÍve field study, thjs rn'ork is inlended Lo provide basic ínformation supported by the more tlelail-ed laboratory sLudies of principally, Chapter 4.

223.2 UAt.-fåel_"" gg( }!.]þdj_

223 "2.1 _SçL{¿ SÉ!æ. Two dams (ponds) (Riddlels Darn and Golf Course) vrere regularly

censused durlng I?BL/82 for both R.diqpql and A":!genqi, while a tl'rird

dan (\^laite Dam) was censused at the same Line, but for Ar_{g1ge.. only. One dam (Ricldlers) rvas selected for a more deLalled study of the spatial

distribution o.f R"dispar and A_"¡þa-J!ei in relation to the changÍ-ng

topography of? the clam as a consequence of drying up and vegetation ri d-istribution. These results however, are not presented in this thesis

as they were not consitlered to be direcEly rel-ated to the mai-n theme of the study.

Ríddlers Dan (Gri.d Ref . UG025I94, l,iap Numbex 6627-L, Bchurrga) is

situated about 3.5 km South- east of the township of Hahndorf (56 km South- easL of Adelaide), South Australia" It is a sma1l darn (< 0.25

ha) constr:ucted il the Lg2}ts by horse-drarvn scoop. As a consecluence

of this mode of const,rucLion and its age, the darn is shallor+ (( 1.0 m deep) v¡ith Lhree gently slopÍng sides (north, south and west), the

easLern slde acting as the rdamr face rrilh the water depth belng aT¡out

75 to 100 cm. Tt is fed by run-off fron the surrounding south and west- q hill-s which are marle up of open grazing land r¡iLh a few ínter¡nlEtent I i PLATE 2

(A) Vieur of Riclcllers Dam, Halrndorl', in latc January 1.982, ì-ool<íng Nortl-r. (Ar.roryed

section shorys the sl-al

dam perimeter urhen l'ull).

(B) Viery of Rj-ddlers Dan, Hahndorl', -looking

South (sanle date as (A ) ) .

36 crops of naLive eucalyptus forest. It reache¡; iLs rnaxi.mum size rapi-dly in.Iune and due to a naf-ural rspí1-1.-overr course on l-he e¿lstei:lL bank, (hrinter:/ear1y Spring) renains at this size throughout July to Septernber "

During Suru¡ner (December to February) the dam has been lcnown to dry up as a resrrlt of evapo::atÍon and stock. ¡ittro,tgtrr this did not happen during the dnration of this study (1979 to 1983) the darn was reduced j.n size by about 50% each Summer through evapoi:ation (no stock being present).

The da¡n has a rich layer (up Lo 50 cm in depLh j-n sorne areas) of

ttoozett or Schlamm (Ruttner L974) sedirnents that are mainly allochthonus in nature, rcrsul-tlng frorn the run-off . The main constituent of the sediments are clays along vrith organi-c matter washed from the surrounding grazing 1and. As a consequence the rr¡aLer Í-s exLremely turbíd, with nuch suspended particulaLe and flocculate material.

presenL.

The dan supports no ernergent aquatic rzegetation, the only vegetation being the litcoral zor¡e submerged marsh gl:asses along wiLh lntermittent clumps of sedges and reeds. Ourfng the Spring and early

Summer (September to November) the litroral zones of all four banks consisL of submerged grasses. In particul-ar the ea.sLertr bank has a

thick mat of svramp grasses that overhang Lhe water and extend dorvn to

depth of about 2Q to /+0 cm into the water. During the Summer (December

to February/IÍa::ctr) the darn dries up to becorne almost devoj-d of lÍttoral vegetation being surrounded by a zone of dry, cracketl sedimentsr(Plates lez).

The rGolf Courser dam (Grid Ref . TG98i+138, Map Nuinber 6627'L' Echunga) is gituaLed about 1"0 km North* vesl- of the Lot'¡nshi-p of llchunga PLATE ]

(A) Close vieui o1" arroved secLion t (Ptate ZA)

to shory urooden stalce and littoral, zone

vegetation l'Jhen dam ( e . the uas l'uf l- i " . l-ate Autumn,/tiinter:/Spring/early Slrnrrner) the area

ol' swamp grasses shovn uould be conrp-leteJy

submerqed.

(B) The easl,ern ual-l- o1' Ridcllets Dam,

photoglaphed in January 1982. l- 37

(64 km soutir- east of Arlelaide), South Âustralia. It is a very srnall

dam (( 0,1 ha) beÍ.ng principally an overflor¡ rlepression from a much

larger dz¡n (* 1.0 ha) situated on the Echunga golf course. The dam is shallorv (( 50 crn), very gently sloping s'ides, with a sandyfclay lÍttoral

zone with clurnps of sr^ranp grass that Lend 1-o become submerged in I'Iinter. As in Ridtllets Dam, during Sumner Lhe dan dries up to some extent, but it is reduced in size by only about 15 to 2O%. This is due to run off from the golf course during Sunrmer follor+ing extended periods of

rvaLering.

The thi::d dan (Grid Ref . TG835275, Map Nurnber 6628-111 ancl pT.6528-11, Adelairle) is situated on the carnpus of the \rlaÍte Institute 5

km South- easl- of Adelaide, South Australia. It is a 1a::ge darn (> 0.5 ha) with steep sides (= 4So) ou the east and north banks and rather less d rP frorn a few ll^å (15o to 20o) on the soulh and rvest" Its depLh ranges I centirneters aL the dants edge to more than 12 metres in the niddle. Tt has a large populati-on of A"4eeqq! and a number of other notonectid species but as far as I coul-d ascertain no &¡!i.gg-. It also supports a

large number of Rainbow Trout (Þ,^þ"- ggtla) that were introduced in

Lg78/7g under the University of Adelaide Sports Association restocking

program.

ì i

223.2.2 _Çpl1qçlrng Technique I f During June 1981, when Rirldlets dam r,¡as fu11, the perÍmeter rvas ! ringed with rvooden sLakes (2"5 x 2.5 x 100 cm) sltuated L meter apart ii" { I along the r¿aterts eclge. Bach stalce bore an inverted whÍte plastic ,t I beaker stapl-ed to the top bearing a sequcntía1 grid nurnber frorl I io 77

--{ (the perimeter disLance of rhe clam). i' 1' 3B

Ten grid locations rvere selecled at randon and l-t¡o sarLnples

(Sirallow arrd Deep) were taleen from each -Locat-ionrc¡n each sampl.e date

The shallorv sample was Lalcen along the tvaterts edge by holding

the net verl-ical to a depth of 5 cm and tovring through a 1 meter

dístance (i"e. beLween 2 consecutive stahes)" The deep sample isas taken

1 meter f rom the water I s edge at a water depth of 35 to 40 crn tor,ring the

neL through l meter.The same collecting technique tvas used at t.he

Echunga Golf Course and \{aite dams, except for the deep sample, the

rt¡aLer clepths ranging fron 25 to 30 cn in the Golf Course dam and 100 to L25 cn in the hlaite darn. Furthermore the grÍd locatÍon rr¡as estimated by tsteppi-ng outr frorn a set pos:Ltion and putting dom two markers l meter

apart, the dam not being marked out wiLh stakes.

$ I The contents of each net tow were ernpl-ied into a large r'rhite tray

for sortÍng . The R.di.spar were picked out by hand, aged by instar class and ret-urned to the sarnple site. The remailrder of the sample was placed

into B0% V/V ethanol, in 1arge, screw top containers. In the laboratory samples were hand sorted, the !*-ggalgi being removed and size c1assed as outlined in Secl-io:n 2:2, and counted. Because of the large numbers of i i osl-racods (Ostracoda : Crustacea) and the time required in counting (even using a sub-sampling technÍque) it ruas clecided Lo estirnate l-he I f relative abundance by. tgrossr method. For very ferv aninal-s thls tsas

t{- done by visual inspection and counting unde:: a binocular microscoper for lX Ì I large numbers of individuals, the animals t+ere decanted into a I { pre-calibraLed, drarvn-ottt tip of a pasLeur pipette, suitably sealed, and

"{ the number directly read off from l-he calibr¿rLion" This rneLhod had an i' 39

error of between 25 and 30%. The seven caLegories of ost¡:ocod riensì-ty

used are shown in Table 2:I2.

TABLE 2:12

CATBGORY NT]I'IBBR OF OSTROCODS COLLECTED PER SAMPLII

1 <10

2 11 to 50

3 51 ro 100

4 101 ro 500

5 501 Lo 1000

6 1001 to 5000

7 > 5000

All collectíng was done with a square bottom aquatic net (F.8.À.

mesh aperture 0.5 mm 20 meshes/cm, net rnonth dimensions 20 crn wide x 25 $ crn deep) throughout November 1981 to July 1982.

The dispersion index (ID) for the various size classes of A.deanei were calculated frorn Southwood'(1978) as the ratio of the

sarnple variance to sample mean. The index is then used to test the null-

hypothesis that the observed pattern is random : ii ¡ s2(r,-r ) i

i fD= t

1

t¡ where n = the number of samples. ID is approximately distributed as ü' f, .t X2 with n-1 degrees of freedom, so that if the distribution is in fact I I I Poisson the value of ID v¡ill not lie outside the limits (taken as 0.95

and 0.05) of X2 for n-l as given in any standard tabl.e. Thus for "{ f l' 40

organisms that are regularly distrlbuted the nean/variance raLio w111 approach zero, rr¡hile a large value implies aggregati-on.

223.3 Resul-ts

Table 2:13 shows the mean number of A.deanei caught per sarnple and the related dispersion índex, over the eÍ-ght collecting periods at

the three sites. Much higher densities rr¡ere found in the shallow

compared wlth the deep samples ín all three locations. Tiuo sample dams (Rirldlets and Golf Course) showed very few significant disperfr{on lr índexes in the shallow saruples suggesting that the animals were

regul-arly spaced in the shallow water. Visual observation made at the tÍme of collecLing supports this with the shallow water zone supporting a large populatíon of smaller síze class of A.deanei round t.he danrs

edge. Where aggregaLions did occur they were present at the earlier

periods of the season. trrlhether this represents aggregatíons caused by oviposition or hatching areas or preferred nicrohabitats around the

damrs erlge I âm unsure. A. deqrrglriln the;¿ft larger l{aite clam, apart

from having apparently a smaller densiuy of A.d_eaqe!, exhibited a constant aggregating behaviour in the shallorr¡ water as shown by the signifÍcant dlspersion indexes. llhether this was due to the síze or

depth profile characteristic of the dam or as a consequence of the nunbers of fish present, I cannot say.

I,lhen conpari-ng the deep lùater samples the incidence of I 1 signifÍcant dispersion l-ndexes increases slgnlficantly. A"4eanei, in

{ all 3 dams, showecl a consistent trend to form aggregations of individuals in the deeper h¡ater. TABLE 2:Il The mean density (N,/sample) and related dispersion index (DI) of A.deand in either shaLlov or deep ruater at three field Locations collected through November 19BI to February 1982. (See text for additional detaiLs)

SHALLOhI DEEP SAMPLE I R IDDLE S ECH UNGA hIAITE RIDDLE'5 ECHUNGA t¡JAITE DATE X DI x DI x DI X DI x DI X DI

& +s lf åe * * 6.1I. B1 ro7 .7 lBl 243.6 r10 76.4 ?5.O 23.5 26 6I. B 32,5 8.4 25 lç It * g )Ê lt 3.L2.BT 30.4 I12 70.7 L39 2r.4 30. B 6.2 20 48 .5 20.8 4.2 26 * 18 .12. Br 2.4 11 59.7 BB.6 * 23.12.81 ?.o 9.2 56.3 50.4 * * 5.r.82 I0 t2 19.6 IO.2 7.6 55 16.5 B .l1 3r.9 Zr.9 L.Z 7r + 15. i.82 1 2,2 29.9 IB.B

24.I.82 4 8.0 4.9 6.0

,ê +r L.2.82 1.4 3.2 L3.5 5.7 2.L 48 8.3 5.1 ?4.1 8.3 0.8 5L

tÊ p < 0.001 (see text for details)

t 4t

I¡lhen the composition of the individuals in both shalloru' and deep waLer sarnples r+as examined (Fig . 222) there r,\¡as a consistent trend exhibited over the three monthly sample perÍods (November, December, January) with the srnallesl- indÍviduals malcing up a signifícant. percent of the samples taken in the shallow ¡+at,er. The number of larger À.deanei collected in the shallow water was significantly fewer. In the deep water the reverse was t::r¡e, with very few small individuals being collecLed while the majority of the sanple lras made up of larger indl-viclua1s.

A similar, íf not more obvíous, trend was observed when examining the spatial distrj.bution of R.dispar in boLh dams (Fie. 223). In the shallow samples the signifÍcant percent of the sample consisted of smaller individuals (Instars I and II) with very few III and TV ínsLars

(< LO% combÍned). No larger instars or adults wêre collected in any shallow waLer sample . As in the A.deanei results , the reverse was found when the composition of the deep waLer sample was examined. The slgnificant. component was rnade up of the larger individuals (Tnstar IV to Adult) with the smaller instars naking up less than 5% of the sample.

Table. 2:14 shorus the relative densities of ostracods in both the shallow and deep water on the varÍous collection date.s. The highest density of ostracods is found in the shallow \{ater. Fiq. 2.:2 The composition oí' ihe A..!-þgrÌgi popuJ,arLit-rn in either shal,lotu or deep uai-er on three ciates during Sunmer ISBI/82.

(Numbels in parenthesis indj-cate total- numt¡er' of individual-s sampled).

See t.ext f or coL-lecting l-eclrn j-que and detaiÌs. htov. '! 981 fìec. 1981

80 BO (1074)

ô0 60 (340)

(1106) 40 (235) 40

20 20

a o 12345 12545 o 12345 12545 ,t- LlJ Shallow Deep Shallow Deep o fE u¡ o. Jan. 1 9B 2

BO

60 (13e)

(521) 40

20

0 12345 12345 Size Glass { Shallow Deep TABLE 2:14 The relative densities of Ostrocods in the shallow (S) or deep (D) rvater of Hahndorf dam on various dates from November 19Bl to Februaty 1982. (See Table 2zI2 for category definitions).

OSTRACOD DENSITY CATEGORY DATI 0 I 2 34 5 6 7

SD S D S D S D S D S D J D s D

6 . 11.81 I B t 2 2 0 + 0 1 0 I 0 00 0 0

3 .12.8I 7 7 I 2 2 I 0 0 0 0 0 0 00 c 0

18.12"81 2 9 0 I 2 0 2 0 2 0 I 0 IO 0 0

'0 23.I2.8L ? I 7 2 2 1 0 I 0 I 0 00 00

5.I.82 0 3 0 5 2 2 5 0 2 0 1 0 00 00

15.1. B2 0 6 1 0 4 I I I I 2 I 0 10 10

24.r.82 0 3 0 0 2 3 3 3 2 1 2 0 10 00

r.2.82 0 0 1 2 L4 I4 40 20 00 10 l'iq. 2zJ The compo sition of, the R.dispar population in either

l-he shallour ol deep val,el in i-ur¡ dams sampfed orr

Lhree occasiorrs from November l9Bl- t-o January 1982"

Numbers in parenthesis jndicale t.otal number sampJ-ed. See text for furl-her detaifs. RlÐÐLES DAftfl (Nov.to.!an. I I 8'l /EZl 80

60 (1221 (67)

40

20

I o Þ lv v A I lt lll lv v A PRED ATOR z, I ll lll ST AGE ol¿¡ Shallow DeeP E tx o. GOI"F COURSE DAM 80 (Nov.toJan.1981/821

60 (33e) (20e)

40

20

o I ll l¡l lv v A t ll ¡ll lv v A Shallow DeeP 42

223.4 Discussiort

I . The resulls shor*¡ that in the snnller clarns (Ri

Course) whereas aggregations of A*5þg¿qt_ are present in both uhe shallort and rleep rvater: during November but disappear iu December eind Jauuary, in the large darn (Waite) the aggregations j.n both shal-low and deep r'rater remai-ned througtrout the study pe::iod (Novenrber to January ). In addition the conposition of the aggr:egations shows a significant. diffc:rence in i-he rlistribution of the small-er and larger size classes; the smaller individuals being found predorninantly in the shallovr (1Í-ttoral zone) rvaLer while the 1-arger indívidual.s r+ere f ound in the deeper water. This age-specific distribution was also observed in instars of Pr.dj-p-par. Tn this case no large instars t¡ere found in the shallorv water, being restrict-ed to their deeper water. Few srnall instars were collected j.n the deep waler, being found predominantly in the shallorss. In additi-on Lo finding the srnallest instars of both Ä"rleane,i and &.Lis-p-€, the shalloru water was also found to cotttain Lhe highesL clensities of ostracods. The effect of water depth and an age-specific spatía1 distribution has been reported for other notonectid species. Murdoch and Sih (1978) showed that the presence of adult or older NotonectÍd instars depressed the feeding rate of younger stages, and that the small juveniles movecl less and spent most of their tine close to Lhe eclges of both laboratory conLainers and large field stoclc tanks. They proposecl our of juveniles in the presence of eaten by adults. Additional work ítj-on of prey to the adults 'lt i 't' decreased the lcvel of infeience by t.he adults. Th.is led Lo the '1 43 suggestiorr l-h¿rt since the magnif-ucle of interfereuce decreases r'¡íth increasing the ¡utitber of ¡:rey, inte::ferel).ce may pl'ay a crj-tical nerliatlng role in determining the magnitude of Lhe predator populationrs functíonal and clerrelopnental r:esponseso More recenLl¡r Sl-reans aud

Shubeclc (1982) shor,'e

My results from A"dearLg! support the above ruorks in relalion to the effect of r+ater depth ancl the associated decrease in density. The continued nonrandom distribution of A.cl.eanei in the \rlaite dam may be due Lo the presence of the fistr predators. The aggregating behaviour being an antl-predation adaptation Ín the same sense as a fish school (see also Chapter 6). Ifhy 4.4æ should aggregate in the shallorv water

during November and early Decernber and yet not do so for the remainder of the collecting period (rnid-December to early February) I am unsure. It may be due to the much larger densil-ies found at the begirrrring of the period. ftnplied aggregatj-ons are very evj-dent however, 1n the deeper water, in both the smaller darns. The fact that no fÍsh are presenl- does not necessarily iregate the anti-predation hypothesis. Bol-h dams support

popul-a tions of P,.dispar, the 1arger i.ndi-vi-dua1s of which, as Fig. 2: shor¡s, are restricted to the deeper rqaler. (ChapLer 5 of l-his f:hesj-s 44 discusses the effect of prey aggregaLion and the resultant decrease in encounLer raLe !¡j-th an ambush predator) " The non--random rlistribuLiolr of A.deanel rnay reflect horvevero Lheir reaction to other conclitions in the dam. For exampleo areas of shadow cast across the tvater surface by a tree or bush seem to attract higher numbers of A¡legle!. Furthermore iL was noLi.ced 1n all t-hree dams, partícuIar1y the Lla:i-te dam, that particular regions consistenlly had maty more A"Cge!É srvimming in then, and that individuals rvould remain in thaL zone for many hours. f coulcl not see any obvious feature (prey density, substral-e type, etc.) that apparently nade these areas more attracti-ve. Obviously additional work is required to identify possible explanations.

In relation to the obse::ved age-sp ecific distribution of A.deanei and R.dispar. it is.believed that both avoidance of cannibalism and the presence of prey, as suggested by Streams antl Shubeck (1982) are the main causative factors. Observations in both the field and laboratory have shown that both A.deanei and Il.dispar are cannibalistic. Unlike Notonecta spp., that. generally hangs from the water surface and forages from that site (Fox 1975¡ Strearns and Shubeck 1982), A.de44e.i- forages Ín the mid-water column. This rt¡ould make foraging in the shallow water difficult for the larger j-ndividuals and nay leacl to a tendency for them to avoid 1t. Thus the strallow waters offer, at 1east, a partial refuge from the adults and larger instars for the srnaller individuals. FurLhermore, the presence of an abundance of litforal zone vegetation (particularly when the density of adults is high) nay offer additional refuge areas. It has been shown that the presence of vegetatiolt very often significantly reduces the efficiency of for:aging predators, both intra- anrl interspecific (see for example Bowen and Allanson 1982; l+5

Frase:: and Cer:ri L982; Heclc and Thornan 1981; Savíno and Stein L9B2; Stoner 19S2). In addiUion to progicling a potential refuge from intraspecific predation the shallou¡ rr¡aLer also contains high d.ensities of osLracods, a corilnon prey of smal-l Â"deanei instars.

Either hypotheses could be used in explaining the observed distribuLion of R.disÞar nyrnphs. In all 1íkelihood both the avolclance of cannibalÍsrn and the availabÍlity of prey are involved in the spatial separatfon, alt-hough I believe the presence of potentÍal prey to be more significant- Although the inter¡nerliate size nyrnphs (II an

A a third hypothesis can be suggested, especially in the light of the results from Chapter 4, that being that the size-dependent distributlon reduces the intraspeci.fíc competitíon for similar size prey (somewhat similar to the interference competj-tive interaction between a large, dorninant species and srnall, subordinate one (Morse 1974)). Ful1 partlculars of thfs hypothesis, with supportive results, ate presented ln detail in the discusslon of Chapter 4. NeverthelessrsufficienL to say that the same reasoning can be applied to explain the distribution found in A.deanei (see also McÀrdle and Lawton 1979; Murdoch and Sih 1978). By maintalning the observed distríbution, the sma11 instars have access to prey that otherwise could be captured by the older, competing colspecifics; the la::ger intlividuals being able to capture ancl handle 46 both large and smalI Preyr the small predators being able to generall-y only handle the srnal-ler prey.

0f course this fielil study has not taken fnto account the resultant consequences of oviPosttion sites and posslble preferred' substrate types on the observed dlstrÍbution. Unfortunately time did not permit these areas of funportance to be examined. Perhaps R.dÍspar affords conteroplatlon as a sultabl-e contender for a fleld and laboratory study of this nature, and the interrelation wiLh foragfng behaviour.

47

3:1 INTR0DUCTTIN To be successful an organism nusL procure nutrients. In order to do this animals have evolved a vast array of metllods enabling thern to exploit every available source of food. Predation is just. one such rnethod or strategy of obtaining the required materials, enalllíng the animal to maintain a steady body state, gro\{ and reproduce. Preclator species evolve more efficienl nodes of prey capture, and prey specíes countef wíth defensive adaptations; the so ca11ed co-evolutionary tarms racet of Dawkins and Krebs (1979). Therefore, the examínation and identification of the mechanisms of prey capture are fundamental to our understanding of predator-prey complexes. Answers to such questions as ttl,rlhy can this predator successfully capture organism A ancl yet not capture organism B (often v/ith A and B being closely related)rt, can often be sought in examining the prey capturing process.

As a consequence of this obvious importance there have been numerous studies on the prey capturing behaviour of predators (for reviews see Curio L976; L{hitfield L978; Owen 1980; Peckarsky 1982).

The study of prey-capturing mechanisms can be approached from a number of different directions. For exampl-e, a Morphological approach was taken by I'lolling (196¿+), Holling et al. (1976), Loxton and Nicholls

(L979) and Kramer (1960), working with Manticls, and Dussault and Kramer

(1981), Rand and Lander (1981), Bauer (1982) and Spence and Sutcliffe (1982) rvorking wíth the guppy (Poecilia retigul-atÐ, pj.ke (Es!x nigqr:), and two ground beetle larvae (Not$lf"g irjggglg$) and NebrÍa spp. ) 4B respectively. Iilhile a morpho-hydraulic descriptior I^¡as given by Tanaka arrd Hisada (19S0). Other studies have been primaril-y concemerl wit-h the physiological interaction involved in prey capture (for exarnple : Mittelstaedt 1957; Roeder 1958; Rilling, Mittelstaedt and Roeder 1959; Maldonado, Levin and Barros-Pila 1967; Maldo¡rado et a1.1974; Copeland and Carl-son 79791 Garton and Strickle 1980; Shelly 1982). Schaller (1972), Kruuk (L972a, b), I,lilson (1972) and Krebs (1979) vrere primarily interested in ttre social intraspecific relationships involved in prey caprure, while Ford (I977a, b), Goss-Custard (1977), llise (1979), Garton and Stickle (1980), Paul and Paul (1980) and NakamT.u (1982) for example, approached the prey capturÍ-ng mechanism by exaruining the met.abolic or energetíc costs involved.

Sorne studies of the behavioural mechanisrns involved in prey capture are listed below:- Richard (1970) and Etienne (1972) used dragonfly larvae to study the vari-ous behavioural components and the initiating stimuli involved in prey capture, while l^lankorvski (1981), working with salmon (Sql¡qo gglq-

L.), Jubb, Hughes and Rheínallt (1983) working with the marine crab

(Carcinus maenas L.) and trtinfield, Peirson, Cryer and Townsend (1983) working with trvo species of fresh water fish, Brean Abranus brama (1.)) and Roach Rutilus rutilue(L.)), sLudied the behavioural basis of prey size selection and associated prey capture. Crowley (L979), also workÍng r^rith damself 1y nyrnphs, investigated the behavioural basis of prey switching in additíon to a detailed analysis of some of the behavioural components of nymph predation on cladocerean prey. Additio¡al important works on the behavioural. basis of prey capture may befoundinArrro1d(1978)(ComnonGarterSnake,Tlr.annopliigs@; 49

Lindquist and Bachman ( 1980) (Tige:: Salamander ' Åqbyq-t-gmq. ligþ11tn.) ; Pastorok (1980) (Chaoborus l-arvae ); Gardner (1964); Jackson (1977);

Lubin (1980) and Rovner (1980) (Salticid Spider, Pþiclippqs -çl-qgus.; Cribel-laLe Spi

focussed, to some extent, on the act of Prey capture per se rather than on the pursuì-t or searchi-ng component of prey caPture. They, therefore,

forrn a unique group for studies on many aspects of prey capture, from neurophysiological to morphological.

Holling (L96¿¡, L966) and Holling, Dunbrack and Dill (L976) studied behavioural and ecological aspects of the prey capturing behaviour of the mantis, H.crãssq, while the actual physiological and mor:phological aspects of prey capture were studied in detail by Mittelstaedt (1957), Roeder (i959), Maklonarlo and Levín (1967)' 50 llaldonado, Levin and Barros-Pita (i967), Maldonado, Rodriguez ancl Balderrama (1974), and Copeland and Carlson (1979). Ilot+ever, the facl- that the majority of mantids use a stalking or pursuit sLage in the predatory repertoire disqualifies them from being consiclered as true sit-and-wait predators contrary to rr¡haL is conunonly thought. Griffiths (1980a) and \dilson (I97h) documented prey capture i.n ant-lion larvae, relating changes in pit morphology and pr:ey capturing techniques l-o prey type and habitat diffe::ences, while Zonnicki and Sloborikin (1966) and

Murdock and Murdock (L972) studi-ed the increase in tentacle length and nemaLocyst content of lyira fjfforaþS- relating it. to f ood deprivation and prey capture. Sit and wait predators may be ai.ded by various auxiliary devices such as webs (see for example Savory 1952; Robinson and Robinson 1971; Kullman 1.97i; Lubin 1980; Olive 1980), sticky

threads (see for exarnple Gilner 1972i ,{lcock 1978 p . 343-'344), traps or

pits (see for example 't{heeler 1930; Bristowe 1958; Buctrli 1969).

Actively closíng snares seem Lo be employed only by certain predatory

nematodes (Pramer Í964) with fast death bei-ng achieved by the excretion

of ammonia (Balan and Gerber L972). Additional importanl- studies of prey capturing behaviour in web*building spiders nay be found in, for example, Auslin and Blest (L979); Eberhard (L967); Lubin (L973, 1980);

Robinson (1975); Robinson and Miríck (L97I)¡ Tolbert 1975).

Predator fhungert, prel size and movement have all been short¡n to be intimately Ínvolved in the prey capturing process (see reviews in

Curio 1976; Edwards 1963; Marl-er and Hamilton 1966 pp. 236-237; Cloarec L972c) although detailed st-udies of the stirnuli eliciting

varÍous components of the prey capture are scarce (Curio 1976). 51

Some aspects of the prey capturing behavÍour of lìanAtra heafrS. have. been studied by Cloarec (197la). AddÍtional work (Cloarec I974a) showed that there llas a correlatÍ<¡n between the foreleg posture, position and amount of foo

Later work (Cloarec 1976) showed that an interaction between visual and nechanoreceptors \ìras involved ín prey capture and that tl're high hit efficiency rate (Í.e. correct distance estímation of prey) could be characterized by two behavioural aspects, reactivity and strike efficiency (Cloarec 1979, 1980a). Earlier r+ork showed that R.linearis responded to a model prey rnoving in an irregular fashion vertically downwards; this evoked the highest response. Stre also showed that, the space for successful capture lies above and ín front of R.lineari-s (Cloarec 1969b).

The aim of this chapter is initially to describe and classify the behavioural components involved in prey capture by R.disPa-r (SectÍ-ons

3:2 and 3:3). Having Írlentifieri these it then expands thern by examining the effects that food deprivatÍon (rhungerr) has on both the predator arousal and prey capturing components (Section 3:4). Finally the

combined effects of food deprivation and prey size are investigated in 52 order to compare their effect on each of the various behavioúral coinpoûenLs invol-ved in prey capture (Section 3:5)"

3:2 DESCRIPTION 0F BASJC_ 3R!jY. c{P:zu&E AND FEEDTNG -B-ryIOuR 322.L I'lateriaþ and Methods

Anj-mals were rnaintained as outlined in chapter 2. For the experiments individual anímals were chosen at random and placed ín a plaslic tank (25 cm Dia. x 20 cm Deep) or a 1.arge glass aquarium (50 x 35 x 20 cm) for observation. Prey ¿1i*a1s (either singly for plastic tanks or in groups of up to 10 for the glass aquariurn) were added and observed: (i) directly whilst a continuous narrative of behavioural evenLs was recorded on a tape recorder; or (ii) by closed circuit video recording using a National- T.V. camera

(Model \^lv-1050 ¡,E/A) fitted with a 1ow light íntensity NBWv-ICON

pÍcture tube and either a Canon zoom 1-ens (12.5 to 77 mrn) or an automatic iris lens (Mode1 KF-54) r,¡ith a focal lengLh of 5.4 nn.

Recordings were made on !f2t' tape on a National NV-7000 video recorder fitted wj-th a pause/stÍll/single frame advance control. Videos were played back through a normal 35 c¡n black and white T.V"monitor, as shown in Fig. 3:1. hlhere necessary two fíbre optic

lamps were used to supply additional lighting. No detectable

change j-n water ternperature \¡ras observed when the lights r/ere on. During playback analysis of video tapes, comparable measurements in both the lateral and plan elevations could be taken wÍth the aid of

waterproof 1 mm graph paper secured to the bottom/síde wa1l of ttre tank together with a previously calibrated transparent acetate Fiq. J:I Diagram of apparatus used in video observation. Thé grid floor vas eithe¡ mm graph paper or a plastic l-D grid. See text for additional_ delails. EÐ ñfiIffROß

TA$iK

ÃhrBlJ$Fr slTE ûfiÅPH PAPER (cm ) 3D GRIÐ FLOOR (Graph Paper)

TR.å.h¡SLUTENT PAPER

V¡DEO CAMERA 53

overJ-ay sheet fÍLted to the T.V.þlonitor. The calibrated sheet r*as necessary to talce into account Lhe disLortion caused by the mirror parallax observed in the plan elevation on the monitor. It rvas

made beforehand using objects of known lengLh being novetl across the tank at various water depths. The sheet perÍlitted therefore accurate measurements to be talcen directly from the i-mage on the screen. Cloarects terininology (Cloarec 1969b) have been used Ín classifying the various components of the predatory process.

322.2 Resulqg_ 1. Prey Capture Posture. (i) In Relation to Support :- Norrnally R.dispar adults and 1-arger instars (Instars 3, 4 and 5) position themselves on some submerged so1Íd object, for example stem of water plant or twig, before taking up their characteristic capture posture. &-É1"-g- nornally holds its body parallel to the object irrespective of the angular position of the support with its head positioned lower than the

abdomen and Lhorax (see Fig. 3:28). R.dis-pes is very sirnilar to R.linearÍs Ín this respect (Cloarec 1969b).

(ii) Body Posture :- Fig. 3:2 shorss the posÍtion of Lhe legs of P..dispar in relat,ion to Lhe body and to each other whi.le in the pre-capture position. The meso- and meta-thoracic legs are supportive leaving the prothoracic legs free. R.d:iepar has also

been observed, fn both the field and laboratory, floating just below the water surface (fig. 3:3) and remaining there for an indefinite period. In such a posítion the respiratory siphon is Fiq. 3 The position of the legs of Fiq. 322 (b) The position of the legs and

R.dispar in relation to the bocjy of R.dispar in relation to body and to each other in the the support. pre-capture position.

A = Angle of Tibio-tarq1r/femur = A = Angle of tibio-tarscr/femur = soo (gl-looo) 9oo (e:-rooo)

B = Angle of femur/coxa = B0o (ZO-gOo) B = AngJ-e of femur/coxa = pOo (gO-tOCo) (a) (b)

VERTICAL l{ôÞ17 I{1AL

SUPPORT

+__ METAIHORACtC

sotHoRACtC

ROTHORAC IC

TI o-TARST CL^W s4

normally protruded thtough the rr¡ate:: surface all the time. (WhÍlst floating R"dispar ís q'.rite capabl"e of capturing smal1 prey that either collide with the raplor:i-al legs (see also Cloarec 1969b) or cone very close" This tvas not reported for R.1-inearis, although Cloarec (1971) did report that R"linearis j-s unable to capture prey

during any stages of the respÌ-ratory behaviour) "

2. Predator rArousalr I{hen a potential prey comes close the posture of &di"pat changes by (i) the body, while remaining para1le1 to the support moves vertícally

away from lt, i.e. the coxa/fernur angle of the rneso- and

meta-thoracic legs increase; and (ii) the prothoracic femorfr are brought closer to the head, i.e. there is a reciuction in the coxa/ferour angle. Depending on the relative

position of predator and PreY, 3-"¿ig¿aå can remain in tlte arouserl position for an indefinite period or return to the eariier described posture. If the prey moves closer to ttre predator, orientation towards the prey occurs.

3. Predator Orientation oríentaLion towards the prey can involve large changes in positlon, in relation to the support (Predalor Orientation I) - (see Chapter 7) in acldition to the normal posture changes (PredaLor OrÍentation II) given belor'¡ :- (i) The position of the body of 3.É*ar- changes by a continuation of the rising of the whole bocly as outlr'-ned in rarousalr" The head is

lifted up between 3 and 5 mm which brings the sagittal axls in 1íne [tg.._¿:z The po sition of R.dispar ruhen fLoatin g just beLour Lhe ryaLer surface. Thc nlet.atholacic

legs have just been moved from Lhe uater surface. Drauing talcen from a slide.

55

tvith the prey. This is brought about by an extension of both the nìeso- and meta* thoracic legs - j-.e. the coxa/fenur angle Íncreases and there Ís some degree of change in the point of articulation

between the ta.r:sf1 segments and the support (see also Cloarec

1969b). (ii) the proLhoracic femora contÍnue to move torvards the head (i.e. a retluction in the coxa/fenur angle) in addition to the fused tihio/tarsi opening away from the fernord.(í.e. there is an increase in the tÍbio-tarst{r/femur angle). I^Ihen open the fused tibio/tarsi are dlrected towards the prey.

4. Prey Capture The prey is captured by the rapid lowering of the femorÇ ' on which the tibio ftarsi are closing. The angle of the fernur/tibio-tarsi articulation changes rapitlly from about 1500 to 0o. The resolving speed of the video analysis did not permÍt timing of the strike to be made more accurately than 0.04 sec. (4/7OO sec.). .This compares with 0.03 sêco (3/100 sec.) for R.linearis as measured b y Cloarec (1969b). The prey 1s held between the tibio/tarsus and femur very close to their point of articulation.

5. Post Capture Movements including Consolidation of Grip. Following capture the femurs conl-inue in the downward movement.

This is accompaniecl by the body moving closer to the support. The femor¿ then move back towards the head (coxa/femur angle reduced) whilst Lhe tibio-tarsi remaíns closerl, liolding the prey. If the prey has been caught wlth one prothoracic J-eg the second 1eg then grasps Lhe prey and consolidates the holrl. 56

6. Exploration The prey is brought- to the head unLj.l it reaches the extremity of the rosLrum. The rostrum ís flexible, especial-1y the third (final) segment. The prey is held between the two prothoracic legs r+hile the rostrum first nakes contact and then proceeds to move over the surface of the prey. Normally the prey is rnanipul-ated by changing the position of the grÍp of the holdÍng legs. This allows further exploration to take p1ace. Exploratory movements continue until a suitable feedíng site is located, normally an intersegmental membrane'

7. InjectÍon of Venom. Tt appears that the mandibular stylets penetrate the prey cuticle first and secure the predatorrs hold on the prey. Ttre maxillary stylets are then pushed in perpendicular to the prey. [In 852 of observations (n = 169) wíth A.{e4n4 as prey, this initial penetration site is located in one of the rear metathoracic legs, norrnally in the fenur/coxa lntersegrnental membrane]. That venom/toxins are injected at this time is concluded from observing the behaviour of the prey (but see also Chapter 5 for supportÍve details).

The Ímmediate effect of injection of venom of R.dispal into the prey is convulsive struggling leading rapidly to tremors, and to flaccid paralysis. The time interval between initial injectlon (as rneasured from the first convulsion) to paralysls is very rnuch dependent on prey 57

sj-ze. For A.degneir pref size 1 (Instars 1 and 2), paralysls occurs

SD 37 secs, Range 52 t-o 190 ll betr+een 1 and 3 minutes (1 = 108.5 secs, = = secs, N = 66) while for adult A.Ceg""i paralysís takes about /+ minutes (L = 272.6 secsn SD - 57.8 secs, Range = 161 Eo 420 secs' N = 196)"

The rapid paralysis, which is also a striking feature of so nany may be due as arthropod predator/prey encounters (see Edr,rards 1963) ' rnuch to the vulnerability of the organs within the haemocoel as to any specj.al poterrcy of the secretions used, for once it is within the body the injected toxins have access to the membranes surrounding nerve and. muscle tissue, and the brief buL convulsive struggling of the prey doubtless serves to speed its distribution through the haemocoel" Ïn addition, the location of a pulsating organ in the neLathoracic 1eg of heteroptera (Chapman 1971, p.667i Locy 1884a) where the initial

ü maI movíng the venom into the r€ injection of toxins occurr aid in rapidly ,f haemocoel in addition to almost Ínstantaneously íendering the tkicking/swimmingt lug inoperaLive.

8. Feeding.

Once paralysed the pretlator inevitably changes the location of the penetration site, normally rnoving to an intersegrnental menbrane on

the abdomen or abdomen/thorax juncLion. During feeding the maxillary

sLylets move deep r,¡Íthin the prey body cavity, and the prey is seen to start rythmic oscillations whilst rspearedr on the stylets. This is believed to be due to the regular alternating forward and backward

shearÍ.ng movement of the maxillae (see also Cobben 1978).

The penetraLion and gri-p the stylets is such thaL after a ll of i: 5B

short rqhile of feeding either one or both protltoracic legs can be

relnoved frc¡rn the prey" This enables, if requr'-red, the captrtre of

ad

9. Finish of Meal.

üIhen finj-shed the preyrs remaj-ns are díscarded usually by e, grasping the cadaylr vrith either one or both pi:othoracic legs' reLracting the stylets aud moving the prey al,¡ay from the rostrum ancl allowing Ít to drop by opening the tibio f Lars't. h¡hen both legs a::e € holding nevl prey the sl-ylets are sirnply withclrawn and the cadavy'r falls to the boltom. Unlike R.línearis, as reported by Cloarec (1969b) ' I have observed R.dispar to clean both its mouthparts and eyes r+ith its

prothoracic legs afLer some meals.

1ì

1|l rl The above description outlines the sequence of events that l normally occur in one feeding cycle. As expected however, much variabílity can and does exist r^rith either parts or enLire sections of the cycle beíng modífied in sone h¡ay or completely absent. Far fron beíng a linear sequential pattern of behaviours, each being dependent on the preceding one, a fat more complex pattern exists. The predator exhlbits a versatile repertoíre of behaviour which permits a great i. i degree of efficiency in prey capture dependent on the prevail-ing I circumstances. Fig. 3:/t short¡s diagrams of prey captures taken from I 1 video recorrlings, while Fig. 3:5 summatizes in a flow diagram the

sequence behavioural components and the i-nterrelationshÍ-ps betrveen ,i" of :17

1i I them. Two principal grorrps of cornporents are ol¡vious in Fig. 3:5; I tarousalt I those associated r+ith the predatorrs inj-tia1 j.nl-erest or the capture. Most of the -{ Lorr,arrls the prey and those associated with

i Fiq. 324 Summary of the sequence of behaviours of R.dispar leading to prey capture. (l) Pre-capture position.

(2) Beginning of Arousal ) ) Arousal (l ) Arousal- ) (4) 0rientation.

(5) ) ) Capture. (6) )

(7) ) ) Consolidation of Grip. (B) )

(e) Exploration commences. (r) ( 4 ) 7 )

I I I I ( 2 5 )

I I 3 ) I

( 6 ) ( 9 ) 59

rest of this chapter r¿ill focus on these sectíons - Predator Arousal ancl order Lo examine thein ntore closely arrcl , perhaps rnore ti. Prey Caoture - in impor:tantly, to study the effect of two previousl-y identified prey capturing etintuli, those of uhungerr and prey sizc (see references in

for example Curio L976i Edwards 1963; Peckarsky I9B2). Firsl. however, Section 3:3 outlines some of these variations found ín prey capture behaviour in more detail, along wíth some additiorral observations.

3:3 VARIATION IN PREY CAPI'URING BEIIAVIOIIR AND OT'HER OBSERVATIONS The previous section, 322.2, describes the sequence of evetlLs that normally occur in one feecling cycle. However' as exPecled, much variability and plasticity or behaviour does exist, with either entire sectíons or parts of the cycle beÍng modified or complet-ely absent. The

úi fa11s into tr+o distínct categories. That of changes in ÌÍ variabiliLy ,1 predator posture duri-ng prey capture and feeding periocls, and secondly

that of changes resulting from the actual interactive process between the predator and the prey, for example a prey animal attenpting to escape. Much of this mutabilj-ty of posture, especially prothroacic leg

I positioning, has been reported in det.ail for the related species R.lineari-s by Cloarec (1969b, 1974a) who correlated it wíth the arnount and position of food in the gut.

To a significant extent the same degree of variability in 1eg position and overall body posturing has been observed in &gsja.-I:-. As a result of this obvious j-nÈerspecific similarity no descriptive studies are reported here apart from whe::e X=d,iSær- showed marked departure froin that reported for R.linearis. !'Ihere behavioural disparity existed or q i t' Fiq. 325 Flov diag.uatn sunmaly ol' behavioural comporrerrts,

arrd interre-laj'-ionships lrel-veen t-frem, in the predatory behaviour of R.dispar. See text. for del,ailed description. AXBUSH LOCATIO PREY CAPÌURE POSIfION

4 an Ð o pBEY r}{ VrCt]ttTY- É

ß: o PREOATOR OR rÊ¡¡TAttOlt o lAboút suppottl u¡ G ô. PRÊY CLOSE BY

PREOATON ORIENTATIOII (For CÀptut.)

PREY CAPTURE

cor{soLtDATro¡l oF GRIP

PREY ESCAPE

u¡ G EIPLORÀTIO'{ Ð È o tilJECf toì oF vEHot¡ UJ Ê À

FEEDItrG Sf ART9 60 rvhen R.dispar displayed a behavÌc¡uraJ. repertoire not previously reported, additíonal observations, and in some cases sinple experi-rnental manipulations, r'¡ere cat:rÍed out, the details of r¿hj-ch are presenl-ed under the appropriate sub-heading be1ow.

3:3.1 Observations on tLe .{¡-qusql- Cornponent of- Predatorl- Behaviour Examinatj-on of FÍg. 3:5 shows that the sequence of event-s in the predatory behaviour of R"clis-Iql fall into Lr+o distinct stages : (1)

AROUSAL/ORIBNTATION and (2) PRBY CAPTURE/FEEDING. As ment-ioned prevíously, Cloarec (1969b, I97/+a) has shorrn tl-tat there exists a strong correlation between the angular posture of the capturing prothoracic

legs ancl the positÍon of food in the gut. Therefore, the intervals between and duration of each meal will be reflected in associated changes in 1eg position and capturing ability (NB Ranat-ra is only capable of capturj-ng prey when the prothoracic legs are in a few part:lcular positions - see Cloarec I969b, L974a for full details). Thus it appears that the initial first stage of the predatory cycle is very

much influenced by the amount and position of food in the gut, which ís thungert also used by llolling (1966) in defínj-ng the 1evel of the predator.

Ihe rarousalr stage of the predator is always present and provides a useful thandlet in identifying particular stages of the capture cycle. Occasionally prey items will collide rr¡ith the predators raptorial 1eg(s) and the tibio/tarsal claw vi11 close on theu. Feeding has not been observed to follow such events, horvever.

Movements about the vertical supporto tuhi.lst still mainLaining PLATE 4

Various behavjoural components of pr:edatory behaviour of R"clispar.

(A) and (tl ) Pre-capture posil-iorr

(C) and (D) Arousal-

(E) 0rientat.ion ( l-o capture )

(t ) Capture

(c) ExpJ-oration

61

rorientationr contact with the meso- and nel-athoracic legs, during the rorientationl stage rnay be absent and/or corubined ruith Lhe additj-onal

movenents outlined in Section 3:2"2. (Further details of movemenLs/orientations about the foragi.ng site are present,ed in Chapter 7)"

3:3"2 OÞqeffetl.ons on the Prey 9æ- conpone4L of PredaçorY. Behaviour The capture success or efficiency of R.d!spa-! is very nuch associa.ted with the thungerr leve1 of the predator and the sÍze of the prey. (See SectÍon 3:6 this chapter for detailed observations and analysis). Ilowever from an additíonal study of 22O caplure attempt.s by adult R.dispar on adult A.deanei" T observed about a 767" capture success for the prey capturecl in a feeding period. This resul-t is cornplicated by R.disparrs ability to capture and hold more than one prey rl i* at a time. Thls unusual phenomenon is discussed more fully in Chapter 5 ,-l where additíonal experimental details are given.

If the capture is unsuccessful, but the prey reruains within strike distance, the predator will attempt repeated captures" Up to three or four strikes have been observed in a time interval of 1.5 to 2.0 seconds. Following unsuccessful capture attempts, R.disPar resumes

I i one of the earlier tposturesr of the capture cyc1e, dependj-ng on tvhether I I prey are nearby or not. t f ¡,

t. After tbe successful caPture of a prey, the exploratory stage .$" r t always fo11o¡+s. (This may be delayed for a considerable time if the 1t I Ranatra is alreacly feedíng on a previously caught prey). -{ lr iI t' 62

During prey exploration the rostrum Ís normally moved over the

,, entire body surface as the prey is manoeuvred by either one ol: both raptorial 1egs" If being manoeuvred by one leg (i.e. the other leg is holding a second prey) the incidence of successful escape by the prey is grealer than lf it ls held and manoeuvred by both 1egs. Table 3:1

summarlzes these observaLions.

TAB.LB 3:1 The incÍdence of successful escape by adult prey during the exploratory sLage of the feeding cycle when held and

manoeuvred by either one or both of the predatorrs prothoracic 1egs.

No. of No. of No. of

animals escape successful attempts escapes

Prey held by both legs 4L 32 2

Prey held by one leg 29 2L 11

)i. i Exploration always occurs and precedes any feeding activity, I

I whether Ít is the initial puncture and subsequent feeding location or I 1 later feeding sites on the same animals. I

ä" ï hlhen tested w1t'h model prey that conferred no nutritional reward I f t and follor+ing various periods of food deprivation, Bdisp".. responded by *lt increasÍng the iniLial exploratory time and nurnber of exploratory boul-s !r t ,l l' 63

before rejectÍon of the prey (Table 3:2).

TABLE 3:2 The effect of food

FAST PERIOD 0-6 hrs 24-30 hrs 48-54 hrs )240 hrs 1. Aver.Time (nins) of initÍal expl.bout 0.84 L.24 L.45 2.42

N 40 L20 LL2 85

Range .26-1. B o.4-2.06 .57-1.90 0.94-4.33

{ 2. Aver" # subsequent $ exp1. bouts 0 1 5 l.v 7

N 11 L4 36

Range 1-3 r-4 t-20 3. Aver. time/bout of subsequenL expl.

bouLs. .2L .34 .7L i.

¡ i

I ,l a 1 The style and techni-ques of collecting data did not perrnit

¡. statístical analysj-s to be undertaken. Nevertheless the influence of ii¡ . f, and nurnber of expl.oratory attempts T food deprivation on the tine budget

,t i { is obvi.ous. The longer the deprivaLion LÍme the longer the initial exploratory boutn the greater the number of subsequent exploratory -{ i' .l l' 64

attempts, and the longer the average time spenL in exploring during each bout before the model is discarded" Follor.ring Lhe exploratory movements by the predator the initial feeding site is selected, ruhich in 85% (n = 169) of the observed cases is one of the preyrs rear (metathoracic) legs. The duration of the inltial feeding stage r.¡as 4 minutes (Î = 4.0 mins, SD = 0.86, n = 68) and it is durlng this sLage that a rnixture of

toxins and digestive enzymes are injected inLo the prey (see Chapter 5 for a detailed study of feeding behavlour and dynarnics). Subsequent manipulation of the prey'enables further expl

i] TABIB 3:3 Summary of nuuber and duratíon of explorati.on or feeding by

I adult R. disoar on adult A.deaneí.

(Time in mlnutes) 3 SD N Range 1. Aver.Expl-oraticxr Time/Site 1.3 0.6 560 1ro3 2. Aver.Total Expl-.Time /Prey 6"8 7.7 LLz Lto26 3. Aver.# of Exp1.Sítes/Prey 5.0 4.L tr2 Ito27 r i I

I

,i I 4. Aver.Feeding Tirne/Site L4.9 9.3 545 4 to 36.5 4 t Ì 5. Aver.Total Feed.Time/Prey 58.2 31 .9 109 5 to 141 ri

¡. i, 6. Aver.# of Feed.Sites/Prey 4.9 3,9 109 Ito27 1l

î.t ,l I { of prey by the -ril These tables emphasize the extent of manipulation I t¡ 6s

predator during exploratÍon and feeding. Although it is to be expected that Lhis confers an aclvantage to the pr:edator (for example furLher nutritional sites in different parts of the body may be found) it also involves a triski factor, in the possibi-lity of losing the prey. This

can happen by the prey acLually escaping (as mentioned previously) or by being dropped (apparently unintentionally, as subsequent behaviour patter¡s indicated) both of r,¡hich can influence the subsequent predatory patl-ern.

3:3.3 Observations on the Defensive and Lvas:Lye Behaviorrl of R.dispar and Lheir Ðfluence on Preqaigry- Uèbqt]quf.. 3:3.3. 1 Introcluction Although not directly linked with predatory behaviour' two

additional patterns of behaviour of B4!Lsp4g- are given below, as they have Lhe potential to affect all of the various stages in the predatory cycle.

The first concerns the predatorrs reactíon to objects thal are

moved in reasonably close proxinity to it. When an object, for example

a 5 cm plastic disc, is moveil near B.dig.par- that is, for example, in the pre-capture posture or feeding, it may respond by extending íts

prothoracic legs away from the head so that they 1ie in the same horizontal plane as the body and in elongation to the anterior end (Plate I d). The body may also be brought closer to the support, r+hílst rernaining parall-e1 to it, by reducing the coxa/fenur angJ-es of I I artlculat.ion of the rneso-- amd ruetaLhoracic legs. Also the predator may respond (either immediately, or followj-ng the 1eg extension response) by 66

swim,ning Lo the bottom of the tanlc (in thj-s case 50 cm depth) and remaining notionless for a variable time.

3:3.3.2 l4aterial eqd_ Uçth"d"-

SUBJI,.IC'I'S: - Sub jects were either classifled as FASTBD (F) , having gone at least 5 days (120 hrs) r+ithouL feeding (Range 5 - 7.5 days), or

NONFASTBD (l'lF), anj-ma1s which had fed within previous 36 hours.

STIMULT:- Plastic disc stilnuli were eíther 5 or 10 cm Ín díameter,

suspended on fine nylon fishing 1íne and weighted with plasticine.

PRESBNTATION:- A1l- stimuli were presented when the predator was situated at about 10-15 cm dePth.

(A) Submerged Presentalion. The disc was brought from as far away as . possible from the predator ()50 cm). and noved directly towards it at a clepth of about 10 cm (a smal1 marker on the line acted as a depth guide) and speed of about 6 .t "u..-1. (B) Surface Presentation. As above but with model held about 10 cm

above water surface.

(C) Hand Movement. As in surface presentation but with eíther a slow

speed ((2 cm or fast speetl ()10 crn uu..-l¡. ""..-1¡

Subjects r{ere scored if they responded either with 1eg extension

or by swimrning to the botLorn of the tank. The duration of the response

was also recorded

I 3:3.3.3 Resqlls anc! Discussion Table 3:4 sutnmarizes the results of ob,servations on 296 adlu]-X TABLT l-4 The Response'of Fasted or Non Fasted adult R.dispal to various modef stimuli. (see text for presentation details).

RESPONSE

TXTENSION PROTHORACIC LEGS SIdiM TO BOTTOM FASTED NON FASTED FASTED NON FASTED

(m) (m) TOTAL N TOTAL N DIJRATI0N (m) DURATI0N (m) DURATION DURATION TISTTD RESPONSE n ï Range nÍ Range n X Range n :f Range

lç + 1. 5 cm DISC SUBI\I. 46 ?4 B (26) 2.6 .5 ! 4 11"120) 4.6 2-IT 1 (26) 42 e +* (zo) 5 4 ='6 )t g* 2.10 cm DISC SUBI4. 65 53 t5 (17) 2.7 .t - 5 19*(zs ) a.g 2-r5 (sl) r+ B 20 rr*(ze) rg 5 =-49

j B (20) B 3. 5 cm DISC SURF. 40 27 t* (zo) I .i -3.8 12 (20) 2.4 2-7 z* (zo) .4 6 3-28 + 4.10 cm DISC SURF. 67 67 zt* (36) 1.5 I -'6 26 (lr) 5.t+ I.5- 9 t+* (lø) tt i 30 rlx GÐ 17.6 9-69

àt (zL) (17) 5. SL0I,'J HAND 3B 11 3 ( 21) 8 .5 -r.3 e*(tz) z.? r-g 0 0"

åÊ * (20) 5e 6.FAST HAND 40 37 7* çzo) 3 9 .B -8.4 13*çzo) B.B 5-13 6 (20) li 2 26 15 24 15 -

lt Number of animals that resPonded. O Number ofl animals observed. (m) Equals minutes 67

R. dispa r (both female nncl rnale) subjectecl to the various stimuli either belor,¡ or al¡ove Lhe rvater surface. The daLa r,ras col-lect.ecl intermit-l,ently over three and a half years and in many cases the r:eproductive staLe and past history of the ilrdividuals r¡as unknown. No statisl-ical analy-sj,s was performed on these data due to Lhís mocle of collecLion attd unequal sample sIz,es. Neverthel-ess the main trentls are obviousì r¿ith the fasled predat,ors responding proportionall-y less and for shorter duratj-on than the non-fasted subjecl-s. Animals ::esponded rnore torvards the large nodel-

ín both sulimerged and surface trials, while no appreciable differr:nce was observed between the trvo smaller discs.

Clearly the thungerr level and si.ze of stimulus have a marked effect on the responses observe'd" Time did not perrnit further experimentation to be undertaken to elucidate these interesting resulls and therefore I can only speculate as to r*hy these trends are found. Hov¡ever, it rvould appear that. for any given threat stimulusz¿(no captu::e attempts were observed throughout the observations), the predat,orrs probabílity of an evasive or defensive response i.s very nruch dependaut on its current rhunger levelt. The longer the food cleprivation the more 1-ike1y lt is that the predator will risk remaining at a potentj-a1 p::ey capturing position irrespective of the threat. Furthelmore, if the predat.or does respond to a threat, the dui:alion of uhe response ís much shorter, and they reposj-tion themselves j-nto a potential prey-capEuring position sooner than non-fasted indívir1ua1s. A somer+hat similar resul-t hras reported by J.ll. Lav¡ton (cited as a personal communicaLion in

Hassell and Southwoocl 1978), for the darnselfly, IscþguIa gleg.ggg, that are at significanL rj-sk frorn predators rvhen swj-rnming ancl will therefore rernain at their ambush site even r+hen t-hreatened by starvation. 6B

The leg extension response described above is also observerl if the prey struggles violenLly. Follorving a capturer prel are often observed to struggle. These struggles lnclude 1eg kicking, 1-eg levering agaÍnst the raptorial legs, body rotatÍon and rsrvimming actionr, especially if they are held by only one 1eg. If these struggling novements are intense, especially 1f the prey is one of the larger

insl-ars or adult A.deaqs:i., the prey is moved ar+ay from the rostrum.

This is made possible by Lhe predatorrs prothoracic legs extending, Lhus moving the prey away from tlie head region.

Prey have been observed to struggl.e inmediately after capture, during the exploratory stage and partÍcu1ar1y when the stylets are being

inserted. The length of time that the prey is held away from the head

is very variable, but agaln the period of food deprivatÍon appears to be related to the observed results.

Table 3:5 surmarÍzes a total of 67 such observations, rnade on both non-fasLed (0 to 24 hour fast) and fasted (5 - 8 days) indj-vidua1s. During the observations the tÍme the prey was hel-d away from the rostrum

was recorded. 69

TABI,B 3:5 The mean period for which a struggling prey rvas held away

from the heacl region of the preclator following either a short fasting (0-24h) or a long fast (5-8 days) (For the t-test 30 individuals were randomly chosen from cach treatment).

TIME HBLD AI^IAY FROM ROSTRIIM (secs.) BEFORE MOVING IT BACK

-x+SD Duration (secs) Range N Short fast predaLors

(o-24 h) L9.6 + 4.2 LL.2 to 57 31 Long fast predators

(5-8 days) 8.3 + 2.1 4.2 to 24 36

t = 4.4, p(0.01, df 30.

As in the previous observations, the effect of food deprivation Í.s very marlced, with the rhungriert predators moving the prey back to the rostrurn significantly faster. Llhether this increases the rriskt element taken by the predator (damage by the kÍcking mel-athoracic legs to the head and eye region could be posslble) is unclear. Obviously further work is needed to shed light on thls aspect of predatory behaviour. 7A

IAROUSAI,' 3:4 ÐU$cBIPr.Igj. 9I rHE PRIÌD,¿\TORIS SPÂCN AND T'IIB IIFFBCT OF

FOOD DBPRIVATION 3:4. 1 Introduc.tion A sj-t-and--ruail- pr:edator lying in wait for foorl rrust be able to detect the approach of potenti.al preyr and it must also be equipped to capture Ít efficienlly when -il- comes r+ithin range. Cloarec (1976) has deterrnined that both visual and to some extent mechanore$aron .tu A utilized by R_anatra- in detecting prey"

As nnentioned in Section 3:2, if an object is noved into the vicinity of lì.dispar it can respond in either of tv¡o h¡ays ,: a characterist.Íc tarousalt movemertt, which nay lead to prey capture, or an evasive or defensive movement. The fol-lor'dng series of experimenLs t+ere conducted to nap the size and shape of l-lie rarousalt space of &.{ispa-L' i.e. that volume of space surrounding the predator wit-hin which the movemenl of an object stiniulates arousal l¡ehaviour.

3:4.2 Material and I'fethods

Pre-experimental Proceclure Adult female &gl"lqf were removed from the holding tanlcs arrd fed abundant prey (Notonectids, moscluito larvae) f.or 2 days preceding the experimental perlod. 0n Day 1 of the experiment they were removed from the feedÍng tanlcs and assigned at random to tt'¡o groups, A and B. Animals in Group A rvere individually housed and tested in large plastic cyllndrical- containers (25 cm Diameter x 20 cm) each, fj-l1-ed to a depth of 15 cm rçiLh filterecl dech1orinat:ed tap h'ater, and fitted wiÇh a . cal.íbrated (n:rn) glass rod r¿hicl.r acted as a support. The animals rernained in t.hese conl-aine::s for the duration of the experiment. 7L

Anirnals in Group B were índividually placecl int-o 1 l-itre plastic bealcers r+hich acted as holding containers between experiments t¡hich tuere performed ln large glass tanks (50 x 35 x 20 cm deep) arranged for video observatlons.

Model Prey Stimulus

The model prey used throughout the experi-ments t¡as made from a

píece of fine wire 1.0 mm in cross section. Tt rr'as fashíoned to resernble (as far as possibl-e) an adult notonectid.. The top half r+as painted a dull orange/brorrn with a non-toxic matt paint, whl1e the lower section of the model was l-eft bare to resemble the sheen created by the wj-ngs with the associ-ated air bilbble" (Notonectids swim upside down -

hence their common name tbackswimmerl)"

A piece of fine gauge nylon fishíng l-ine was used to suspend and

move the rnodel durlng experiments.

Notonectids swim in what 1s best. termed a rjerkyt motíon, short bouts (<0.5 sec) of propulsion can be separated by períods of notlonlessness ranging in duration from 1-10 seconds (see also Chapter

6). To represelt this type of movement in the model, the thread suspending it was slowly rolled between thumb and forefinger to produce a rjerkyt roLational action as the prey was presented to the predator.' l^lith sufficient practÍce and a long enough piece of thread (about 50 cm)

repeatable lnovements could be produced. 72

Test Procedure

,¿lnima1s nere observerl bclLh late::ally and from above by ej-ttrer video camera or dlrect observalion. For vídeo recorcling the apparatus was arranged as shorun in FÌ-g.3:1. The use of the angl.ed mirror pernltted both lateral and vertical fields to be observed sinultaneously on the T.V. Monitor. Both the floor and back wa11 of Lhe l-arrk t¡ere covered rvith mm graph paper (the cm lines outlined in black Ínk) to permit relative measurements to be made. For nore acctlrate measurements a calibrated transparent acetate sheet was aLtached to the T.V. Monitor screen to produce an overlay from ¡rhich distances could be read and measured.

PresenLatíon of Model Prey:- The direction of approach of the model prey could be along any one of eight predetermined directional paths 1n either the horizontal or vertlcal plane, as shown in Fig. 3:6. For Group A predators (he1d ín plastic conLainers) the animal and direction of approach vere chosen at random without replacement for each presentation until all animals and dlrectíons had been tested. For Group B predators, (virleo observation) one animal r,¡as chosen aL random and put :[nto the glass Larrk and allowed 15 ¡dnutes to acclimatize. Normally on reachlng the support the predator t.ook up Lhe characteristic tpr"y capturet posture v¡ithin a few minutes. Presentati-ons were never conducted unless the predator was in this position. Follorv1ng this rsettling inr períod the order of the direction of approach of the model prey 1ra,s chosen at random without replacenent until all the directlons had been testecl. The predator was returned to t-.he holding tank and another animal chosen at random" This procedure was repeated until all Group B individuals had been tested,. Fiq. Jz6 Tlre djrection paths used for ¡rr.esenting rnodel- pre), to R.dispar to del-ermine the aror-lsal predator distance of the " Point 0, beLveen predal-orrs eyes, uas used

as the reference position l'or dj.stance

measutements. See t.ext fol addilional detai ls. H#ffiåtrürurÂL

Ð =1 80' PilAruË

o 6 -1 35

Point 0 @=90'

@=45'

& 0"

VffiffiT¡TÅL

o PtÅNffi €-9t Ð=45' Ë= t SS"

G=1$Ð o 0=0/3&0o

ê=ãäS 6-ät0 €.= 3"1 5 73

In both Groups A and B each anirnal t*as testecl /+ times" The entire procedure of 10 (anlmals) x 16 (directions) x 4 (repeats/animal) normally took between 5 and 6 hours.

r\nimals were tested on Day 2 (fo11-orving 24-30 hour fast (hf))'

Day 3 (48-54 hf), Day 4 (72-78 hf), Day 5 (96-102 hf) and Day 6 (120-726 hf).

The tarousal distancer (as defined belorv) vlas measured beLween a point 0 (Fig. 3:6) mid-way between the predatorrs eyes and the anLerior tip of the nodel prey.

Arousal Distance - (as outlined in Section 3:2), when a potential prey moves in the vicinity of 3.!i-spar- the predatorrs posture changes in 3 dist-inct, measurable ways. All 3 changes were usecl as indicators of predatorts rarousalt although changes in coxa/feruur angle r,¡ere used primarily. Definition:- The arousal distance v/as defined as that dÍstance of the model prey from the predator that was associated v¡ith a reduction from

90o in the coxa/fenur articulatÍon angle of the predatorts prothoracic legs.

NB. It has been short¡n wíth a related species, Ruqgqr:a. lineel::Þ' (Cloarec 1976), that durlng normal predatory behaviour mechanical st.funulÍ from prey, in addition to visual sLirnulation, play a part in the perception of prey. Receptors j.nvolved, situal-ed on the prothoraclc 1egs, and theÍr area of stÍmulationo below the foreleg 74

femurs, have been rvel1 documented (op. cit.). In the experiuents

reported in this thesis no additional mechano-stimulation was providecl apart from Lhat associated with the roLational- movement of

the model, as ouLlined above. Therefore it seems appropriate 1-o

point out that the resul.ts obtained in tliis section (and that'of 3t5 &.

3:6) could be due to a combination of both visual and some

unquantifiable mechanical componenL. Every effort was ma

standardj-ze Lhe movement, of the model during experimentation to reduce any variation caused by it.

324.3 Results As the variance withj.n and between groups A and B v¡as found not to differ signifícantly (F( 1.5, p)0.05 in all cases) they were pooled for analysis.

IIORIZONTAL PLANE: Figure 3:74 shorvs the syrnetrical shape of the rarousalr fie1d, in the horizontal p1ane, of R.dispar at- 5 different food deprívation 1eve1s. The overall shape is ellÍptical, with each ellipse being essentially Lhe same shape for each fasting period.

Two tendencies were noted in the resul-ts (1) the arousal dÍstance between Ranatr:g and the prey increased as the period of food deprivation increased; (2) the dÍstance was greatest for prey in front of the predaLor and decreased as the model was moved to the sides anrl rear.

To test the validily of these apparenL tendencies, the mean liq. 327 The arousal l'ields o1' R.clispar as seen jn the horizontal and vertical- plones follcuing 5 pcriods of food cJeplival-i-on.

(@) = 48h rast (@) = J2 h rast (@) = 96 h l'ast

Interval-s betueen dashes on 0o axis = l- cm. ANOUSAL A FIF¡-D S

FIO R lZ0f\¡TAL 13 135 't 80

o

4 45

B

VEËTI çA L o' 45

90 (Cos O 7 f ) (Cos o) t3

(Ços- O-7 f )

lBO o (Cos - 1-Oi (Cos 1'O)

27 0" (Cos O) 315 225 (Cos o,71) (rlos O.71) 75 arousal distances (AD) beLr¡een the þgq!-ra and the model prey were plotted agaÍ-nst fasting time for each of the angles of presentatÍon,

In all cases the slopes were significantly posltive (Fig. 3:8 A to E). The t values, degrees of freedom and associated level of probability for the angles of presentation j-n Tabl.e 3:6.

TABLE 3:6 Tab1e of t-staLisLics, degrees of freedom, and 1eve1 of significance for regression of rnean arousal distance on fast time. (t-statistic tests nu1l hypothesÍs that the slope is. noL dífferent from zero).

ANGLE OF PRBSENTATION t-value df Siqnificance

0o 5.99 3 p( 0.002

4s 11.31 3 p( 0.001

90 7.62 3 p( 0.001

135 4.69 3 p( 0.005

180 6.87 3 p( 0.001

These results shor¡ that the distance at which a Ranatrq ís aroused to a model prey increases as food deprivatlon tine increases and that this is irrespeclive of the angular position (ln the horizontal plane) of the model prey.

Analysis of covariance, to test the nu11 hypothesis that the five slopes are equa1, showed that significant differences j-n the slopes did exist : F = L6.36; df 3, 15; p(0.001" fiq. l:B Regress-ions of Arousal- Disl-ance (AD) against fast.

time (FT) for R.dispar vhen nodel- p:ey are present-ed along 5 dil'ferent directionaJ. paths Lo the body axis

in the horizontal- pJ-ane. (A) Presentation path 0 = 0o

(B) Plesentatíon path 0 - 45o

( C ) ['resentation path 0 = 90o (D) Presentation patlr 0 = I35o (E) PresenLation path 0 = 1B0o

Angle 0o is directly in front of predator. o Angle 180 i" di""ctly behind the predator. lj 50 (A) 30 (B) o=d o=4 5' 45 26

40 22 t

35 a 18 AO=26 O4.0 l4FT 5a.O l0Fl ^O'5 30 R= I6 14 R" I e

T=5.9, 3df. p.O OO2 T.lr 31. 3dl. Pr0 O0l 25 10 24 4A 72 96 120 24 48 72 96 120 ô

(c) (D) l5 a lo E a E â=9 o' O=135'

UJ 12 a 8 o a z I 6 t-

2 6 4 o AO=3 66+O,lFT AD:5.29.O O3FT J R--94 3 R=.9 7 2 U' Þ T=7 82. 3dl. p.O O0l l=4 89.3d1. p.O O05 o o o ú. 24 48 72 96 120 24 48 72 96 120

lo (E)

O=1 8o" I

6

4 ÀD=2-33+0 06FT

2 R= 97

T=6.87. 3dl, Þ.O-OO I o 24 48 72 96 120

FAST TIMË(hr) (FT) 76

As Èhe slopes rrere not equa1, a rnultiple comparison procedure rvas used to determine whlch of the slopes were different from tr¡hich others.

The Student Newnan KbS" multiple range test (MRT) was used to test differences between each pair of ß values, by Ho: ß1 = ß2 and HO: ß1 = ß, r+here 1 and 2 represenl any two of the regression lines (Zar 1974). The test staLístic is given by

b I -b 2 q

SE

AsIx 2 are the same for lines'l and 2 the

1s2Y.x¡ pooled

SE

2 T x

Table 3:7 shov¡s the comparison of each paír of S values rvith the calculated q statisti.c and probability level, and suggests the proposÍtion that although a rhungerr effect does exist in arousal distance around the Ranal{'q (Fig" 3:74) this effect is not the same in front as behind the Ranatra, 77

TAULL _3;f Table to show the cornparison of regression slopes using the I ^ Student Netqman ttþ$fs MRT (q statistic). Regression 1, 0 = 0o; Regression 2, 0 = 45o; Regression 3, 0 = 90o;

Regression 4r0 = 1350; Regression 5, 0 == 1800.

Regressions to- Þq b values g Statistic -P-:-

compared

L&2 0.1458,0.1875 -2.085 Not Signif- icant (NS)

1&3 0. 1458, 0.10c0 2,4LO NS 1&4 0.1458, 0.0354 6.133 <0.05 1&s 0. 1459, 0.0596 4.790 <0.05

2&3 o. 1875, 0. tr_000 5.833 <0.05 2&4 .0.1875, 0.0354 L2.675 <0.001 2&5 0.1875, 0"0596 9.840 <0.001

3&4 0.1000, 0.035/+ 5.872 <0.05 3&5 0.1000, 0.0s96 4.040 <0.05

4&s 0.0354, 0.0s96 -3.025 NS

Both Ho11Íng (1966) and Hardman and Turnbull (1980) found a similar increase in response di.stance with thungert for the malrtÍs, Ilierodula cËassa and ttre wolf spider, þ!g Yancouverl- respectively. In the case of the wolf spider, a1-though the increase in reactive distance straight ahead of the spider (0 = 0) t"as found to increase with hunger, the react.ive distance directly behínd Lhe spitler (0 = 1800) did

not vary with thungerr, remaíning close to 5 run frorn the spiderrs face.

Ilar

VERTICAi, PITANB Fig" 3:78 shows the strape of the arousal field in the vertical pl.ane after 5 dífferent per:iods of food depr:Í-vation. Observations on presentation path 2250 proved extremely difficul-t to acquire without disturbing the subject; therefore these results have been discarded. The overall shape is al-most circular with each field being essentially the same shape for each fasting peri-od" (Unlike Lhose fou¡d in the horizontal plane however, the shape of the fiel-ds are not syrnmetrical, in relation to ttre axis of the plane).

As the sarne two tendencies that were observed in Lhe horizontal plane were again noticed ((1) Arousal clistance increased wiLh fasting time; (2) Distance was greatest anterior to the predator), a sinilar program of analysis was undertaken. The mean arousal distances were plotted against fasl-ing tine for each of the angles of presentaLion (Fig.3:9 A to G). In all cases (except the 2700), slopes were signifÍcantly positive at the o = 0.05. The t values, degrees of freedom and associated leve1 of pr:obability for the angles of presenlatÍon are shown in Taù1e 3:B. Fiq. 329 Regressions of Arousal" Dj.stance (AD) aqainst Fast

Time lrT) fol R.dispar urhen prey are presented along 7 dil'ferent directional. paths in reLation to the longitudinal body axis of the predator in the vertical plane.

(A) Presenlat.ion path 0 = 0o (B) Presentation patlr 0 - 45o (C) Presentation path 0 = 90o (D) P,resentation path g = If5o (E) Presentation pat-h 0 = 1B0o (F) Presentation path 0 = 27Oo (G) Presentation path 0 = ll5o (A) ê=0' (Ë) ø=4s" 40 40

30 30 a t

20 ?o AD-26 o4+o 1 4FT ÀD-26 73+0 lOFl

R- 9l 10 R= s 0 fo T-5 89, Jdl, p.0 o02 T'3.82.3d1. q.o oã o o

(c) ê=so" (D) ê-i 05" 40 40 Àg'5 5l+O llFf ñ 30 30 Ê: 90 4 f.32 22,3dr. P.C oot 2Ð 20 Ë Ë O3+O | îFl to ^O.r6 10 u¡ R".I S Ð l-e 25.3d1, D.O,01 ÌP o o q{ þ (Ð (H) Ð='t Bo' (F) ë=2?0 n ÀD=3 77+0 O¡tFT 40 ÄD-3 ÐS+0 02FT J 40 4 R: 17 R' ô I g) l=6 3 t, 3dl, p-O 0 t t:2 97.3d1.0 1>Þ>0 o5 ä 30 30 Ð 4 20 20

10 to

o o ?¡¡ 48 't2 Sâ I ?O ?4 48 72 06 120

(G) g=G i 5 4

30

2t

AD-r2-12ÌO 1{FT 10 R- I I T=lr 36, iìdf, p.0 Ol o 24 48 72 96 12A F',q$T TIMH (hr) (F'T) 79

TABLE 3:8 Tabl-e of t-stat.istics, degrees of freedom, and associated l.evel of significance for the regressions of mean arousal \ distance on fast time. (t-statistic test null hypothesis

tTraL the slope is not different from zero).

ANGLE PRESENTÂTION t-value df Si-enif icance

0o 5.89 3 p( 0.01

45 3.82 3 p( 0.05

90 8.25 3 p( 0.01

135 39 "22 3 p( 0.001 I 180 6.87 3 p( 0.01

270 2.97 3 0.1)p)0.05

315 11 .98 3 p( 0.01

These results support those found for the horizontal plane and likewise suggesL that the arousal of Ranatra to a model prey increases as food deprlvation time íncreases, irrespective of the angular position of the presentalÍon.

¡

Analysis of covariance to test the nu11 hypothesis that- the seven i slopes are equal showed that a significant difference in the slopes did i i exist; F = 6.13, df. 5,19, p(0.005 ancl so the student Newman Kuel MR

I shows the t lest was again used Lo test for differences. Table 3:9 l, ! comparisons of each pair of ß values with the calucated q statislic and t- I t associated probability 1eve1. f

I

I

q

T' .l t' 80

TÂBLE 3:9 Comparisons of each p values usi-ng the SNI( Multiple Range

,N Test. T'ab1e shows regressions compared, the calculated q statistic and level of probability (p).

Slopes to be Compared Þr@. g st-atistic P

I & 2 (0o vs. 45o) . 1458-. 1 179 o.996 Not Signi- ficant (NS)

1&3 (0 vs. 90) .1458-.10s8 2.00 NS

1&4 (0 vs. 135) .1458-.1113 2.O3 NS

1&s (0 vs. 180) . 11+58-.0596 4.79 <0.01

1&6 (0 vs, 27O) .1458-.0229 6.93 <0.01

. 1459- 1383 0.39 NS 1&7 (0 vs. 315) "

2&3 (45 vs. 90) . 1 179-. 1058 0.50 NS

2&4 (45 vs. 135) .1179-.1113 0.30 NS il 2&5 (45 vs. 180) .1179-.0596 .2.53 0.1)p)0.05 ï 2&6 (45 vs. 27O) .LL79-.0229 4.32 <0.05

2&7 (45 vs. 315) .1179-.1383 -0.89 NS 3&4 (90 vs. 13s) .1058-.1113 -0.61 NS 3&s (90 vs. 180) .1058-.0596 5.13 <0.05

3&6 (90 vs. 27O) .1058-.0229 8.29 <0.01

3&7 (90 vs. 315 .1058-.1383 -2.17 NS i I (135 180) .1113-.0596 8.62 <0.01 i 4&s vs. 4&6 (135 vs. 270) .LIr3-.0229 14.73 <0.01 I 1 4&7 (135 vs. 315) .1113-.1383 -3.37 <0.05

¡. 5&6 (180 vs. 27O) .0596-.0229 4.59 <0.05 ,¡" î .t 5&7 (180 vs. 315) .0596-. 1383 -7.89 <0.001 I 6&7 (270 vs. 315) .0229-.7383 -11 .54 <0.001

-rt

t' i t' 81

No significant differences were found bet-rseen the slopes anterior to the predator suggesting that the effecl-s were Lhe same along those particular angles of presentatíon. IÌowever, the fact that significant differences were Present between the slopes of the anterior and posterior.presentation paths, and also a number of the slopes of the pos¡erior presentation paths, indÍcate that the food deprivation effect

was not the same in front as behind the predator.

Holling (1966) and Hardman and Turnbull (1980) fornulate,l different functions which combined both the angular position of prey and the food deprivation time of the predator to describe the reactive field of the predator in the horizontal plane. Neither attempted such a description for the vertical p1ane, which to some extent is

I understandable for the wolf spider, in particular, is normally assocÍated with a flaL substrate on which any reacLion between it and poLential prey will take place. The same argument cannot be used Ín connection with the mantis hos¡ever, for it is apparent that nantises respond to prey in the vertical plane, (this nay of course be true for the wolf spÍder).

model to the I Before comparing Lhe results of fitting Hol1-i.ngrs

I data obtained for Ranatra it 1s perhaps useful to reiterate his

I

1 methodology bri-ef1y.

rì

¡. :, .il' T

I {

4

T J 1' B2

3ee_.lire" qf- 3:4.3 " 1 Hollinsrs ¡tq,lel" _f9¡:- ¡þlaima.EæA of _tle -b1.4-

II . c::assa . Ilolli-ng replotted the original polar co-orclinaLes of reactive [email protected]

nantid moved its head to fixate the fly, using Cart.esian co-ordinates.

The maximum distance of reaction declinecl in an S-shaped manner as the angle to Lhe body axis increaserl. This pattern was found at 4 different hunger levels of the mantid (Fig. 3:10 Column A, Diagrarn 1). In order to produce linearíty a double transformaLion lras necessary. The first. transformation, concerned the dj-stance of reaction itself. Thís distance, Holling argued, must in part be a function of the size of the

object viewed, since a larger object, he presumed, could be noticed at a greater distance than a smaller one. The important determinant of awareness is likely therefore to be the angle subtended by the object, t measure nurnber ommatidia that- are ï1l since this angle is a of the of I stimulatecl. Thi-s angle Ho11íng termed Ithe angle of vi-siont and defined the rminímum angle of vision for awarenesst as the angle subtendilìg an object at the extreme boundary of the visual field (Fig.3:10 Column A, Diagram 2). (It is considered that the term rextreme boundary of visiont is regrettable in retrospecL for we have no evidence that the object being subtended is indeed at the fextreme boundaryr. The mant"id,

i ¡ as may other predators, may be quite capable of piercing objects at a far greater di.stance, but simply not rreacLingr to them in any ,{ f identifiable ruay). If the size of the object and the maximurn reactive !

,1 't " r I

I {

-"{ I' B3

dista¡ce are known then the minimum visual angle for atvareness can be

\ calculated frorn the expression :

Lan a/2 L/2'a r or q 2 arctan (L/2 ru)

where d mínimum visual angle for avareness

L length of prey

and r maximum distance of awareness. a

Thís transformation changed the prevíous1y S-shaped curves to curves with slopes that change in one direction (Fig. 3:10 Column A, Diagram 3). Since the graphs so produced depicted angles of vision rather than disLances, the relations were reversed, that is the minimum angle of visÍon increased as the angle to the body axis increased. This .I ffi size of the ommatidial angle may Iti suggested to Holling that the effective I change in the same way frorn the front to the back of the eye, although no sections \.rere taken to confirm this, and that the curvi.linear form of the relationship found resulted from the architecture of the mantid eye. A second transformation, that of squaring the angle to body axis, was necessary to produce linearity. As a resulu the relation between o and.

the angle to body axis could be expressed as : i. 2 ¡ 0 =Q + m0 (2) i o

I I 1 where 0 angle to bocly axis

tt .t 0 minimum angle of vision directly in front of the mantid ( ,t o I o=oo) size or hunger. if anrl m the slope of funct.ion of field

I.l l' Fiq. 3:10 Summary of the various methods used in calculating the arousal fj.elds of the

mantid, H.crassa, (Column A, after Holl-ing 1966) and R.dispar (Columns B to D ).

CoLumn A. Figs. I to 6. Summary of Hollings method to describe and model the reactive field of H.crassa

Column B. Figs. l to 5. The application of Holling's model in describing the arousal field of R.dispar.

Column C. Figs.1 to l. The modifiecl HotJ-ings model (the use of 0 instead of 02) to describe t.he arousaL field of R.dispar.

Column D. The Multiple-regression method.

See text for full details. ..-€È¡:_,/ - .. j.+2.- - --- A- ^ - -

Column D Column A Co¡umn B Column C

a o E 30 ¡- e ¡ E o

ao E E

!ôc th.l.llrO) o ala T 2 I o rÐ ao: t0 at+ .o9Ft I ã I ¡-.eô l_i.----i r¡

E f 200

a¡ar. r. aody a¡1.(cl d.¡....

ñl o Fâ¡l flmo TULfIREGRESSION SUUMARY (Ð 24 4S 72 96 120 ¡ lãblâ B F-rêtlô Sla^¡l P o .al .98 .Ê2 ,e8 .94 zo Nal¡oang -ú.ta laat.2a t 0.Ol .at 200 o' .61 .86 .77 .92 .o4 o Faal o.tt l¡7.07 t o.ot .ae o añtr. ro aod, ¡¡r.(e¡l d.sr... Conatanl ¡e.t¡ l2e t 66 r o-or

(4) Fa.l liñô lnlercept(4) Slopo(m) C (m/4) Fst T¡il htdcop(¡) slopo(m) C {n/¿) Faat TLrÞ htrcepte) slopein) C lñ/¿l À 174.€O o.0446 0.ooo256 24. 1A 67 8.0¡ 1O 6.91 1o-ð 24 12.36 o,19 oorS a f 41.OO o.o415 o.o00294 48. 13.22 9.1¡ lC 4.7¡r0 " 4a 7.89 0.14 o.0231 AD. 29.74+(lnOx -5.t4) 4{FT x 0. t I ) @c 98.95 o.0s23 o.ooo327 72. 11.12 t-ilto' 6.5xlo " 72 0.s2, o.1 5 o.0222 D s2.14 o.o219 o.ooo266 9ôo 8.43 6.9rlO 8.2x10' 96 4-f7 o.r3 o.o2a 1 lot 24< FT< l2Ohi 124.45 o.o350 o.000241 l20. 8.to 6.1 ¡ 10 7.srro-' 4.61 o,12 0.0264 .2)^120

@ o

(

¿o ¡o

ÀâOUSAL FIELDS B4

The equatioD ruas made more useful by expancling m, the s1ope. Since, Holling ar:gued, the relatÍon bet-ween 0 and a j.s. a function of the strucLure of the eye, and this sLructure is not subject to changes induced by hunger, it is 1ikely that the ninirnum angle of vision at any angle 0 bears a consLant relat.ion Lo oo, irrespecl-ive of hunger:. That l-s

q. a K

0, o

where 0. ruinimum angle of vision at 0 l_ l_ and K a constant.

In order to fulfil this condítion, the slope m, ín equatíon (2) must be a functlon of q such that. o

c m =0'o

where C a constant, independent of hunger, that describes the way effective orunalidía1 angles change from the front to the back of the mantid eye.

I,lhen C l¡as calculated by dividing,cointo m, using values obtained from regression analysis of the dal-a in Fig. 3:10 Column A, Díagrarn 4, the values obtained for C were reasonably constant (see Fíg" 3:10 Colurnn i\,

Diagram 5) and shorvecl no consistent trend r,¡Íth field si-ze. Therefore 85 equation (2) was rewritten as

([=0 + 0oc02 (3) o ...

It was now possible to describe the reactive field of the nantis' Ilolling used the previously derived expressing

or L tan a/2 Ll2t a

ra [2 tan (a/z)]

vhich, takingoas sma1l, was simplified to r" Lla, . (4)

This equation, proposed Ho1ling, had general application to any visual predator, since it sirnply involved the geometry of vision. Equarion (3), whích described the speclfj-c way in which the

minimun visual angle of mantids changed at various angles to the body

axis, could not,¡ be substituted into (4). That ls

L

ra (0 + soc 02) o

or L

a 0 + c 02)l o tL/G 86

From equation (4) L

o ([ o

l¡here r the maximurn distance of reaction directly in front of the o rnantidwhere0 = 0o.

therefore

ro

a (1 + C02¡ r . (5)

Equation (5) provided Holling with a description of the shape of the reactive fie1d, independent of hunger, pr:ovided ro and C are knor'¡n' Fig. 3:10 Column A, Diagram 6) shows Lhe response fiel d for ll.crassa based on this equation (5), usÍng the empirically determined values for r ancl C (as shown in Fig. 3:10 Column A, Diagram 5) - after Hollíng o 1966).

3:4.3.2 Models for the Bstimallon of thq Arousal Field of- R.dispar In order to compare how well the Holling model fitted the data obtained f ot @!g, at. least in the horizontal plane, the data was processed as outlinecl above. Figure 3:10 Colurnn B shot.'¡s a comparative

sunmary of the results calculated from Hollingrs node1. 87

Ho1-liDg failed to provide any data on hotr precisely his model rthe fiLted the observed data, aparL from j-ndicaLing that shape of each field sÍze clearly is very well rlescribed by Lhe linesl (Houing 19ó6 rfitr p.zg). Using the Holling model for the 8..4:.Plg- data the aPpears to be good although the rnean square r:esicluals for all fast períods are quite large (Table 3:10). A better fit, in terms of srnaller residuals,

was achieverl by using the untransformerl angle to body axis term, 0, rather than 02. Comparison of the R2 values in the regressions of s

(minimum angle of visíon) on either 0 or 02 i¡dicates a better linear relationshi.p using 0, as shown in Fig. 3:10 Column C, Diagrarn 1.

The resultanL calcul-at.ed distances for each angular position of

prey at each fast 1eve1 of predator give significantly smaller MS residuals when cotnpared wiLh tTre observed da.ta' as sholJn in Table 3:10" This improved rfitt can also be ol¡served in the re-plotLed field shape for the predator as shown in Fig. 3:10 Colurnn C, Diagram 2. Hardman ancl Turnbull (19S0) constructed the shape of the reactive field of the rrlolf

spider, Parclo-sa vancouveri by following llo1l-ingts (L966) basic

rnethodology (as outli¡ed above) but then using a multiple regression

equaLion r,¡hich related the cosine of the angular position of the prey to

the bocly axis of the predator and the fasting period'

The trends described by Harclman and TurnbUll for P vancouv rl- an relation to the effects of foocl deprivation and angular posit-ion of prefr are very similar to those clescribed fot &diupql- here. This being j'f so it secmecl approprÍ-ate to examine their nlodel in order Lo see it IABLt l:10 lable ol CalculotecJ ÂrousaÌ Distance (AD), ResicJuel SS, encl ResiduaL l.iS, betveen the 0bserverJ ancl CalculaLed Arousal Disl-ances at various ongular posiLions of the prey ond loJ-louring 5 dif,feient periods oF Food deprivation For tlro predator. Tho calculated AD uere erriveC at. From J diÍl.er.ent mocjels l-lral ere desctibcd in the text.

FAST ANGLT OF HOLLINGS TIME PRESINTAT ION ORICINAL I,IOD f4ODIFIED HOLLING ¡IODEL I'IULTIPLE REGRESSION

lcula ted CaIcu la t.ed CalculoLed AD Resid.SS n ltS DF AD Resid . SS NMSDF AD Resid.SS n I,I5 DF oo 27.' 62.5 10 27.5 62.5 Ì0 28.2 742 10 45 2t.B r257 7 tl. 5 252.5 7 l0.l t94.6 7 90 17.0 892 - B 72,5 J2 ' " 8.9 80.5 8'12:2 6.3 51. J. B 9.7 32 rt5 1I.5 ?TT '2 4 6.7 ll .6 4 4.0 4.0 4 IBO 7.9 42 5 5.3 0.5 5 2.t t6.4 5

0 J4.9 107 l0 t4.9 106 :9 Ì0 t4.7 107.4 l0 45 to.2 1074 . B l0 17 .1 384.9 l0 12.8 256 t0 48 90 'lt.l 2r.5 1556,2 9 T9 4t II4.' 9 15.2 43 8.9 49.t I IO.4 4t 135 14.5 460 0 8.5 52 .B 6.5 44 I lB0 10.0 179 B -8 6.8 24.t 4.9 9.7 B

0 t7.7 lr.4 l0 17.7 ll4. I l0 17.2 It6.l l0 tl5 t2.6 2369 l0 lB .5 240.5 l0 15.4 299.9 t0 72 90 2J.2 llB0 46 10 98 12.2 60.6 J.0 ll.l 46 tr .4 59 IO I2,5 46 735 15.7 7A6 9 9.r 78 .5 9 9.t 79.5 9 tBo l0.B t44 9 7.J 50.2 9 7.4 50 .4 9

0 40.J 102 t0 40 3 102. L t0 J9.7 t05.8 l0 45 34.9 ll00 l0 l9 7 J76,9 10 lB .0 552 t0 96 90 24.8 tl6B l0 69 48 T' I t2.9 l0 l2.t 48 74 23 l0 r7.l 4B 135 .t6 .8 594 l0 9 I 14,6 l0 tr.7 76 IO TBO lr.6 205 l0 7 I 67.9 l0 l0 111 t0

0 42.1 92 l0 42.J 92,1 t0 42.2 90 L0 lt5 36.6 712 IO 20 696.5 IO 20.6 712.4 l0 I20 90 26, r n45 4B r0 59 T7 7 30.5 l0 t7.6 4B 16.6 62.4 10 ?5 4B rt5 17.6 695 l0 I0 J l0 .5 l0 14.2 252.8 l0 lB0 t2. r 10 lr5 2 3t l0 12.6 I47.4 10 88

\ìras an ímprovement on the rnodified Holling model used for R"díspar.

Linear regressions seemed just.ified, on examining the data for R.clispar, between 1n0 (the natural logarithn of the angle to body axis) and the arousal distance of the predaLor (Fig. 3:10 Colunn D, DÍagram 1), and Fast time with arousal distance (Fig. 3:10 Colurnn D, Diagram 2)' The multj.p1e linear regression relating angular position of prey in relatiou to the predator body axis and the food deprivation tine to the arousal distance of the predator is :-

AD = 29.78 + (1n0. -5.74) + (Fast Tir"(hru) 0.105) for 24 ( Fast Time ( 120 hours.' Ful1 regression statistiis are shor+n in Table 3:11. The calculated response fields for R.dispar deprived of food for 24, h8,72,

96 and 120 hours are shor,'n in Fig. 3:10 Column D, DÍagrarn 5.

Examination of the MS resÍduals for each fast tirne (Table 3:10) indicates that the multiple regression model gave smaller residuals for tj¡e 24 and 48-hour fields, but larger residuals for the 72, 96 and 120 hour response fields when comparecl rvith the tmodifiedt Holling model"

The overall mean MS for the modified model is some 10% less than the nultÍple regression nodel, and therefore indicates a better fit to the daLa. TABLE l:1I Summary of Multip1e Regression Analysis of Arousal Distance of Predator (mm) on the natural logarithm cf the presentation angJ-e (o) of model prey (in the horizontal ptane) and food deprivation time of predator (h)

ANALYSIS OF VAR. DF SUM 0F SQs MEAN SQ. F

olt Multiple R RegressJ-on 2 29925.3815 14962.69r 874.62

R2 .89 Residual 224 3832.r3L9 17.108

Adjusted R2 .88 o7 00, Std. Deviation 4.r3 Coeff. of variability L). /lo

VARIABLE g Std. Error of B F-ratio Siqnif.

Natloang -5,7430 0.1¿r1.0 1657.?60 p <0.0001 Fast 0.1051 0.0084 r57 .369 p <0.0001 Constant 29.7782 0.8256 I29r.548 p = 0.01

SUMMARY TABLE 2 VAR IABLi F-ratio Siqnifieance Mult eR R Simple R 0vera11 F Nafloang 1657 .256 p <0.0001 .898 .807 -. B9B 874.616 Fast r57 .369 p <0.0001 o./! l . BB6 .2L6

Equation: AD : 29.778 + (Natloang x -5.743) + (Fast ''}ê 0.105) f .or 24 hrs

VBRTICÀL PLANE rarousalt AttempLs to describe mathenatically the vertical fi.eld, ll in the light of the lIollíng (02) or modified l{olling (0) models were unsuccessful.

Examinatíon of Fig. 3:114, the change in tarousall distauce at

each angle to the body axis tested, shows a cyclic change over the 3600 examíned. The sinusoidal nature of this curve suggests a cosine transformation of 0 to produce linearity when regressed agaÍnst tarousalr distance (see Fig. 3:124).

However, regressions of 0 (tlìe minimum angle of vision) against

Cos$, for each of the fast times examined, resulted in non-significanl: slopes in every case (t(0.85, p>0.1 in each case for testing observed an ü slope i-s di.fferent fron 0). l,lhen the 3600 field was divided into r upper 18Oo and a lower 1800 section by bisecting'a1ong the 0/3600 axis (the predatorrs l.ongitudinal body axis) (Fig. 3:78) and separat-e regression analysis for Cos0 r,¡ith 0 undertaken, signifícant slopes r'rere observed for each fast period in boLh the upper and l-oi'¡er sectíons (t>3.08, p(0.05 in all cases). Table 3:12 shr¡ws the regression surunary for boLh the upper and loler areas with the calculated const-ant, C, as described previously.

È I I Fiq. 3zII (a) Tlre clrange in Arousal- Distance of R.dispar vhen

model- prey are presented at differenl angì-es Lo the body axis fol.Iouring 5 different food depriva- tion periods.

Fiq. l:11 (b) The minimum ang les of vision ol' R.dispar foJ-lovinq different food deprivation t.imes, in relat.ion to the angle to the body axis.

Only the 24 hr and 120 hr fast curves are shovn. The remaining 5 intermediate fast periods follour tlre sane patt.eln, but have l¡een left out to avoj.d overcomplicating the f"igure. (a) r 1?Oh¡ Fast

Ë o 96hr Fast ¡ Ê 40 Ð 72hr Fast o TU o ü o 4Bhr Fast A n 24hr Fast g 2 Õ 20 () u)

O Á 't 4 û

$- (b) ttJ J $ z, 4 40 4 ä * ä q 2t ; ff ^

t s0 't 80 27ç 360

AhåGLH Tü #ÜÐY AHË$id} 90

TABLE 3:12 Surnmary of Regression Statj stícs from plotting o (the

mj_nimum angle of visj-on) on Coso (vhere 0 equals the angle to borly axis on Lhe model prey in the vertical plane). The 3600 field has been divicled into an Upper 1B0o and Lot,¡er

1B0o fiel_d th::ough the predatorts longitutlinal body axis.

Y-intercept (co) Slope (M) R Constant (C = co/M)

IJPPER 1BOO

24 h î.ast 24.18 -22.94 -.84 .94

48 17.15 -14.6/¡ -.88 .85

72 13.83 -LO.2B -.90 .74

96 L2.77 - 9.50 -.87 ,74 L20 11 .21 - 7.89 -.85 .70 .79 = Average

LOWER 1BOO

24 37.76 -28.77 -.98 .76

48 31.59 -18.97 -.76 .59

72 24.77 -L3.72 _.69 .55

96 22.34 -L2.93 -.75 .58

120 19.15 -10. 87 -.78 .57 .61 = Average

However, when the average constant values a::e substituted into Hollingts equation for estímating the tarousal disLancet (AD), and solved, the results are both erroneous and self contradictory. It woul-d appear therefore that, the assumptions made by Holling, in formulating 91 the equation concernÍ-ng rvísua1r continuity from the front to the baclc of the predator, when observed in the horj'zontal plane, do nol- hold true in moving from the top Lo the bottorn as observecl in the vertical plane, at leas t not for R.dispar. Possible reasons for this incons:i.sl-errcy will be discussed at the end of the chapter"

In order to describe Lhe vertical tarousalt field of B.dispet' wíth the obvÍous effect of food deprivation, a sirnple nult-iple regression equation was developed. This related the angular position of rhungert the prey to the prerlatorts longitudinal body axis (0) anrl the leve1 of the predator (as measured by food deprivation tine) to the resultant arousal distance.

As menti-oned prevíous1y, the sinusoiCal nature of the curve shown in Fig. 3:11, suggested a cosine transformatÍon of 0 to produce linearity when regressed with the observed arousal disl-ance (see Fig. 3:124). A second regression, that of fast tine (hrs) wiLh arousal distance (Fig. 3:12B) seemed to be justÍfied (see also Ilardman and Turnbull, 1980). The combined multiple'regression is :- * AD = 13.393 + (Cos0 * 13.088) + (FasL Tito"lhr"; 0.086) for 24 < FAST TIl"tE < 120 hrs.

The ful1 regression statlsti-cs are shown in Table 3:13 and the resultant calculatecl response fields for R.dispar deprj-vecl of food for 24r 48, 72, 96 and 120 hours are shorrn in Fig. 3:138 Fiq. 3zI2 (a) Regression of the ArousaL Distance (AD) of,

R. dispar against cosines of the angle of' model- prey presentation (0) in tlre vertical plane. The cosine is 1.0 directJ-y in f,ront of the

predator (0 = 0o), is 0 at 0 = 90o ancj is -l-.0 directly behind the predator.

Fiq. 3zI2 (b) Regression of t.he Arousal Distance (AD) folJ.oruing dif,ferent periods of fasting. 40 (a) c

(Ð

e o ,

ð H 4 ADe18.O9+12.Ð3COSO R.g2 þ o T-3.17, 5df, pcO'OS

¡¿l Ð o úÉ - 1 .O o 1.O 4 þ e0srt{ETA(') (cosêl #)

40 (b)

Ü)p 0 tr ({

o

6 2

ð AO=16.32+O.O7FT R .97 T=$.37,3df, P

o DlL 48 72 96 f 20 FAST TsMË(hr) (FT) TABLE l:11 Summary of Multiple Regression Analysis of Arousal Dístance (AD) of Predator on t.he cosine of the presentation anqle (cosrang) of model prey (in the VERTICAL plane) and food deprivation time of PredaLor (hrs)

ANALYSIS OF VAR. DF SUM 0F SQs MEAN SQ. F

Multiple R . B0l Regression 2 294s5.A43 I4702.522 2BB . B] R2 .645 Residual t17 16137.756 50.908 Adjusted R2 .643 Std. Deviati-on 7.134 Coeff. of Variability 3t.o2ó

VAR IABLE g Std. lrror of B F-ratio Siqníf " Cosrang 13. 08 B .562 540.787 p <0.01 Fast a.86? .492 53.rO3 p <0.01 Constant L1.393 .975 180.528 p <0.01

SUMMARY TABLE 2 VAR IABLE F-ratio Multiple R R- Simple R Overall F Cosrang 540.687 .76 .; .76 2BB . B07 Fast 51.108 .80 .64 ,20

Equation: Ad 13.393 + (Cosrang. 1l.0BB) + (Fast A.862) for ZLt hrs < Fast < 120 hrs Fiq. 3zI3 (a) The predicted arousal- f,ield of, R.dispar on the hcrizontal plane, 1'ollorving 5 food deprivation times as cal-cufated using the modified

(0 instead of 02) l-lollinds moclel-.

(L:) The p redicted arousal field of R.dispar in the

vertical- pJ-ane f"ollouring 5 food deprivat.ion o times as calculat.ed using the multiple- regression t.echnique.

Both axes are measured in mm. (See text for details). 40 HORIZONTAL PLANÉ

FÀST PEFIOO 21h¡ o 3 FAST PERIOD 4ah, a FAST PÊRIOO 72lv O

FÀSI PEFIOO 9€hr . FASf PERIOO l2O hr ^

¡¡¡ ra.. OooooOooÒoooOOooO .¡rtaoðoooa.a.aaaaaaaa oooooo ral¡ .¡¡.oooa..a.a ooooo Ä. ¡ @oaa a ¡1. ooa. aa a a aaa ooo .. il S.. 'r' ¡t to. . 600 a aa oo I a. ooo 'r

INT O

o -15 -30 9 21 33 45

I VERTICAL PLANË

20 oÔoooooo ooo ..¡ oôo ¡ ¡ a o o ¡.^¡ .. o a - ¡ ¡ a¡ o o a Ù oo " 10 .¡ . a o a a oo

o oo aþ o . ¡ ¡ ,l o o o aa o Dao ooo

oo oã ooo a.oo a . ôô " aa coD o- a . o o a -10 ¡ ¡ < o aa o r¡ !o a¡ -J o -a a a. O o ¡ ¡ .¡ a a a a. a aa oo o ! I lil 20 ,i -25 -l I o3 17 3f 45 il

.'r

il I { , -ql ï 92

3:5 DESCRIPTION OT'AND EFF'ECT OF PREDATOR FÄSTING ON THE 'CAPTUNE

sPAçEr .

3:5.1 Methç,ds_ The experiment-al cletails outl.ined in Section 3:4 rvere repeaLed except as outlined belorq.

TEST PROCEDURE the movernent of Lhe model prey was continued along the directÍona1 path after tarousal movernentsr had been noticed until the predator attenpted to capture the mode1, í.e. a STRIKE took place. The Strike l)lstance h'as defined as that distance from the rnid poínt (o)

betr+een the predatorrs eyes to the anterior tip of the model prey when the strike took place, í.e. there was a rapid lowering of the femora on whÍch the tibio/tarsi are closÍng.

A Stríke r+as classified as :- l. A HIT (+) - Contact was made between the femur/ttbio tarsi aod the nodel prey. 2. A MISS (-) - No contact was made. 3. CAPTURE - A successful Hit - contacÈ was made and pr:ey was held beLween the femur/tibio tarsi articuiation.

3¡5.2 Results

Fig.3:14 shows the shape of the strike field following 5 different periods of food deprivat.ion of the predator. It isr as probably expected, restrictecl to the region anterior to the predator and is delineated by the +90o to -45o lines, in relation to the 0o line running through the longitudinal- axis of the body, when vierving the I t prerlator ín the vertÍca1 plane (see Fig. 3:14). In the horizontal plane the predator can strike (capture) on1-y at prey 45o either side of the '[he Fiq. Jz14 shape of the strllce space in t-¡oth tlre

horizontal and vert-ical- planes l'ollouiing 5 dil'ferent. f'oocl de¡:rivation periods. [:or

claril-y orrly the 24 hr and 120 hr fasts have been shovn (i.e. tlre shortesi- and .Iongest).

(@) 2/+ hr fasl- (@) l2o hr rast Htffiru0NTA[- FLANE

gCI

45"

Ð" VERTIEAL s0 FLA NË

45

t

sls 93 body axis, although prey brought to rr¡ithin 5 nln along the 9Oo path evoke a strike rvhich, however, i-s not directecl at the prey. To test for a food depri-vation effect a one way ANOVA, with repeated neasur:es design' 6pss, statistical Package for the social Sciences, update 9), rvas performed for each of the angular paths of presentaLion where strikes occurred, in both the horizontal ancl vertical planes. Tn all cases rhungert except one (90o, Horizontal) significant effect of h'as observetl (see Table 3:14).

The changes in the mean strike distance along the various paths. of presentation are shown in Fig. 3:15 A to F. Cornparison of means was carrj.ed out using the StudenL Newman Ke-r'tls Multiple Range Test, the results of which are also shown on Fig. 3:15.

TABLE 3:14 The effect of food deprivation on the dístance at r¿hich R.dispar struck at a model prey when moved along a number of different angular paths of presentation in either the horizontal or vertlcal p1-ane. Fast periods were 24, 48, 72, 96 and 120 hrs.

ANOVA TABI,E

HORIZONTAL PLANE SS DF MS F p

Presentation Path

00 140.71 15 4 35.L779 7.43 <0.05

450 131.5626 4 32.8906 9.69 <0.05

900 0 40 0 NS

.../(contd) 94

p VERI CAL BLANE SS DF MS F Presentation pal-h

00 LOi.279L 4 25.3198 8"36 <0.05

4 14 4300 2.97 <0.05 450 57.7200 "

900 36.2761 4 9.0690 2.89 <0.05

3150 90.3512 4 22.5878 3.43 <0.05

For each of the different presentation paths in both vertÍcal and horizonLal planes the effect of thungerr was about the sarne, i.e. there r{as an increase of about 5 mm in the overall strÍlce distance" (i = 5.6'

SD = 1.2 mm in vertical plane; T = 5.5, SD = 1"4 in horizontal plane). There \rrere some dif ferences, depending on the angular path of presentation although they were not significanL as Table 3:15 sho¡ss.

TABLE 3:15 The increase in strike distance of the predator for prey offered along various angular paths of presentaLion in both the vertical and horizontal planes following a I2O hour fast of the predator.

VERTICAI, PLANE ANGULAR PATII OF PRESENTATION (o)

oo 45o 9oo 3150 1 sD x2 p Increase in

Strike Dist. (rnn) +6.5 +5.9 +4.9 +6.2 5.6 !.2 .7 4 NS

HORIZONTAL PLANE Increase in sdi/ùt" Dist. (mm) +6.5 +4.6 5. 5 r.4 .35 NS l Fiq. 3zI5 The change in mean (a959í' Confidence Interval) stlike distance aJ.ong various arrgular paths of

presentation o1' nlode-l prey in br:th the vertical- (a to d) and horizontal planes (e and f) folloruing 5 diifelenl- l"asl- periods.

(a) 0o Vertical Pl-ane

(b) 45o Vertical Pl-ane

(c ) 90o Vertical Pl-ane (d) l15o Vertical Plane

(e) 0o Horizontal Plane (f) 45o Horizontal Plane

Means vith same letter on any one graph are NOT

signif,icantly different (SNK Multiple Range l.est, q = 0.05). €t A

16 I Ä

14

12 (e) 4 16 BB B offic (b) ^ 14

A d òe 't4 A 12 tn g) i-l +t lx ! TO 10 6æC Æo u"l ( O c ) æ, (f) 4 14 Ä : þ I {Ð 10 A I õ 12 A ltJ g a I 10 þ- (d) (å 1 6

4 24 48 72 9ö 1?O 12 AB

I 24 4B 72 06 120

h FA$T TltulH{rrr) + t x Jì

¡. t ,l I {

{ i¡ I rl I' 9s

To describe the strike space pe! se of a sit-and-wait predator ís to iclentify only a minor, although significant, component of the predatory process. lrlhat is of paramount importance hor+ever, is the preclator t s strike success or capt--ure ef f iciency within this dynamic space. In order to dissect this relationship two questions can be

posed:

1. Are their partícular regions within the strike sPace thert, given an equal number of prey presentations, evoke a greater number of strikes or caPtures? 2. hlhat is the effect of food deprivation on these regions?

In order to answer these questions the previous data were analysed ín relation to the number of presentations, and subsequent- strikes, hits and caPLures.

Because of the extreme difficulties in repeatedly ident.ifying a point in a 3-dimensional space, the 5 angular paths of presentaLion were used to identify regions where strikes, hits and captures were highest.

Fig. 3:16 A & B shorvs the dífferences ín the proportíon of either strikes, hits or captures rvhen the prey \4¡ere presented along theSe different angula:: paths in both the vertical and horizontal planes' following the 5 periods of footl deprivation.

I t Tn the vertical plane (Fig" 3:164) those prey presented along either 0o or 45o path appear to evoke the greatest response witl-r respect Fiq. 3z16 The effect ofl arrgJ-e of prey (to predator body axis)

and f ood deprivat..ion on the striking, hitting and capturing behaviour of, R.dispar.

(A) Vert-ical Pl-ane

(a) The number ofl strikes is expressed as

a ,"ó ol' prey presentations.

(b) The number of hj-ts is expressed as a

9ó of strikes¡

(c) fne number of capLures is expressed as

a ,'á of hits.

(B) l-lorizontal- Pl-ane

The number of (d) strikes, (e) hits and (n)

captures expressed as in Vert.ical PIane (A)

above.

Angle of Prey to Predator Body Axis.

(tr) oo

(c) 45o

(o) 9oo

(c) 3r5o (A) VERTICAL (B) HORIZONTAL

(a) (srRrKEs) ( d ) (srRlKEsl

100 I 0-o-o I ¡

-a o\-o z oUJ É o u¡ o- ( b ) (Hrrs) (e) (Hrrst ø 100 o a UJ o U' ø t-t l! E c- t -ar- o. x o ul É, UJ o dtç G (c) (c¡prunes) (f) (c¡prunes) zÐ too tr æo a o-o o o/ il". a o/ o aæa o

a I o 24 4S 72 96 120 24 48 72 96 120 FAST T!ME (hr) 96

to capture attenPts. Those prey presented along the 45o path promote the 1-argesL nurnber of strikes and hits whilst those mor¡irtg directly

t.oruards Ehe predator (0o) are capt-ured more often. Tn the horizontal plane, Fig.3:168, prel prersenLed a1-orrg the 0o evolce the largest number of strikes, hits and capl-ures. Prey rnoving in Lhe horizontal plane greaLer than 45o to the predatorrs body axis are not capturecl, although at very cl-ose dislances (e.g. 5 mm) they evolce occasional- strikes- The results of 0o and 45o support Lhe clata from the vertical- p1.ane wj th respecl- to the number of captures. It woulcl appear chat prey noving within the 0o to 45o arc, in both Ehe horizonLal. and vertical planes,

evol

The same pattern is observed in the horizontal p1ane, with prey

I presented along the 0o path evoking the largest number of capture attempts. Again there is an,obvj-ous thunger effectt in both the 0o and

, capiures. i 45o paths, increasing Lhe proportÍon of strikes, hiUs and t Prey that are presente

i, presentation ang1e, even follovring the 120 hour fast; the model prey tÍ mm evoke a strike anrl no capl-ure was ever t must stil1 come to within 5 to

I

4 I i 97

observed. As a measure of the leve1 of success or sLrike efficiency for fcapture each angular path of presentation the efficiencyr 'tl. /f CaPtures CE= x 100 # Strikes

was calculated for each path and fast period'

Table 3:16 shows the mean capture efficiency for each angle of presentatÍon of prey and fast 1eve1 of predator in both the vertical and horizontal plane. Analysis of variance reveals no highly significant effect of fast (thungeri) or angle of presentation' x TABLE 3:16 The x capture efficiency of &gigIg- (# captures/# strikes 100) for each angle of prey presetltation antl food deprivation tíme of predator in both the vertical and

ü horizontal plane wlth the associated ANOVA Tables. t& I VERTTCAL PLANE

cÂPnIRE_ BFFTCTENCT f %)

ANGLE OF EASr PBRI9D 24h. 48 72 9tL U9-

PRESBNTATION

00 100 75 70 70 80

450 71 72 60 64 70

i o 50 66 70 i 90 050 I o 50 66 75 55 I 315 100 t MS F p 1 ANOVA Source SS DF

). Total 8078.2000 19 425.1684 iir . 'il 2.0618 NS T Angle 2671.4 3 890.4,667 ,t I Fast 224.2 4 56.05 0.1298 NS

Resirl. s182. 6 T2 431 .8833 iir l¡ I i t' .../(cont. ) 98

Table 3:16(cont.) I

HORIZONTAL PLANE

CAPLURE EFFICTENS. (Ð-

ANGLE OF FAST PERTOD 24 48 72 96 I2O

PRESENIATION

00 100 75 70 70 B0

450 s0 43 62 62 66

¡

ANOVA Source SS DF MS Fp

Total 2249.6 9 249.9566

Angle 1254.4 I L254.4 7.493 = 0.055

Fast 325.6 4 91.4 0,486 NS

* Resid. 669.6 4 L67.4

Ttl

The capture efficiency is noL affected by food deprivation lrrespective of the angle of presentation of prey" Although the ANOVA did not reveal a signifÍcant effect due to angle of presentation on the capture efficiency, in the vertical plane (p = 0'15) in the horizontal (p t'lith thi-s' an i plane the effect was not quite significant = 0'055)' i examination of l-he mean capture efficiencies over all fast períods for ¡, ,t capture is higher in some parts 1 each angle shows that the probability of

!

¡. of the strike field than others.

tt. f

,t rl { 4 ITf ,'.¡ 99

Table 3:17 surnmarizes the mean capture efflciency for each of the angles of present-ation in both vertical and horizontal p1ane, irrespecLive of fast Lime.

TABLE 3zL7 The mean capture efficiency of &-d. Lar when the prey are presented along various angular paths to the predator in both the vertical and horizontal p1'anes.

VERTICAI, ANGLE 5¡ CAPTURE EITFICIENCY (7") + Std"Dev.

0 79 + L2.O

i 4s 67 + 8"2 I i ' 47 + 28.0 i 90

31s 69+20

I I t l{

J HORIZONTAL

0 79+ L2.O

45 56 + 10.0

I

.,.1 .J

ì prey to the i clearly then the angular position of relaLive i on capture probabilíty. Ïn i predator has a profound influence the I one would intuitively expect a 1 additj.on to the angular positiorr ri tdistancet effect on tlìe strÍke and capture parameters' due to Lhe T, ,t physical limitations of the raptoríal 1egs. However, due to the design

,t I (anrl the smal1 sample sizes) no correlation analysis { of this experiment betweel the distance of the strike and subsequent capture success \^/as *.{

I, { 100

undertaken at thls poinl-, alttrough the data and experj.mental observations sug¡lested a strilce distance capture success relations'hip to exist (see Sect-íon 326.2).

fn sumrnary ì-t can be seen that the overall strike/capLure space of R.díspar is restricted to the space anLerior Lo the predator and

bounded by approximately the 45o axes eíther side of the horizontal plane and the 90o to -45o (3150) txes in the vertical. The 1eve1 of rhungerr) food deprivation (as a measure of preclator significantly affects the distance betrr'een the predator and prey l¡hen a strike takes p1ace. The longer the fast period the greater the dlstance from rvhich the strike takes p1ace, up to a maximurn of about 30 nun for a 120 hour fasted predator. Presentation along some paths evokes more strikes, hits and captures than presentation a1-ong others, indicating that there

is an opLimum region within the slrike space where the capture efficiency is highest. In particular, prey rnoving along the Oo path in the horizontal plane or either the 0o or 45o path in the vertical plane,

evoke the highest number of strikes, and hits, whí1e the highest capture efficiency occurs when prey are moved on the 0o path. Following long periods of fastin g (>72 hours) prey moved j-n the regj-on bel ow the 0o axis (Vertical Plane) along the -45o (3150) presentation path evoke a sinilar level of sLrikes as both the 0o and 1+5o paths.

Clearly then, for a predator like R.dispql, both the position of the prey (i.n relation to the predatorts longitudinal body axis) and the

ir :1 time since the predator lasL fed significantly influence not only the 1 ,l atLempt but the {r proporLj-on of encounLers that wíll result in a capture dj-stance at which the attempt will. be mo.de. llowever, anoLher important -t 101

variable (as mentloned in the introduction) in examining predatory behaviour, i-s that of prey size. Therefore to investÍgaLe the combined influence of prey size and predator thungert (as measured by food deprivation time) additional experimenLs rvere carried out, the results of which are presenterl in Section 3:6.

3:5.3 Disucssion The sequence of behavioural- events leading from initial arousal to prey capture and finally prey tiiscard that were observed in R.clispar are slmllar to those of R.linearis (Cloarec 1969b). They foll-ow the predatory pattern that has been previously reported, for exarnple in mantids (Ho11ing 1966), salticitl spiders (Gardner L964)' ghgbor.,t" larva

(Pastorol< 1980), a number of orb-weaving spiders (for example Jaclcson I L979; Robinson and Mirick L97l; Lubin 1973) and ant-Iion larva (Wilson t974; Griffiths 1980a).

The similarity in overall shape of the aro usal field of R.di-spar

and R.linearis (Cloarec 1969b, L976) tends to suggest that, as in R.linearls, both visual and nechanoreceptors (located ou the prothoracíc tibial spur) are involved in perceiving prey. In addition Cloarec (L976) reported that both these sensory structures combined to elicíL a strike. The size of the strike space is believed to reflect the n relationship between the length of the raptorial- 1egs, the stike path and the overall functional morphology of the legs. The shape of the strike field for R.linearis r+as not reported and so a comParison cannot be to { be made, although one would imagine it to similar &r-Liæ.. to2

The effect. of food deprivation or rhungerr in lnc.reasing the reactive or arousal fields of predators has been reported for a vari-eLy of aninals, for example mantids (llolltng 1966), r+olf spíders (I{ardman and Turnbull 1.980) and paradise fish (Mathavan, Muthukrishnan and Heleenal 1980), although no sultable mechanism of hot" this shoul-d happen has been proposed"

In R.dispar. food deprivatíon j-ncreases the distance of arousal alorrg all directions oi prey presentation in botli the horizontal and vertical plane, although the results tend to indicate that the effect is not as pronounced for presentatÍon paths coming fron behind the predator. The strike dístance also increases rvith fasting, but not so dramatically as that observed for the arousal distance.

Attempts to describe the shape of the arousal field and effect of thungert using the model designed by HollÍng (1966) proved reasonably saÈisfactory, after slight modífication, for the field viewed in the horizontal p1ane, but totally unsatisfactory for the vertical p1-ane. This was probably due to the structure of the eye of RanaEr, with íts two distinct regions (see Cloarec I97Lc, 1976; Pell-erano and Carlo L977; Carlo and Pellerano 1977), and the effect this has on the assumptions of Ho1-lingts model in calculating 0, the mininnum angle of vision, and perhaps more significantly, C, the constant, which e independ/nt of hunger, clescribes the way the effective onmatidial angle changes fron the front to the back of the eye. It v¡ould appear thaL these assurnptions cannot be met in moving from the top to the bottom of the eye. 103

Ilol-ling (1966 p.27) assurnes that the structure of the eye ì.s not subjecL to changes induced by foocl deprivaLion; thus it is liltely that the minimun angle of vision (o) at any angle of prey presentation (0) ís consLanL. ïf thj-s is so what causes the mant.id, or any other predator, thungryt? to react to prey at a greater distance rvhen It is thought by tlre majority of workers (for reviews see Barton Browne L975; Bernays

and Simpson 1982; Dethíer L976) that t--he sensit-lvíty of the sense organ(s) themselves change i+ith the f ¡notivati.onalt staLe. Although the suggestion thaL selective attention is medj-ated by centrÍ-fugal control- of activity in the sensory palhways is not well substantiated (see Hinde thungerr 1970 pp L2O-I/+4) the long-t-erm changes in arousal could stil1

depencl on sensory structures. The evidence' ho\\¡ever, ís against such a view. rn the blowfly, the time relations involved in the adaptation of the taste organs are quite different from those of hunger itself (Dethier and Bodenstein 1958). In rnammals' more dírect eviclerrce Ís available. Meyer (Ig52) found that 34 hours of starvation produced no

changes ín the sa1t, sweeL or bitter thresholds of hunan subjects'

N¿rchman and Pfaffmañ (1963) c.oncluded that taste preferences are

governed by a central mechanism although the possibíliLy that there is a

change in the patterni.ng of the peripheral discharge, even when the overall frequency remains constant, remains open. In addition, in the case of visual sLinuli, where shape, sÍze or patl-erlt relevanL to many dífferent types of behaviour are received through the same sensory organs, motivatÍonal changes in responsiveness can hardly depend on' the

changes in the sense organs themselves. In general, therefore, the effect of rnotivational factors on changes in responsiveness Lo particular stimuli is usually described as a sensitizal-ion of particular stínulus-response relations (Hi-nrle 1970). Á food-deprived predator 104 responds to a potentíal- prey when a satiated one ignores it. Sensitization of sl-inulus-responsc relations irnplies tÌraL there is an inverse relationship between the strengLh of the motivatjonal factors and ttre si:rengt-tr of the stimulus required to eliciL a response of a given strength. Such relationships have been found by Baerends, Brputuer

and Llare::bolk (1955) and lleiligenberg (1965a, b, 1966) (see also Barton Ilrowne 7975i BarLon Browne, Ifoorhcluse and Gertven 1975; DethÍer I9-l6i

Dethier ancl Flanson 1965; Dethier, Solo¡ron and Turner 1965; Nel-son Ig77). Often the mot,j.raLional factor influences not just one response but a group of functionally relaled oiles (see examples cited in Ilinde 1970 pp 60/+.-632). The results observetl in -&rj!!g- are cousistent v¡ith this thene for, iu adclÍtÍon to increasing Lhe response and strike disLance, food deprivatj-on also affected the other connponents of prey capturing behaviour; it increases the proportion of arousals, attempted captures (sLrikes) and successful captures per unit encounters r'riLh prey (see also Chapter 7).

The ramifícation of the thungerr effect have be.en reportecl in a nunber of different- vrays ancl for differenL organisms" Beukema (1968) reported that rhungert causecl an increase in the range of objects recognized or accept.ed as food, r.¡hile Swynnerton (1919) discussed the

relationship beLween palatabil-ity and hunger. 'ltlhen hungry, prerlators accepted a varieLy of over thirty insect species indiscrj.ninanily, but wit¡ satial-ion only the most palat.able. Sirnilarly, search behaviour has been founcl to be affected by food cleprivation (see for example, Mech L97O; Schaller 1972; Sanclness and Mcl'lurtry I972i Banlcs L957; Dixon

1g5g) " 105

For a sit-a¡d-wait. predat-.or l-ike R"dispgLl, where searching for prey is not important, the effect of food deprivation i-nfluences those components of predatory T¡ehaviour Lhat will increase lts ltkelihood of capturlng a prey. selection wil-l act on those componerlts'that w-ill íncrease overall the fitness of the organisn" However, there musl- be a compromise or trade-off in the energetics of prey capture belween, for example, the distance at which a strÍke is made or the nurnber of sLrikes thungerr and the relative success of the outcome. Thus given a certai-n motivational 1evel there wj.1l exist an optimum predatory response incorporating the strike disLance and the number of capture attempts'

Clearly then food deprivation time (thungerr) has a significant effect on the predatory behavÍour of R.dispar, in a rvay that will naximise the overall capLure efficiency of this predator.

Cloarec (1969b) showed that both shape anrl sr'-ze of prey ¿ffected the predat.ory behaviour of R.l-ÍLearis. Therefore, it would appear that, as in other predators, a combinatíon of internal (hunger) and external (ptey size) stimuli are necessary for the sequential ordering of components of the prey capture behaviour. Sectlon 3:6 sets out to investigate the interrelationships between food deprivation, prey size and predatory behaviour. 106

3:6 EFFECI oF PREY- SIZB AND FQqD- !BIvA'll$- 0N tqEp¡Jp[y.

BEHÂVIOUR 3:6.1 Effect o,l Prev Síze and Food- Peglvatioq on Prev CaDturing Behaviour 3:6.1.1 Material and Method The pre-experimental procedure was adopted as outlined ín Sectíon 3:4.1.1 for 20 adult female R"dispa!. On Day I of the experirnent 5 aninals were allocated to each of four treatr¡ents (T1 - T4), each treatment being a proposed different fasting tÍrne as outlined bel-ow.

TABLE 3:18

TREATMENT 1 T2 T3 T4

FASTING PERIOD (hrs) 0 to 6 24 to 30 48 ro 54 >240

Modqf Prev:- A collection of model prey were made as outlined in

Section 3:4 and vere 2, 4, 6, B, 15 and 20 m¡n ín length. This range was decided on as it covered the size of the common prey that R.dispar encounter in their environment.

Procedure:- Predators r+ere observed by closed circuit video T.V.(see Fíg. 3:1). Each moclel prey was presented to each predaLor a total of ten times over the entire experimental period (norma11y 6 hours). The predator and order of prey presentation r+as chosen at random for each series of presentations and a model prey lras presented only trvice to any 107 particular predalor during a series. This enabled the toLal of 10 presentations of any model size for any predator to be staggered over the entire experimental period. Individual predai-ors were allor"ed a ninirnun 15 minutes to accl-i-rnatize on transferring to the observation tank, norrnal-ly r,¡hilst. a second predator was being Lested'

All prey vrere presented along the directional path 45o to the predatorts body axis in the vertical plane. This path rvas chosen Ín order to promoLe the greatest number of clearly measurable behavi-oura1 responses (see Secrion 3:4)'

The outsone of each presentation was categorized as fol-l-or+s 1. Predator |AR0USALt to prey i.e. An Arousal 2. Predator ISTRUCK! at prey i.e. A Strike

3. Predator macle contact with prey i.e. A Hit 4. Predator grasped prey i.e. A Caplure 5. The predator-prey distance when an arousal took place í.e. Arousal Distance

6. The predator-pr:ey distance when a strike took place i.e. Strike Distance

A one-way Analysis of variance for each of the behavioural categories at each hunger 1eve1 showed no signÍficant effect of time in rel-ation to the duration of each experiment (about 6 hours) and therefore the results frorn each lndividual predator were treated as a contlnuous data set and a mean proportion or mean distance was caLculated for each prey size presented. 108

Statístical- Analysis Statistj-ca1 procedures r^¡er:e conrplet.ecl using SPSS (StatisLical package for the Social Sciences Update 9). Data analysis for t-he effect of hunger and prey size on each of the six behavioural categories followed the design procedure of MAN0VA rvil-h repeated mee,sures. Due to

t-he blnomial rather than normal- distribution that proportious normally follow, the deviation from normal.ity being parti.cularly great for sma1l or large proportions, an arcsine transformation tuas performed on the raw

mean dat-a.

Comparisons between means was performed using Lhe Student I'lewn¿rn

Keuls Multiple Range Test (Zar 1974) "

3:6.1.2 Reeg_1-!e and,Dj-scussion

The arousal- of the predaLor to the model prey is significantl-y affected by the time of food deprivation but not the size of the prey nodel (Fj.g.3:174). Follorving a short period of fasting (0 to 6 hrs) the predator is arouserl to about 502 (1 = 4L"3, SD = 7.5) of prey presentations. No significant difference in the Percent arousal was found betveen the three other fasting regímes, the mean responses being

86å, 9/+.7 and 99.6 Lor tine 24, 48 and 240 hour fasts respectívely.

Iolhereas prey size had ninirnal effect in deterrnini-ng whether the

predator was aroused to Lhe rnodel prey it had a highly sígnificant effect in influencing the proportion of these arousals that led to the predator striking at the prey. Fig" 3:178 shor^rs that prey size in arldÍtion to fasL period both influenced this behavíour although Lhe significance of the interact-.ion term suggests thaL they do not act Fiq. 3 (A) The effect of prey size on the Fiq. 3:17 (B) The efflect of prey size on the

number of arousals expressed as number of strikes expressed as a

a proportion of presentation proportion of arousal-s after 4 aft.er 4 different food deprivation different food deprivation periods.

periods. Value given is the mean \/alue given is the mean (+ 952(,

(+ 959ó Confidence Interval). Conf,j-dence Interval-) .

In the comparison of Means they are ranked from the loruest (Rank 1) to the highest (Rank

4 or 6). Subject means underl-ined by common Line are not significant. (n) oto 6hrfast (O) 24 Lo l0 hr fast (s) 48 to 54 hr fast

( O ) Z¿rO Lo 246 hr fast :,--j ,{ <¿¿d--

É e ë (g t¡ 1.O f.o o g, 4 t, c.3 o o.g

0 .9 a.6 o o o.s Ct 4. o cq g .i1 o,4 o.4 ú þ( o1 f{ û+l ø ¡x o 6 o,2 ø.2 É î ñ g, dt f, o 0 o 2 6A 15 20 o 2 4 oú 15 20 Proy Longìh (õm) o Pr€t Length (mm) o

Sou¡cE oF vÁRlarloil ss ÐF È1S F-Rar¡o Src. So!åcE oF vaRlaTIou sû¡4 s¡uaRss DF hS t SIG

r87 ,27 ConsÌÁt¡Ì CoilsraNf i90,71 I 190.71

l. J6 l5 0.20 ÊÊRoR 1 ERßoR I 2.lt 16 0,11

5,34 P < 0,001 q. Hü(sER rs,02 25,tt6 Hut(ì, 3,67 r.22 18 P.0.001

80 0.0¡r ERROR 2 1,77 EÂÂoß 2 5,2\ 0,07

5 0,1.(l P . 0.06 t{S PiEY sIZE 0.51 PñEY sizE \,17 5 0.e7 12,54 ¡.0,000.1

0. L,7 ¡ - 0.0û tls q.0q ì!s¡ù,8Y P,stzE i.5 t¡ t HUI]6, Z PREY sI:E q.25 t5 0.2c F.0.05

Cotr^R¡sori oF l'lcÃils r- {d = 0.05) cofir/úrsoN ùr fìEÀ$s r- SΡK tlut.l|PLE R^flGÊ TEsr (¿'0.05) R^T( il SÄfiPiE I¡]EÂil

l 2 5ii RAflÍ OF SANPLE ¡.]€ÂNS

'| Í^sr PERio! I q 2 3 56

PRtY s¡zE F-R¡T¡O NoT sIGN¡FICA¡T F^sr I'ErtoD ) 3 ¡l

t'RËY stzÉ ?0 66 109

independently. The trend of the larger: the prey the less liltely that the predator vroulrl strike at-. it was presernt over al.l fasting condit--ions, being par:ticularly obvious in the 0 to 6 hour fasL. Hor"ever as t'he fc''od

deprivaL-i-on tfme increased tirere was a corl:esPonding increase in the propo::tion of strikes at the largel: preye rintil for the longesL fast time ()240 hrs) the proportion of strikes vlas about Lhe same for all prey sizes; almost 10oz of arousals (r = 98, SD = 2"5)' As in the tArousalr category Lhe x number of strikes \{as significantly less for the shorLesl- fast period, over all prey sizes, Ì{hel:e¿s no such differences \sere observed betv¡een Lhe other three fast periods. Àcross all food rleprivation tirnes there rvere sj-gnifi-cantly fev¡er strikes at the l.arger prey model (20 nun) than the other models"

0f paraurouut inportance to the preclator in any elìcounLel: rvith a potenLial prey item, is rvhether conLact is nade with the prey and more importantly that contact leads Lo a successful capture. FÍg. 3:184

shor¿s that although boLh fast tíne and prey size have a significant effect, no differences between the mean proportions could be identified

usíng the Student Newman Keuls MR Test. In a1l likelihood the probability of the predator makj.ng contact with the prey once a strilc-e has been initiated is very much dependent ou the characteristics of the

ì movement patEerns and i prey it-self" Factors such as stvimming speedr i evasion efficiency will all play a vital role in deternining r*'hether a

I I hit occurs. Fig. 3:1BB shorys that r,¡hereas the prey size has a

rl significant effect in the proport-ion of prey capture, hunger has no ,i. "3 olÌ this predator' The x ï signíficant effecL on the capture efficiency ( capture efficiency for the srnallest Prey, over the 4 fast perlods, Ís signif-icantly less than the 5 rerlainj.ng sizes. No such clifferÚìnces lferc --tc

i' Fiq. l The effect of prey size on the Fiq. l:18 (B) The effect of prey size on the

number of hits expressed as a number of captures expressed as a

proportion of strikes after 4 proportion of strikes after 4 diflferent food deprivation periods. diff,erent food deprivation perioCs,,

Value given is the mean (a 95c,á Value given is the mean (+ 959í, Confidence Interval). Confidence Interval).

As Fig. 3:I7 (¡) 0to 6hrfast (o) 24 to 30 hr fast (r) 48 to 54 hr fast (o) 240 to,246. hr fast (Â) û 1.o 1,O

t) ; 'o = ù.¿ o o.e ö- C) .: :. v 0.6 lo ll Õ o êil ø -. +t rL I ln o.t ø flo tu r9 rrl ñ û,¡ o.2 9,2

= (ç = o o 4 rt a 15 20 2 6 s 16 Prày L€Âgth(mm) Prey L€Ggth(fl6)

0 cF VÀÂlalion DF fS

Coftslatr 68,lB íe,tB CoNslÁNT | 2A3.ß iRftoR I J.2¡r 0.20 r:mùr 1 1,09 i6 0.ú7 HUilG. 0,10 3 0.lc 0.50 P"0.6¿ ¡lS ¡luilc, 1.51 , 0,50 1,18 P < C.005 TRRÛR 2 6.C4 71 0.08 SRROR: J,09 71 0,0E PßEY st zE r,5t 5 0,tI 1.6 P. 0.rlûi IR€Y sI:E 0,¡¡9 5 0,10 P . 0,05 HUI¡6, T PREY S¡ZE t,29 0.c9 I.D P.C.49 l{urc x PâEy srzE 2,17 15 0.t4 P . 0.(105

(:oÌpåRl5or¡ oF l1Ëatrs r- SN( flULTIPLE ihsc[ ÏEsl

Railx o¡ S^ÍPLE liEANs

3 456 RÀrr oF llf^{s

FAsr PERiot 23 t 23¡¡ 6

fiußG F- RAI¡o ilot stGiltFlcrûi PRÉY sI IE 22it5 áß PREY sIzE 2 i5 20 ¡l a 6 110 observed over Lhese 5 means, although Ít' 1s to be remembered that the proportÍons for the 15 mn and nore particularly the 20 nrin prey size are based on a very sma11 sanple and thus the variabil-ity of these ntrnbers is hi-gh. The Igg@ struck at the larger prey sígnificantly less than the other prey rnoclels (see Fig. 3:178) although iL appears that fr:r' those few strikes t-he proportÍons of captures is quite high. Fig. 3:19

shows the nunber of captures as a Proportion of the initial nunber of arousals of the predaLor to the model-. Although not a Lrtte measure of capture efficiency, it better emphasizes the trend of the effect of prey size on capture efficienc.y. In addition Ít clearly shot'¡s Lhe greater proporLion of captures of the 6 nn rnodel prey.

As show¡ in Seciíon 3:5, food deprivation time had a significant

ínfluence on the distance at rvhich Bqqatl:a was aroused to and struck at a prey. Figs. 3¡2OA and 3:208 show the additional effect of prey size on these distances.

There ¡,¡as a significant effecL of both food deprivatíon tíme and prey size on the distance at which the predator r¿as aroused to the model prey. A comparison of Lhe fast period distance means across all prey sizes revealled that animals Lhat had fasted longest uras aroused at a significantly greater distance than the other 3 fastÍng periods' No difference h¡as found belween the trvo inlermedj-ale fasting Periods (24 and /+8 hrs) or the satlated/short fast and Lhe 24 hr fasting time. Over all hunger 1eve1s the predaLors were aroused to the smallest prey model (2 rot) at a si-gnificantly shorter dj.stance than the other 5 models' Although the conservative nultíp1e range tesL used did not identÍfy any sig¡ificalL differences in ar<¡usa1 díst¿rnce beL.ween the relnaining 5 Fiq. 3zI9 The effect. of prey size on the number of prey captures expressed as a proportion of predator arousals after 4 different I'ood deprivalion periods.

Val-ue given is mean t (959ó Confidence Interval).

As in Fig. 7:I7 (n) 0to 6hrfast (o) 24 to 34 hr fast (r) 48 to 54 hr fast ( o ) 240 to 246 hr fast Ø o an 1.O 2 o

o.8 o C o

L o.6 oe CLòa O'o L0D o' +l o.4 øÞ< o g, o o.2 J

CL ct o o 2 4 68 15 20 Prey Length (mm)

F-RATIO SIG, SouRcE oF VARIÂTIcN SS DF IIS

CoNSTANT 53.59 1 53,59

0.15 ERRoR I 2.Ut) 16

2 e='0/ NS HUNG. r.32 0,lrq 2,Bg

ERRoR 2 ¿{ ,61 75 c,06

6, tlJ p.0,05 PREY sI zE I ,98 5 0.39

0.111 L, )/ p=0,006 HUNG BY PREY SIZE 2 ,18 15

(o('0'05) coÌ4pARISoN ot'llenls :- SîlK lluLrlPLE R^llGE TEsr

ß¡ur, or - llE¡tls 123 q 5 6 HUNG - F - RATIo Nor slGNlFlcANT q R 6 PREY sIzE 20 15 2 111 rnodels, what is of particular lnterest l-s the rank orcler of the fneans" The predators were aroused to model size 6 at a greater distance tilan' any of the oLhers, even though it is less than a third the length of the largest model used (20 nun).

The trend of responding at a greater distance to rnodel size 6 rsas repeated over the first Lhree fast periods (O to 6, 24 to 30 and 48 to 54 hrs). Itlhen subjected to the longest (>21+A hrs) fast the response dÍstance increased 1ínearly wíth prey size.

The duratíon of the fast period had a significant effect on the distance at which the predator struck at the PreY, i.e. an attempted capture took place. No such effect was observed for the size of nodel prey offered. The predaLors struck at prey at a signíficantly greaLer distance after the )240 hr fast and significantl-y shorter dj-stance when subjecLed to a ninimal fast ((6 trrs¡. There l{as.no signifícant differences observed 1n the strike disLances between the 2 Íntermediate fast perlods (24 and 48 hr fasts).

Having identified the significant effects thal- both for:d deprivation tÍme and prey size have on the arousal and strilce dístances of the predator, the next obvious questíon to ask concerns the poss1ble relationshlps between arousal and strike distance and the subsequent capture success of the Predator.

Figs. 3:204 and 3:208 show the significant correlation bett+een the arousal distance and the strike distance. That 1s, those pre

Value given is mean (+ 959í, Va1ue given is rnean (+ 959(, Confidence Interval). Confidence Interval).

As Fig. JzIT (u) 0to 6hr:fast (o) 24 to 34 hr fast (ø) 48 to 54 hr fast ( o ) 240 to 246 hr fast '- ¡ -- i-4tûv:.- .1 + : ,--*J -.Ä :r -4 = - -.

65

E

Oj 4 È. E o-ù- ãxJI 3 Ëo ;¡ .e c É¿1 o 2S *-@ al, l5

6 15 2C 464 15 20 2 ô ô L6nglh (ñfi) Proy Long(h(mú, Proy

Coñsr¡dT 17q5S,89 i3459.89 ùcrsiÀtT I i29882.62 F-arofi I 227 ,25 l6 r{.20 6¿04,97 16 42t,lâ ÊFroÁ I F.0.00q P.0.00t 5rilG, 597 ,70 ¡99.21 1t/,2t NuriG, 2\Aâi,93 l râ,10 EKÂOâ 2 tn\8.u 60 Én¡oR 2 525,15 6,56

r55,39 c.08 ?4),002 P?Ê\ ltiz: 776,74 5 Ê¡EY stzE 66,79 i3. t5 2.00 P-.ot Ns

ì5 1.23,C0 t,?3 P-0,ocl¡ iIUNG. X PREI S¡ZE r¿{5.0¡ lìuNG. f PnEY sl zE !2\43 1.26 P-,2U ilS

Co¡PAR¡toN !F :- SñK ÈiuLr¡FL; CotspÁFisofl o: llEANs r- sltl tluLT¡ÞLÈ n^{eÉ fEsï ( ' 0.05)

RÂ(( oF llEANs 1234 5c iì^{K oF fE^Ns \2511

tluil6. J2t\ HUNG, R^il{ r2t\

(ü) .Z- . . . 52 i v^L!É is.s l¡j---15:& X VALUES J_4_ i0 2¡r 11 14 i-?

PREY s¡zE RaTto xoT S¡GfltFlcÀxT. ?iEY S¡ZE ¡¡15205

34 15.S,6 i vaLUEs 2ì,89 32.27 32,81 33,¿76 69 It2

at the prey at a greater distance. Table 3:19 shor+s the trend remainerl sig¡ificant even r,'hen allorvi-ng for both the fast perlod and prey size effects Ín l-he correlation eqttaLion.

i TABLE 3:19 Correlation coefficients of Arousal distance with Strike distance allowing for the tinne of fo,rd deprivation and sÍze of prey presented to the predator.

Corr. Coeff. Signif. A.Dist" with S.Dist. 0.66 0.00001 Controlling for fast 0.37 0.001 Controlling for p.síze o.67 0"001 Control-ling for: botlì 0.39 0.001

In relation Lo this trend one should therefore pose tltlo questions:-

1. Do those predators that are aroused to a prey at a greater distance increase the proportion of successful captures; and 2. Do those predators that strÍke at a prey at a greater distance increase the proportion of successftrl captures.

I i {

--t 113

Tabl-e 3:20 shorus the resultanL correlation coefficient maLri-x of the arousal. ancl strilce distance r+ith either the proportÍou of successful captures or the capture efficiency (No. of successful captures/No. of StrÍkes x 100) taking i-nto account the fast period and prey size used.

ru 3.?O AROUS& DIST¿.NCE

CAPTURES CAPruRN EFF.- R value sie R value Sig

Simple .54 .001 .13 .20 Controlling for fast .28 .003 .08 .38 Controlling for prey size .56 .001 .L2 .20 Controlling for both .30 .002 .o7 .46

STRIKE- DISTANCB

Sinple ,40 .001 .11 .25 Controlling for fast .15 .116 .06 .53 Controlling for prey size .39 .001 .11 .25

ControllÍng for boLh .15 .1-2l .06 .51

It ryould appear from Table 3:20 thal- those predators Lhat are aroused to prey at a greater distance catch a greater proportj-on of the prey that are encountered. This trend is stil1 significant even afLer Laking into account the obviously inportanL food deprivation effect and appéars to exist for I the slze of the prey. Although this relationship I strike clistance the significance of the correlaLion disappears rvhen 1r4 conLrolling for fast t-ine" Neither arousal distance lloi: strilce dj-sl-ance appear to be correlaLecl r,'iLh overall caplure efficiency as measur:ed by number of captures/nur¡ber of strikes, horvever Lhe design of the experlment did not perrnit- conclusive staLemenLs Lo be mado because distance, 1j.ke other parameLers measu::ed, was a dependanL varíable. To invesLigate the effect of distance beLween the predator and prey model on the prey capturing parameters, the folloving experíment v¡as carried out.

326.2 The Effect of, Disla_qce. betweeg P::eJL U"q.l an{ PreÈatoli on lhe Prev Capturi¡g Þ*""iour. 3¿6.2.L Methods

TwenLy five adu1t, non-reproductive female Rr_dig¿ql were fed abu¡dant prey and then fasted for 48 hours. The prey stimulus used was model si-ze 6 uun used ín the preceding experiments. As before the model was presented by hand along either of two direct'iona1 paths, Oo or 45o to the preclatorts body axis in the vertical- plane. Anj-mals were observed in large glass aquariums fitted with a ve¡:tlca1 dowelling rod at one end, on which the predator rested. A graduated (in nm) glass rod fixeil to a moveable clamp llas fixed at ri.ght angles to the piece of dowelling to aid in positioning the prey. Horvever, esLirnatiou of distance was not without error (see belorv).

Each trlal consj-sted of 50 presentations of the nodel (2 per subject) at 5, 10, 15, 20,25,30, 35,40,45 and 50 mrn distance from the predator. The shorLer distances, 5 to 10 mn, proved difficult to use as the predator often responded before the prey was in position" Cloarec (1976, IgTg) experi-enced the same problem in a similar series of 115 experÍments rvlth !-Å+¡gglrs-. A number of presentations were reco::ded by use of video T.V. as outli-necl in Section 3:5, to enable an esLj-mate of error associ.ated in the distance estimati-on to be calculated. Tabl-e

3:21 shorvs the degree of error associated with the estimated rlistance and the subsequent range of distances that may have been used. '

TABLE 3:21 Est.imation of Error in Distance Estimation.

ESTIMATED ATIUAL ERROR POSSIBLE

DIST. (mm) DIST. (nm) (7") RANGB (nnm)

5 4.O 20 4to6

10 8.5 1s 8.5 t.o 11.5

15 12.5 L7 12.5 to 17.5

20 23.2 L6 16.8 to 23.2 22.5 to 27.5 25 27.5 .10 30 32.0 7 2B.O to 32.0

35 36.2 5 33.8 to 36.2

40 42.O 7.5 38 to 42

4s 43.1 5 43 to 47

50 52.6 5 47 to 53

At each presentatÍon the following behaviours were scored if they occurred: 1. Predator aroused to prey model i.e. ÀROUSAI. 2. Predator struck at the prey model i.e. STRTKB 3. The predatorts sLrike hit the prey model i.e" HT-T 4. The strike result,ed in a capture i.ê. CAPTURB (See Sectlon 3;6 for definitlon if requlred). 116

326 "2. 2 Results and Discussi-c¡n Fig" 322! a and b shorss Ehe reactlvity of ttre p::edator.as measured by arousal (response) rnovements, strikes and associated hits, elÍcited by a given number of presentaLions of a model prey at a different

decrease r,r¡hen the prey comes closer than about 10 mn. Tlre greatest

number of strikes is found to occur aE 2O mm in both the 45o anil 0o presentation paths, although little difference is observed in the number of strikes occurring between 15 and 25 run from the predator in tþe 45o presenLation path"

srRrKE Et'FrcrENcY:- strike efficiency (# captures/# strikes x 100) is

expressed in terms of captures and strikes for 50 presentations of the nodel prey at each dístance frorn the predator. The strike efficíency is

shown Ín Fig. 3:21 for both the 0o and 45o presentation paths, and is the probabiliry Lhat a strilce elicited by the model prey presented at that dlstance wÍl1 be successful.

As Fig, 3z2L shows the probability of capture is low at fÍrst (where closer to predator), increases to a maxinum at 15 rnm frorn the Fiq. 3:2I The effect of distance of prey on the gó reactiv-ity

of .R.dig:._ar,. Reactivity ruas measured by the number of arousals, strikes and captures eÌicited by a gì ven number ofl presentatio¡rs of, rnodel prey

at di1=ferent dist.ances fronl the predator along the dil'l.erenl- directional paths (A) oo, (B) 45o,

to the predator body axis in the vertical pJ.ane. The horizontaL l¡ars indicale associated error in distance estimation. See Lext for additional- details. (A) 100

rousals 80 Strikes ðs 60 t"- Captures =o

TU 40 æ,

2

o 10 20 30 40 50

10 (B)

I +---4rousals --û- òe Strikes 6 -!- =t- O Gaptures

UJ 40 E

20

o 20 o 40 50 DISTANCE (mnr) LL'I

predat-.or arld then clecreases abr:upLly at 25 nm. Al-though stil1 ablc' to successfully .utrtrre prey at a dj-stance of 5 nn the loirer: probabilit-y is

belleved Lc¡ reflect the morphology ancl relatecl ¡nechanistn of the gr:asping

action rather than visuerl acuity" E*di"pAn- is particul-arly efllicie¡nt in

capl-uring prey of the 6 nm size class at distances betr+een 10 and 20 mm

r+hen presenLetl along the Oo path (i.e. comj.ng head on), and bel-ween 10

and 25 nm for prey approaching a1ong the 45o path above the longitudínal body axis.

MAXII'{UM MOTIVATION ]]IS.IANCE ¡,ND MAXIMUM EFFICTENCY DISTAI'ICB:-

An additional way of expressing strilce efficiency ís 1-o examine l-he distríbution of sl-rikes and captures in relation to distance. The term rrnaximun moLivation distancer was first used by Maldonado, Levin and Barros-PÍta (L967) and later by Cloarec (1979), and v¡as defined as the

.ú distance al- r+hich prey ilems elicits the most strikes. It is an ry I unfortunate tern because there is no tuay of knowing (from the ternt Ítself) rvhether tma,ximumr refers to notivatíon or distance. Á betl'er

term vould be perhaps rthe distance of maxi.mum notivationt. Exarnination of the rstriker curves in Fig" 3:21 shoi^rs that the naximurn number of

I strikes occurred at 20 nun distance. Therefore, 20 mm is the distance of fmaximum maxímum rnotivatlon of adult R.clispar. Similarl Ir the

I rthe i efficÍency distancer or as argued above perhaps better stated as

distance of maximum effíciencyr, i.s Lhe dj-statrce at whÍch most captures t rcapturesf I occur. The curves l-abel1ed in Fig " 3'.2L show that the maxi-mun number of captures occurs at 15 mrn, therefore the distauce of ,1" I t maximum efficiency of adult R.dispar is 1-5 mm. I

4 l' J 118

327 DISCUSSION The previous Section (3:5) showecl Ltre significatL effecL l-iral- food depr:ivation had on :- f . increasing the distance that both R.d51-p^a-r". \{as al:ousetl to anrl sLruclc at, a model- prey; and 2. increasing tlie number of init-'r-al arousals, sl-rilces and, as a result, the nurnber of capLures.

SectÍon 3:6 set out to demonsl-rate t.he added effect of prey size on predator behaviour in order Lo see if, as in other predators (see for

example Connell 1961; Farrner ancl Beamish 1973; and ntulìel-ctus references in Curío L976)r pre¡I size is a significant capture elicit.ing stimulus in R.dispar, and if so what components of the p;:ey capturing sequence are affected. In arldition, the daLa collected pernit the overall effect of thungert when gi.ven a range of fr{ on the subsequent size of prey caplured iP ,r different prey slzes, to be examined and discussed. For R.dislqr- it appears that the iniLial rarousalr or tpreparationt for prey capture is significantly affected by an inLertlal of rhungert state. Prey size, apparently, has no effect on t-he proportion of aniluals that are aroused. thungerr Holvever, for those predators that are aroused, both ancl prey size sig¡ificanLly affect the distance of arousal, the significant I F-ratio of interaction ímplying that they do not act i-ndepenrlenLly. Ïn all likelihood thís reflects Lhe visual capacity of R.clj-spar and the

,t (but see a1-so f fact. ttrat l-arger objects can be seen at a greate:: distance i Maiorarr¿ 1981). 0f course the possibility of a combinatíon effect ¡. l{' "t betr,¡een visual and nechanoreceptors cannrlt be ruled oui, although T I Cloarec (L976) dítl show that rnechanoreceptj-on tvas restrictecl to areas It close l-o the predator, especj-all.y belor+ the fr:ont legs. As thc prey 4 i' i 119

thurrgerl moves closer Lo the predator both and pr:ey síze deiermine

wheLher or not a sl-rike r+i11. take p1.ace" Tli:Ls is brougirt- abottt, iL 1s proposed, by the prey organism movi[g into both t.he visual b-1.nr¡cular

(anC mechanoreceptor?) fielcl where, dependill8 on l-he nurnber, position

and frecluency of omrnatidj-a and mecharo-sensil1a stÍmulated aud l-heir relative intensitj-es, a siz,e/distance estimati<-in can be marle (see Frantsevich and Pichka 1975). ltlhether the str:Llce talces place or not thungerr depeirds on the observed size of prey al1d the internal state" clear:ly a relatively compl-ex decision nnaking process takes place incorporating, not only the obviously important prey size component-, but: also the influence of the thungerI notivational l-eve1 and the assocj'ateci prey size/risk factor (see also Cerri and Fraser 1983; and Griffiths lg8ob)" The outcome of the sLrilce, i.e. tvhether or not it is a success' is depenclent on prey size and not on Lhe internal state of the preclal-cr" Thi.s ís thought to reflect the relationship betr+een size (and shape) of morphology. Horuever, prey size has no ti the prey and the preclatorrs leg effect on the strike dístarlce, this being deLermined by the inLernal or motivational state of the preclator. On reflectj-on this seens reasonable forn as rnentioned previously, B.rlislAq has a restricted capture field' determined o.¿erall by 1eg length and no pursuít or lunge compouent (see copeland and carlson 1979) in their predatory repertoire.

, i i Holling (1966) reported the absence of a irunger threshhold of the

'{ I tawarenesst in the manticl, ancl interpretetl it (op. ciL. p"23) ¡, response

! to be adaptive, ttfor urhile it Ís only necessary that hungry predators be i(. 't important that both hungi:y ancl satiated' I ar\'are of prey, iE is extremely

I predators be ar,,rare of potent.ially clangerous objectsrr. In my r'rork wi-th

R"dispar j-L vras possi-ble to clifferenti¿ii:e between preclat-ory arousal and -nc I 1' t20

rawarenessr or evasíon behaviour as reporLed in Section 3:3" The lack of an tawarenesst response occurring in the model prey experiments is probably due to the range and rnaximum size of lnodel used. The range was

chosen to cover the size of prey iterns t'hat lì$ffË had been seen 1-o catch in natural (and laboratory) sltuations. As mentioned in Chapter 2 of this thesis, adult R.dispar catch and consume prey ranging j-n size from srnall water fleas (DaphnÍa spp.Ï 2.0 rnm in length) (see also B1oís and Cloarec 1983) to tadpoles (LimnodJqasles Bsgæ-' l' 25 mm in length) al.though, obvÍously nìovenent and speed, amor^gst other things' will have a pronounced ínfluence on capture success"

Holling (Lg6/+) preclicted the optirnurn prey size for the mantis' H.crassa, from anal-ysing the geometry of the rnantid l-j-nb and relatíng this with the size of prey that it can grasp. By offering model prey of various sizes and recording the percentage of sLrikes provoketl, Holling apparently confirmed, quite impressively, his predictions. Recently, horvever, Loxton and Nicholls (1979) have critized Hollingrs mcdel on the basis of Hollíngrs suggested mechanism of the functional morphclogy of the 1inb. Loxton and Nicholls (op. cit.) proposed a new n¡echanistn and

arguecl that Hollingrs original equation gíves a predicted prey síze r+hich is largely determ.inetl by the length of the tibia. However, this

does not necessarily negate the idea of leg morphology being highly correlated wíth an roptimumt prey size that can be caught. In acldition

LoxLon and Nichols (op. cit.) provided a number of exaroples of different species of mantises whose 1-eg morphology, they suggestedr were relaterl

rì :1 I to the type and slze of prey they v¡ouLd probably catch. In addition' I ,l I (1.977) discussecl the 1eg segment proportions of sorne aquatíc { Gittleman bugs in terms of their adaptive advantages in capLurj-ng prey of ..,1

I LzL

clifferent sizes, and suggesl-s, in analysirrg the relationship betveen prey size aud ease of capture, that-. the lnechanj-sm 1ay in the mechanics

of grasping (G-Lttleman 1978) "

C1ear1-y, as shor,rtr in Fig " 3:21, the greater the strike dj-stance the less likely the prey rvill be capl--ured (probabj-lity of capture r'ri11 eventually be zero). As díscussed earlíer the strike distance is clearly affected by foocl depri-vation (and the associated notivational staLe (Hinde 1970)), and therefore the greater Lhe íntertlal or hunger 1eve1 the more lil

thungert Sone similar ramÍfj.cations of the effect in pre'datory

behaviour have already been discussed -i-n Section 3:5. Additional related examples Lhat are more pertinent to this sectÍon j-ncl-ude Kniprathrs (1969) study of Kingfishers (Al.qeqo- atthis I{e found tJraL

when starved, these diving birds attacked fish thal were longer and sv¿am a¡ a greater clepth than they normally vroul.d" Beukerna (1968) found that rhungryt stickLebacks (Ga.stergsteug qlculggtttÐ suapped at, grasped and ate a greater proportion of encountered PreYr arrtl that they swam more actively than sati.atecl fish and therefore had more prey encounters" lHungerr has also been shorrn l-o increase the intensÍty of peckJ-ng in

young chicks (Hogan 1971), and a mantis will activel-y pursrre flies only

I any t when they have þeen starve

other component of predatory beÍraviour: (IIo1.1i.ng 1966)" Thi-s i-s r+hat, in t22 f unctional tei:ns, would be expectcd f rorn an ambu*;h preclator compared wilh one thaL actively searches for prey"

The data set collected in these experj-menLs allor'¡s some Lenla[ive

cornnents Lo be nade on the effect of hunger on the resuliant relationshíp ru:Lth prey size as measured by a unmber of predatory behavÍours. From thj-s ít was hoped to idenLÌf)' either a particular size tselecLivet or range of prey sizes that, as a result. of particul-ar behavÍours, would be caught in Lhe natural setting.

Food deprivation clearly extends the size range of rnoclel prey that R"dispar res pond to (Fig" 3:174) and wj-l1 strike at (Fig. 3:178). tarousalsr Irrespective of hunger the highest propoiîtions of that 1ed to a strike were directed at the smallest prey si-ze, and linearly decreased

tor,¡ards the largest" Although not significantly different the trends identified in Figs. 3:1BA and 3:1BB are, it is thought' i"mporLant. Firstly, rvhen one examines Lhe number of resultant hils as a percentage of strilces the smallesl- (2 run) model, althottgh -tnitiating ttre highest nrmber of strilces, records the lowest núnber of hits. This is followed by the two largest models, that evcllcetl f ew strikes so the values presented are associated wlth large varíance (see Fig" 3:18À). hlhat is of pa::ticular interest horyever, is the fina1 rank orderíng of Lhe prey

size means for captufe. SignificanLly less Prey Size 2 vere caugl-rt

compared with the other prey sizes. In acldition, although the conservative nultiple range test rlid not identify individual dÍ.fferences, the orrlering of prey size models is considered important

with more Prey Size 6 being successful-1y caught than any of the otherst this bej_ng fo11or,recl by Prey size B then Prey Sj-ze 4. Again, the L23 srnallesL model used, Prey Si-ze 2, was caught signifícanLly less than the other models.

As the food deprivation level of B¡l!ÐlaJ- increases, so the upper size limits of model prey increasen Drees (1952) found a similar result for the jumping spÍ-der, Eplblemurn (Sa1-ticidae), when black, round, flat dummies of increasing size elicit capture movements with increasing food tleprivatlon, until fina1l-y the nodel size extended into a range which, l-n norrnal spiders, releases f1-ighL resPonses (see also Hoppenheit 1964 quote

This chapter has demonsLrated the signi-ficant effect of both thungerr and prey size on the subsequent components of predatory behaviour of a sit-and-rvaiL predator, 3gæ-diital. Tn additj-on, from the resulLs oblained on capture success and model- prey sizer once could tentaLively hypothesize a relationship such as thaL shown :i-n Fj-g. 3222' Similar relationships have been suggested for mantids (Ilolling 1964) and l'iS. 3222 Ã ltypo'circl-ical reÌatioriship bel-veen pr.ey size

anci capl-ure success based on eitlrer t,he nunrber o1' successful captures as a proportiorr of

initial predator arousal-s or number of

captures per J-00 strikbs (*l -._;,Á=

(@) Çapturee As A Proportlor¡ Of ¡nitla¡ ArossalE

o c) o Ð o g¡ 0 ;," i, ô

lr #

ð * @

*o * s , {( t gt ilo$ @ o # J

(¡ @

¡J o *

@ Ê0 }\' 'Þ o o 0 0 o o

Capt¡¡re Elflclency{ Nunnber Of St¡ccoetlul Ceptures o (strtro + 124

Chaoborus larva feeding on Daphqlq (Pastorolc 1981).

In additíon to prey size, the resultanl- capLure success will also be relaLed to Lhe fr:equency with which the prey encounter the predator.

The next chapter seLs out to investigate the effecl of boLh size ar.ld encounter frequency on the predatory behaviour of &.4j.g-ee,{. by examining the functional responses of all clevelopmental stages of the predator, R.dispar, with the complete range of sizes Of 1ts conrnon natural-ly occurring prey, AnisoPs. deanei."

3:8 SUMMARY In sumnary, this chapter has described the sequence of behavioural conponents involved in the predatory behaviour of a typical siL-and-wait predator, &.1!!5¡g¡.n paying particul-ar attenti-on Lo the effect that food deprivaLÍon and prey size has on the various identified components. The results idnlcate that -

1. The sequence of behavj-our fo11ow the previously described predator pattern, i.e. Tnltlal Precapture Posture, Arousal, Qrientation, CapLure, Consoliclation of Grip, Exploration, Injection of

Venon/Enzymes, Feeding, Discard, although much variation atld lnterrelation betweeu components exi-st" a

2. Both the tArousalr and tstrike? space surrounding the predator tcapturer íncrease with food deprivationr.althougþ thq. space i.s restricted to the morphology and reach of the råptori.l. 1egs, there being no lunge or pursuiL component. t25

3. The internal hunger or motivational 1eve1. determines luhether R"

and n¡ech¿rnorecepl-or, sensory organso '

4. Once additional, more accurate, assessnent of Prey can take place the decision to stríke at a prey is, although again influenced by hunger, significantly affecLed by prey si.ze. The distance of the prey when tlre strilce takes place is affected by the lhungerr sLate not the sÍze of the prey.

5. The outcorne of the strÍke is determined by the size of the prey struck at, noL the thungerr leve1 of the predator. This is believed to reflect the relatiotlship belween strilce Lrajectory, leg morphology anrl prey slze.

6. tllungert apparently affects al-1 components of predatory behaviour leading up to prey capture, by increasing not only distance of rresponser but the nunber of strikes, hits and captures per unit presentation of prey. It does not iucrease capture efficiency

however rvhich remains at about 70-807". Hunger also Íncreases the range of prey sízes that R.¡lispq responds to and attempts to capture.

7. The effect of food deprivation is considered to reflect a rnotlvational change in respousiveness to partÍcular prey stirnufi usual-ly clescribed as a sensitization of particular sl-imulus-response L26

relations, rather than the food deprivatlon affect.ing the sensory mechanism. There being an inverse relation betr¡een the strength of the stimulus and the strength of Ëhe motlvational factors required for a given response.

8. The predatory success in relatlon to slze of model prey suggested an toptimunr sÍze that could be captured, irrespective of predator motLvational level-, which is based primarlly on the relatÍonship between.the shape of the grasping 1eg and size of prey.

t27

4:1 INTRODIICTI A sit.-and-wait pr:edaLor is one that, bj' definj-tiou (see Chapter 1) is normally dependent on the movetne'-nL of its pr:ey rather than its ol¡n searching movernenLs, to br:ing about an encounter with tire prey' The outcome of an encounter will depend, arnongst other things, on the current rnotivaLional state and captur:ing efficÍency of the predator (see chapter 3) and the evasive or escape efficiency of the prey. Both of these variables are the result of a number <¡f different selection pressures operating on them. The previous chapter exanuined the effects of predator thungert and prey size on the various components of the predatory behaviour of &rtliÞi4-. This Chapter seLs out to examíne the effect of prey density on the capture success as measured by the number of prey ki11ed. In addition, by observing all developmenLal stages of the predator and the range of prey sizes that would naLurally be available, it is hoped that changes in capture success in ::elation to the age structure of the populations of both the'predator and prey will be identified.

The rlensity of Lhe prey in the predatorsr vicinity will directly influence the frequency of encouneers (see also Chapters 6 and 7) and clearly then will influence Ëhe number of prey that can be successfully captured by a sit-and-rvait predator. It is therefore logical in an analysis of predatory behaviour to examine in detail the effect of prey density on the subsequent number of prey kill-ed.

Solornon (Ig/+g) first acknowledgerl the twofold naLul:e of the response of predal-ors to changes in p::ey density. IIe applied the term rfuncLlonal responser to Lhe change in the nunber of prey ki11ed by r28 intlividual- predators, and the term rnurnerical responset to the change Í-n the clensity of tire pr:edators. T'he term functional response has cor¡e Lo be used to describe hor.¡ individual preclaLors respond to changes in the density of a single prey species that occur within a tÍme interval that is short relative to the predaLorts life span" Such functional responses reflect changes in the attack rate of a predator whose physical characteristics (..g. size and age) remain essentially constant during the interval (Murdoch and Oaten 1975), and rshich search at random for a homogeneous prey population, itself clistríbuted at random (Hol1ing 1959a, b, 1965, 1966¡ Hassell et a1.1976).

Intuitively, one woul.d expect a functional response to take the forn of an increasing number of'prey eaten per predat-or as prey density increases, at least up t.o sone limiting value representing maximurn prey consurnption within the prescribed time ínterval.

Ilolling (1959a) discussed three basic types of functional responses (see Fíg. 4:1). three possible responses 1eve1 off at .411 high prey densities, because the pretlator becomes satiated and/or tras no more time in which to eat more prey. In the Type 1 curve the response rises linearly to a plateau and produces denstty-inclependent mortalÍty of the prey up to sat,iation_; Type 2, the response rises at a continually decreasing rate to an upPer asynptote producing i-nversely density-dependent rnortality over the entire fange, while in Type 3 the response is sigmoid and this produces density-dependent mortalÍty over the lot,¡er part of its range (Begon and Mortimer 1981; Ilassell 1978)" Fio. /+¿7 HoÌlingrs thlee basj c l'undamenta-l respor"rse

cul'ves : -

the number' killed (or eaten) l¡y a sìng1e predator per unit time. a HOLLINGS three basic luncllonal rosponae curves:-

thc number killed by a rlngle predator per unlt tlmo

I

i TVPE 3

o Itu o= - o o !E TYPE 2 lu G E Ð =

TYPE 1

PNEY DENSITY L29

The usual response for arlhropod predators Ís the Type 2 curve (tlrough Hassell et a1. (1977) have eviclence which suggests that signroid functional responses mây be more frequenL than was once thought). Hassell et al. (L976) reviewed these rtypicalt Typ" 2 arthropod functional responses for a variety of predators and parasitoids, while Hassell (1978) modelled all three responses in a general way and then consídered their effect.s on l-he outcome of prerlatot-ptuy models by c.onsidering each of the three types of response in Lurn.

Several factors, notably the developmental state of the predator and prey (Thompson 1975; Mci\rd1e and Lawtor' 1979), the predatorrs mode of search (Akre and Johnson 1979), hunger and satiation (Nakamura 1974; Mills 1982), the availability of prey refuges (Hildrer+ and To¡vnsend L977) and temperature (Messenger 1968; Thompson 1978a; Gresens et al.

L9B2) are known to influence the functional response of invert-ebrate predaLors.

The object of thj-s chapter is twòfold. To determj-ne the form of the functional response of a sit-and-wait predator using a range of densities that it normally encounters in the field (in rnany previous determinations r¡ith other predators it would appear that unnaturally hÍgh densities of prey have been used, which nay lead to artificially higtr encounter rates (see also Hassell g! -4. 1977)). Second, to examine variOus aspects of, and identify any al-terations in, the functional response in ::elatÍon to temperature and developmental stage of both predator: and prey. Sectíon 4:2 describes the statistical techniques used to derive the 130

pararneters which characLerize the fuuctíonaI response, togeLher with

some of the ¡-rossible errors Lhat can arise in so d.oing, while Section 4:3 describes ttre basic exPerirnental procedure. The effect of temperature (Section 4:3) and predator and prey size (Sect-'Lon 4:4) are dealt with next.

422 rHE PAP:AUçrBRS 9Ì_ TIIE FUNq]IgÚu RBSPONSE

422.L Revielv o f FuncLíonal Response Equations.

Models rvhich purport to

changes ín prey density have becone familiar in the behavioural and ecological literature (e.g. Thomp son 7924: Nichol"o., und Bailey 1935; Holling 1959b; lrlatt 1959). The most important in the present contexL is the trandom predator equaliorrt of llogers (1972) used to rlescril¡e the funcLional response" Detailed revÍews of models for predation can be Oaten (1975) &1 found in Royama (1971), Ilassell- ancl Mry (1973), Murdoch and $:r! t and Hassell (1978).

Holling (1959b) narie the biologically rea-sonable assumptÍon that a predatorls searching efficiency was dependenl- upon prey density. He pointed out that predators always require some time Lo consume their prey and this time would not be available for searchj-ng. This so-ca1led handling time progressively reduces the time available for searching as

more prey are eucountered. Ilolling argued that as prey density

increases rsearcht becomes trivial, and handling takes up ¿rn increasing proportion of the predatorfs tirne. Thus, at high densities the predator: effectively spends all of its tirne handling PreYr and the predation-rat'e thandlíng-tintesl reaches a maximum, cletermined by the maximum number of l-hat can be fitted int.o the t.otal time available. E i, i 131

tdj-scr (equation .l Thus he clerived the noru familiar eclttation 4.3)

(Holling 1959b):

T ... 4.L S Tr-ThNe

N e aN'f,,s T "'4'2 where T time spent searching for S PreY¡ T. total time that prey and predator are togelher, Tt handling time

I a attack coeffícient (or attack rate) Nt prey density and N number prey eateu e of

Combining equations 4.1 and 4.2, we obtain

N aN T e t t 1+a Tt N, ... 4.3.

fhis equation is an rinsLantaneous equationr and as such makes no allo'.¡ance for any reduct.ion 1n the prey numbers during the course of the

i ¡ experiment (1.e. exploitation). Consequently, it should be used only I

¡ when an experiment ís run for a very short period (see, for example, t 1 Visser l9B2), or where the predatorrs search is systematic, so that

¡. areas that have been searched are not re-visited, or when a prey item is t' r replaced as soon as it is eaten. In the present rvork, the prey rvere not rt I replacecl rluring the course of the experiment and the total tlme that predator and prey were togeLher rvas reasonably long (24 hours), so Ehat "{ i' .l l' i L32

I the tdiscr equation was consldered not str-Lçtl.y nppltcable. The most .l the func[,iotrtt'[ response under these | ) appropriate equation to describe ,ut fl ,] condÍtions is the random predator equatlon of Rogers (L972) r'¡hich can be derived from the disc equatl-on as follows:

dNt -aN t

aaa 4.4 d. 1+aThNr.

tíme È' I This 1s the instantaneous rate of change of the populatlon at

Let N at time 0 = No anrl N at time T = NT

Then:

I I

T

1+aTnN dN dr 4.5 aN

N o o

I

I i fntegrating, we have i

li NT ,t tt I 1og. N+ T.N ,tì h T ¡, - [t] 4.6 1, o T a ,t I N { o

{ ii t { t. 133

and solvÍng wÍ,thin limits gÍves :

N, + Tn Nt -!1o8" No - Tn No = -T l1og"aa ... 4.7

which sinplifies to

1og Nt, aT-aTn (NT-N.) 4.9 e -

N I o

I

I ¡l where N, is the number of prey left after T, and No is the initial prey number at time 0.

I *r. hlith the substitution N", the number of prey eaten is sinply Io - rearrangement, of N-eoI' = N_ - N*, elimination of logariLhms and some equation (4.8) becomes the random predator equation :

N. = No (1 - exp (-a (T-Tn Ne))) t+.9

4:2.2 Random Preda_tor Equation i. ¡ Equation (4.9) allows for exploitation and can easily be amended i to include a predator densiLy term (Rogers L972). 't I i

¡. Most workers rvho use the random predator equaLion for estj-mating

li (Attack anrt (Handling Tirne) have done so using a linear I a RaLe) \ rt i {

'lil T ¡ t- 1_34

regression model based on equaLion 4.9 (Rogers L972; Evans L973i

Hassell et a1 . 1976).

1og ... 4.10 e N_N e -oTt+aTON"

N

Th-Ls rnethod however, is statistíca1-ly incorrect as the terrn Nu occurs on both sicles of the regression equalion (see ecluation 4.10), and one of the basic assumptions of regression, i.e. Model I, analysis is that the independant variable (urhÍch is Nu in this case) should be without error, which Nu obviously is not.

If this inc.orrect method is atlopted the estlmates of a in particular and r"t be unrealistic due to trqo factors. First, irr a ä I normal Type 2 functional response rshich rises with an ever decreasÍng

slope t,owards an asympLote, there is a prey dens'ity beyond whlch the predator eats a constant nurnber of prey. In the regressíon of the logarithrus of the proportion of the survivors against the nuruber of prey eaten, any points beyond thls prey density merel-y provide more variance about the plateau, resulting in an increa-se in the slope of the line and

the 1ov¡er intercept. This lvill lead to an increase i-n the estimate of g (aLtack rate). Since the slope of the regression line is g times T'rt Tn (handling tine) will be correspondingly reduced. i the estJ.mate of '{ hor,,'ever does not greatly alter the I Thls first Lype of discrepancy points are on Lhe ; relative values of q and Tn Provi

rt / I experiment. I careful design of the

-(

l 13s

The second type of discrepancy (ruhich resulLs by iirvalidatj-ng thís statistÍcal procedure) *ay be of greater inporLance. If the level of exploftaLion is too high at any prey densil-y, the logar:ittln of the

number of survivors becomes very sma1l; at complete exploiLation it :ts of course the logarittun of zeto. High exploiLation has thc effect, as discussed previously, of increasing the estímate of a and reducing that

ot (Thompson (1978a) has shor¡n by simulation that any exploitation ä. greater than about 757" causes a sÍgnificant bias in the estímate of a

and \).

Glass (1970) pointed out that a logarithrnic or sinila:: transformation may produce a bias in the results because logarithnrs give greater emphasis to data points near the origin than to those further ar{ay. In addÍtion, such a transformation is only valid j-f ít resulLs in

I honogeneity of the error variance, vhlch is a necessary requirement for regressíon analysis. If the error term j-s additive then a non-linear rnodel such as equation /+.9 should be fitted by an iterative least-squares parameter estimation procedure, such as that described by

Glass (L967, 1970) or Conway, Glass and'lrrilcox (1970). fn the

least-squares meLhod, values of the constants in the equation are found

that ¡nÍnimized the surn of the squared deviations of the observed values from those pr:edicted by the equation (Daniel and Wood 1971).

To avoid the errors discussed above a non-línear weighted least-squares procedure has been adopted throughout. This invol-ves viewing equatíon 4.9 as expressÍng the number of prey eaten (N.) as a

It 136 non-linear funcl-ion of prey density (N) and the tr,¡o paraneLers a and \ (attack raLe and handling time), plus the exper:imental error. i.e. Nu = f (N; a, T¡) + error ... 4.11

Given values of the parameters a and value of the functi-on, ä, " corresponding to any value of N, can be calculated by solving equation

4.9 for N^.e- This equatÍorr can be rearranged as :

Nu - N+N't'exp (-a(T-Ne Th) ) = 0 ... 4.L2

This can then be solved by the standard Ner+ton-Raphson iteratiúe method using the observed number eaten as the init,ial approximation.

, Follorving the ¡nethod of Visser and Reinders (1981) to compensaLe for the variability of the error in equation 4.11, the functÍon to be minimized is defined as :

2

N e(fit) -N e w2_ . ... 4.13

N S.E. e where the summaLion extends over the prey densities used, and Nu and Ne(fít) are respectively the observed and the calculated values for the number of prey eaten at each of the prey densÍties. (Ne(fit) is the function specified in equation 4.11.) By including the standard error (S.8. Nu) Ín the denominator of 4:13 the correcL weight is given to each poì-nt.. r37

To starl- the iteration process initial- esl-imates of the trr¡o parameters. a and T, were obtained from the linearísed forn of equaLion 4.9 (see equation 4.10). The iterations continued until the relative change in the weightecl residual sun of squares , X2, (see equation 4'13)

¡n¡as less than 0.01.

The final estirnates of a and Tn can then be used to estimate Lhe final values for the fitted number of prey eaten (Ne(fit)) at the given densities.

The computations r{ere carried out using the GENSTAT (General

Statistical Progran) (Alvery et'al. 1977) on the CYBER 173 computer at the University of Adelaide cornputing centre.

4:3 BASIC EXPERIMENTAL PROCEDURB The fur-rcLional responses ín the following experiments rsere

measured by placing single predators in containers with different densÍties of their prey. The predators were allowed to feed for no more than 24 hours, after r¡hich the numbers of prey killed and eaLen were noted. The number of additional dead prey was also recorded if necessary, although, due to experimental design (see lat-er this chapter)

this occurrence \{as \rery uncommon.

Prevíous work (e.g. sê€ references in Bernays and Simpson 1982)

has shc¡v¡n that. noultlng can signific.anLly influence the feeding behaviour of insects. In addition Cloarec (1969b, 1980b, 1981) has 138 sho\trn thaL the predaLory behaviour and perfornance of Laqgtr4 lilgd"- is also altered by rnoulting behaviour. To overconnr: thís difficulty, only indivÍdua1s of Rrqslar_ LhaL had moulted at leasL 48 hours

previously were used :i-n the experimenLs. This ensured that they had recove::ed from the nroult and seLtlecl dot+n bo Lhe nornal st.eady feeding rale" Prc+y r,rere l_.d.gËqi.

0rr day 1 of an experimental run, ttre R._!!sAar krere remcved from the holrling talks and provided wÍth an excess of míxed naLural. prey

(n"Uotg -gggtgi, other Notonectidae, Corixidae, ostrâcods (0st.recoda : Cruslacea) and DSpllttia upp. ( phni.idae tac ). Folloi'ring 24 hours of feeding the predators r{ere removed and fasted for 24 hours" During the fast period anj-mals rvere housed in the experiurental containers. These were 1 litre plastic beakers fi11ed with B00 ml of

filtered, dechlorinaLed tap r+ater, with a length (about 15 crn) of 2 mm

dÍameter woode¡r cocktail sticlc weighted aL one end with a lead sinl

embedded ín bees wax, which acted as an ambush site.

Prey anirnals ruere fed abundaut water fleas (!æ[þ. spp.) or

mosquito l-arvae, up to the size grading (as outlined in Chapter 2) and couuting immediatel-y prior to the beginning of an experimenL, in order to reduce any cannibalisn that may occur (see Fox 1975c). Any prey

individuals that appeared darnaged due to the sorting or counting proceclure were replaced. The prey anirnals were tratrsf erred to the experínent-al containers, the waLer 1eve1 made up to 1000 ml and the

predato:: allorn'ed to feed fot 2.4 hottrs. Â11 exper:iments were performed at either a const-ant temperatttre of 2-2.f-2.0oC or iIr nore closely

controllecl cr:nstarrt temper:at-.ure rooms under a light regj-me of 1/+L:10D 139

(lights on 0600 h, off 2,000 h). The experimenls sLarted betvreen 1700 arrrl 1800 h. At l-he entl of :ube 24 hour- feeding pcriod the pretlators rvere removed and the ,l.niÞqÆ- recounled " Both Lhe nurnber of live and dead -r-ecordecl" been at-tr:qw" were Dead &ii_s_qp._s_ coul-d have cied naturally, kÍlled but not eaten (see Chapter 3, Sect.ion 3), or killed and eaten. Control containers of varíous prey clensÍLies \{ere rull at inLervals throughout the entÍre e>lperimental program. The results showed the incidence of naLural mortalíty or cannibalisn to be less than 1% and it

was therefore ignored in Lhe analysi-s. The number of þjgg.c- eaten duri¡g ¡he experj-nent. was obta,ined by counting the number of díscolour.ed

thuskst. RaqaE_rq, lilce other lleteroptera, is an exterodigester, and sucks ouL the preyts liquidifj.ed tissues. The very characterisLic discolouration caused by enzyrnatic activíty permJ-ts the eaten prey to be readily disLinguishable from the occasional natu::a1ly dyíng ¡trnisEls."

4;4 THE EFFECT 0F TBMPBRATURE 0N l'Ìm IUN_CU9NAL RESPONSE 0q ADULT

R. dispar .

424 L fntroduct.i.on Very 1iut1e is knoln about the changes of the key parameters of the functioual responser fl and Tn, with respect Lo changes in the physical conditions of the en'vi.ronment, despile the obvj-ous i.rnportance of , for exarnple, ternperature. Only a few investigaLors have ex¿rmined this relationship beLv¡een temperaLure and functional response, (Mack et q1. 1981; Messenger 1968; Thompson I97B; Gresens eL a1, L9B2). Since ternporal synchronization is necessary if ¡rredators are to exerL an effect on their pref r seasortal- differencês, 'sspsaia1l.y temperatul:e' can I/+O alter the magnitucle of pr:edatj-on (Thompson 1978a). The f acL Lhat adtrlt R"clispar: are rel-atively long-l-:ived ancl overt'¡inter as arlu-l-ts rneans l-hnt they experíence higtrly variable euvironmental temperal-urt:s, espec:ially in the shal.lorver or surface ruater 1ayer. The follorving sectj-on therefore sets out to describe the effect of temperature on the functional response of R'clisllqr- adul-ts and j-ts ÞreY, A¡lq ltgi'

424.2 Metjrc¡d The basic experimental procedure has been describecl above" Eight' replicates were used at each of four prey densit--ies rangiug fronr 4 to 50 Åni"rÆ/litre. The experiments were perforned at four temperatures,

name1y 15.5 + 1"0, 20.0 + 1"0, 25.0 + 0.5¡ arrd 29 t l-.0, ín cotlsta.nt tenperature roorns. IJoth predators and prey r¡ere acclírnatized at the appropriate experinental temperalure for trvo days befo::e Lhe start of the feeding perio

424.3 R_esg_ltq: Following exarnination of Lhe resul-t.s of this sect-lon and l-hose referred to in the follor.rj-ng Section 4:5 the most generaLLy applicable

response seemed to be the Type 2, although a Type 3 curve tnay have been

fitted in some cases (see i-n particular 15o Curve of l'ig " 422). Accordingly, the Type 2 response model was fi-tted to tlie darta, as outlined in Section 422. The obsen,ed arrd fitted daLa are sho'-rrr:Ln Fig'

422. The curves alj- shc.¡w a typical Type 2 response and Lhe degree of fit of l-he data i.s good as sho,,vn in Table 4:1" The asytnp'Lote i-ricreases j-nteflnecl:-ate from 8.2 at 15"0oC to 25.0 {g!ryg1 eaten aX 29.0ocn wi-th values of 16./+ anrl 2Q.2 al 20,OoC ancl 25.OoC respect-:Lvel-y. The liq. t¡;2 The l'unctionaf rcisponse of, ¿rdult female

R. di.spar- l-o clran qes ìn pley densit;, ol' pr:ey

size cl.ass 5., ô-._{e^T1.e_j, at îuur' cliiier.enL

vat-.ei: temperal-u tes .

Ileans , 9526 confidence intervals and l'it.ted curves are plolted. 2d

2A

to

þtå¡ 4 t o u¡

m'JJ Ësd &. 30 30

2

!J

o o 2i) o PffiHY ÐgNSlTY (N pe¡' litre) 7/+L

attack-rate (Ð and handling-time (\) n.tuneters are shown in Fig.4.3. ' gver the temperaLure range investJ-gated the atüaclc-rate c.hanges ín an hi. almost linear fashion.

The handling-time curve shorus a dramatic decrease from 15oC to

2OoC from which it appears to reach a constant value between 20oC and

30c.

TABLE 4:1- Values of the functional response parameters, attack rate (g) and handling-tine (Til Lhat provided the best fit to the rnodel, with the associated ¡2, and df.

Temp a SLd.Err. å Std.Err. X' df. sig. ofa of T. d al iú Ì 150 .05 .01 2.50 .34 1.55 3 NS

200 .03 .005 0.69 .14 0.48 3 NS

250 .08 .02 o.76 .16 2.45 3 NS

300 .11 .007 0.71 .04 0.34 3 NS

(The probability level on the chi-square distrj-bution was set at ot =

I value indicates that no significant i 0.05). A non-sígnificant X2 I difference was observed between observed and fítted v¿rlues.

4 I rì' 424.4 Díscussio¡r ,\' !r [" The constant handling time between 20 and 29o suggests that at 1l I I these Lemperatures B-¡!!,æ. was feeding at its maximum possible rate at the higher prey density and that the processes which linit the speeti at 'q I' I F j.ct. 4z 3 lhe effect ol tuater temperature on the atLacl<-

rate (g) and the handling time (L) of= adull-

l=enlafe R.dispar feedin g on size-cl-ass 5

A . " cleanei- Means and Standard [-rror plot-ted. Ð.12

s.10 2.5 @ 4 þlÊl 2.O {wl o.o I tu ß¡ H s- h- 4 1.5 ff o.o 6 Ð æ, J (} Ê 4 1.O þ 0.04 &. þ {( 4 å o.0 o.5

t o 15 20 25 30 TEMPffiffiATI"IFE ('ç} L42

whích t.he prey :Lndivicluals are ttprocerssedît (c1lr:ect handling of the captured prey during colì5uïnplioir, the exl-ract.i-on process, dj-gestion, gut cle¿rrance, etc) wel.e rìoL apparenl-1.y speeded up above 20oC. The Inean grrt-clearanc.e tjme for B.,Ljipcl at 22oC was about 3 hours (l = 3 h 16 m, gg = 27 m, N = 5) fol-1owi.ng a 5 day fast. No ol-her data on gut clearance rates ¿re avail-a ble for R.dispar. Lar,¡ton (1970) showecl that for larvae of the dansel fIY, Pvr rhosLolna nvmphul.¿r gut clearance times increased significantly fron 5 to 15oC, but above this temperature Lhe rate of increase became greatly reduced. tr{hether a sj-mil-ar paLtern exists for R.cli.spar is of course uttlcrtorvn, although the constaut hanclling-times observerl between 20 and 29oC is cousistanL with this possibility. The asympLote increases dramaticalfy (I'ig. t+22) frorn 15oC

to 20oC but thereafl-er increases aLnosL linear1y, whích shows i!:self in the increasecl values of a tr¡ith temperature. One v¡ou1d expect thaL at p1-ateau off as the I higher t.emperatures the atl-ack rate r gr should also

water becones unnaturally ruarm and lies'outside the normal range of R.dispar" It was believecl that 30oC would have been warm enough to shor'¡ this, but it is not uncomtroìl for the upper surface waler layers in the

farm ponds to approach 3OoC and slightly higher in shallor+er regions'

Thus it may be necessary Lo subject Rr--cliFÌar Lo liigher: temperal-ure to see if the attack rate reaches an asympl-ote. There are signs of this happening at the 1or,¡er (15oC) en,rl of the curve as shov¡n in Fig. 423. Handling tines should behave in a reciprocal manner, decreasing vrith

t ternperature and then increaslng under slressful conditions. Messengcr 1 (1968) reported such a pattern variation in the parasitic klasp' 3Bo-ry j 'f while Thornpson (1978) reporl-ecl thaL hantlling tine of a darnsel 1r e-ægþt3rL,

I f1y, decreased exponentially, 1evelling off at l-6oC' { Þl¡îUqtqS--Leåæ,, v¡hile the attack rate (a) increaserl signoidally, 1eve1líng off at -.rt i 11"r3

range that was 27 "5oC. In ::etrospecL, the temperaLure 8"-d-i-qllqr- subjected to cloes not. fu11y cover ttte natul"al upper arrcl lorser lirnits experienced in the field, although 29"5oC is very close to f-he upper linits.

The drarnatic Íncrease in handl-ing-tirne observed at 15oC probably reflects the lower rrrpbabolic rate of Ru-ttegA at this temperature and the thungerr resul-ts associal-ed influenc.e on the 1evel of the predaLor' The 25oC cloes in Chapter 6:3 shotr, that increasing the ternperaLure from 15 to not significantly increase t-he actj-vity 1evel of the prey rvhile increasÍng it to 30oC does. Therefore it is proposed that the observed j-ncrease Ín the ntunber of prey eaten shown in the functior¡tlrespollse of results for 15 to 25oC indicate an increase in the rnelabolic activity thungert rather R. dispar (r+ith Lhe associated increase in rnotivation) than an increase in prey activit-y and therefore encotlnter rate'

However, at 29oC" the additíonal prey eaten may have resulted from both

predator metaboJ-ic and prey activity effects'

0N 425 rHB EFE]ICT. oF 49E SrRUCruLq qF åÔË P8EDA:lOB ÄND -PREY ru"

TUNCTIONAL RESPONSE 4:5.1 Introduction. All size classes of both R"dispar and A.lqelel are found together

durÍng the surnmer ancl autrlmn periods in farn ponds. The functional responses (witir the associated a and Tr. Parameters) will vary markedly, the depending on Lhe st.age of the preclator developrnent and the síze of prey taken (Murdoch 1971; Hasse11, gL a1. 1976). Tdentification of into population { these changes permi-l-s Lhe inc.orporation of age strucf-ure

moclels, whj-ch in tu::n provides a more dyrtani.c predator-prey model 144

(Hassell 1978). The j-dentificaLion also allorvs chzrnges in the behavioural slrategies employecl by the predator in all stages of its devel-opment to be identified. To do this we must first lcnow how the successi've parameters of a and \ (assurning Type 2 responses) vary among developmentarl stages of the predaLor rvhen Lhey encounter the same size of prey, and troru they vary nithj'n the same pre

Despite the undoubted importalìce of such age Jtructure effects (Âuslander, Osler and lluttaker !974; Oster and Takahashi 1974) very few studies present us wiEh such informaLion, gathered for a single predator species. Notable excepLions are provided by Thompson (1975) worlcing (1977) with nymphs of the damselfly Ischnura -el.Jg"q. and Fernanrlo wÍth the preclatory mil-e 3,]Utos-qiÆ- persirni-lis" Thompson (1975) obtained functional responses for a number of the nymphs feeding on rt¡ater fleas (!q4Ð of different sizes, and from each response he absLracted values of a an.l Tn which r,¡ere found to depend upon predator and prey size. Although his resulLs are incornplete because he did not examine sma1l predator instars, they indicaLe a clear tendency lot a to decu-ne

and T, to increase as prey Trecomes larger or predators sma11er. These -lr trendl are also largely supported by Fernando (1977) and by several other experiments, reviewed by Hasse1l, et a-1.- (1976), where predator or prey size alone have been varied.

The only sLudy whích I am aware of that reports changes over a1l instars of a pretlator feedl-ng on a fu11 range of prey size classes is that of McArdle and Lawton (1979). Working wíth the Noto necta/Daphnia system they were able t-o show that the changes in both attack rate anri 145 handling tine r,rere far: more cornplicaLed ttran those docurnented by

Thompson (1975) for the I3f:ggg/Dglft"ig system, r'¡ith rnaxinum atl-aclc rates for small prerlators attacking sma11 PreY¡ arrcl large preilators attacking large PreY" Aclu1 t Notonecta had lower attack raLes tlran the two previous juvenile instars (4 and 5). Fron their data and those of other r+orkers, which they revievred, they suggest thal small predaLor lnstars will usually cornpete wil-h large instars for food, unless there is spatial or temporal separatiotr between them. In addition, røhere predator and prey sizes are plotted agaÍ-nsL either a or Th, complex surfaces are to be expected vherever a wide range of prey and preclator sizes is Ínvolved.

The followÍ-ng section sets out to describe Lhe changes in the functional responses and their associated attack rate (g) and handling time (\) parameters for all instars of R.dispar feeding on a fu11 range of size cl_asses <¡f A.deanei under controlled conditions in the laboratory" As such ft constitutes the first cornplete surface to be described for a sit-and-wait predator.

425.2 Method The basic experimental procedure has been ouLlined in Section 4:2, while the collectlng, síze grading and prcì-experinental holding period for both predators ancl prey are given in Chapter 2. (Additional experimental details are given, where appropriate, be1.ow).

Orlginally the experimenl was desi-gned to perrni-t each A. deanei lnstar to be vierr¡ed as a separate prey size cl-ass. I{owevern due to difficulties in both capturing and maintai-ning suffic'ient numbers of r46 first-instar prey to allorv this, Prey Size 1 is a combination of both Instar 1 and 2"

The experimental design, as shown tu Fig. 424, \tas treatecl as a 7 x 5 x 5 randomized block with norrnally 5, but in some cases 8 replicates.

Only those predators LhaL had moul-ted aL least 48 hours before the experiment r^¡ere used" The first i.nstar predators ro'ere observed for moulting for 48 hours after they were userl brrt this never occurred" 0n rare occasions predators tlied during the experimental period" Ttrese inclividuals t results rrrere discarded. Individual predaLors were used only once during any particular instar but sorne were used a second time in an older instar.

4:5.3 Re..sults As outli¡ed in Section 4:3 the mosL appropriate response nodel Lo fit to the data seerned to be a Type 2. Ilowever, as before, it would appear that a Type 3 response may have been just as easily fitted in

some cases (see for example Figs. 4:,5a (Prey Size 1); 4z5b (Prey Size 1); 4:5c (Prey Size 5); 4:5d (Prey Size 4 and 5); 4:5f (i'rey Size 5)). The functional response curves obtained for each of the predator instar (aclult.) /ptey size cornbinations are shor,'n in Figs. 4:5 a-g. Generally the degree of fit of the data to the model is good, as shorrn by Table 4:2. ltre majority of curves c¡btained follow the typical Type 2 pattern although several exceptions al:e worth noting" Figs. 4:5 e-g

shorr that the functional response obtained for the 5t,h instar and both

the male an

functÍonal resp onses of, R.dispar. FF{EÐATTR$

AÐULT AÐIJI-T ¡S¡ $TA R t¡¡sïAR iNSTAÊ IF¡STAR SNSiAF ¡¡¡ I'd FËMALE +JA.LE SËF|-¡CåTEl{c.rtg¿s

4 t¡J N 12 ã 2û lü 30 È

eü

l¡.t N

a)

rt ul T n, u ¡- å, t

ur N ú- <ì7

t¡) tìl t.¡ r, Þ

E, Fios " 4:5 A-G The functionaL response of each develop- nrental- stage of, R.dispar [eeCing on 5 size class of A.dga¡ei. Graph (A) is for I inst.ar ti.dispar, (B) II instar, (C) rlr instar, (D) IV instar,

(E) V inFtar, (r) Adult 1'ema]e, and

(G) Adult male.

Means , 959ó confidence intervaL and f-it-ted curves are pJ"otted.

PSI = Size cl-ass I, PS2 = Size cj.ass 2, PSf = Size class ), PS/+ = Size class 4 and PS5 = Size cl-ass 5 of A.deanei Note the changes in the vert.ical- axes. .(A) I INSTAR

12 PSl 6 PS2

6 3

o o

z PS3 PS4 u¡ 6 6 Þ l¡J É, UJ 3 3 6 E I zf o o 4 o4 o

PS5

4

2 + t o PREY DENSITY (N per titre) (B)

I¡ INSTAR

1S 12 PS2

tt

o o

pss ¡t 10 3 PS tÀ¡ þ" 4 f tÁt 2 flt 5 u¡ m # Þ w s o o o?o3040 o o ¡[o

3 PS5

2

,|

o o PRËY üËñdS¡TV (È{ per t¡tre} ! I (c) III INSTAR

I

PS2 3 PSl

20 15 10

I

I o

z PS3 ul F lo 1 ¡J¡ ¡E ul o 5 5 =Ð z o o 'to o o o

6

I

3

i ¡ { o 1 4 l\ (N + PREY DENSITY Per litre) I t

1'

ì, t I

I q

T' 1l ..1

'! rr,I tÞ

t i

'tf î i I (ðrt!! .rad ¡¡¡ À"!"¡SfdËfi ÅãUd T ! o o o Ì

I i v

I

ssd

o o I

o or g w nr , zl- m Þ -{ ffi Þsd csd

l: ,l o o I

I t ç1. $

þ 0e

11 l

.l

l

I '1 4

I .t (E)

V INSTAR

PS1

5 50

2

I

I o o

z, PS3 3 l¡¡ 30 Þ u¡ 20 E trl 't5 o I E zÐ o o3 o

PS5

1 J

5

r o Il (N + PREY DENSITY Per litre) r { ì. r ,t 4 { 4 r (F)

AÐUYT FEIHALE

psl 5û 5

2 2

o o

ps3 PS4 ts 30 1-d ¡¡t E u¡ 15 dt tr z o o 4

to i I I o o ( PREY Í!Ëh¡$¡TY {N por tltro) 't I T

Jì

,[

,l

,t t {

{ I (c)

ADULT MALÉ

I PSl

5 5

25 25

o o

z PS3 t¡¡ 30 F ut 20 I E t l¡¡ o E I zÐ o o o

PS5

2

f

o 10 o PREY DENSITY (N Por lltre)

,t {(

I { i

.tI t TABLE 4:2 Values of the attack rate (a) and handling time ($) Raramete¡s that pror,,ided the best fit to the Type 2

shourn by the v"lres. Functional. Response Model, es """o"i"lud XIA

.PREY PREDATOR STAGE S IZE CLA I II III IV V ADULT ADULÏ

2 I -2 g T. a g a a 5 9 å X g h {t x2 s h h x2 c .l-7 2.4 10 r.4 tl 7 14 .15 .Z 10 .I7 l0 .T I 4.97 3.62 r.94 2.85 0.75 4.83 r.38 0l .24 o2 2 02 .90 ot .It .01 ..05 01 .06 01 I

q 12 4.6 o6 2.0 13 t.0 '), .I9 .J 1I 22 11 5

2 J.29 2.t6 2.24 r.89 1.40 4.0r. 0.54 oJ .L6 o2 4 02 . .09 0z .04 .o2 .05 oz .r4 01 .r7

.02 4,I 01 1.0 06 1.8 ,I7 I.3 .t2 .9 20 .69 22 .6 .Ê 6 J 2.27 o 26 0. Bl I.69 9.42 6.7r I.42 .007 1.1 001 I.5 0l .24 0l .D 04 .2r 05 .11 08 .I7

.01. 6 .6 01 5.8 0B 3.9 07 I.6 .15 1.4 14 .76 11 1.0 4 1.21 7 .39 r.55 5.37 r.04 .75 2.24 .00? 2.5 c02 2.t 02 ,53 ot .as .02 .1 .01 .11 .0I .09

.01- 16 . B 01 15.6 02 4.8 07 2.6 .11 2.2 1l .99 1¿r 1.1 õ B ( 7.46 3.59 9.7L lr.Ls 0.16 2.6L 0i ,.2 004 2.6 0l_ 1.5 Ot ,55 .01 .06 03 .11 0l .16

1. Attack Rate value given, stendard e¡tor underneath. 2. l-iandling Time val-ue given, standa¡d er¡o¡ underneath. A. A Chi-square vaiue iover than 7.815 (q = 0.05) implies no significant difference betveen observed and fitted values. 0.01

Values for the herndling tirne (f) and attack rate (a) for all prerlator/prey comb:ilalions are given in Table 4t2 artd Figs. 4:6 ¿rnC. 4:'/' It is obvious frorn Fig. !+26 that, in genelal, the handling t'ime increases aS prey si.ze increases and decreases as predator size increases. This pattern supports LhaL of Thornpson (1975) r¡ho also reported handling time ':hanging monotonically in tire opposite clirecLion for Cragonfly nyrnphs feeding on ÐaPhqþ" The atLack-rat-e surface however, is very different frorn that obtained by Thompson (1975)' rls the predaLor size increases the attack rate surface changes i1 a complex way (Fig . 4:7). For the first two pretlal-or instars (I anrl II) the As the maximum attack ral-e occurs on the smallest prey sizes (1 and 2) " predator size continues to increase so the att-ack rate on each of the prey sizes increases. Unlike the previ.àusly published al-tack raLe rrolls surface of McArdle and Lar¡ton (1979), rrhere the surface c'vert, i.e" the maxima are found in opposite coruers v¡ith sma1l predators atLacking sma1l prey and large predat-ors aLLaclting large PreTr no such change is observeri here. In the present sLudy the maximum attack raLe' rather than trollingt completely over, slops mid-rvay, i"e. aL prey size 3.

Examining the surface in Sreater detail reveals that the maximtlm attack-rate for predator instar III is almosL the same fc'r prel' size 1

and 2, that of predator instar IV is greater for prey sj-ze 2, while in the three largesL predator sj-zes (j-nstar V, ariu'1-t fenal-e and male) the and maxj-muln attac.lt.-rat-e ís found for prey size 3. As ir: Lhe lfc'Ardle F íq, 4:6 The effect of' predator and prey size crn the

handling time of n.-{+Sg*f l"eeding on A.de_etrlej,. [xact numerica]- values are given in Table 4:2. HANDLI${G TIME SURFACE

15

E to u¡

5 =l-

il 5 ll 4 v 3 v PREY SIZE INSlARS 2 AF PREDATOR ltM f-iq " 4:7 The el'f'ect. of predator and prey size on

the attack-rate (a) of R. dispar feedin q on A.dçanpj!.. Exacl- numerical val-ues are given in Table /+:2. 1----rL +þg: ::-:à

&Þ h; o' å ùÛY +' Cô À' Þ €D

Þ

Þg

(¡l 4 ù (r¡ "s ¡\) úr i+ ? o C} I ?o 'u 6¡ ATTACK RATE 148

Lawton (Ig7g) sturly, predator instar v has the largesL attack rate values over all prey sizes and the adult @.3. have noticeably loiuer attack-rates for various prey sizes than insLars v, IV and to some degree III.

425.4 Discussion The apparently 1ínear funct.ional responses obLained for the fifth instars and adult Rana.t.!q, feeCing on Lhe sm¿r11est p':ey sízes, requires and conment befo::e considering l-he overall changes in the attack-rate handling-tíme for the various predator/prey combinàtions. It is interesting to note that for these larger. predators exposed Lo t'he various prey densities that are.found in the field (including the very and ín dense aggregations).no plateau is reachecl in the response curve, fact resemble a Type 1 functional response'

The Type 1 response, nuch less corunon than Type 2' is not normally associated with insect predators, usually being characleristic of filter feeSers. In a Type 1 response the predation-rate 1s directly proportional to the food concentratJ-on up to a particular concentration low value which ls a function of the predators handling efficiency. At prey concentrations the hanrlling componenL of the predatorrs behaviour

does not interfere with the predation-raLe, because iL happens sufficiently quickly to remove all the food accumulated. Above a cer¡ain prey density however, the predator is unable to process the prey any faster. At such densities, therefore, it, ingests food at a maximum rate, limited by Í.ts handl.ing l-j.me. The Type 1. response is, therefore, t49 an extreme form of the Type 2 response in rvhich the handling-time exerts its effect not gradually but sudclenly.

The present. study shor,'s that the larger RanaEra predators, lrhen fee

As mentioned previously, very few daLa coriparable to those in Fig. 4:6 and 4:7 have been published. The most complete are those of Mc¡rdlets and Lawtonts (1979) attack-rate and handling-tirne surfaces for

each inst,ar of Notonqcta Rlauca attacking four size classes of Dgpþgi"q

naP.na as prey. For R.dispar the handling LÍne surface is very sj.nilar to that reported by Thompson (1975). The attack raLe surface hovrever, Ls simílar to that of Notonecta described by ìlcArdle and Lawton (1979) except ratl'rer than the maxi¡na being in opposite corners' the uraxima are found about rnirl-way along the prey size axj.s for the 1ar:ger predators (see Fig. 4J). Slices through partial surfaces in Lhe prey-size or predator-size planes for other species tend Lo suggesL simílar relationships (Hassell et a1. 1976i Hassell 1978). In general, these results are to be expected. The larger the predator and the smaller the 1,50 pref, the more the preclator should be able to attac.l< and eaL, result'ing in high atl-ack*raies and short handling-tirnes for large preclators feeding on sma1l prey. The reverse r,¡ill also be Lruer w-tt.h long handlÍng t,imes and 1ow aEtack rales for sma11 predators attaclcing and eating large prey. llor¿ever, there must be a lirnit to t-his process, because all predators ignore prey tirat is very sma1l or very large relative to their own size (Griffiths 1975; Sa1-t L967; hTilson 1-975)"

The effect of prey size on the proporLion of e.lcottnters that initiated attack by adult female mantids (Holling L9i¿4) arrd R. dispar (this work Chapter 3) are shown Ín l'ig. 4:B À & B. Obviously Lhe attack-raLe is deLerrnined not on1.y by Lhis proportion but al-so by the arousal distance of the predator to the prey and the encounLer-rate

between predator and prey (Holling 1965). Typically' the subcomponents (see Holling 1963; anrl later this chapter) contribute to the atl-ack rate in different h¡ays. Large prey may initíate a sma11 number of attacks, few of which are successful- (reducing g), buL n¡ove about quickly, thus increasing the number of encounters (see Gerritsen 1980;

Gerritsen and-strickler 1977; Vita et_.+_l_ L982), and being easy to observe (bor:h of which increase a). McArdle and Lawton (1979) put forwarcl the plausible assurnption that although 1t cou1cl not be expect-ed that changes in any one of the subcornponents would predict exactly nhat

happens to the attack-rate, we may reasonably expect a values t-o first increase, plat,eau and then decrease with decreasing prey size, follovring the general pattern observed in Fig. 4:B A & ts" Furthernore,' the turnover point might be expected to be at a surall I optítnuml prey size j-u earlier (sma1-1er) Ínstars of the predator" Pertinent data suppor:ti-ng these inferences are provided, amongst others, by DÍ>lorr (1959), Dixon

ìr- . Fiq. 4:B The effect of prey size on eit.her' (A) the percentage of' encounLers that jnitiated

attack by f ernal e nranti ds HierocJula cl:assa or (B) Lhe pr'oportion of alousals resuJ-ting

R in a capture by l'enlal-e " dispar.

(A) afLer l-loJling 1966 (A) Hierodula crassa ( after Holllng 1966)

70 U, UJ @'@ zU' o À 60 U, t¡I o G, V o 50 @ e

òa 40

30 2.5 5.O 7.5 10. o U' RADIUS OF PREY M O DEL(mm)

.n :f 3 o.u l! o z o.s 9 F É o (Jt o.¿ É À U) o o.o .t, lll G 3 L o.z o 2 4 6 I 15 20 LENGTH OF PREY MODEL(mm) ls1 and Russel (Ig72), Glen (1975) and l\lratten (1973). l'lc¡trd1els and

t surf ace also Lawton s or,m rvork ( IgTg) rr,Íth the Nogoqçg-çg" atl-ack-rate ín supported this hypothesis. A sonewhat si.milar pattern is ol¡served R.dispar (see Fig. t+27). For instars I to III' and prey sizes 1 to 5 the maximum attaclc rate increases almost monoLonicall-y witir increasing toptÍmumr predator size and decreasing prey size. If there is an prey beiow size beyond rvhich the attack-rate sLarts to decline again it lies (i.e. rto the right oft in Fig. 4:7) Anisopg prey-size 1" The piateau appears to be reached in Tnstar III however, and the decline in aLtack-rate for prey size 1 is very evident for instar IV" The shape of the surfaces for instar V and both aclull-s is very much as predicted by of McArdle and Lawtonts (Ig7g) hypothesis and also from the prediction Fig. 3:22 in Chapter 3. 0bviously the actual capturing paltern for different sized prey that is observed using live prey v¡ill not depend only on prey-length as Fig. 3:22. implles' Rather, a number of dj.fferent characterístics of the prey itself will signif,icantly Ínfluence the caPture success.

The 1or,¡er aLtack-rate observed fòr both atlul-L sexes compared with lnstar v was also reported by McArdle and Lavrton (1979) for Nct*oieq@ g1auca. They suggested that food requirements of the last instar, which

grow rapidly, are much higher Lhan those of non-reproductive adults' El1is and Borden (1970) also found that adulr' &tonecte. undul.a-B feedj-ng As on Aedee larvae ate considerably less on average than did instar V' far as possÍble only non-reproductive Rana'q-rq aduLts r'¡ere usecl in the present experiments, so these dat-a rnay support McArdlers and Lawtonrs usíng hypothesis. Hor¿ever until comparative experiments are carried ortt rs2

bol-h non-reproductive and reproductive adul-Ls fl.o conclusive sLater;ienLe

can be rnaden

Attack rates and handl-íng t:Lme surfaces are also useful to preclict Lire relatÍve proportiorrs of seve::al sizes of prey that each

pretlator i-nstar ur:111 take from a n¡ixture of differenl sizes' \'Iithout exploÍtalíon, the nurnber of prey of size class 1 eaten will be :

N 4.L4. e a1N1Tr/(1 + rliatTh.Ni)

rvhere there are n s:lze classes, and the N. is the number of prey available in the j-th size c1ass. lz \ and T, as defined previously (McArdle and Larçton 1979).

(The equaLion can be easily :i-ntegrated to allots for expl-oitatiou' as shown by Lawton g! 4. 1974). ü I If R.¡Lþper are given equal numbers of òach of tl-re five size clas'rses of É, equatiot 4.:rt+ predicts the number of each size class eaten by each predator stage will be as sliov¡n in Fig. 4:9 (A)" The mean length

of prey capLured and eaten by each E-¿jgB1t- stage can he calcuiat-ed from the numbers ealen and is sho',,'n in Fig. 4:9 (B)" Although the mean length of prey talcen increases with predator size, the larger predators

I prey. The patte::n of predicted sizes talcen is i will take the smal-ler

I siuilar: to the resulrs of McÂrclle and T,¿twton (L979) for l!o*[qngqþ-

{ f predator. In addition, other worlcers have presenLed results fron real

(as opposed to predicted) data on prey-size selection by a vari.ety of t- 'J preclators (see for exarnple Aclams 1980; Davonport and t differenL I I lrlinterbount 1976; Malnquist and Sjostrorn 1980; Pearson and Steinbe'r:ger 19BO; Sheldon 1969, 1980; Thompson 1978b; D"I.l11'son 1.975; Wj-rtLerbr:urn { I J Fiq. 4:9 (A) The predicted number of each prey size

cl-ass capl,ured uhen equal'numbers (n = 19¡ of each size class are presented at the

same l-inle to each predal-or instar and adult of R.dispar.

(B) The predícted mean length of prey captured

by each predator instar or adul_t. (See t.ext for adcJitionaL details and predictj-vc: equaLion). c1 o lÀ) c ûa ! o o ! ø

q0 I o âø I É 5 2 E û I I 2. ! I o ô Y SIZE GLASS i' ¡{t 2!¡¡ t ¡la t tttat I tt{l t2lat v ADULT þßEoAToR srÀGE I rl ut ¡Y

7 FnEt gtzE cLA88 I

t þ ?hEY tlEE Ct^98 a Ë

5 '-Ð fc PNEI AIPÉ CL¡g' Þ lil 4 G 0. FSEY âI'E CLÀ55

3

tÍtt s¡¡E cL^tt 2 t. ADULT i ll ilt tv v i PËEÐ/ITOR STAGE i + t ù

J: 1(

¡, t ,t 4 {

{ r 153

predal-ors irave 1974 and addition¿¡l references i-n Peckars;lcy I9B2). Large the much broader niches in Lerms of prey size than srnaj-ler predal-ols of (1975) hes same species (Ho1ling et al.. 1976). This causes, as \^iilson suggested, an asyrnüeLry irr the compet-iLive irrteractions of clifferent sized preclators" Äs McArdle ancl Larvton (1979) have pointed oui, large preclators eat th-ings ruhich are unavailabl-e to srna1l predators (1arge lsimpler prey) but the reverse is ¡nuch less trrre. If attack-rate surfaces are found, as by Thonpson (1978b) the effecl- i+il-l be particularly obvious" Bven a more complex surface, for example that found fy Uc9/dte and Lar+l-on (1979) for Nqto¡ec'!4.' ancl R"di.spar as found in this st-udy provides only a parti-al relief frorn competition for tlie snallest predator instars, because the large predators sLill f-eed on the smallest prey. The sarne will prcibably be Lrue for small insLars of R.dispar. Size differences Lretween suc,cessive instars may therefore provide only a poor means of reclucing exploiLation competí'tion (Hor:n and

May 1977) "

Hov do the sma1l insl-ars of prerlaiors avoíd intra-specific competitiol with larger ind.iyicluals, apart from by simply beÍng small-er? insects Two conunon features of the life-hist.ory patterns of predatory have been reported for l{otonectidae. Successive instars of [*o.¡SSIg. glauca are sLaggered in time, so that large and smal-f indiviclual's very rarely occur togetirer (reported in l{cArdl.e anrl Lawton L979, P"2-13)' teinpor:al-ly) whilst srna11 insLar s of Ì'l.hofi:rnani ma y be spatially (buL not of separ:ated f rom t-l'le adults (l{ur:Coch and Sih 1978; Sj-h i9S0) " Instars R.ciispar are nol- sLaggerecl in time , aIL clevelopmental stages being (Deceirber to presenL in the same local.ity for: most' of Lhe suinrner rnonLhs February, in Australia). Hol,,e\rer, ex¿rminaLion of tÌ-re srlatial 154 distriT¡ution of boLh R.clispar and A.,lqane'!" ínstars in Lhe sLrtr:ly area shor.¡s a very characLerislic pattern, tvith the s;rnal1est in*;t-ars concentrated in the shallolr'er water around the perirueLer of tl're pond and the 1-ar:ger j.nstars founcl j-n the deeper'l{aceï. Tilese field lesults offer support.-lve evj-clences to Murdochts and Sihts (1978) laboratori' resul'ts concerning spati-al distribul-ion of Notonectidae, but pe::haps more importantly provi

These results demonstrate the multi-faceted selective forces that

can operate in causing the observed spatial dist'ribution of different

devel-opmental- stages of predators. Tt is to be expected tl-raL selection will be operating to rerluce the intra-spec.ific competit'ion in both the preclator and prey species (the prey species usecl, Ân-igo-p-.1 -Lgq:t4' i.s itself a predatory insect) in addition l-o it maximizing the probability toptirnumr of the l,arious preclator instars encoulìtering the prey size 1_55

that it can capLure and eat. This highlights the care that musL be 'tal."n in our ridentiflcationt of proposed selective forces v¡hicl'r resulL in the observed situation.

obviously, as l,fcÄrdle a¡rd Lawton (Lg7g) Point out' equatÍon 4'14 ís a simplifícal-ion. ln a naLural mixture of prey of different sizes' the predator nay not hunt or capture randomly (which is irnplied) ' Instead, the predator may rselectt profitable and discard or i'gnore unprofitable prey, usin; a far more complex ¡nechanism tlrarr can be (Krebs prerlicted- /ut"ty from each functional response in isolation Lg77).TheresultsfromChapters3ancl5suggestthattlrisis (1983) trndoubtedly true for Rrjl:Spê!. In addition Blois and cloarec prey sel-ection, have shown that R"linear:Lq exhibits a density-dependent' while rvith first instar nymphs shoving a preference for small Daphnj-a in larger predator nymphs the preference for big Darrhnia increased' The selection was evident first when the reiafive densities for the high' preferred size-class were 1ow, then when absolute densities were with Lhe choice generally being made before striking'

Further discussion on prey-size selection fa11s outside the a solid-data bounds of this present study, due in parL to the lack of classes base of selecLion i n R.dispar when offered various prey size simultaneously. Time did not permit the completion of planned preclicted experiments, although inÍtial results appear to support the shed ones to a certaiD degree. Obviously further exper:irnentat'ion wil1 1_ight on the tselect.ivityt of prey by a sit-and-waj-t predator. 1s6

has This chapLef set5 ouL to examine the effect thaL prey densit'y \, on the numbers of prey eaten. C1ear1y, as the results show' progressively more prey are eaien as density increases until finally reaching a maximum. This being so, the questíons that automatically atl-ribuLes of corne to mind are concerned rqlth identÍfying part'icular both predator and prey that are j-nvolved in nraking this possible' describe As tli-scussed earlier, the two paramelers widely used to the functional resPonse are pr the attack-rate, and .\, the ,It handling-tine. The overall values of a and are a cornposj-te pÍclure of the individual sub-components into which they can be tiivided (llo11ing 1963, 1965, 1966). Holling considers a to be a function of: 1. The reactive distance of the predator for prey, i'e' the maxinun distance at which a predator will react by attacking preyt

2. the speed of movemen{át predator and prey, and

3. capture success, i.e. the proportion of prey, coming close enough tr-r be attacked, that are successfully capl-ured'

Handling time, Tn, was sPlit into

4. time spent pursuing and subduing each prey itemt

5. time spent exploring and eating each prey, and

6. time spent in a ttligestive pauset after prey had been eaten during

whj-ch the predator does noL at-tack J-urLher prey' i

I L57

obviously a wirle range of differeût variables rvi1l impinge on ,!:. a utd T,r_' these facLors, causing significant variation ín both likely changes Nevertheless, usÍng these sttb-compottents, one catl predict t, wíth prey and pretlaLor size more easily Lhan similar predictions &. of var:y in the sarne direction for ¡lr because all the sub-components å. prey that are (Thompson 1975)¡ this is unLrue for a. Thus, whereas (a11 causiug rel-atively large will take longer to subdue, eat and tligest or an increase in TO) (see Dixon 1959), these same prey may be seen move faster recognized further away (giving a rise to an increase in Ð I (leading a (again increasing a) (G1en rg75), but escape more easily to decrease ir q) (see also trlratten 1973; Brotrn l97t+).

/ chapter 3 has exaníned factofs involved in the reactive distancel't (5) exami-nes two and capture success of R.dis-pgq. The following chapter rl that' have been r features of R.dispar prey-capturing and feeding behaviour on the number of observed and, it is belj-eved, have a significanL effect preyrs prey eaten. As mentioned above, parLicuiar features of the determining behaviour, in addition to the predatorts, are involved in out to examine the outcome of the interaction. Therefore chapter 6 sets play a and ident.ify sone attributes of the preyts behaviour that predaLor' ì the preyrs eocounter raLe with the i significant part in i

L t

1

ß

ì. ,¡' T f

'l

4

I.l 1' *'-.- æ -¿r* \r.. ;.# -_.t. =T.-*' 158

5:1 INTRODUCTION:

The way in r,¡h:[ch a suckíng bug lilce R"dispar feecls on a single prey can be líkened to the way in which a predator util.izes a patch of

resource. As food is renoved, so the quality of the patch decreases as

the amount of food left in the prey (patch) and thus its ease of extraction (harvest.ing) declines" I

Recently, two classes of nodels have been developed to exarni-ne

the amounL of food extracted fron Lhe prey (patch), the time spent in feeding and the effect. of prey densíty.

MODEL 1 : OPTIMÂL FORAGING (FEBDING) Optimal foraging theory predíct.s that natural selection rvill favour those behavioural processes that maximize the neL rate of food i il intake (Pyke, Pullian and Charnov L977). One important, feaLure of this 's I theory that has received a great deal of aLtention i-s the allocation of

tÍme to patches, Í.e. how long a foraging predator should spend in a patch of resource of certain profitability (Krebs 1978, L979; Pyke, e-!

aL. 1977; Schoener 1971) and what are the controlling fact-ors. Trvo

hypotheses have been suggesLed with respect to these factors. One suggests that the predaLor can monitor the relative resource qualities

i i of patches in the environment and the subsequent selection of, and tin:e i

¡ spent in a patch is based on this informat.ion (e.g. Cha::nov L976a; i I ¡, Krelrs L979; Krebs, Ryan and Charnov l97l+). The ot.her, which has been t. the subjecl of various models, predicts that the time spent in patches i' {t I is dependent on some ínnate responses the predator makes to each patch ,t I I t on its own, and does noL require the predator to have overall it l) t .i t_ 1s9

information about the environmental profitability (e.g. I-lassell and May 1974; l{aage 1979).

Recently, Cook and Cockrell (1978) exlended opt.imal foraging

theory to consicler the amount of food extraci-ed from a prey and the time spent in the exLracti-on. They proposed that if- sorne parts of a prey

r¿ere easier (or more nutritious) to consurûe than others a preclator could

maximize the net. rate of energy intake at high prey clensities by

selectj.vely feerling on these parts. The mechani.sm suggested by Coolc and Cockrell (1978) involves the prerlatorfs ability t9 tmeasurer the time bet¡veen prey captures, i"e. the ínLercatch-inte::va1, the handling*ti-rne (feeding-time) for each p::ey being determined by this va1ue, such that each prey should be discarded rshen the ingestion rate reaches the average rate of Lhe energy harvest (i"e. similar to the marginal value

I Lheory of Charnov, L976a). Since Lhen this hypothesis has been further used to explain the variance in Lhe allocation of tirne per patch and lhe partial consunption of prey in a number of different organisms (Gill-er

1980; Griffiths 1980a, b, L9B2¡ llodges 1981; Hodges and trrrolf 1981; Kruse 1983; Sih 1980). I

MODEL 2 : GUT-FILLING

i i The variability in feecling times anrl ttre partial consumption of

prey carr also be explained (Cool< '{ by a gut-filling model and Cockrel.l I ¡, i 1978; Gelperin L97Ii Johnson, Akre and Crovrley 1975). This model suggests that the predator continues feeding until parLi-cular regions of i{ 1r I ,t the gut are'fu1l. During the intercatch-interval food passes through I { the gut into a second region. The space j-n the fore-gut at the start of

.,i4

T' .¡ l' 160

the next. feerling deterrnines horv much food can be taken in ancl hence the feedlng time.

The previc'us chapLer sholecl t-he effect of prey density on the number of prcy eaten. It is expect,ed that nean intercatch-int-erva1 should fa1l as prey density increases" Both models predíct thaL the

rnean handlíng-time (feeding time) should decrease r¡j.l-h increase in prey

density. The Lwo rnodels, however, will predict dj-fferenl- relationships

bet-ween individual inLercatch-Ínl-ervals and t.he subsequenl individual

feerling-times. No correlation belween t,hose tÌso parameters is expectecl in the optimal foraging model, as the feeding-'time for all pr:ey iterrrs

caught at each density is seL at a value determined by the AVERAGE

intercatch-interval. The gut-fil1ing mode1, however, predicts a

positive correlation, as the feedÍng-time on each prey is determined by

the length of the intercatch-interval whj-ch determines the ravailable space! in the gut.

The experiments reported in thj.s chapter were designed to explore the relationshÍps between feeding-tirne, i-ngestion-raLe, prey-deplet.ion and prey-density in the light of the above models; then to identify

possíb1e controlling mechanisms utilized and to compare these wil-h cther predators.

5zZ rl{q ruQ DYNAMICS QE R. dispar 522.I Methocls

General Adult female predators, R"dispar: and adult - PreY ' { A.deaneio were used throughouL Lhe following experiments. (Tj-me did not 161

pernÍt an analysis of other instars of the predator or sizes of the prey).

Before any feeding experiments predators were rernoved frorn the holding tanks, having been collected fron the field during the prececling

48 hours, and fed a mixture of nosquito larvae, Notonectidae and. various other aquatic invertebriates (Odonates, !g¿!1gþ. etc.) for 24 hours. "pp. They were then removed ancl deprived of food for betr¡een 48 and 5/r hours,

depending on the durati-on of the experiment. No predator !¡as used for more than one sel- of experiments,

Adult prey were removerl from holding tanks after being coll-ected

at the same l-ocation and tirne as Lhe pr:edators. During t-he holding

periodr prey individuals had access to a rnixture of comnon food types (Mosquito larvae, Daphn:lq spp. srnal-ler notonectid instars - normarlly II or III) at all times. By doing this it \{as assumed.that irrespective of Eheir past feeding history the prey animals would be at a coraparable state as a food source at. the beginning of the experiment.

Prey were removed from the tanks, gently blotted on absorbent

tissue and weighed ancl measu::ed as outlined in Chapter 2:2. Following

the experiment-s, dead, partíally digested prey \,ùere renoved and weighed

and measured by the same procedure. Prey carcasses-".rrrerê placed into individr¡ally marked glass vials and dried at 105oC for 48 hours and ttren reweighed. Because of the strong positive correlatj-on between animal

lengtlr and dry rveight.(see Chapter 222) the amounL of rlry weight I t extracted could be calculated (+ error) by subtraction r62

Both preclators and prey were randomly assigned to any of LLre treatmenLs and all- experimenl-s \ùere carried out in an insectary roon of a constant temperature (22 + l.5oC r,¡ater temp.) in lighL supplied by a bank of 3 tdaylightt fluorescenl- tubes 1.5 metres above. A diffuser filter provided an even flooding of light of íntensity 1200 1ux at lhe waLer surface.

The tern rfeeding timer and rintercatch j-ntervalt are based on

Lhose of Coolc and Cockrell (1978). rFeeding timer is defined as the time from actual onset of feeding to termination on a single prey and has been adopted rather than the term rhandling tirner of Cook and Cockrell (1978) to avoÍd confusion (see below). The fintercatch intervalt ís defined as the timà between successive captures, rather than the t,i-ne from the termination of one feeding to Lhe beginning of the next, because of the multipl-e prey capturing caparbility of R. dÍ.-spar

(see Section 5:3). In addition, vrhen holding nore than one prey, rexploret feeding can be interupted, thus enabling Be4atra to the second prey. Normally the term rhandling timer includes these periods of exploration, änd therefore to avoid confusion has not been used.

All tines were neasured with an electronic stopwatch. The terms ringestion ratet (weight of prey ingested/unit feeding tirne) in reference to predators artificíally interrupted during their feeding activities, anrl rmarginal ingestion ratet (amount of prey i.ngested/unit feeding t.ime) in reference Lo predators l-hat v¡ere allowed to feed until they naturally discarded the prey have also been used as clefined by

Kruse ( 1983). 163

BXPERIMENT 1. To lnvestigal-e the initial feecling behaviour (injecl-ion of toxi.ns/digestive enzymes) and subsequent extracLion of prey tissue, predators were all.owed to feed on individual prey for fj-xed periods.

/D¿ Similar-sized prey an/rnals (f = 1.3.37 rng., SD = 2.98 mg.) were sel-ected and indivÍdually hand fed to adult R"rlisper by car:efu1ly placing the notonectid in close proximity to the Rqna-t¡:a with the aid of a paj,r of fine, soft forceps. Interruption of Lhe meal (after 5, 15, 30, 60 and

105 nj-nutes) r+as achieved by either grasping the prey carcass vi-t,h forceps and removing iL from the rostrum or gently grasping the predato:: and rernoving it from the water. This treatment always terminated feeding and the prey carcass could be removecl. (Care was Laken at all times in handl-ing dead prey to prevent darnage. Thj.s is particularly noticeable following long periods of feeding when Lhe carcass has been exposed to the digestive enzymes and can lÍteraI1-y tfall apartr. Dur:ing a few experiments this unfortunately happened and although the remains were collect.ed and processed for other purposes, the rcsults have not been includecl in the following analysis).

EXPERIMENT 2._ The effect of initial prey-density on prey-depleLion or utilizatlon (percent of total available eaten) was also determlneC. Six densities (8, L2, 20, 24, 32, 40 prey/container) were tesled"

Notonectj-ds were capLured and the experiment terminalecl afLer the first I prey ín every trealrnent had been consumed. Prey vrere removed, rneasuredn rveighed and dried as outlinecl prevíously. There r*'ere three replicates at each densit,y.

EXPBRIMEIIT 3. As prey-densiLy increases, the intercatcb-interv¿rl sborrltl decrease and both the optimal foraging and Llie guL*liniLin¡; hypotheses 164

\{ould predict a decrease in the feecling t,irne per prey item" Therefore the effect of prey density was exaninecl. Four: pr:ey densities (1, 10,

30, 60 prey/container) were tested. Predators treated with density 1 were hand fed as outlined in llxperiment 1, r+hile &anatlg. treat-ed wÍth densities 10, 30 and 60u captured their prey naturally" As prey individuals r"ere captured they were immedíate1y replacecl r*'ith a sÍmi1ar sized individual to maintain a constant densi.ty. As prey were discarded they were immediately removedr'measured, vreighed and transferred to Lhe glass vial. Data was collected over an 8-prey catch seguence at'each density.

522.2 Results and Discussion Experiment 1. For an exterodigesLer 1Íke R4natlg, feeding may be able to be l¡roken down into three stages. (1) Injection of venom/enzymes (2) digestive pause r¡hÍle enzymes act and (3) extraction of liquidifieC food

(Griffith 1982). The results are consistent with this hypothesis when examining changes in weL weight of prey, Fig, 5:1. There is an initial weÍght increase rvhen the tcocktailt of venom and digestive enzymes is injected into the prey, (norma11y Lhrough an íntersegrnental membrane of one of Lhe rnetathoracic legs - see SectíonJ:) but within a fer" miuuLes Lhe aninal begins to extract food.

However, when examinÌng changes in dry weight of t,he sane

animals, Fig. 522, prey body contenLs are apparent-ly extracted frorn Lhe onset of feeding" The ingestÍon raLe decreases with Lirne and supporis the fj-nctings of Cook and Cockrell (1978), Gi11er (1980) and Kruse (1983). These results suggest that j.nj-Li.ally Lhe pr:ey contents are Fiq.5:1 The change in vet rueight of indiviCual orey durinE feedíng by adult R.dispar.

(*) Increase (-) Decrease

Value given is mean + 95ió Con.fidence Intervai. Curve is f,itted by eye.

Figure in parenthesis indicates number of repl-lcates. I - o àe I,) o) 2.5 +l lx

Et 2.O E

UJ .E 6. ll o

; 1.O =t- ¡¡J 3 o.5

=tlJ o z o+I 2 o o 60 o I o 90 100 1 o FEEDING TIME(min) o.5 (8) FjS" 5:2 The curnul-al-ir¡c dr;, ucigl-rt o1" l"ood e>lLracted f,ronr i¡rdiviciu¿r1 pley ruìi-lr i-jme spent Íeeding

by R. dispar.

Val"ue given is mean +_ Standard Error.

Figure in ¡tarenthesis. indicaLes nunrber of repJ_icates. ÐffiV WT. EXTRACTËÐ {*e} Õ l\¡ P 6 Õ Ð t

ôb Ð T ¡T rft LJ & fr) l@ ffio ffi s'

e¡ ö 16s qulckly ingesLed, buf- thaL the yielri per unit feeding Line d.ecrcases as the pr:edator contj.nrles to feed. r\ one-wny ANOV¡' suggested thal- Lhe mean rates of exLraction at different sLages of feeciing \,¿ere not the sarne

(tt,r6 = 6.72, P = 0.000/+) (Fig. 5:3). The SNK Multíple Range Test (c = 0.05) revea.led that extraction rate did significanLly i-ncrease during tlre first 15 minutes before decreasing, although even af.ter 30 uiuutes feeding the rate of exl-raction was stil1 marginally, although not signifícanl-1y, higher than the initial extraction rate. The rate of extraction then decreased almosl linearly as the feeding session continued" This phenornenon Ís quÍt.e differenL from what has been previously reported for sucking ínsects and the significance of it will be discussed shortly.

Fig.5:4 shows a comparison of Lhe amounts extracted as measured by both wet veighL and dry weight deLermÍnation. Ini.tially an impossible situati-on seens to exist, with the dry r*eight. curve exhibi¿ing higher values at the different interrupL times. This suggested that external water nìay have been passÍ-ng.into the prey as the feeding session continued, especially as .L"{!:-g- changes the l"ocaLion of the feeding site in Lhe preyrs body (see Section 3:2) on nunerous occasions. This phenomenolr v¿as later confirrned as outlined in Section

.!. J.A.<. o

Coòk and Cockrell (1978) found thaL in the back srvímrner,

Notone-ctq Elauclr_, feeding on mosqulto I arvae, there appeared to be irnrnediate extract-j-on of food from the prey vit-h no eviderrce of a digestive stage. Tn contrast to this, Grlffiths (1982) measuled changes in wet r+eighl- of prey, follorvi-ng capl-ure by ant--lian l-arvae, anri argued Fio. 523 The change -in meern rate of dry rueiglrt extracted

f rom a single prey witl-r time spent feeding by

R . d:'-s¡rar

Val-ue given is mean + Sl-andard frror. Means vith same -Letter are not signil'icantly

difl'erent. (SNK fïultiple Rangc Testr o = 0.05). Figur:es in parenthesis indicate number of, replicates. o.o 6

A (7) iC o.o 5 E ot E z o t- B o o.o4 (7) É, B Þ (8) B x (81 r¡J IL o ¡I¡ o.o 3 Þ-

Ê, BC (s)

c o.o 2 (5)

o.o 1 o 40 80 120 FEEDING TIME (M¡N) FIG. 524 A comparison of the rueight change in individual prey during feeding by R.dispar as measured by both ruet and dry rueight.

Val-ues given are mean + 95ií Confidence IntervaL. Figures in parenthesis indicate number of replicrtes. Both curves are fitted by eye.

( g ) Change in ['Jet t^Jeight ( ¡ ) Change in Dry lleight (See also Figs.5:l an.d 5:2 and text for additional details). ût E

LrJ É. L ç o 2.5 f- = E 2.O

(5) ÉoL) 1.5 (8) ot^ o\ o+l vX 1.o t- lrl LrJ 7 =É L¡J o.5 -t-- H hl I Êz. 20 3 o 5 60 70 80 90 100 110 [J + c5 z. (¡r) (min) =(J o.5 FEEDING TIME 166 that the observed :Lncrease irr prey ruej-ght r"as due to the injection of venom and enzymes" In additj-on, he poinl-ed out LhaL Cook arrd Cockrellrs

(1978) conclusiorr of immediat.e extraction of prey conlents lry -N*Sl",tca- was probably partly due Lo l-heir use of dry weiglrLs r*'hich r"ould r'rake enzyme injection difficult to detect; although he did conceed that the

Lwo anj-roals may use different extraction techniques.

The results from R,dispar. using bot"h ruel aud dry extracLion weights, shour t.trat the two pal-te::ns observed i-n separaLe anirnals (wet weight in ant-lio:r, Gríffiths 1982 and dry weight in NoLonecta Cook and Cockrell 1978) cao occur i¡r the sarue anirnal " hlhat is belÍeved t.o be happening i" R.üi*ar. is that in aclditj-on to pumping enzymesftoxins into the prey, samples of the prey tissue, probably haemol-ymph, are rr'ithdrawn at the same time. Because of the design of the experiment, vrith l-he first interrupt.j-on not happening until 5 minut,es after the onset c¡f feeding, the injecti.on/extract.ion dynamics over the first. few minuLes is unknown. It is quite possible that prey contents are extracted r¿ithin the first few seconds (see references in Polla::d 1973) and continue in a cyclic fashion, the changes in prey r*eight being either masked by the injection of saliva or well vibhin the bounds of the error factor associated with weighing (see Chapter 2). However, t-he observed ext,racLion of dry material may indicate a modified probing behavíour response, which i-s common in some other sucking insecl-s, (Ilennig 1968;

Mclean and Kj-nsey 1968; Zert1'er 1967) that can be likened to a pre-pouring wine taste, to ascertaj.n the type and quality of the prey. In additÍon; it has been shown (see references in Miles 1972; Pol.l-arcl

1973) ttrat for many sucking organisms the release of quanLJ.ties of enzyûìes is stimulaLecl by the pr esence of foocl. The fact thaL R'dispar: r67 is subrnerged under the v,'ater may require this predator: flrst to sample the pot.ential food source, not only to a1lor¿ idenLifÍcat,ion via appropriate chemoreceptors in the mouth parts but also to cause adequate salivation. This hypothesls is speculative, although examination of Fig. 5:4 does sho',v, in additlon to an increase in wet weight of prey at the beginning of Lhe mea1, that prey conLents (as measured by dry weight differences) are being extracted from the beginning of the mea1.

EXPERIMBNT 2. Tl-rere v¿as a negative relal-ionship betr+een increasing prey density and prey depletion (Fig. 5:5). Follor+ing.an arcsine data transformation, a one-way ANOVA showed that l-he mean percentages eaten were slgnificant.ly different (F = 7.745, df 5,12, P<0.05) among Lhe six densj-ties. Mean comparison tests (SNK Multiple Range 'Iest, o = 0.05). revealed that !r.!iqpar- feeding on prey at the two higher densitiês wêrê '¡ : significanLly more lwastefulr than those fed prey at t.he lower densities, whereas the remaining mean percentages eaten al- the lower densities were not significantly dístinguishable as Fig. 5:5 shows.

EXPERIIÍENT 3.- It was suspect.ed that the resul.ts from Experi-ment 2 rel-ated to differences in feeding-tirne per prey among the six densÍties; for this reason, the effect of density on feeding tíme was explored.

Fig. 5:6 shols t.he relationships betr+een mean feeding l-ime, nean intercatch interval and prey density, where mean intercatch int.erval and feeding time per prey both decreased as prey densit-.y lncreased" These findings suþport those of Cook and Cockrell (t978) and Gíl1er (1980).

In addition, Fig. 527 shorys the subsec¡uent expected relatlonshJ-p between the mean dry weight extracted per prey, Lhe feeding time ancl the Fic1. 525 The relat-ionsh.ìp betueen jnitial prey densj.t¡, sr,6

p::ey deplct-ion as measured by percent consumed.

Val-ue gjven is nean + StandarcJ Devi¿rtion. Fìgure in palerrttrcs-is is nunbel of replicates.

Mearrs uith same .lel-ter are nol- signif icantJ-y dil'f,erenL (SNK, Mr:ltjpJe Rarrge Test, cr = 0.05). The percent. consumed uas cal-culated flom t.he first B p:rey eal-en at each density. _l I

to

90 A àe (s) w 80 þUJ 4{ å¿l AB þ 7t (3) Jgr tJ,l Ð sc ffi u¡ 60 (3) ñ l o c I s)

50 D I D (3¡ T (31 4û I '¡2 20 24 32 40 PffiHY nËruËlTY (N/#ontainen] Fiq.5z6 The el"1'ect of prey density on the mean feeding

time (O) and mean intercatch interval (O).

Value gÍven is mean a 9.596 Confidence Interval_. (For clarit-y only t.he upper half of the conf idence

interval- is given for the feeding time mean, and

louer half of tlre intercatch interval_ mean). å: --<*¡¡Ê-? -____å-_rê .t *aa-

a INTERCATCH INTERVAL (MiN)C (tl @ (o o (¡) È 6) o o o o o o

o l- o-l ! Il m o zG)m Øo l:¡ o-l { z o o = g. o

o) o l-< o-l

(¡) Þ (¡| o) @ o o o o o o o o o o FEEDING TIME/PREY(MiN)O Fie. 527 The efl'ect o1'[1ean interca'uch illterva]_ or-t (A)

the tirne spent f'eeding ( S mins) ancl (B) tl-re

dry ueight of, prey ext.r'acl-ed from each prey

( n mg). Values given arc tncarì .+ Sl-ancl¡:rd Lrror" Figure in parenthesis indicates prey

density (number/container). Both sìopes are

siqnificant-ìy clifferent f,ront Zët-o. (For A; t = 30.6, p<0.01 ; f or B; t -- D..33 , p<0.01). '(6r¡,r) GSLCVULXE i¡-tС34 ^ÂS0 ul lf) ul c¡ o \ o

Ë \ J 4 \ þ ourffi @ @ €¡þ g ff o L?þ \ $ \ Êffi f\ uJ \ F" \ &. \ ; ö \ (Ð 4 [!.i qK À, d€ s¡ Õ rÕ co

€) o o Àl Õ @ 1Ê o{u¡ue} ãrSB¿ SNg#33i 168 intercaLch interval. As the prey clensity decreased, frorn 60 to 1 prey per conLainer, so the resultanL intercatch interval and feeding t:Lrne increasecl, and in conjunction with this the average dry weight extract.ed per prey also increased.

Both the toptÍ.ma1 foragingt and the rgut-fi11ingt model predict these quantitatÍve trends, hourever. In order to distinguish beLween them, the indiviclual. feedíng tir¡es are plotted against' l-heir corresponding inl;ercatch interval. Iühen the densiLy trea-tments are broken down into pairs of observatlons in this rday, as suggested by Cook and Cockrell (1978), there is no correlation between int-ercatch Ínterval and feeding tíme as found by Lhe Spearman Rank Correlat-ion Coeffíci.ent

(rr) Testr . âs shown ín Table 5:1.

TABLE 5:1.. Correlation between indivitlual intercatch interval and

individual feeding tlrne as assessed by Spearrnan Rank Correlation Coefficient TesL.

Density p rs 10 0.0175 .47 )

30 -o.L077 .29 ) NoL SignifÍcant

60 0.1464 .2r )

!ùhen' exami-ning the feeding time of each captur:e t-hrough the capture sequence ít r,¡as found that- the mean feeding t-.ime per prey

decreased throughoul the capLure seqllence ¿rt. al.1 prey densiti.es tested. 169

Unlike that reported by Giller (1980) analysís of variance indicated thc: prey density did significantl-y affect the values for the mean feeding times. Flg. 5:B shows the mean feedíng time decay curve for the various prey densiLíes.

The relationship betrreen mean feeding time and prey densit.y (Fig" 5:6) can be pre

a high mean feeding tirne for thät density. As t-.he density increased,

where l-arger numbers of prey were caught, correspondingly lower mean feeding times resulted. Similar declines j-n feedÌ.ng times through a by and Borden (1970) sequence of prey captures have been reported Ellis ' Fox and Murdoch (1978) and Gill.er (1980) for notonectids.

Fig. 5:9 shows the overall effect of the extractÍon rate and duratÍon of feeding on one prey item at dífferent prey densÍti-es. Àt the higher prey densíties, although spencling less than half the time on

a prey iLem, compared with the low density, the predator is sti11 able

to obtain almost 6O% of. the available prey contenLs before disca::riing the prey.

DISCl]SSION

The results presented above are very much in support of the optimal foraging mode1. Overall, the ingestion rate of P.={iÐgt adults Fiq.5:BA The decay in mean feeding time + 95,06 Fiq.5:BB The relationship betveen the number of prey

Confidence Interval through an B eaten during the first 5 hours of the prey catch sequence. experiment and prey density.

( @ ) Prey densÍty of 1 per container. Value given is mean + 95i(, Confidence Interval-. ( O) prey density of l0 per container. ( ñ ) Prey density of 60 per container. For clarity only the top or bottomrhalf of the confidence interval has been given

for each mean. (The curve for the l0 prey per container

density fofloryed the same trend but has been Left out of the figure because of

the same sample size vith the associated large variance that overcomplicated the

f,igure ) .

a .---€é -.¡ì--.-"d.*.--

(A) (B)

100

ro 8 80 o

E I t IJJ ui 60 6 = l¡l o I -]. IJJ o É, 4 l¡l 40 j È lü lt lt I o '1 ï 1 E Ð20 t¡J 2 @ =Ð

o o 12345674 10 30 60

CATCH SEQUENCE OF PREY PREY DENSTTY (N/ContAiNEr) f iq. 5:9 The change is rnean (+ Standard Erlor) amollnt-

ol' dly rueight extlacl-ed f lom a s-inçle pr.ey

tLrit.h feedrng tÍmc of fì.clis¡rar. tJci r¡ht extlacl-ecl

is explessed as a percent of total clry veight of'

prey availabl-e. The arro\us jndicale the mean feeding time per prey'at- four dif'ferent prey ciensities. Prey density shorun in parenthesis. {} Õ ß¡

Õg ffi trl & f,dd¡ w d þ

UJ & ffi q.i;Y Õ ffi"¡ s tÉ.¡ u-

Õ Õ # Õ æ æ q' tr{ 31#VÌIVAV -!VÅ0å Sû oA $v frãJ"3HH.!-XE'åffi ÅHG 170

feeding on arì lndivj-dual prey iLem dec¡:eâses as t-hc prey is consumed (although to begin rr¡ith t-here is an increase as Fig.5:3 srhows). 'l'hj.s decrease is j-n agreement r+Íth t-hose reports for the other heLeropteran

pre dator Notonec-t¿r by Ccrok and Coclci:ell (L978) anrl Gi1ler (i9ti0) and for theco1eopteran1arvae'D-yLj¡.¡cgs-&"c_@(Kru-se19B3)"Becausethe ingestion rale decreases r+ith time spent in a pal-ch, optinial foragíng theory predicts that as the probabil.ity of prey captr:re increases (increasirg prey rlensity in this case) f:he feecling tine on indivi.dual prey items r+-ill- decrease and the ultinate result r,¡ould be an overall increase ín the rate of food intake. Predators fed inrlÍvidual prey ítems had consi.stently longer feed-lng l-irnes (and ther:efore, lovrer marginal Íngestioir rates) per prey iten l-han dÍ.d predators rt'hj-ch captu::ed prey iterus nat-urally from constant densilies of 30 ana 60 per: container (see Fig. 5:B). These results supporL Lhose of Kruse (1983) and are in agreement with one of t.he predÍ-ctÍons of oplimal foragj-ng

theory (Charnov I976a; Parker and Smith 1976), thaL is, the richer Lhe hal¡itat, the shorter the handling or feeding tirne per patctr. Unlike

Kruse (1983) however, the results shorrn in Fig.5z7 and Table 5:1 ì suggest that-the mean inLercatch interval may very well be the mechanisn responsible for this optimization for 3*3i"juf . It suggests that the predator Ís reacting to the average profitability of the environment rather than the specific leve1 of food in the gut. whí-ch has resulled

fron the lengLh of the inLercatch interval. The optimal feed-irrg model.

therefore gains support over the guL lirní.taLion hypothesis" This does

not necessarily mean Lhat Lhe gut is never limiting and that hunger ís not affect-ing the feedÍng-t:tme; rather guÌ- capaciLy defines Lhe lj.mi.Ls within which t.he optirnal feeding st-.rategy can occllr (Cook and Cockrell

1978). 17L

Because the ingestíon rate at first j-ncreases and then decreases (Fig.5:3) with time sperll feeding on an lndivl.rlual prey i.tem, the amount of food taken per unit feed:i-ng tirne is j.ncreased when each prey iEem is not fu1ly exploiled. One rray that the predator could assess the prey density is for it to monitor the prey that are sw-lmming in the close vÍcinity. This has recently been suggested by Kruse (1983). The results tend to suggesl- that at, the lorser prey densitíes R.dÍsp_ar- is able to dj-fferentiate between encount,er rates but at. higher densities it is unatrle to do so. This may of course be due in parL to the experimental constraints imposed on the predators. !trhen they were hand-fed individual prey (Density 1) no other prey h¡ere present and the optí.rna1 straLegy would be to exploit the prey irrdividual more conpletely.

The ínfluence and assessnent of encounter rates rvith prey by

R.dÍspar is explored more fu1ly in the next l-wo chapLers"

5:3 EXPEEJW. T0 DI+{oNSTRATE ItlE PASSÄGE 0F WATEB INTO TÍTE PIIEY

DURING FEEDING BY R.Dispar 5:3.1 fntroduction As mentioned in Section 5:2 Experinent 1, examination of l¡oth the wet r+eíght and dry we:'.ghLs exLracted fron a single prey showed an apparent.ly contradictory sl-ate to exist with higher values for dry weÍ.ght being found. This suggested Lhat as the feeding period continued on a single preyn water from the external environmenL was passing into the prey and artificially inflati-ng the wet weight" This of course L72 rr'ould lead to 1or,¿ estirnates of Lhe anounL of wet rueight extracLed frotn the prey.

In addition, other observations tended to support such a hypothesis :

(i) Nornally notonectids are buoyant and musL sw-im against this natural buoyancy. (ii) Notonectids killed artifÍcially in boiling r+aLer always f1oat.

(iii) Notonectids that have been fed on fo:: long periods by R. tlíspar

tend to sink rvhen released.

(iv) The carcass shor,'s no shrlnlcage or distortion following long

feeding per:iods, quite the opposite in fact, with the abdomen normally showing signs of distension.

To test this hypothesis of external water moving into the carcass, possibly through prevÍous feeding sj-tes, the following experiment using a fluorescenL dye, Acridíne Orange, was carried out"

523.2 EXPERIMENT L 5:3. 2.1 M"gþdS. A. Pilot Experiment : In order to ensure that the fluorescent. clye r+ou1d not naturally pass into and be taken up by tissues, a number of animals were tesLed as controls. From a 12 stock solution of Acri

Orange lg/fOO rnln three dilutions (0.1, 0.2 and 0.5U) were made up. One adult A.igglei r+as placed int.o 10 nl of each dilution and left for 6 hours. Folloruing this the anímal was removed, careful-1y blot.ted on absorbent tÍssue, taken through a series of ten r^rash.es of distilled water, killed Ín boilíng rvater and inmediately dissectecl and examined 173 under U/V light. Five replicates of eac.h treat-rnent r,¡ere tised.

Concurrentl-y 6 adult R"clispar were placed in sepa::aLe 1 lÍtre bealcers filled rr'ith either a 0.1 or 0.5% solution of Acridine Orange. Int,o each l¡eaker one aclult A"deaneí was placed, and observed continuously for a¡y noticeable variation of the normal swimming behavÍour of Å._¿_S.+"lleL or the capLuring behaviour of R.dj€!g!. Following capture the p-r:edators were allov¡ed to feecl until they naturally discarded theÍr preyr upon whích the carcass rvas imrnediately carefully reraoved r,¡ith a pipette, bloLl-ecl on absorbent tissue, rvashed as before and immediately dissected and examined under U/V 1ignt. There r,¡ere three replicates for each dÍlution used.

5:3.2.2 Rgsq-lLq

(1) Control Prey Animals followíng examinaLion under U/V light no obvious fluorescence was observed in the internal Lissues or haeniolympli for any of the dilution treatmenLs. There r¡as fluorescence, horvever, in parts of the exoskeleton, especially on tl-re legs and arouncl the poste::ior tip of the abdomen. The animals treated r¿ith the higher concent.rations of dye showed more intense fl-uorescence.

Prey animals thaL had been ferl on, all shorued signs of ínternal fluorescence, especíall-y those pr:ey that had been caught. j.n the 0"5% sol.uLion. Again, there was fluorescence in the rirain body cuticle and

1egs. There were r1o obvious changes in eil'-her predatory behaviour of

R.dispar or swimrning behaviour of A.deaniÍ. The rnean ('l SD) durat.iorrs of feedíng r{ere 117"r- 21 nÍn. and 106:f-_ 31 rnin. for the 0.L7" and O.57" dilutions r.espectively. A L-t.est- showed tha.t they rvere not significantly different (L = 1,96, df 5, p) 0"05)" \7/+

It-. r,¡as concluded that although noL permiLtin[¡ a quanLitative assessmenL Lo be carried ouLo the technique allo¡ved a gross identification of rsater movement.

5:3.3 EXIERIMENL ¿ 5: 3.3. 1 Mqt¡9.4,"

The design of the experiment was the sarne as Lhat outlined j-n Section 5:2.1 except thaL predators were held i.n 1 litre plastíc beakers fi11ed rvith a 0.5% solution of Acridine Orange. Single prey v¡ere introduced and, following capture, the predators were allowed to feed for a pre-determined period before interruptÍon. The location of all feeding sites r¡as noted ancl the duration of each feeding period r''as recorded using an electronic st.opwatch. Folloviing Lhe removal of prelr the carcass hras treated as before. tn: interruption tínes røere 5, 10, 2Or 30, 60 and )100 mÍnutes and there were 3 replicates for each tirne.

5:3.3.2 Results Fluorescence was observed in all prey animals that had been fed on for longer than 20 minutes. 0n1y one of the replicates ínterrupted after 10 ninutes showed very slight fluorescence. Table 5:2 summarises all the results which suggest that as the ¡neal continues, in conjunction r¿ith the change of feeding siLes, exLernal wal-er passes into the preyts body.

5:3.4 Discussion The results are consisLent with tbe trypothesis Lhat drrring feeding on a prey item by B*4iæf r¡ater from the external environmenl: TABLT 5:2

FEEDING INTERRUPTION TIME ( mins )

5 IO 20 30 60 >100

Replicate 1 2 t 4 5 6 7 B 9 10 1r rz rJ 14 15 16 r7 18 Fluorescence Present + ++++## ++.{+.H.{+.{+# No" ofl feeding site l-ocations 1r11 I 12722 3 2r45911 7

LocaLion of lst Sitel RL RL RL RL RL RL RL RL RL RL RL RL Ab RL RL RL RL RL

Duration (nins, ) t443 3 24566 3 46043423

1. RL = (Rear) Metathoracic Leg. Ab = Abdomen.

The effect of meal length on (r) the number of feeding site changes on Prey body and the subsequent presence or absence of Acridine 0range Dye in prey bociy. The fluorescence

rating u/as a subjective classification by on the spot comparison betueen prey contents. (-) indicates no fluorescence present. L7s

can pass into the Freyrs bod3r" Fl,¡wever, ít appea::s l-haL this is ver:y depentlent on the tumbcr of feeding site changes duri-rrg Lhe meal that the predator malces. As previotisly denonstraled in other studies in t.liis rvork, there is a st-rong correlaLj-on beLween durati"on of meal arirÍ the

nurnber of feeding sile changes. Pr-'evious clat.a collected on 60 tlifferent individuals rvere re-anal.ysed to provicle the number of feecling site

changes and the duration at each site, (these observaLions were made irhile conclucting other feeding ex.perimenl-s rvhen a prey item rsas fed on until naturally di.scarded). Fig" 5:10 short¡s the wet weight extractj.on curve (::eporterl earl-ie:: Fig. 5:1) wif.h Lhe rnean site change índicat-ed.on the feeding time axj-s. As the duration of the meal increases Lhe rate of site changes increases, this being parLicularly noticeable aft.er about 60 minutes of feeding. It would appear that during feeding rsater

may be passing into the prey carcass eitÌrer l-hrough Lhe feedittg sites or

]lJ l,r damaged i-ntersegmental membranes that result fron puncl-uring or tearing I by Lhe stylets or by the combined effecL of rnanipulat-Lon and the

digestive enzymes present in the prey. This in turn ttould cause Lhe

discrepancy betvreen the observed extraction weíghts in the wet and drY calculations.

The fact thaL water couLd be passing j-nto the preyts body Curì-ng

I i feeding also introduces sone interesting physiological a,nd behaviclural factors. The obvious diluting effect following long periods of feeding I 1 l, could be used to pr:ovide a stj-rnrrlus to cease feeding and dj-sc¿trd the i{" prey. Obviously, due to the design of the experiment, r+hich vras tl I I purposely left simple, no quantitatj-ve measurements could be taken and I used to calculate raLes of flor¡" In a1l- l:Lkelihood, l-he use of stich a

-"{ crude met-hod of measuring such small- quantifies of water flow r*oul.d be

t,l I .l Fiq. 5:I0 The change in vet veight of individuat prey during feeding by R.dispar alon g ntith the number

of feeding site changes during the feeding period. Value given is mean + 95?(, Confidence Interval; figure in parenthesis indicates number of repricates. The mean duration from commencement of feeding at. each site uas cal-culated from additional experiments. (See text for additional experiments).

Feedinq Site Number Y Duration (min) RanE n ( flrom commencement of feeding) 1 4.2 Zto 6.4 60 2 l6. B Bto 3I 51 J J6.9 17 to 42 44 4 58.2 11 to 7I 44 5 62.O lB to 90 40 6 70.0 6l to 11r 28 7 72.6 63 Lo 105 17 B 77 .5 70 to ot 17 9 B0 .0 70 to 95 I6 10 BI.2 74 Lo 10r 15 11 85.0 75 Lo 107 l0 12 9t.B B0 to ro7 4 lo

òa to 2.5 o, +t lx

g' 2.O E (5) u¡ É è 1.5 l¿ (8) o Ë 1.O 17l =1- (7) lrJ B o.5 ; a z ? ePW9 I o + o.5 176 quj.te unsuitable, and \{ou1d probably require radioactj-vc label.s anrl a far nore cont--ro11ed exper:iment.

Ilowever Lhe fact rernains that Lhe evj-dence is vcry much Íu agreement with the idea that as &"jiry. feeds on the rnore complex prey (in overall- shape and struct.ure) the opportunity exists, over tirnr:, for waLer to pass into the prey. The rate of f1or" of water rnay in turn be used indirect.ly as a physiologi-cal stimulus on the currenl quali-ty of the prey, a topic very much in discussi-on at Lhe present t-ime. Gibb (1962), Krebs (1973) and Charnov (1976a) have each formulated differ:ent hypotheses concerning the nechanj-srns that govern the al-locaLion of tÍrne to patches. Both Gibbs (1962) and Krebs (1973) prerlict thaL the marginal raLe of food intake (tire rate of Íntake at the Lime predaLors opt. to deparl.fron þatches) will increase with parch quality. They differ however in that Gibb (L962) proposes that. the amounL consumed per palch remai-ns a constant, regardless of paLch quality, whereas Krebs (L973) suggests that. the tine spent per patch remains consLant. fn contrast, Charnov (1976a) predicts that both the Li.me spent and the amounl- consu¡ned per patch are posiLively co::relaterl rvith patch qualiLy. Consequently, the rate of food int¿rke at the time a preclator leaves a

patch is suggested to remain a constant over ¡ratch quality, i.e. all

patches should be reduced to the same marginal rate. Charnovrs hypothesis has recently been supported by lioclges (1981) and Kruse (1983).

irlhile the present work does nol attempL to offer evidence for any of the above hypotireses (different patclt or prey quali.ties were not

offered¡ ê.g. snal1 anrl large prey) j.t is felt that it provirles sorue L77 evidence of a possible physiological mec.hanism by which R.rlispa:: can assess the state of the patch and, depend-lng on oLher associaLr:d variables react to the st.irnulus in the optinal way.

524 MIILTIPLE PREY CAPI'IIRB AltrD HANDLING 0F R.dispar 5:4.1 Introduction

Three distinct buL interrelated forns of prey capture have been observed in Ranatrq ¿iqp-gl (Figs. 5:11 , 521.2, 5:13).

TYPE 1 A single prey is caught and once feeding has commenced, no furLher attempt is made to catch arrother. 0n1y after discarding the old carcass is a ner,r prey caught.

TYPE 2 2 prey are caught, the second while the predator is feeding on the first. The seconcl prey may be caught by ei-ther one or both raptorial 1egs, the firsL. prey remainíng rspearedr on the sLyl-ets during feeding (see Chapter 3). If both legs are used the prey can remain held by both or transferred to one leavíng the second 1eg free" 0n cessatj-on of feeding oir'the firsL prel¡ t.he carcass is discarded and the second prey brought to the rostrum for exploration and feeding. Once Lhe stylets are in posÍtion another prey can be caught.

TÏPE 3 3 prey are caught, boLh the second and the third whilst the predator is feeding on the fÌrsE. The third prey is caught by Lhe second free raptorial leg mentioned in Type 2. Às in Type 2, as each prey is uËilizecl and discarded subsequent exploratory/feeding niovemenLs take place wiüh either of the prey held. (In some cases predaLors have been seen to svitch betr+een Lhe truo cluring both ex¡rloralory and/or Fiq " 5:1Ì Diagramatie representation of Ìyoe t prey capturing behaviour of R.dispar. É -4 {s rffi *å Fiq. 5:12 Diagramatic representation of Ty¡re 2 prey capt.u-ing behaviour. TYPE 2

@ PREY

o UTILISED PREY n Fiq" 5:I7 Diagrarnatic representation ofl Type 3 prey capturing behaviour. AgHd o4st7lt,., c

/(3Hd

E ldÀI

1eg.

The inciclence of suclt behaviours rvas be1íeved to be more 1íke1y at high prey densities (in assocÍatÍon vith the related increase in encounter rate) " Obvlously nore than one prey must be encotlutered duríng a feeding peri-od for Lhe behaviour to be exhibited. However, it was considered to be oi importance in investigating this phenoinenon to see if these behaviours were purely random in any one feeding period or if they follov¡ed a sequential pattern and if so was Lhe beháviour correlated with any identifiable variable. Food deprivaLion has previously been shown t.o signifÍcanLly affect capture and feeding behaviour therefore the fo11owíng experinent r+as carried out.

5:4.2 Materials and Methods As the behaviours trad been observed in a number of different

instars of R.dispar, 3 different deve lopmental stages tvere chosen for investigation.

Predators : lnstars IV and V and adult fenales r'¡ere selected fron individuals that had moulted at least. 36 hours (normal1y /+B hours) before being used in the exper:ineuts. They had access Lo an excess of natural prey of various sizes f.ot 2 to 3 days and then k¡ere renoved from tlre food tanks and fasted for eÍ-ther 0, L2r 24 ar 48 hours. PLATI 5

l'îulti-prey capturing behaviou r of adult. femal-e R.disoar. A. Type I

B. Iype 2

C. Type 3 @ @

.D \

@

\

\ L79

Prev : Adult Anisops deqnei were used ín all experiments" They t+ere chosen to be of comparable size (Í = 6.8 nrn, Std. Dev = 0.4, llange 5.7 to 7.9) .

Following fasLÍng the Índividual predator was introducecl i.nto a 1 litre translucenL plastic beaker filled rvith 800 ml- of filtered, dechlorinated tap waLer" A wooden satay sLick (2rnn diameter x 200lun), weighted at one end, provided a support/foraging site. The predator was allowed 30 minutes to seLLle, but normally took up the characLeristic prey capLure posture (see Sect,ion 3:2) after a few minuLes.

Prey were added carefully along with water to rnake up the quantity in the beaker to 1000 rnl. Two densities v¡ere used, anil eíther L2 or 20 prey were introduced, r+hi-ch iniated the beginning of ttre experiment. As each prey was captured a new indívidual of s1milar size

was added, maintaining the density. Observations rùere carried out in

either of two ways :

(i) By a narraLive recorded onto a continuously running tape recorded, with times being taken by an electronic stopwatch; or (j-i) By time-1apse movie photography. 'fhe camera hlas posì-tioned vertically above 4 cont-ainers that could be monitored sinulLaneously. One frarne was taken every ten seconds. Prey were not replaced as they were caught in these experiments. Replayj-ng the film frame by frame permitted analysfs to be carrietl out to the nearest 10 .seconds, a negligible factor considering previorts observaLions. 180

The availability of subjects restricLed the number of replicates, the ntunber observed is shown in Table 5:3. The duration of each (540 experiment v¡as nonnally nine hours minutes) "

TABLE 5:3. Number of Replicates for each developmental stage and fast treatment.

0h 12h 24h 48 FasL Time

Prey Density L2 20 t2 20 12 20 L2 20

Pred. InsLars

w 2 3 4 22334

v 6 6 8 81010L2L2

Adult e 32343333

524.3 Resul-ts

The same basic pattern of behavíout" Lu" exhibited by the 3 predator stages and, in addition, the truo densitÍes showed no significant. effecL between them. As a result of this and Lhe 1or,¡ number of replicates 1n both the fnstar IV and Adult treatnents; only the results fron the V Instar are presented.

Tal¡le 5:/+ surnmarises the effect lLhat food deprivation has on whether the predator captures more than one prey at the same Lime. 181

TABLE sJî. The effect of fasl-1ng on the likelihood of the predator exhibiting multi-prey capLure. The ntunber in parenthesis indicates Lhe number of replícates observed.

FAST TrME (h) Type 1 Type 2 Type 3

0 (6) Yes (6) No No

12 (8) Yes (6) Yes (2) No 24 (Lo) Yes (10) Yes (B) Yes (2) 48 (L2) Yes (12) Yes (6) Yes (6)

Clear1-y Lhe hunger leve1 of the predator has a significant effect on whether the predator, given the opportunity, will capture more than one prey. Figs. 5:14 to 5:17 shov the mean sequence of behaviours for the four 1eve1-s of fasting tested. They show Lhat the probabi.lity of attempting to capture more than one prey is not only influenced by Lhe food deprivatlon time but also on the rate of encounLer with the prey.

Once feeding has c.omnenced there appears l-o be a critical- period during whÍch, if an encounLer takes place, the predator will attenpt to capLttre Fiq.5zI4 The prey capLuring behatriour exhibited by satiated V instar R. di ar. ft signifies beginning of feeding. HT = Time in minutes spent handling prey. CT = Time in minutes from beginning of feeding to capturíng next prey. N = Number ofl repì-icates. (For Figs. 5:I4 to 5:l-7 see text for additional- details). TY E I ú * 1OO"/, + --+

c H.T x-= 37- 5 m C.T. x= 90-6 m

Satiated N=6 l2-2roo h H T Handling Time C T Time to Capture Fiq. 5z15 The effect of a 12 hour fast on Lhe number of V instar R- dispar exhibifing

either Type J. or Type 2 prey capturing behaviou:c.

Abbreviations same as Fig. 5zL4 TYPE ¡

v

i'l T t-47nr C T r'-54¡ì1

zsltzl

---Þ TYPE I -x.18tm crï.î*mffi ffi l-l T x-62rn

TYPE ¡¡

12 h fast N=8 Fiq. 5z16 The effeci of a 24 hcu¡ fast on the number of V instar R.dispar exhibiiln g either Type I, Type 2 or Type 3 prey capturinç behaviour.

Abbreviations same as Fig. 5zL4 TYPE ll ( duration Í.2lOm) * A----A CT x.17m

2o7" (2) (duration TYPE lll (duration -& lOOm) TYPE lt -n 3lOm) ___ft ¿.4 C T x.4om HT ir-86'2-A-A m HT x-118.5m TYPE I A A--A 12 HT 14 cT 24 h fast N=lO TYPE I A_-1.&_-A HT 75 cT 13 Fiq. 5z17 The effect of 48 hour fast on the number of V instar R.dispar exhibiting either Type I, Type 2 or Type 3 prey captu:rinç behaviour.

Abbreviations same as Fig. 5214. TYPS iI TYPÊ II¡ TYPE ll (dwðl¡M x.226m

+ Ïl ¡ltet - ¡gOmn

HlI.7lúffi-A-A-Ä.-fl\C T T.{m CT aa¡2'2m H Ti.3üo

tz'¿(21

TYPE III TYPË tl (rlurati6 t.35¡ñ

+ Tlalld 46ôm¡n

(?)

ÏYFE II x.24Om

+ Tlåt¡d d2 ñin

rt/. í1t Hf¡.88m Clt.20m

¿18h fåst t{- t2 TYPÉll(durðlid x.æm

¿ flaltôr - 2OOm¡n

Hf7'7io CT F.2O Hl l'79h L82

the prey. Once pasL that period prey are ignored even if they cone r*rell within strlke/capture range (see Chapter 3) Thj.s cr1tical ? " periotl is apparently longer the hÍgher the moLivational or hunger leve1_, as evi-denced beLwcen. the 24 and 48 hour fasLs in particular.

No statistical analysls was carried out due to t.he lorv number of replicates. Figs. 5:14 to 5:17 show that the multi-prey capLuring follows a seguential pattern through the feeding period, the exhibited

pattern dependent on the ÍniLial hunger leve1 of the preclator. The end

I of the experiment (abouL t hours) always saw the predators exhibiti¡g a

typical Type 1 behaviour, irrespective of prey encounLer rate. Type 3

behaviour was always followed by a period of Type 2 ruhich then rnerged into the Type 1.

ü e Table 5:5 shows Lhe allocation of tine to each of the behaviour ,l types durÍng the ent.i.re 540 minut,e experiment, followi-ng the different fast periods. Clearly the predaLors spend a sÍgnificantly greater time in Type 1 afLer no fast (predators satiat.ecl) aud a 12 hour fast.

Follor'ring a 24 hour fasL Lhe predators spent about equal time between

Type 2 and 1, both of l'¡hich were signiflcantly longer than the Type 3 duration.

I I

I

I

!r l' : 183

'1.

TABLE å31 The t.inie spent ln each of the 3 nulti-prey capturing behaviours by V Instar R.díspar follor,¡in g four levels of

food deprivation. The value shot,rn is the mean + Starrclard ù\ù Error, the number Ín parenthesis indicate replicates-"'

{. Mean tine spent in each behavioural type

Type 1 Type 2 Type 3 Fast Level

Satiate

* Total tlne of ExperÍmenL was t hours (540 minutes) ** Prey densit'y was kept constant at 12 per container - see Metirod.s SectÍon, 524"2" -I .l p<0.05,t-test t. i i predators more time 4 After the 48 hour fast, spent slgnificantly

1 tl' 1n Type 2 behaviour while about equal tírne was spent in Type 1 and Type

L 'lt 3. Obviously the probabÍlity of a predator exhibiting the different å' ï prey capturing technÍques is very much affecLed by i-ts current hunger I leve1. In asbociatlon with the mean time spent in each of the 4 i' 184

behavlours, Table 5:6 shor+s the mean time spent feedlng on each prey whilsL exhibiting the dj-fferent behaviours" iri

TABT,E 5¡6 The mean tlme spent by the predator feeding cn one fndfvidual prey while eÍther holding one, truo or three prey (i.e. Type 1, Type 2, Type 3 behaviours), following four l-evels of foo

predator. [Nurnber of replicates sanne as Table 5:5].

Mean TÍrne spent Feeding on each Prey

Type 1 Type 2 Type 3 Fast Tine Ìl 55 12.1 (1.5) s Sabiated + ,.ï l2h Fast 67 + L9.8 (2) 81 + 31.5 (1.2)

24h Fast, 68.3 + 6.7 (2"L) 86 + L4.6 (2.4) 47 + 20 (2"o)

48h Fast 60 -r Ll.g (2.7) 81.7 + 6.3 (2.6) 35.6 + 7.L .(t+"2)

During Type 3 behaviour the prey individual is processed muc-h

i faster, being discarded before it has been totally utilized" As the i feeding session continues, about the same time per prey Í.s spent feerling 4 1 in the Type 2 behaviour and TYPe 1. !

¡. 1¡ ,¡' î .f 5zt+.4 Discussion I I The ability of lnvertebrate predators to capture rnore than one grbups. Cook -{ prey at a time has been reported fo¡ a number of differetlt lff .t t' 185

and Coclcrell ( 1.972) shov.,ecl that the rE¡ductiorr in feeding time by of the Nojonecta &Lg-u_çg- was Jrrobably due F-o an increased ability predaLor to handle srnall llerus, enabling it to caLch a nel'¡ item r'rithout first havíng to abanrlon Lhe previous one" Sim-ilarly, both l{ardman and Turnbull (19S0) and Haynes and sÍsojevic (1966) reported rnultipl-e prey

catchtüg :tn the wolf spider, Pqrylg-sq Enc-o-ll-vq1:å and t-he crab spider, Philoclromus ry!æ. respectívely, and rel-ated it to Lhe i-ncrease in the nrrmber of prey caught at hlgher prey densities" Nakamura (1977) observed in a nuruber of tlifferent species of r¿olf spicler Lhe abil-ity to capture more than one prey. ïn such cases tl-re spider \'¡as able to easily handle two Lo three flies (DrosoPllila n@"Jfq"-tsÐ at the sarne tj-nte, but rarely more than four. The nt¡mber of flies a spicler tvas able to hold at any given momenl, seemed to depend on l-he body size of the spider. Thi-s is due Lo the spider having to store the prey ttnder j"ts

$ abclonen. Spiders were observed to caplure a fly, feed upon i-L, disca::d ,i the remains, aud then rrecapturer the dead f1y a sec'ond or subsequent time. For each subsequent trecapLuret of a dead fly the spider repeated the process of feecling upon it and t-.hen discarding the remaj-ns. In l-he case of mrrlriple prey capLure, the spicier discarded the flies either all

aL the same tine or singly at j.nlervals. The first prey to be cauglrL nas always tl're firsL to be dlscarded.

predat-or was of ten seen mornenta::i1-y to explore a ,t In RaqgEra, the f second prey that had been caught. On only one occasion t¡as a pr:edator ! t' seen to alte¡:nate feerlÍ-ng between two prey" Obvlously, r+hen ho1'riing 3 "tf Ï prey this ¡uou1d not be possible, as both legs are holcling Prey ¿tt6 ttt" I { third is on the stylets" A1so, in B-.__4!ugr once the prey is discarded trecapturedr. '"{ it sj.nks Lo the bolLom ancl is never ¡,i I i t' 1B6

The effecl of body sfze on multiple pr:ey capturitrg was also díscussed by I,lilliams (1979) workÍng with the aquaLíc splder, Dolomedes

aquaticus" Idhen feeding on smal1 Pre¡lr adul-ts were observed to galher nore than one Índividual. (One adult D"aquatícus was observed to hold in its nout-hparts and eight crane f lies ' AIlUgPbilþ neozelandia pedipal_ps). However, rvhen capl-uring larger prey, for example a 3 cm

1-ong coclcabully fish, Gobiornor hus it required not only the nouLhparts to seize the prey bul also ttre legs to secure ít. lJí11iarns (1979) suggested that this multi-prey capture behaviour increased the size of

each meal, and therefore enabl-ed the spider to feed l'ess frequentl'y.

Thís rsas probably a rlirect consequence of prey being reaclily avaiJ-ab1e for only a very short period rvhil-e the spíders wer-e active, Lhe numbers of insects in flighL decreasing very rapiclly after twilight.

1

It i-s suggested that a sorner,¡hat sj-milar pattern exists for

R.dispar. Ma ny of its prey move in discrete groups. lgþLíe and other

nacro-zoop1-ankton, which in adciition to forming a large componenL of l-he diet of small- insl-ars, can also be eaten by older instars anci adulLs,

move in dense aggregations. Furthernore, the co¡nnon prey of 8.4i,Fjl,a.q Ín loca1 haL¡itats; A"dea-nei and other notonectid species, tadpoles and

I i move groups or schools Lhrough the t+aLer body (see I small fish all Ín Andorfer 1980; Braclen 1982; trnlassersug 1981); as well as surface I I ¡, rlr,relling species, for example water-striders, GerEiå, (see Riley 1-92L;

Spence 1980) and whírligig beetles, 9y¡:L-qt1q-, (see tleínri-ch and Vogt ü" r r ,l I

'r{ I 187

19S0)" Therefoi:e, R*.4ispql indÍviduals can be exposed, on occâsions, to a group of potentj-al pi'ey al-most simultaneousl-y or over: a very short- period. Thís capability of multl-prey caPture allor¿s RÆÊPar to tcapitalizer on the si-tuation and to capture adclitional prey'

The multi-prey capl-ure has been observetl ín all insl-ars of R.dispar but wj-th the smaller indivirluals talcing proporLlonately smaller prey. This supporLs Lhe observations of Radinovskv Q.964) on Lhe early instars of &.fugg-, In addition, adult and a number of smaller instars (I and TI) of U!!þæf have been observeci holding rnore tha:r one prey Ín Lhe nattrral habÍtat. Therefore il- appears that this prey-capturing behaviour is not an artifact of l.aboratory experi-nentation but represents a good example of horv selectÍon can act to improve tire prey capLur:ing capability of a sit-and-r+ait predator.

I

The clesign of the experiment did not perntit- the mechanism involved in controlling the mul-ti.-prey capture to be Í

where the predator did noL consume all of a prey item. \tlhether such a I I be I mechanisrn j.s invoh,ed in the rnulti-prey capl-uring of &-dåÐal cannot /r with t ru1etl out. The very evident hunger effecL is certainly consisl-ent rvorlc j-s .{ Lhe possibj-U.ty of a sinilar: meclranism" Ilot¿ever further

I .l 1BB

requlred before more concluslve staLements can be made.

5:5 DISCUSSION This Chapter seL out 1-o examine related behavÍours of the predator that coul-d possibly be involved in the lncrease ln the numbers of prey eaLen at rlifferent prey densities as presented in Chapter 4" The first is the way individual- prey are utilized, wilh the associated

feeding t:Lme and extraction dynamics' aL different prey densities' The

second is the behavioural capabillty of R"dispa¿ in capturing and processing nore than one prey at a time, and the tine budget' associated wlth each prey. Both of these phenomena arer as is to be expected, interrelated to some tlegree.

Both the above have been shorrn to be density dependent wíth the than one feeding tj.me per prey being shorter at .high densit-ies and more prey being caught where prey density is high enough, with the associated lncrease in preY encounter rate.

Cook and Cockrell (197S) nodelled the possible relationship

between the prey extractj-on curves and the functional response curves

obtaJ-ned, arguing that as prey density increases search costs (inLercatch-interval) will decrease, so will optímal feeding tine' The nunber of prey items which may be eaten in a given tlme will thus increase but at an ever-decreasing rate, producing a typical Type 2 funclional response curve. The results ol¡tained in Section 5:2 and in Chapter 4 are consisLent with this hypothesis, particularly for { functional response curves of the larger predator and rnedium sized prey. 189

In adrlition Section 5:2 also demonstrated the reduction in feeding t¡ne Lhrough the catch sequence. As mentionetl prevÍously this would have Lhe effect of having high feeding times at, lora prey densities and progressively shorter feeding tirnes at hlgher densfties, as the predator moves through the catch sequence. In conjunction with the shorter feeding time nore prey will be seen to be eaten at the hÍgher densities.

190

6:1 IITBQUru. The observed nurnber of prey ealen at differenL densities is (/ dependy'nt on nol only the preclator?s feeding behaviour but also the A preyts behaviour (see Discussion Chapter 4). This is parti-cularly true Ín exanj-nÍng the changes in the predatorìs attack raLe a. Furthermore, in a rsit-and-r+aitt/mobile prey interactiont those particul-ar behaviours of the prey for example, speed of movenent-, movemenl patterns, aggregatíon behaviour, rul11 play a signlflcanÈ part in deterroining the frequency or rate of encounters with the predator.

It has already been shorr¡n ln Chapters 3, 4 and 5 that a nrrmber of

the sub-components of the handling time (S) and attack rate (e) change with length of tirne since last the predator ate (hunger effect) and that predator age (see also Griffiths 1969) and prey si.ze (see also Thompson, 1975) will have an effect on Lhe number of prey eaten. 0f particular inportance, however, is the way in which tiese componenËs vary with prey density.

Chapter 5 discussed changes in the predatorts behavÍour that wíll j-nfluence the nunbers of ¡:rey captured as a result of changes in prey densily, this ChapLer examines the preyrs behaviour in more detail, in partlcular prey density effecLs, 1n orrler to identify those

particul-ar features thaL will effect lts movement and resultant encounter rate. 191

6:2 DESIIUoII. _0F BÁSIC s\iryINg. ]E$AIIOUB Qql &l.qq"- 4e4IL% ÆuI'rs.

6¡2.I Material.s and MeLhods As shown Ín Fig. 6:l- a vicleo carnera was positioned vertically abcve an observatÍon tank in which rsater was mal.ntaíned at a constant temperaLure (22 + 0.5o) and depth (5 crn).

Fed A.degggl adults were placed ín the holding tatks and held overnight. AfLer about 18 hr. fast individual animals ì.rere transferred to the observati.on tanlc, allowed 15 nlnutes to settle and thelr tswimmingt movements observed continuously for 15 minutes via the video moniLor. A totaL of 20 rarrdonly selected adults rvere observed and recorded" Reanalysis of some video tapes and direct observations showed that there lrere at least 4 categories of swinlning movement that could be recognized ancl measured (Types 1 to 4).

Having identified lihe 4 movement patLerns, the remaining video tapes were scanned to isolate a suitable example of either oue of Lhe movement categories. Once located the lengLh of tape was replayed frane by frarne Q/24 sec. = 1 frane) while the position of the animal tvas recorded on a transparenL acetate sheet fixed on the rnonitor screen. The dlsLance travelled and t.ime taken could then be calculated.

Movements by tl're animals in the vertical piane (i... water column) could not be measured. However, where possíble only animals rnoving horizontally wer:e selected for neasurenent, but nevertheless an unknot'rn error \{as unavoidable, although every atternpt was nade Lo keep 'this to a Fiq. 6:I Diagramatic representation of the apparatus used in exam,'.ning the svimming behaviour of, A.deanei.

Not to sca.Le. l----å ,& i. -- æ¿¡É*-

Video Camera

Watcr Heater/Cooler

ïl Water Mixer

Clamp- Observation Tan Holding Tank Grid Base-

Water Bath L92 minímun, by using swallor,t \raLern

For each category a total of 15 complete movemenf:s were neasured using 5 different animals"

622.2 Results : The four novement categories identified were :

A. TÏPE ! Animal remains j.n same location but moves about its vertÍcal axis either to left or right. This is accomplíshed by only noving one of the metathoracic rrowingr 1egs.

B. TYPE 2 A tswirnningr movernent of a rjerkyt fashion. Each sLrolce resulted 1n the anirnal movj-ng no rnore than 5 rmn (X = 3.5, SE - 0.3 m¡). The animal can remaj-n staLÍonary between each power sLroke' The duration of the stroke lasts 0.11 + 0.04 sec., which gives a theoretlcal speed of about 50 mn observed speed however, was 10 + 3.3 mm "..-l.The _1 sec. -, the difference exisl-ing because of the Pauses between power slrolces. The angle that the netathoracic leg nakes with the longitudinal body axis (in the horizontal plane) changes from a 45o to 90-1000 during the stroke.

C. TYPE 3 As Ín Type 2 but distance travelled at each strolce is

between 5 and 10 nun (i'= 8.5, + SE= 0.7 nLrn). The duration of the stroke is about 0"25 sec. (1 + SE = 0.27 + O.Ct6) rvhil-e the metathoracic l-eg angle changes from 45o at the beginning of the stroke to ) 1000 at the end. As in Type 2. there may be pauses between sLrokes. Estimated Speerl _1 = 25 + 5.2 mm sec. 193

D. IYII å Frc¡rn a statfonary position the airintal moves with a rapi

50 cm How accuraLe a figure this represents is unknor,¿n. about "u..-1. Due to the 1ow l-ncidence hor¿ever Ít has been disregarded from later observations and analysis. Having identified three clear categories of

movenenL in Anisops the next step vas Lo invesLigate the effect of both water temperature and density on the proportion of tirne spent 1n each category and the overall effect this has oR erìcounter raLe with the predator.

623 EFFECT 04 I^IATER TmæEtu\TURE l\ltlq DEILSITT_ 0N THE MOVEI'IENT PATTERNS

0F ADULT Anisops deanei.

6:3.1 Materials ggg. Ugth"_gE_ The apparatus was arranged as outlined in Sect:Lon 6:2. The desired temperature t{as selected and the apparaLus and experimental aninals were

gradually acclimalized for beLween 16 to 18 hours before Lhe experiment (ttris was also the anÍmalÎs fast time).

AnimaLs were carefully introduced lnto the experlmental tank, left for 15 minutes and t-hen their novenenL patterns were sirnultaneously

observed (via the'f.V. Monitor) ancl recorded on video tape for 5 19/+ m-'r-nuLes. Once observati-ons r^rere finished, the anÍ¡nals r{ere removed and a fresh group of 4¡SoÆ" introducecl.

Four prey densities (1, /+,12 and 30 prey/contalner) and four experlmental temperatures (15o, 2Oo, 25o and zBoC) were used. Each pr:ey denslLy was replicated a minirnurn of 5 tirnes al- each temperature and animals were used only once.

During a video tape anal-ysÍs one animal r¡as c¡rosen at random and continuously observecl for the 5 minutes, recording on an 8-channel multi-evenL recorder, the nurnber of Límes each movewruent c.ategory was seen, and its duratj-on.

623.2 Re_sul-ts and Di scussíon

Because of the unequal and in some caÞes 1ow sarnple sizes the time spent in each of the movement categorì-es was co¡nbj-ned to produce a tot,al time spett moving per 5 minutes observaLj-on tÍme.

A 2-r+ay analysis of variance demonstrated that both Temperature and Density had a signj-ficanL effecL on the tirne spent moving, as shown in Fig. 6:2. The two overall trends shown in Fig" 622 are that animals move more at the higher ternperaLure but less as density increases. It appears therefore that intraspecific interactions are significant in groups of A"deanei and this is expressed in a significant decrease in movement.

made l-he support these f i-ndings ObservatÍons during ex¡leriments ' j-n that A.deanei individuals at Lhe highe¡: clensities tended to form Fiq. 6:?. I he ef f,ect o1' uiai-el tempelature and density orr

time spent rnoving per 5 nin observal-i orr period. Means uitlr some fetter are noL s-ignil'icantly

dií'í'elent (SNK l'1ultiple Rar-ige Test, o = 0.05). Va-1ue given i s mean I 959¿ Conf j.cJence Interval-. ( E ) trraler 1-emp. I5oC.

( O ) I,Jater temp. zOoC.

( E ) VJater temp. 25oC.

( @ ) l¡Jater tenrp. 2BoC. 60 ffi u) tr 50 cLIJ

q) q¡ a, 40 (5 z, o 5 30 F ûIJJ tr¡ 20

TU Ë Ë 10 I ¿t 12 30 ÐËNSITY (N/Êontai¡rer)

ÐF ss MS F-ratio $IüN ¡F. { sCIufrcË

TEfuT P 4 1046.43 348.81 22.S3 p=û.0O01

ÐEF¡$ITY ù 58O.62 193.54 12'v2 P=o'oo"l

Ërror I 136.88 15.21

Trf,l'AL 15 17$3.S4

i i I Conrparison of &{eans SNK-Fdultipte Range Test {o(*O.O5)

t { A A A 20 25 e8 1 TEh#PS. t5

rì' c D ÐENStTrË$ 1 4 12 30 å' ï

I i t

qt/ !l T r¡ t' 195 aggregation, i.e. thcy forrued a group ou gr:'oups of j-nclivíclr¡als

(dependj-ng on Lhe density) tiræt-. morred arou'irl the experj.niental t.anlc:Ln a fashion resembling a school of fish" In erdclì-tion, a,G tììc:nl-ioned Ín

Chapter 2, Ä,cleaneí fo¡:ln very dense a.ggr:egat--ions in the:Lr natural habitat, rnoving around the porrd or darn Ín groups of up to hundreds of lndÍviduals. UnfortunaLely time dicl not- pernit the collection of additÍonal daLa on l-he sLructure and dynarnics of these aggregal-ions in order to nore fu1ly corroborarle their probable si-gnifictrnce. Examinaticrn of the literature suggcsls thal- thj s behaviotir has evolved as an anli-predator defence nechanì.srn (see fo:: exarnple Bertrarn 1978; I{amilton L97L; Pulliam L976; Rubenstein 1978; [Iilson 1975) as that found in bird

flocks (see for exarnple Bertra-m 1980; El,gar and Cat.terall 1981; I(r-:nward

L97B; Lazarus L978, 7979: Povell L9743 Pulli¿un 1973; Siegfried and

Underhill L975i Vine 1971) or, perphaps more pertínent to [de-qrre-!, fish schools (see for example Ândorfer 1980¡ Aotci 1980; Broclt and Riffenburgh

1960; Ne1ll and Cu11en L974;). llheLher this behaviour may confer an

advantage to the group as a ',qhole (see for example Vine 79-/I;

1^lyne-Bdr*ards 1962) or Lo the individual (see for exampl.e Dar¿kins 1976;

Hamilton 1977) ís still sorner¡hal- of an unansroered question. Although

nany we1.1 lcnown bi-ologist.s rvho subscriberl, outspolcenly or by

impllcation, to the former have, aL the same tirne, for example FÍsher (1953) and Lorenz (1966), admitt.ed that the nature of Lire group

advanlage renains obscure in many cases"

r{nti-predator aggregal-.ion behaviour has been reported for a number of aquatic insecl-s (see for example Brown and Orians 1970;

Deshefy 1980; Heinrich and Vogt 1980; lllldrew and Totvnsend 1982; l4i11er I966i Treirerne and lroster 1980, 1981, l9B2), autl Ín particular

in possible response to ambush or siL-and-r,¡aj-t pr:edators (Taylor 1975; 196 see also Gerrítsr:n 1980; Gerritset ¿rnd Strick,Ler 7977i hril-lj.ams 1966 for adrlitional discussion)" Þlil"ler (1.966) usecl ttre tern rfloc-lcr for the aggregaLÍon of a relaLecl species &f!g-qgf. p_el,L¡cçns, and reported a synch::onized clÍvi.ng anrl ascendiug behavj-our, vrhere indj-víduals of el"l.ucens rfoll-or,¡ a leaclerr to the water surface for rener'¡a1 of the airsl-ore" Mi1ler argued that, on ascending, the leading bug ls ruore f.ikely to be caught by a rr'ait:ing predal-or than are those r+hich follor¿ lmrnediat-ely after, and therefore selection rvould be expected to favour rfollowingt, lrere it not for the fact that the renev¡¿1 of airsLores j-s essential and there ruust therefore be rleaderst" fn opposition to Millcrrs conclusÍon, using the tilar,¡lcs and dovesr argument of

Maynard-Smith and Parlcer (1976) and Dawk:Lns (1976), natural selection vould favour followi.ng in every"case - the anÍna1 símp1y developíng a larger air store and thus alraays being able to fo11ow ( see also

Maynard-Sm1lh and Price 1973; Ken.,sard L978).

Synchronized ascents represent a behavíour patter:n that can only be developerl Ín gregari-ous specÍ-es ancl he cit.ecl a second species,

À..dg.llfliu_, that do noL form aggregatÍon, where this behaviour does noL occur "

In the present work Ít has been observed that A" dru! certaÍ-nly form very dense aggregations of mixecl j.mmature ancl mature individuals of boLh sexes that rlo not appear to be feeding aggregations" Whether synchronized diving behaviour in ,¿l"deanei occurs ís unk¡ror¡n , no detailecl observations !¡ere nade. r97

Examining the aggregat:i.ng behavÍour of À.d-e-:+ne! in the 1-i-ght of an tanti-anbush predation strategyt one can hypothesize the selecLive advantage on ruoving in a group, r+ith the averaþe rate of encouuLer with a sLatÍonary predator being reduced. Ilor*ever, unEl-l further rvorlc ià done ln this fteld of A"deanei group behaviour no conclustve statements can be ntade.

' The effect of change of temperalure, either high or lorv, on the activity leve1 of aquatic insects is wel.l- knorvn, as it is for most poikilotherms (see for example Cofrancesco and lloruell L982', and review by hlard and Stanford 1982). In the present study no significant effect of temperaLure was observed between 15o and 250, but a rapirl rise in probably due this act.ivity 1evel ¡vas found at 2Bo. Thls ís to temperature approaching non-optirntun li.mits for this aquatic Ínsectt

although during mj-d-sumner (January/February) the waLer temperature in

the upper strata reaches 27o to 28oC, some times higher. The following figures represent the teruperal-ures at. whi.ch there was a 50 per cenl: nortality after 24 hours exposure in sone Ephemer:id aquatic nymphs. Bcdyoneurus from sLones at si

The effect of ternperature on movement has also been used to invest.f-gate the prey capt.uring behaviour of predators. HollÍ.ng (1963,

1965, 1966) showed that tthe speed of movernent of preclator and preyt was

a significant componenL of the tattar:k raler of the predator (see also l9B

ChapLer 4 this sturly), while Thompson (19784) dernonstraLed that- ì)4b]lig" spp.¡ the corilrnon prey species he used in pr:edaL:Lon experirnertt.'; r,¡il-h

0donaLa predal-orse \rtüre more acLi\re àt 160 than BoC, and l-hÍs had a significrrnt. effect on Lhe over¿r11. predato::y success.

Clear1y, this present experiment deruonstrated that both anj-mal density and rvaler Leflnperature have signi-ficant effecLs on the acti-vily leve1 of &d_qal.e.!" I{otuever the r¡uesLfon that needs Lo be asked is v¿hether these variabl-es have a corresponding effect on Lhe encounLer rate r+íth a sit-and--rvait p::erlator líke Bt-5!-i.sgqg. Accordíngly, the

followíng experimen| r,¡as carried out to test the effect of both prey density and t+ater temperature oiÌ the encounter rate wj.th R.dislg!.

pEN:slrI_ w4fER rEI'IPERATURE g. 6:4 IIIE: EqIECT Ag -P.REY. ¿lùD ru

SUI}SBOUENT ENCOUNTBR RÂTE \rITH AN 'ÀMI]USH PREDATORI 6:4.1 Method

Three large glass aquaria (50 x 35 x 20 cra deep) were filled r¿ith 20 litres of fill-erecl, dechlorinated tap waber, and positÍoned out of direct sun.l-ight on a work ben.ch" ïtrorki-ng frorn the shape of the rÄrousalt and tStriker fields described in Chapter 3, approximate 3-D

model-s of tire combined shapes werè constructed from fine fuse wire. One

moclel l+as fixed to a piece of cocl:tail sLi-ck (2 nrn diarneter x 200 rnn) that hacl previously been wefghted wj-th a cube of bees r'¡ax and a l-ead sinker. The cube of wax fitt.ed into empLy ice-cube blocks arranged in a grid pattern on the bott.om of the tanlc, thus secur:ing it. The model

t preclator I was arranged on l-he verLical sticl< so that it was about 10 crn

below Lhe water srrrface anil posiLioned, as far as possible, in the same 199 angular locaLion as found in a normal predator. Three models were made,

1 each beÍng placecl :Ln the ceulre of t-.he gr:j"d of e¿rch taulc"

Prey anirnals, caught the pr:evious day, r,¡ere intrccluced into the tanlcs and left f-or 24 liours. The t¿ater temperature was continuously nonitored every 15 minul-es by a l-hermfstor fitfed to a calibraLed Rustralc event recorder.

Qbservations r4rere nade conLinuously for one tour on ea.ch tanlc in turn, using a mnl.ti-channelled event recorder" Bach tine a prey anirual- srr¡aÍvinto the rArousalt or closer into the tstriker fiel-ds iL was scored as an appropriate encounter. All observationrs v¡ere conducted between 0900 h and 1500 h under the naLural light and air temperalure 1eve1s prevaiU.ng at the time. Four prey densíties : 1, 4, 12 ancl 20 animals per tank (equivalent to 0.05, 0.2, 0.6 and l-.0 animals per litre) and three water temperaLures, 15 + 1.5o, 19.5 -r 1.5o and 25 +-1.0oC, were observed, By observlng the water Lemperalure from the Rustrak recorder at different tinnes of the observation period, the required water temperatures for the experiment could be identified. Tlrrough necessity this normally meant thaL observaLions for the 15oC treatnenL were

conducLed aL the beginning of the sesslon, (i.e. between 0900 and 1100

h), 2Oo mÍil-ruay Lhrough (i.e. between 1100 and 1300 h) and 25oC towards the end (i.e. between-1300 and 1500 h). There were no obvíous signs of any changes in overal-l srvirnmirrg beiraviour as a result of I diurnal

futhrnst although adrnÍttedly they cannot be totally discountecl. The positíoning of the table and tanlcs rvas such that there was no appreciable change in the light int-.ensity d-uring the observatj-on segsion. 2go

Each l-reatnrr:nt \r'as replic.aLed t.wice aucl no aniinal lüas u.'jed mol:e than olce" The duration of the cornpl-eLe experíment -[asl-ed o\¡er a 16 ctay

perlod clurl.ng May 1983"

624.2 I{esul-ts and DÍscussi.on

BoLh prey density anrl ternperature have a significant effect on tArousall the rate at v¡hich prey encounter ihe ¡nodel field of È"CåP!.gt" Fig.6:3 shows gr:aphically the Lrends observed and ¿lso gives the

resulls of the 2-way ANOVÄ anaLysis" Simil.arly, Fig. 6:/+ shor¡s l-he prey

1 density and teilperature effects on t.he increase in encor.tnter rate of pr:ey wi-th the nodel tStriker fieId. In this case ternperature apparently did ¡ot significantly influence. the encounter rate, althc,ugh examination of Fig, 624 shows the encounter rate at 15oC rsas much lower at tlie trígh densil-y. d It I

These results appear to contradict the findings of Section 6:3 in so far as it \ùas suggested thaL at ttre higher density the rate of

encounLer would be lor+e::. Ìlowever, the significant effect of the lower leve1 of acEivity anil intraspecific grouping of PreY, at the higher densities, can be seen clearly ruhen one predícts the expecLed encouuLer i. for a group of prey based 01ì one individual-. If a uniforn i rate

I dist.ril¡ution of prey exisls the encounter raL.e for 4 ptey individuals t I should be four tines that found for I individual. L:Lker'¡i-se for i densities of L2 ancl 20, al.l ol-her things, for exarnpl-e srvimmlng speed, ,1 " !ll behavi.our, eLc. being equal . rI ,

,t I

Table 6:1 compares Lhe number of encounters Per 60 ninutes l-hat -{ l' Fiq. 6: J The eff,ect of density and watel t.emperatute on

the numbel of encountels ryitl'r a 'rnc¡del-' predator.rs arousal l'-ì.cld in a 60 min obscrvatíon period.

Value given -is nle¿:n.+ Stanciard Frror.

( D ) ldatr:r temp" -l5oc.

( O ) l,Jal-er temp. 20oC.

( @ ) I,Jatel temp. 25oc. .= E o (o o 300 g IL

o o É, 200 I ì (¡, Ê l¡l t- z f o o 100 z UJ lr o cc l¡J dt =f o

1 4 12 20 PREY DENSITY (N/Container)

SOURCE DF SS MS F-ratio SIGNIF.

Density 3 7 6429 2547 6 25.12 P=o'ooo2

Temp. 2 97 44 487 2 4.8 p=o.O5

I Error 6 6083 1013 i ,{ Total 11 92258 1 'l I r rt I /t

-{

ll 20r rìrere observed conrpared i,Jith the expecLed number assumin8 a uniform disLributÍon of prey inclivíduals o '

TAB"LE 6:1 The observed mean number of encounters per hour by prey wi-th

(A) the Àrousal. or (B) tlìe Strike Fields of R.dispar rnodel

at 4 dlfferenL prey densities and 3 dlfferent rual-er

temperal-ures comparecl w"ith the expected number of encounters assirming a uniform distribution of prey Índividuals.

A. AROUSAI. FIELD

PRBY DENS]TY (N/contaÍner)

1 4 12 20

150 0bserved 15. 5 23.5 78.5 L62

Expect.ed 15.5 62 186 310

200 0bserved 2t 44 L29 202

Expected 2t 84 252 420

250 0bservecl 19.5 66 166 306

Expected 1.9. 5 78 234 390

B.- STRIKE FIELD

15o Observed 3 n5 4 18. 5 30.5

Expected 3 .5 L4 42 70

2Oo 0bserved 4 6. 5 47 67

Expected 4 16 48 80

25o Observed 4.5 13 4s 85

Expect.ed 4.5 1B s4 90 tjq" 624 The eFl"ect of'density arnd uater temperat-ure

on the number of' encour-rl-ers r,ui1-h a 'modell

predat.or's strike f iel-d in a 60 min.

obs;ervaition ¡rer j od.

Value given -j,s mean + Starrdald Error. ( O ) l^/aLer t.emp. l-5oc. ( o ) lrlater temp. 2ooc. ( 6 ) t¡Jatel temp. 25oC. .= c, 80 o (o o J UJ E l¡J v E l- 00 Ø !f ! tt, Ê l¡¡ z 40 J o o z u.l tr o É ul co Ë 20 =z

o 20 1 412 PRËY DENSITY (N/CONtAiNET)

S IGNIF. SOU R CE DF SS MS F-ratio

p=O.OO5 DensitY 3 6411 2137 13.18 p=O.1 (NS) Temp. 2 1119 559 3.4

Error 6 972 162

Total 11 8503 202

The effect is qu:Lle obvious, befng repeai:ed over Lhe tÏrr:ee temperaLures, ,rhorving the ovrlral-l ef1lect of densi,t-y (and temperature) on reduc-ing Lhe encourrter ral-e with the predator. Tt is suggestecl that the aggregation bel-ravicur and densiLy effecL in reclucing aclivil"y of

.A"deanei. significanlly reduces the enc.ounter raLe \^Iith Lhe pretlalor" The type of behaviour r,¡j.11, in turn, effecL the incidence of prey

captur:ing behaviour of -lL*d!fla1.

However, ín cons:Lderirrg the fuiictional response (see Chapt,er 4)

it Ís often assumed thaL the number of prey ki.11ed is proportÍonal to the number of prey encountered by Lhe preclator. Älthough it is clear thaL the predatÍon raLe might not rlirectly depend on the eucount-er raLe

(Hasse1l 19783 Mur:doch and OaLen L975), the encourter r:ate has only

rarely been investigatecl separately in functional- response experi.rnents

(see for exarnple Everson 1980; Sabclis 1981)" Furtirermore, recently ü ,.1Ì Katser (1983) has shor+n that tl-re functional response curves and the

encounter raLes differ marlcecll-y in several aspects. He shor'red the assumption that the predation rate is proportional to the encounter rate

is clearly not justified, at 1-east nol in the mite species thaL he

tesled" Hor*'ever, it seems obvious that for a s:Lt-and-wait predator,

like It"dispar. if there is no encounter between predaLor and prey, then

I i can be no predation. Therefore although not atLernpting Lo relate the I funcf-ional response of R.dj-spar so1e1y to the encounler raLe, it can be

{ 1 seen l-hat ínrlirectll., bearing in mind the other comporrenLs of the

predatory process discussed in Chapter 3u encount--er rate rvill ':{ - ü { I significantly affecL the prey-capt.ure of ttdfgpg1.

I

{4 t' 203

In addftion, woul-d be expected tlìat the prey-eucounter rate n ft woul-d not only influence the nunbers captured and eaten by an ambush predator but signifi-cantly affecL other components of the predatory behaviour. In particular, the choice and duratlon of stay at a foraging site, it may be hypothesized, may be very much affected by the extent and frequency of encountering prey.

The next chapter sets out to Lest Lhis hypothesis by observing I the effect of t I (i) prey denslty on duraElon at and rate of movement between foraging

l-' slLes; and (ii) the effecL of encounter rat'e wiLh prey on foraging slLe selectíon.

I I i

(

+ t I Ji 1,

ì. t ,t I

4 r -_:-; .l q*-¡*r- 20tt

7 ¿L IN'IÌìODIICITION Surprisingly, one of the rnosl- obr¡ious questions conc-erning selection of an optirnal ambush site, by an ambush ¡lredator* has not- received the aLtent.ion it deserves (Curio 1976). Clearly orlè of the mosl important criter:ia for any sit-and-rrait ¡-rredator in evaluattng an anbush site is the assessment of prey availabilj-ty. This is in turn a basic assumption of current optírnal foraging theories, in that an optimall.y foraging prerlator nust be able to lneasul:e or assess prey availabilÍty anrl therefore deLerrnire an expected raLe of food int-¿rlce for a given environnent or patch (Pyke, Pulliam and Charnov 1977; Krebs L978i Krebs and Davis 1978; references in Karnil. and Sargent 1981; grians 19S1). This assessment of expectecl food intal.ce fcrms the bas;is for the predatorts evaluation and subsequenL decision rnal

time, opt-irna1 choice of diet, choice of searcTt tacLics (Charrrov I9':/6b3

pyke, Pulliam an

i, nrunber of hypotheses have been put forr¡ard in atternpting to explain a ú

1r clensit-i' I possiþle mechanisn by rvhich predartors assess the availability or I predatcr { of prey. Johnson, Alcre ancl Crov¡ley (1975) suggeste

frequently nhen surroundlng prey rlenslty is l-or¿" Bviclence for this hypothesis exi.sts from Zonmiclc'i and Slobodlcin (196ó) vorking vÍth Hvdry! 7 ¿ littoral-i.s !trhen the density of prey was reduced the liÉr.È floateil free fron the substrate and noved to another sÍte. Adrl:LLional r'¡ork on

various r+eb building spiders (see for exampl-e Riechert L974' L976; Janetos 7982; JaneLos and Cole 1981) have shot,¡n that when prey availability is 1ow spiders move and reconstrucl, their webs aL another location. In adrlítion Morse and Fritz (1.982) and Morse (i983) rvorking with the crab spiders, !@e-nê l'ati.a and XJgliçqå9@9Ir, reporLed. that the spat.ial distribution of the spiders most .c1osely resernblecl that predicted if spiders responded to the number of Eimes insect prey visitecl a¡r umbel raLher than more direct measuresr e.g. attack or capture frequency. They ul=o sho"ed that, for those spiders that noved, thelr probability of eventuall-y occupying a high-quality stem markedly

increased. Horuever, the tr.'o species, althorrgh cot-a.nonly found on ttre

same plants, responded to very dífferenL densitj-es of prey, resrrltï-ng irr If.vatiq spending truice as long on milkweed plants compared with X.emerËoni. More recently, Formanot¡icz, Bobka and Brodie (1982) reported theib studÍes on an rextremer ambush predator, the r+ater

scavenger beetle, Hydrochara -oblgsetq (Coleoptera : Hydrophilidae). Larvae of this specles were found to nove significant'ly less at high

cornpared with low densiLies, although there l,¡as no significant difference in the number of prey capl-ured at the four experinental

I densities. From their results they concl-uded that capture rate and ì rnechanj-sms i, saLiatíon (gut fullness) could be ruled ouL as possible of if r I assessing prey density. Addifional work by Forunnotricz (1982a) on rt I { another beet.le species, Dltigcq!¡_ verticalis suggesLed thaL this sPecies

,.{ utilized an encounter rate mechanism itr assessing p::ey densi.l-y.

I 2C7

The experiments reporl-ed in this chapter sel- ouL to Íuvestigat-e the effect of prey density on the duration of stay aL and subseeiuent

nurnber of changes between ambush or foraging siles, fn particul-ar I proposed to test the hypothesis that lì-"(.igpol would change the poslt.ion of their a¡rbush síte more often antl stay for shorler periods of tj-me when the prey density was low, and r'¡ill clecrease the frequency of

changi-ng and therefore remain at a site for longer r,¡hen prey density is higher" Secondly, experj-ments r¿ere under.Eaken to try Lo identify

factors involved in the mech¿ruism used by R.clispa.r in assessing prey density or availabilÍt.y in adtlit.ion to the f mean intercatch intervalr reported in Chapl-er 5.

pl¡.N_srrY 7¿2 rHq I_ru. _0F PRBY 9N. MOVEMBNIS_ BËT\^/BEN AND AROUIID AN AMBUSH !JåE- BY R. di-spar 722.L MaterÍa1 and lfethocls Fifth instar R.dispar: were collecLed and maintainecl as outlined

in ChapLer 2 during January to I'farch 1981. After rnoulLlng, ma1.es and

fenales !ùere separated but rnaintained as before. The animals were usecl 5 to B days after the ¡noult, Lhus standardizing (as closely as tire experimental condítions and availability of anj-mals allorued) post-moult.

behaviour" Each anlmal ruas used once only"

The basic unit of apparatus consisLed of a glass aquaríum (33 x

17.5 x 26"5 cm deep) with Lhe vertical wal1s covered on the exterior surface r¿ittr neuLral Lranslucent paper. A false floor of cLear perspex

{ with six foraging siLes located vertically, 1 each 5.0 cm from each

corner, the other 2 individually p1-acecl rni-d-way along each of the 208 longest sicles, 5.0 crn fron the aquar:iun rvall- The foraging sites lrere vertical rloi,relling rods, 0.5 crn cL:iameLer x 20 cm 1ong. ¡1 3-dimctsj-or:al- gr:id rnade froln plastic p,ratilìg \{as then loç¡ered c¡ver tire foraging sites onto the perspex floor (see Fig. 7:1,4,). Bach square of the gi:Í-d rvas 1

.^3" This provicled, not only a normal spat.ial idenLifÍcation aid, but: the depLh component, of the grid acted as ::eceptacles int-o whi ch discarded prey could sinlc. This prevented them fron being clislod¡1ed and relocated by the movement. of Lhe predator or prey and Lherefore could be used as a visual marker of foragin¡1 actÍr'ity of the precìator.

Three such tanlcs lrere ar:ranged as shor,¡n Ín Fíg . 7 ¿Il3 on o 1 ,n2 *

5 cm l¡hick sheet of polystyrene. Four vertical. Climplexo rods, 1 neter in length, located at each cornêr of the po1-ystyrene sheet supportetl a

Itentr of black photogr:aphic curtaining that surrounded the apparaLus o4 the four sides but remained open at the top. The tops of rhe verLical C1Ímplexo also acted as anchor points for adrlitional framework thal: supported a camera and flash attactrmenLs.

A Bauer cj.necamera r+Íth tiLne-lapse attachment was situatecl 1"5 m dírectly alrove the tanlcs, along rr¡íth a rechargeable flash gun (Þfecabli[¿ j-t 215 telecomput.or rnodel) " The positioníng of the camera v¡as such that pr:ovirled the besL field of vier'¡ of all three tanks and a srna11 electronic digital cl.ock fixed to ¿'.n ouLsirle ¡¿all of one tank"

Preliminary trials identifie

l'Ìool uli't-ll rLloc¡den ambush sil-es.

Fiq. 7: LB Arrangement of 3 glass tanl

obse:.'vat-ion " (Ë)

(w) 209

The time inLerval betrveen exposures v¡as 60 secs" Super-B reversal fi.[n (Kodalc Tri-X) was usr:d Ín a].1 experimetrl-s, with the standard 15 rn (50 ft.) carLridge of fílrn providing about 3600 exposurerr in 60 hours" Flash duration \{as set at 0.00C02 sec" Controll-ed experiments beforehand in normal light (>2500 lux); dim J-igl-rt (15-20 lux) and darlcness (<0.05 lux) using the characLeristic 1.eg extension or dive responses of B.dÆlqf (see Chapter 3) as an indicator of rfrighLr fail-ed to show any observable effecL of the rflashr on the behaviour of the predator, likewise rro significant effecL ou the pre)r \,/as observed.

Experimental Procedure Âdu1t female B*.þqIe! were fed abundant prey acl !t ltilrlt. for 2 days preceding the experiment. They were theu subjected to a 48 hour fast, to rstandardizer their alírnentary conrlition. For the last 4 hours of the fastj-ng period Lhe anlnals r+ere transferred Lo Lhe experinental- tanlcs, one individual per tanlc, ivhich had been filled with 10 litres of filtered, dechlorínated tap waler. Animals v¡ere observed to rseLtl-e downr r'¡ithin a few minutes and norrnally took up their characteristic prey capture posture (see Chapter 3) within 5-15 mitluLes"

Prey anirnals were adulL A"!-grnef that v¡ere collectecl up to /+8 hours before being used and rnaintaíned as outl.ined in Chapt.er 2" Three

prey densities were used : no prey present (N = 0), medium density (N =. 3/1Ítre) and high (N = 15/1itre).

The cinecamera \ras normally srvitched on 10 minutes before

introducing t-he prey anJ-rnals into the tanks, which j.n turn initiated the onsel of the experinenl. This rvas arranged to be as close to 1200 hours 210

as possible. The llghting r:egirne was 14L : 10D r¿ith lights on at 0600 h, off at 2000 h, r+hich approxirnal-ed the naLural- lÍght conditions during the experimental perí-od. Â bank of 4 fluorescerrt lights (Phi1lips r 2 x 40 rvatt tl^IhÍtet, 2 x 60 i.¡att rDay Li-ght.r) covered by a light diffusing panel was positioned 3 netres above the apparaLus.

The tanks nere inspected every B hours and ttre number and

location on the grid of dearl prey was not,ed and then an ec1ual number of adult Á"deanei r¡ere added. A fol-d-back v¡indow ín the curlai.ning permitted easy access to the tanlcs without causing any apparent disturbance or fright reaction in the experimenLal animals.

Observations during darlcness ((0"05 1ux)'were carried out under Red Light (Phillips TL 40 !¡att/15 fLuorescent tube). PrelimJ-nary studies in din light (<2.0 lux) with and r+ithout. the red lighr on levels of activity were nol significantl-y different (¡2 = 18.6, p(0.01) and

Èherefore Ít was assuned Lhat the red light did not adversely affect or: dÍsturb the anirnals (see also Bailey 1981; Elliott 1968).

The duration of each experirnenL r¡as 60 hours at the end of whlch the predators were reLurned to the holdlng tanks. Prey Í-ndividuals, boLh dead and alive, were rernoved and counLed in order to check against recorded observations.

The experirnent was repeated flve t.imes beLween 22 ,Ianuary and 19 March, 1981, using new preclaLofs and prey individuals in each case. 2LT

Analysl-s of Fi-lms

Films were project.ed from a Kodak Ektagraflc l,lFS-8 Projector fltted with a single frame advance/reverse control, onLo a screen.

For assessing movements between foragíng sites each film was vie¡+ed and the number of site changes ancl duration of sLay at each si.te was recorded. During the viewing of the film it was noticed that the angul-ar position of the predator around the foraging site (in the horÍzontal plane) changed, and that Lhere appeared to be different patterns exhibited in each of the treal-ments. In orrler to quantify this nnovement the following analysis schedule was adopted.

Movement around the foraglng siLe was defined as the angul-ar change of locatlon of R.dispar each ninute of the sample peri.od (see below). It r+as by rneasured directly fron the projected image by taking the cornpass location of the predaLor in relation to its own longitudinal body axis and the centre of the foraging site, a fÍxed point in each frame. ThÍs was done for frarne txt and Lhen re-neasured for frarne rx *

1r (i.e. 1 minute later) and so on over a sample of 2O consecutive frames" By simple subtractlon the angular movement could be calcul-ated.

Due to the vol-ume of available data obtained (over 18000 frarnes collected from the 5 filns) it was decÍded to subsample each film by analysing a bloclc of. 20 consecutive frames (i.e. 20 consecutive minuLes) every 360 ninutes. This therefore provlded a total of 10 samples of 20 frames for each filrn, and r+ere arranged to be talcen from about the sane time of day.to allol¡ for any diurnal ryLhms, etc. Table 7:1 shows the 10 time lnLervals used for each experiment. 2L2

TABLE 7:1 The tirne of the 10 observation periods for each experiment. (Starting at the time inclicaLed a sample of 20 consecutive fra¡nes ruould be analysed for angular posi-tion change in R.4!gpqÐ.

OBSERVATION PERIOD NUMBER t234s67 8910

DAY NI]MBBR 1 2 3

Expt. 2 t636h 2236 o436 1036 1636 2236 0436 1036 1636 2236

n 3 ls15 2LLs 0315 091s 1515 2L1.5 0315 0915 1515 zIL5

n 4 L620 22ZO 0420 1020 1620 2220 o420 1020 1620 2220

il 5 1738 2338 0338 0938 7738 2338 0338 0938 1738 2338

+J Unfortunately the film from Expt. 1 was destroyed before this analysis could be undertaken.

7¿2.2 Results

(A) Movement and Duration of Stay between Foraging Sites. Tables 7:2 and 7:3 show the number of site changes for each of the prey treatments and the mean duration of stay at each site

respectively over the five experiments. A l-way ANOVÀ showed that a highly significant prey density effect r+as eviclent for both nurnber of slte changes (F = 83.6, df. 2, 12, p (0.001) and duratíon of stay (I'= 2I.2, df 2, 12, p <0.001). ÏJhile a one tailed t-tesl- of

comparison of means showed thal predators changed foragÍng sites signiftcantly more frequentLy when no prey were present comparerl 213

vlith 3 prey per litre (t = 8.13, 4 df, p(0.001) or t,rhen only a fetv prey were present (i.e. 3/11tre) comparecl with 15 prey per litre (t = 2.1+B¡ 4 df , p(0.05).

TABLE 7;2 The nunber of sJ-te change s of adult R"dispa:: when eithef no prey (N = 0), ferv prey (N = 3 t-1¡ or nany prey (N = 15 1-1) are present over a 2.5 day observation period.

EXPERIMENT NUMßER

PREY DENSITY 1 2 3 4 5 x SD

(n.1- 1

0 39 47 31 33 48 39.6 7.8

3 6 3 1.1 2 8 6 '.6

.3 15 2 1 4 r 2"2 L.3

In addition the absence of prey caused the predator to remain at a site for significantly less time compared to when 3 prey per litre Ìrere present (t = 3.42, 4 df , p ( 0.05), r,rhlle, conparison of means between the 3 prey per litre and 15 prey per lltre treatnenLs showetl

that. the predator remained at the arnbush síte for less time when the prey density was low (t = 2.63, 4 df, p(0.05).

Figs. 72A-C shows the mean duration of stay at each foraging slte over the 5 replicates of each treatment. Initially, predators remaln at foraging sit.es for significantly 1-onger than later stays, TABLE 7:l The mean duration (min) of stay at each foraging site by adult R.dispal uhen either no PreY, feru prey or many prey are present.

EXPTRIMENT NUI'IBER PREY DENSIÏY t 2 7. 4 5 X SD (n. r-1) x 6I.9 7B 107 99 B7 87 ]B

0 SD L42.7 I49 17l_ 153 I6t

RANGE I Lo 696 I La 722 I Lo 6?4 1 to 750 1 to 648

X 5r4.? 900 100 1200 400 663 377

3 SD 1010.7 r235 922 685 793

RANGT 97 to 21Cl 158 to 2807 27 Lo I9I9 10 to l4l8 2II Lo 262I

1 1537.2 19r5 i216 834 iB00 1469 394

L¿tq SD 1611 . B 2505 801 511 240I

RANGE ZCO lo 265I I23 Lo Jr+77 79 t.o 1900 296 Lo IJ15 100 to ll00 l'ig. 7 :2 The mearr duration of stay (mins) at each ¡Lmbrrsh síLe before moving on to ne>lt síte. (A) Prey abscnt. (B) Prey densíty 3 per 1irre.

(C) Prey densí ty 15 per litr:e.

Verti-cal bars indicate Standarcl Error. Nunrbr:r:

in par:e,nt-hesis i s uuntbei c:f rep'l lsates ine¿an arrd

S"E. are based ou" (A)

80 X= 27 rnln PREY AÐSENT o ul t- 60 õ r .t) l d 400 ã l¡J l- 20 f= o 'to X= 14 mln u, zU' 10 o(, t- 80 .5 5 60 ttt- tt o z 4 9 F fl f a 2

ll+ 1{l-ll g7 o r 3 5 7 I 11 .t3 15 17 1S 21 23 25 27 29 31 33 35 39

(6 5¡4¡+------+(,rl

AMBUSH SITE NUMEER (B) (c) 2000 1600 PREY DENSITY (PD)=3/IITfo PD = 15,/lltr.

u, ul 1 000 E IJ' r ooo Í u) Þ I 600 c0 Ë 800

trl Ë 5 60 O l¡l a, I 200 o O 40 l--

.F 200 E 100 t- 't oo 800 U' l! o 80 9 t"-

cÉ f ô 6 400

4

20 200 1 2 3 4 5 6 7 I I 10 11 1234 (s) (s) (4) (3t (s) (3) (2) (2) (1) (1) (l) (5) (3) (2) ( r)

AMBUSH SITE T.¡UMBER

{ 214 irrespecLive of the presence or absence of prey. In all three Lreal-menLs there is a tendency for the duraLi.on of stay at a sit-e to become shorter as Lhe experirnent progresses"

Because of the design of the experiment the hunger effect r'¡as continuously changing and could not really be cont-ro11ed. To a certain extenL this problem was alleviated in a second series of experimenl-s discussed in Section 7:3. Nevertheless the results obtained over the 5 replicates are consisl-ent anJ permit, it is believed, reasonable concl-usions to be drat¿n.

When no prey are present R"disÞar appears to fo1low two distinct movement patterns between foragi-ng sites (Fig.7".2Ã). Initially, over the firsL 46 hours it remains at a sil-e for a variable amouut of time (Í = 277 r SD = 217.8, Range 39 to 720 mins., N - 50) ít then srsitches into a second pattern of rernaining at. a site f,or a sígnificanLly shorter time (7. = 14.2, SD = 11.4, Range 3 to 54 mins., N = 30 sites). lJhen prey are present the predalor remains at the initiel site for significantly longer cornpared with the cluration of st-ay at. later sites. At the lower density (i.e. 3/1itre) (Fig,7:28) the variability in length of stay is more evident compared v¡ith the higher density (i.e. 15/1itre) (Fig.7z2C), which exhibits a consistent l-rend of reducing the duration of sLay at each consecutive síte, although the lorv number of actual site changes, in parlicular the high density treatment, ¡nakes it difficult to identífy long term effecLs or patterns" Idea1ly, the experimetrt needs to run over a much longer time to identify any real,patternt 2'15

especially afLer l-he flrst 6 or so site changes in the high prey

density LreaLmenL. significantly nore prey were eaten (t = 2"59,4 df, p(0.05) at the higher prey density than the 1or*, r+hile the distribution of dead prey carcasses on the grid floor of the tanks mírrors Lhe rnovements

between foraging sites by the predators.

(B) Movement Around the Foraging SÍLe. For each of the 10 periods per fil-m a mean angul.ar change per

minute was calculated over the 20 consecutive frames, the clal-a was

1og transformed and a Z-wqy ANOVA incorporating a repeated design rtime tptuy (SPSS Uptlate 9) was usecl to test for either a of dayr or densityr effect on the mean angular movement per minute around Lhe foraging site. Tabl-e 7¿4 is the resultant ¡1N0VA table and shows that the time of day had no effect on movement r*rhereas prey densÍ-ty did.

ttin¡e TÂBLE 7:4 Z-vay ANOVA to t-est the effecl of either of dayr whon

sample was taken or prey density on rnean angular novement per minute of adult R.clispar around a foraging siLe.

Source of Variation SS DF MS F Signif

Time of Sample 1.8 9 0.20 0.784 NS

Error 1 18.4 7l a"26

Prey Densily 23.O 2 11.s 9.768 p(0.05

Error 2 7.O 6 1.2

Time x Density 3.00 1B 0.16 0.641 NS

Total- 53.2 L06 2L6

This bei¡g the case the dat.a over the different períods -,"el'e combl-ned to form one clata-set. FÍ-g. 7."3 shorts tire frequency distribuLion of the angul-ar movenent about the foraging site for predators at 3 different prey densiLies. The null hypothesis that there lfas no difference in the frequency of angular move¡rents at each prey density was tested r,¡ith a simple X2. A 3 x 6 contingency tabl-e tv¿rs usecl to test the effect of densitY.

The results are summarized in Table 7:5. When no prey vfere present the predators renained stationary sÍgnÍ'ficantly rnore often than

when prey \4'ere present, while the incidence of moving through relatively small angular changes occurs significantly more often when prey are present. Although not significantn the. higher frequency of large

angul-ar movements observed when p::ey were absent' merÍts sorne corulent' After remaining stationary at a particular position for long periods (over one entire 20 minute observation in one instance) the predator

would move through a large arc about the foraging site and thris be in a position to recelve stimuli frorn a previously unscanned region of the surrounding r+ater body.

Because of the resolution of ttre projected inage it was noi: alrvays possible t.o identif y when a capture took place, and f:heref oi:e Ï could noL ascertain, with confidence, whether these smal1 angirlar

movements (rvhen prey were present-) t¡ere ascociated with or:ientatÍon leading to a capture or whether a post capLure behavíour which producerl a tsweepingt or tsearrchingt pattern of mot'ements ruhich caused the Fiq. 7 z3 The effect of prey density on the lrequency of angular movement around

the ambush-site by adult R.dispar. The values given on the absicca are the means of each angular mcvement cLass. See text for addi+.iona1 details. 40û

200

100 ffi nnev ABsÊNT

pnev DÉâ{s¡rY 3,/lltre 8û ü f] ene v DEÊ{srrY 15/lltre

6 Þ o

4 tJ w É l¿ 20 çpF ro

6

2

q c¡ ¡o q q I 1 (v) o q \ q 1 @ a9 (o to r¡¡ a o, Ð @ ¡o ¡'. o ñ¡ o ø 9, o G¡ €\¡ a(, ri 1¡)

AI,¡GULAR MOVËMET.IT CLASS (') TABLF- 7:5 Effect of Prey Density on the frequency of the mean angular movement around the foraging site, as measured by degrees moved min-I. See text fo¡ details.

MEAN OF FREQUENCY OF MOVEMINTS AT X2 Analysis

ANGULAR I4OVEMTNT CLASS I 0 15 prey per J.itre I I t 3 0 3 0 15 (degrees moved min-l)

0 (Predator stationary) 464 219 235 X2 = 43.9 p<0.05 X2 = 37.5 p<0 .05

p<0 I 220 396 435 ,y2 = Z5.I p<0.05 X2 = 35.3 " 05

2 10 .6 40 B6 57 x| = 8.4 p<0.05 X r.5 NS

20.r 7 ?0 IO x', = 3'r NS 2 t NS

2 ?9.7 7 2t 9 xi = 4.3 p<0.05 .2 NS 2 l0 22 I6 II+ x|, = '48 NS ¡. .BB N5

Effect of Density (t x 6 continqency table) X' = 236.61 p<0.05r l-0 o-. 2L7 predalor to be rest,rlcted to a fsmallr area. Additlonal work ís requirecl \{Íth R.dispar in orde:: to ldentif y aûy significant. pat'ce.rn of behaviours.

In summary, the above experiments shor¿ed that both the number of site changes and the duration of stay aL each site was significanLly affected by the densiLy of prey present" In addiLion, prey density also affected the extent of angular movements around the foraging site.

One question that arises from these resulLs is whether the duration of stay aL a foraging site (and the associated number of changes betrqeen sltes) is dependent on the successful capture and subsequent nutrÍtional reward whil-st at a site, or does just encounLering a prey affect the time spent at a site

In order to answe:: this the following experiment was carried out.

7:3 THE EFIECT. 0F PREY WOUNTE$ VBRS-U-S FOAD- INTAKE 0N DURATION OF

STAY AT AN 4UBUSH IITE BY- R" díspar 7:3.1 Material and Methods Adult femaLe R.dispar were fed a mixture of prey types and then fasted for 48 hours as outlj.ned in Section 7:2. Individual predators were placed in a lar:ge glass aquari-um (50 x 35 x 2O deep) fill-ed v¡ith 20 litres of fllt,ered dechlorinated tap water. The floors of the tanks were fitted wlth a plastic grid bottom (see Fig. 3:1) and 12 f.otagíng

sites construcLerl from 2 nrn Dia. x 200 mm satay sticlc embedded into a 2tB v¡elghted wax block that fitted int.o the grid floor. The 12 siles were allocated at random in the tanJc" Three strch tanks \ùer.'e seL up.

Predators were exposed Lo one of l-hree treatmeuls : (1) Tank r¡ith No prey (NP) (ii) Tank with Model prey - encounters only (EP) (iii) Tank with Live prey of knorm density (FP)

Trvo prey densitíes (with theÍr assocj.ated encounler rates) were selected, these being liigh density (1.O/litre, i"g. 20 animals per tank) and Low density (O"2/\itre, i.en 4 animals per tank).

The encounter raLes l{ere calculated from the results of Chapter

624, poo1ing the data from the 2Oo a¡rd 25o experiments over the 4 densities. Although not strict.ly applicable, it rvas deciCed to plot encounLer rate as a linear fun-ction of prey density to provide the theoretical encounter raLe for use with the model prey. The resul-tant

linear analysÍs is shov¡n ln Fig. 7:4.

By substÍtution into a polynonial equation a prerlicted encounter

rate + error could be computed for any prey density over the range used in the linear model. The calcul-ated encounter raLes for 4 and 20 prey per tanlc (0.2 and 1.0 prey/1ítre) were 1 and 4 encourrters per ninuie for

the arousal field and 1 and 10 encourrters per LC ninutes for the prey capl-ure fj-eld respectivel-y. These encounter rates r.¡ere used for l-he rnodel prey to resemble, as closely as possib1e, Lhe encounter rates that woul.d occur with the l1ve prey. Fiq.7z4 Regress-ions o1" tlre number of' prey encounl-ers

ruith eit-hel the predator's atousial. f iel-ci ( O ) or stril

fields ancl addit.j-onaf detail-s). Regress-i-on statist-ics shovn on l"igure. NUMBER OF ENCOUNTERS WITH AROUSAL F]ELD /60 m¡no Þ úr o o o o

cD@@ aa o

ct) Þ E+!

oo o o o Þ o o a a aa a o o o NUMBER OF ENCOUNTERS WITH STRIKE FIELD/6O mln. 2t9

PRESENTATION OF }ÍODEL PREY AND BXPERIMBNTAL i'ROCEDURE

Ifodel preyr 6 run in length (see Section 3:6) r.rere suspended on fine nylon fishlng line and presented to the ¡lredator as outlined in

Chapter 3t4 at the above rales in such a fashion that the predator was clearly aroused towards the prey (in some instances predators struck at the prey) but rvas not permJ.tted to capture or, in relatlou to the strike space, the predators were allowed to strike at and capLure the mode1.

Predators were cont.inuously observed f.or L2 hours and there were 3 replicates of each treatment using new predaLors ln each case.

7 23.2 Resu_1ts

Table 7:6 summarizes the results of Lhe nunber of siue changes and duratÍon of stay over the three replicates for each treaLrnent. It would appear thaL the mere fact of encounterÍng a prey will result i-n the predal-or remaining at a rpotentíal1y rewardÍngr foraging site. To test. the null. hypothesis that prey encounter does not significantl,y increase the duration of stay at a foraging sÍLe the raw data were analysed usirrg the Mann-trrlhitney Test betweet each Lreatment. Because each experirnent hras run for a set period (i,e. 12 hours) it was decided to lgnore the final duratlon of stay fo. u..n replicate, as this simply represented the experimental period minus the sum of the earlier duratíons, and therefore had no real biological meaning in connectíon with the duration of stay at a foraging site. The result analysis sunmary 1s shor,rn in Table 7:7. TABLE 7:6 The number of site changes and mean duration of stay at a site by R.dispar in relation to density of prey.

REPLICATE NUMBER PREY DENSITY I 2 3 X 5D

(n .1-t /l Site Changes 3 I 4 2.7 r.5 0 ï+SD Duration J.B0 a 145 360 + 76 I44 + IA7

MODTL PRIY /É Site Changes I 2 l r.3 6 ï+SD Duration J6O + 209 240 + I94 36Q + 3t+I 4 LIVE PREY /l Site Changes t 0 2 1.0 1.0 7r+SD Duration j6O + 396 72O+O ?40 + l0B

MODEL PREY # Site Changes 2 I 0 L.t 1.1 i+SD Duration 24Q + 216 J6A + 56 720+O 20 I-IVE PRTY /É Site Changes 0 0 I o.3 6 ï+SD Duration 7?O+Q 7ZO + 0 36O + 487 220

TI\ÞI=E 7:7 The cornparison of the duration of stay at each foraging site

when either no prey were present or when the precl.ators were stfuuulated with rnodel prey at two clifferent encounter rates"

]]WN. OF ,TÂY (minsl

L0\4r ENCOUNTERS PREY ABSBIüT HIGH BNCOUNI'BRS

508 372 489

410 208 130

281 40 400

601 306 720

129

297

4L

200 lJ=29 u=29 p(0.05 p(0.05 U=9 Not Significant

Clearly encounters cause the predator to remain at a potentially profitable site, even though no nuLritional. reward is forthcoming. The fact that the experiment l¡as run over a relatively short period does affect the conclusj-on that can be drawn. In all lilcellhood, the patLern that. has been observed r.rould disappear after some Líme due to the predatorrs food deprivaLion tj-me increasing, and the need to capture and 221 feed becoming par:amounl. Ne'uertheless ít is believed that Lhjr-s experinent l1lustrai:es that pre)r encourrtcir, as disLinct from feedlngo is an írnportant element in determíning l-tre tj-me budgc+1-ing of si.L--anrl-v¡ai"t predaLors at forerging sites"

7zh DISCUSSION The results of the experirnents described in l-iris chapLer support the hypothesis that prey densi-ty significantly affecl-s the ¡rrrnher of tines l-hat a sit-and*wait preclator changes its foraging si[e, anrl tire subsequent- durati.on of stay at each si.te, and as such support,s the findíngs of Forrnanor,¡icz eL al" (1982). 0vera11, more prey \¡¡ere eaLËn at the higher density and so capture-::ate, or the anount of foocl Ín Lhe predatorrs gutr may be useci t.o assess the availabilir-ty of prey.

Hol¡ever, the seconcl series of experiments, us:lng model prey, c1ear:ly de¡nonslrate that predators remain significantl-y l-onger at a foraging site afler encountering a tpreyr, compared witll the mean sjLe duration when no prey are present or when captures actually occur. llhe rate of rnodel presentation neverl-heless does not appear lo affect the duration of stay.

CJ.early this type of behaviotr::al response should be adapl:ive, fc¡:-- on changing ambush sites, when prey density is loi+, the probabil-ity of encountering new prey increases (åornrnicki and Slobodltin 1966), At higher prey densi.ties the preclators remain longer at a sibe and h¿:ve therefore rnore tÍne to engage in other activil-ies, for Éixanple nate findi¡g/matíng. In addition, the lessenlng of oiren water swj-rnrníug or movements amongsL vegetatÍon r¿ill reduce their exposure to p::edaLors

(see Chapter 2), Ânclretss (L979) reporled just such a câse j-n ihe 2.22.

Lízar:d, CorytoDhanes crisl-atus, vhere Lhe risk of pr:edati.on l*as nj-ninized by .idopLing au exl-reme sit---¿rud-ruail- predaLiou Leclurique coupled with the poLential- to capture and handle large arthropod prey¡ thus ::educing the bouts of feeding activil-y. Âs nentio¡recl ín the int.roduction, a basic prerequísite of current optimal foraging theory involves the predaLor?s ability to assess prey denslty, but there exists

1ittle enpirical data on the possible rnechanisms adopted by pretlator:s enabling then to do thi.s.

It has been shoryn in Chapter 5 that the mea¡r tirne l¡etr'reen prey captures (i".. mean inLercatch interval) plays a significant role in the lray that &.¿iuJ4f. subsequently utilizes individual prey iLems, whích resutts Ín partial consumptíon of the prey. In addÍtion horvever, the results presented in this chapter indicate that capture and feedi.ng .LqL se are nol the only criteria used in assessi-ng prey density but l-hat. stinuli from a prey encounter sigrrificantly affects the tine that a predator will remain at a site. It r+ould appear likely therefore tirat:

R.dispar utilj-zes rnore than one technique in assessÍng the prey deus:lt.y and that lt can utilize Lhi.s irrformation in at l-east t¡¿o behavioural responsesr i.ê. durat.ion of stay at a foraging site and prey uti-lization.

Jaeger and Barnar:d (1981) suggesl-ed that the salamander,

Pl.lt_hg.É"Ê cinergus Green, may use an encounter rate nechanism in determining search tacLics and optimal di"eb, ruhil-e recently FormanowÍcz

(1982) repo rted that larvae of Dytiscus vert.icalis ma y be utilizing an encounLer raLe ¡nechanism in assessì-ng prey density and subsequently changing its sea::c.h tacl-Ícs. The initial select:Lon of a foraging site 223 by &"4.ig"=E_, as an exarnple of a siL-and-r¿ait pr

If prey are noL in Lhe vicinily the predator adopts oue of l-wo movement patterns (see Fig" 7:3Â) depending on, probably, the tiine since the last meal and Lherefore its present- motivaLional sta.te. It rvil1 move to another site and either remain there for between 40 rrr-inut-es and 12 hours (7 = 277) or from 3 to 54 rninuf.es (ií = 11+"2) ancl t-trerefore, ín the second casen rapidl-y rnove betr,¡ee:lr foraging sites, inc.rezrsirig Lhe 224 probability of locating and encounterlng more profitable ambush sites and thus prey. A sírnilar hunger-'dependent change in foraging si"te has been repori:ed by Griff-lLhs (1980a) who found that anrlion larvae changed locatiorr of their arnbush p1-ts more often rshen starved than well fed,

In summary it can be seen thaL the assessment of prey densit-y may involve a lat more complex series of factors than have hitherto beeu reported, and that dependlng on the circurnstances, predators may si*ritch between different nechanisms or utilize information from several simultaneously. Clearly, for &.éis3af, boLh prey encounter rate and prey capture rate appear to be inportant in determíning the durat,ion of stay at a foraging or ambush site (with the subsequent related rate of movement between sites) while the mean inlercatch interval significanLly affects the feeding time on an individual prey.

To r,rhat extenl these factors are lnterrel-ated i-u so far as providing an overall rpict.urer to the predator of the surrounciing profitability of the habitat is concerned, cannot at present be determined. Recently, Formanowícz (1982) suggested that the encounter rate between predator and prey may be an overricling mechanlsm by which Dytiscus verticalÍs larvae decíde which search tactic, either active search or ambush, to adopt at varíous prey densÍt-ies, rvhile Inove and Matsura (1983) workÍng with the rnantid, Pêrat-eno-dera ag¡qstip.qnn-Lq' successfully correlated a swilch in search tacLics, fro¡l ambush to acLÍve forager, with an increase in trunger, erlthough they were uuable to r)tq identify the mechanlsms underlying the rever:se swÍtching, fron active search to ambush. Perhaps an encounter raLe Lhreshhold may be involved?

Unlfke !.vetticath. and !-r.ryr;gligennís, Rr_clj.s?al cannot capture prey while swimming and the::efore the ractlve searcht component of its complete foragíng response is concerned r+ith locating an ambush sÌte rather than prey items lgr se. hlhether R"*diepar_ randomly chooses arnbush sltes or uses far more subtle envi.ronmental cues is unkn<¡wn, as is the relative energy expenditure. Morisrl-ta (1952) showed that ant-lions expend sígnificant amounl-s of energy and tj.rne in locating suitable sj.tes for their pitfall traps. Therefore, it can be seen that in addition to assessing the potenLial profitability of an ambush site the costs involved in movlng between sítes should be taken into account ln some way by the preCator. One would expect that selectÍon v¿ou1d favour those pr:edators that, when prey become scarce, move to a new, more favourable slte rather than starve to death. However, 1t could be hypothesízed that after prolonged periods of prey absence the optimal strategy to adopt, taking. inÈo account the cosls involvecl in moving between sites, would be to remain sÈatÍonary. This hypothetical adaptation ¡:emeins tr¡ be tested.

226

This sludy has atternptecl to exarnine the ethology of a siL-and-wait predator, R.disoar. that Ís found ín farm dams and other lotic water boclÍes in ¡luslralia. Ernphasis has been placed on the inportance of both the lnternal motivationa1- leve1 of the preclator (as measured by food deprivation tirne) and the density of the prelr Ín affectÍng the varlous behavioural components that rnake up the preciatory repertoire 1-eading to capture; in addition to examining their effect on the way that individual prey are utllized during thr feeding períod.

Each chapter of this study has its own discussion section, where data are discussed and compared rn¡ith the .pub1-ished f.iteralure. Hor.rever, lt would seem ruorttrwhile at this poinu to examine several general points that have not been considered previously in detail and to nention others that may be of general inLerest to workers contenplating slrnilar behavioural studies on such predators. In conclr:sion, aspects of thiÈ study that. show promise as Eopics for future research rvork are discussecl .

As mentÍoned previously, sit and wait predalors are, by definition, rellant. on the movement of the prey to bring about an encounEer and a subsequent capture. Therefore, j.t is to be expected that naLural selection would most certainly have Lhe effect of concentraLing the predatory effort in (1) the capture process -g- s_e_ and probably (2) the selection of a foraging site that increases the probability of an encounter takj.ng p1ace"

DaLa presented j.n this t-hesls shov¡ tiraL _&.(i-g.+r., as a sil and wait predator, certainly exhibits behaviours pred-lcted from (1) above. 227

Ilhe actual beh¿rviours involverl in the selecLi.on of an anl:ush site (2.) were not exanined h<¡wever. WhetÌrer R.di.spar chooses siLes at randc¡m wÍLhin the pond or uses a far more el-aboraLe s1'stem of cues is unlcnol¿n"

Horvever, once positionecl at an ambush si-teu its duration of stay is affected by the number of prey encountered/ and, i-n all likelihood, tl're number of captures and subsequent f:eecl:Lng. Clearly both the hunger J-evel of the predator and the prey density play a signif.icant part in deLermining Lhe observed llehavlours associat.ecl r,¡ith lates of movement at and bet¡¡een ambush siles" Píanlca (I97/r) characLerized pretlaL<¡rs as ambush foragers or act-Lve foragers. Using this deflni.ti-on R.di-spar would be categorized as an aubush forager. Inove ancl l"lalsura (1983) shor"ed that in the si-rni1ar1y calegorized manLid, Paratenodera anqustioenni,s " hungry individuals sLart actíve search. They conclr:ded that there Ís a phase of active search even 1n so-called a.rnbush foragerso lì.dispar does not have a searchíng component (for prey) in lts predatory behavl-our" Horvever, the increased rate of rnoving bett+een ambush sites, as demonsLrated in thj.s thesis, ín association with hunger, is be1-ieved Lo const,itute a t searching I phase in the preclatory process; . The I searchj-ng I corf,ponent being for a suit.able ambuslt si.Le rather bhan for prey.

ff prey were randomly distribuLed and nor,'ing wÍ.Lltin the waLer l¡ody the selection r¡f anolher ambush site by R"dispar would not necessarily increase ttre probability of encounte¡:ing prey. I{or+ever, the fact that prey are not dístri.buted at randorn but. tend to form aggregatior:s often cont.aining hundreds of individuals, and that certain ar:eas of the dam seem to be tpreferreclr by the prey means l-hal: Lhe probabil:i.Ly of encourlLering prey can be increased. Thís is achjevecl by 228

R. díspar by increas-Lng the rate of movirrg beit+eeu am'bush sites, l-hus increasing the liltelihood of encountering a favor:rable s-ìLe flor ltrey encoumters.The interrelated prey encounLer--r¿rl-e and Lhe predaLo::rs curren¡ hunger 1eve1 are used as stimuli to esl-irnaLe the profitabiJ-ity of a site" Tf prey are absent, and the hunger 1evel hj-gh, the predator remains for a short peri-od before moving off to anoLher site; if prey are present and feeding cotilnences, the hunger 1evel dro¡rs and the predator rernai.ns at t.he sÍte.

Bearing the above in rnind, il- would be expected that the sit-arrd- wait. predator, like other predators, will at times be faced wíth an over-supply of Þrey¡ and that it could be exliected that selection rvould favour those predators that can capl,talize on such circumstancesn

R.dispar appai:ently proves t-o be just such a predaLor, having the capability to capture and hold more than one prey al- l-he sarne tirne, Ín addition to being abl-e, via its unusual extraction dynarnics, to ut:llize prey in an optimal manner.

I have tended to rliscuss much of the data in t-he light of current optimal foraging theory. Thls is ttot Lo say that I believe optimal foraging to be ryj.thout fau1L. Indeed, even if the ruode1s are successful in Lheir prerlictions, they do not necessarily l-ead to the conclus.Lc¡n that animals are perfectly adapted to the natural environment. As has been pointed out, evolutionary adaptation lags behl-nd environment-al change (see Krebs et a1. 1981; Maynard-Smith 1978). Nevertheless it has proved to be a vahrable tool in the quantiLative study of adaptat-Ì-on, especially i-n the field of decision'-r,rak:i-ng processes by anj-mals. A recenL corollary in examining, foraging patLerns is the 229 influencc of oLher acti-v:ttles ancl. how these activitics are associ¿rLed with foragj-ng" For exanple, M:iJ-inski- ancl Heller (1978) shor,uecl that the efficieucy of foragirrg sticltlebacks (GqË.tæfg-q!-ut¿q- ggLsaÇJls-) was reduced by the i-mposition of the neecl 1-o l*¡alch for a predator, ancl there are numerous exarnples of birds int-ersper:sing theÍ.r feecling wíth scanning to detect danger, at l-he expense of thelr foocl-gather:-i-ng rate (for exarnple,

Barnard 1980; Bertrarn 1980; B1-gar and Catterall 1981; Greig-Smith 1983; Inglis and L,azarus 1981; Lazarus 1978).

In R.rlispar, iL v¡as shcwn thal both the defensive or evaslve behavÍour associated rr'ith either predator threat or prey defence signi.fic.antly irrfluenced the predatory behaviour, principally the tÍrne-buctgetLing at foraging s-i-tes" I believe that Lhe ínfluenc.e of such risk facLors may prove worthr'¡hile avenues of future research in pre

This sLudy has, it is belÍeved, provicled a number of potentially sti-mulating and useful areas of research t-.hat either may be continued or act as a starting point for future research. Tn addlt:lon to those already mentionecl I believe of-hers are tvort.hy of note" Firstly, the age-dependent distribuLion in the field needs to be exarnined for evid-ence of the stgnificance of ovipositi.on- sÍte selecti.on, andfor support for Lhe Ipresence of preyI hypothesis. I'his rnay prove dj.fficult since R.clisÞar proved to be a difficult animal to deal. v¡ith in the field. Perhaps A*tþS.ngi ruay pr:ove a betLcr canclidate for such sLudies" 230

The un.usrral- feeding behavíour in r:egar:d Lo the exLraction dynami.cs, ancl the associat-ed diluuing of prey contenl-s by exLernal waLerr mây prove useful- in investigating the rvay in which pretlaLors assess the quality of ttre prey" The rvhole arca of pr:ey'- size selection by arnbush preclators, alth.ough only rnargÍna11y i-ouched on in this l-hesÍs, r,¡arranLs further examirral-ion" Can ambush precla'cors raffordt to be as selectÍve as acLively pursu|ng predators, especlal-ly if prey are encountereil infrequentl-y? This 1s only one of the questious thaL come to mind in considering prey size selection"

DaLa show tltat boLh prey encounter and hunger level sÍ.gnificantly affecl the rate of change betr+een, an.d the dural-ion of stay atr alì

arnbush slte. Extensiolr of this approach to examlne the effect of feeding on1y, in isolation fro¡n a previous encounter, will probably provide valuable data in identífying the methods that predaLors use in

assessing the profítabilÍty of an ambush site. The íncorporation of a longer experimental period, in conjunctíon with the use of model prey

will, no doubt, all-ov rrteJ:e conclusive sLaLements to be made in assessing the importance of encounters only, r.rithout subsequent feetling . R"dispar rvould be a useful subject for such studies.

Finally, in a concluding statemenL to this slrrdy as a who1e, I believe it appropriate to say thaL enquÍry aboul- preCatory behaviour in general, and sit anrl wait predaLors in particular, is in it-s infancy.

Th.e Í-mportance of such knowledge :Ln deternining cornplex predatorprey

relations has already been lnent:loned. In acldition, suclt an appnoach rnay be useful noL only in explaini-1g the relationships br.tL provideg a betLer utrderslanding of the way such assclcj-ations may have clevelopetl. 'æ4- t -'--+r' 5:';=b;+--:ú - =/-'â----: 2il

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CARLO, J.Il.rle and G.N. PELLERANO (1980). Ì'lj-croscopic anatomy of nax:l1lary glanCrs of Ranatra (Herniptera : .'JeteroPtera : Beunos 39:9-15. Ranatridae). Iþ¿":*- Ses* .E-t- Ait.eÐ.. CERRI, R.D. and D.F" FRAZER (1983). Preclation and risk in fora8,-i.ng rrrinnor*,s : balancing conflicling demalrds" A"g.-. NqL- l2lz 552-561.

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CLOAREC A" (1973b)" Variations de 1a structure de 1a patl-e ravisseuse de lìanat-ra linearis L. au colrrs clu devel-oppenent larvaire I. Uorffie.,resã"- þll.-.- So.c". g.Hq": -Ere.tu,Ug. 4Bz 16I-178.

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GRIFIUTHS, D. (1982). Tesl-s of alte::nate mr:del.s of prey consunpti-on by preclat-ors, using Ant-lion larvae. ,|" !3Ðn- Eqql' 51: 363-374 " GRIFFITHS, K.J. (1969). The importance of coiucidence in the function¿rl and nurnerical responscs of tr,¡o parasites of t-ire European pine sar+fly, Neodiprj,oL seJ-tf-f.ef . ÇîtÌ* l[t!ç¡:gL 101: 785-81B "

HAGUENDER, l-1. 97978). The aquatic Hemipt-era Ranatra. loJy¡-" .Ngtt._ 53: 35-36.

llALE, H"1"1. (1924). Studies in Aust.ralian aquaLj-c llerniptera" III-. Nepidae. Rug=. $"Àt¡9!-:- t'{i¡q. 2z 503-5:'20" HÂl'ÍILT0l{, hl.D. (1971). Geometry for: the self.i.sh þet:d, J"- !lfp-*" }i-,?-l* 31 : 295-'-311. and O.J. JÄCOBSEN (1978)" Net'¡ records of Ranat-ra 1r¡le¿;rr s IIANSEN t J,.P" j-o1.ogy L (llem" llet") front llorway, rvith notr:s on its b arrd

IIARP, G"I-" ¿rncl P"A. IIARP ( 1980). Aquati c macro-j-nvertebratcs of I{apanocca National- hrildl.ife Refuge, Arkanr:as, u"s.A" IIg-c* Ärkans., Acacl. Sci. 34:115-117" systems' Theorr,:ti ca1 HASSELL, l.I-P. ( 1976) " Ârthroporl predator-prey Jg Ecl " Iì.I'f .IIa1' ' " 71-93. s\i stems IIASSEI,L, l'l"P. (1978). The -4¿rc-'yg=- ef g¡[þrygd. Ilggelgrllçi: " Princeton lJni.versit-Y Press.

HASSELL, ì,{.P. and R.l'1. t'lÁY (1973). St-abil.ilf in insect host-parrrsite motlels. J:Alfu. I9q1. t+2'" 693-736" IIASSELL, ll.P. and lì"M. I'ÍAY (1974). Aggregation itt predators anql insecr: par:asites ancl its effect-on siãbifit'y. L.A¡1tl,"- -Ei9].t- 432 567 -594. of HASSELL, I'f .P. and T.R.E" SOUTI.iï'JOOD (1978). I'oraging strategies Insects. dn-¡t-" Ìey= Eçg!' rfu.ut. 9z 75-98' BEDDII{GTOI{ (l976) The col,rpotlents HASSELL, l,l.P. , J.ll. LAI,/TON ancl J.R. " of arthropod predation I. The prey death-rate" .ü- ¿'iilL- Lq-ol. 45: 135-164.

IIASSELL, M.P., J.ll. LA\,ITON and J"R. BEDIJII{GTOÌ{ (1977)" Sigrnoid funcl-ional responses by inrrertebrate predators ancì paras:Ltoids. J" Anim. Eco1. 46: 249-2'62.

HÂYNES, D.l-n ancl P. SISOJEVIC (1966). Predatory behaviour of Phj.loriromus lgf-llg (Ar:aneae : Thomisidare) . Çell, Ent-o¡¡r:rl-. 98: 113-133, by HEII,IGEI',IBERG, \,I. (1965a)" The suppressi.on of behavio'¡ral- actjvities -L 50: 660-'o72'' frighteníng stimuli. -rS,Æ=- SlfVS: tL HEILIGIINBEIìG, kl. (1965b)" The effect of e>:ternal stiinuli on t-he atiacl': reaclincss of a Cichlid fj-sh. Z* -Elg!* fiLvs-,i.gl' 4()z 459^h64"

HEILIGENBERG, 1,/. (1.966). The stimulatjon of territorial singitrg Ín house crickets' (Aghe-H. 3gjregtiEg) . 7-,o ]|ql]r,i,.- bgglt 53: rL4-129. HEINRICII, Il. a¡rd F.D. VOGT (f 980) " Âggr:egati.on and foraging beliav-iour 7 z of tr'/hirl-igig Beetles G inidae Belrav " lÈql $,o9ql¿kJ* 179-186. tt!'robj'erclntt HEI{NIGn E. (i968). Uber ì}eziehungen zt'¡-Lshen dem d¡it: schrvarzen Bohrenblattlaus (¿\p!+.Si Jg.]'-t*" Scop") und dcrn Stol'l't.ransport bei Vici-a. lg!¿q t-. Ai1.qjf.- P f'l-anzcnschilLz-. 4-. 15-76, on HTLDREW, A.G" anrl C.R" TOI,\'¡lSliND (1977)" The irrfl-uence of sttbsL::ate I1tIu:l9 the function¿rJ- response of. iLlç5![-9-¡¡q1i,¿- .qU'1¡l--€ìfiXL (Triclro¡rt.era : l'olyccntropoîìîlàã). lþ...'t-gg:t'iì, 3Iz ?'1*26"

IIILDRE\.I, A.G" ancl C.Iì. TO\'JI'trSEND (1982). Predert':rr:s aud prc:y itt a paf-chy envjroninent : ¡\ freshwat'-er study. ù- A¡j1u Ie{. 51: 797 -816, HINDE, R"A. (1959). Unj-tary Drives" .B-gf:g&U:," 72 l-30-141"

I.trNDE, R.A" (1970). : 4. qn!ilç.-:å-s- qf -e-!-[glsgr a¡.A conrparative iv-iìi11 Lìoclk Co. Ner,' y.crk. 2 Sydney" ition " g-Uç-f tllt{Dll, R"A. (19S2). åLh-çlg,:¿ : J-t-g. åqtp{--e- e¡-{ fc?}-jil-ig11q. -S1-:À sci.ences " Ironflra^lø-steî€îiïà-s-Ser-ies " Fontatta Paper. baclcs. HINTON H.E. (1961). The st-.ructure ancl funclj-on of the eg,g*shell i-rt 224'-257 the Nòpirlae (l{emi¡rtera)" J.- lgfu-.1þJ"i.g1*.- 7z "

HTNTON II,B. (1962). A key Lo the eggs of th.e l{epidae (llemiptera). PtqA" B* Ent-.- .Sgg- l*å{"" $)* 37: 65-68" by IIODGES C"l,l" (l.98l ). Optimal foraging in bunrblebce,s : Hun';i-ng expectertion" &t¡t" -Bglt4llt- 29t I166-l l.7l-.

I{ODGES, CnM, and L. L. \\r0LF (1981)" OptLi-rnal foraging i n bumbl.ebees : \{hy is nectar left behind in flor'¡ers. ßehav" Ecol. Sociobiol 9z 4I-44"

ITOFN'fAN , lnl.E. (1930), Notes on the life hist-ory of I'ggagf* S',þi-ngg. ¿g' 3L-:r7 Prog- Nît. IIr-="-t*" S9q- Ful<"tSgr æJ*- !ú" 3z " young IIOGÂN, J.A. (1971). The development-' of a hunger s¡r5¡ç¡¡t in chicks. Behavi our . 39: L2.B-2OI.

UOLLING, C.S. (1959a)" Tlte components of pr:edartion as ::evealed by a sturly of smail manrnal piedartion of t"tre lìuropear Pine Sar+fly" Cat]- JjtI.lgI}gL- 9Lz 293-320. HOLLING, C.S. (1959b). Some characLeristics of sj.niple types of preclation anrl parasili-srn. Cq¿r &:Lo:g4.r- 91: 385-'398"

HOLLII'lG, c.s. (1963)" An experinent-a1- com¡ronent analysis of population Drocesses. l4em" Ìintonol-" Soc. Can " 322 2 -32, I{OLLING, C.S. (1964). The anal-ys:'.s of conrpl-ex population proce'sses" Cu&- Entomol. 96 z 335-34'l

HOI,LING, C"S. (1965). The functiona] ::esponse of predators to prey dehsity an{ its rol-e ín niimicry and po¡lulatioit r:egtl1-ai-icn" Mqg. Þlg¡feL. S-o-q* çe].- 452 1-60. l-lOl,LfNG, C.S" (1966)" The functi-onal response of inverl-cbra'te 482 1-86' preclators to prey rlensity" U.çg" lit:Räf.q.L.- -$-p-q* þ['- HOI.LING, C"S"; R.L" DUNIIRACIi ancl 1,"¡f. DÏf'L ( 1976 ) " Preclal-or size anrl prey size : preslllned r-elationship i n t-he lnanLid llier<-rdu1a ðqg¡S .É". Saussure" C-e*- J*- ZogJ* 5/+l 1760-17

IIOLI'fES, S.J" (1905)' The reactj.otts of .!ggq-t¡g 1-o ligirt' -ù Çqg*- &it1{,- aUÈ Lü:ç!:- 15: 305.

H0LI'18S, S.J" (i906). Deathr-feigning in lÙ¿n-a,!r:í1' ¿. Çglp-t". Neur" anC .IlsJch, 16: 200-216" ll0l,l'1ES S"J" (i907). Observatj-ons on the young of Lql3g,tf+." !¡þ!* Bqll-- .tjgg$S" Il.qle. I2z 158-164 "

HOI,ST, B. von (1948)" Quantitatjve untersuchungen uber umsti.mmungsvorBange in Ze¡ntral-nel:vensystent, I" Der influss des Apperir uui clãs Gleic¡ger*ic¡tsuerhalterr bej. I,_tolo.¿l:y]åUl. Z. llfl-8l:- Phys;i c1. 31 : 134-148" I{0ppElÌl{EITo 1,,1. (1964). UnLersuchung,en uber clen Eini.luss vou }iunger and das ßeut-.efanguer:hal.ten tler larva von ô.9 ilrng^ SaLLigt-urg uuî -1'..!iå"-r clg.¡lqq- I'tlrtt. (Oclon:rta) . Z

INGLT.S, I.Rn and J. LAZ.,\RIIS (1981). VigiJ-ance and f l.ock size in brent geese : the eclge effect" 2.. Ti9.¡2-ÐSJto-lt 572 I93-2OO'

INOVE, T. and I'f" I'Í.ATSUIìÄ (1983). Foraging strategY of a l{antid, j.sms Par¿r tenocìera aup.ustiDennis S. : Ifechan of slui Lching tactics betr+ee¡r ambush and act-ive seer ch. Qw-g.Lgg+.e. -G-*:J¡-¡)-. 56 : 264--2tr.

JACKSON, R.lì. (L977). Comparative studies of lig!:ng. uld.Iþ-l-.-þq (Araneae, Dictynidae) : IIr Prey and liredatory Behavi'our" . Pst'c-be. 83:267-280. JACI(S0N, R.R" (1979). Predatory behavj-otrr of the social spirlcrr [*lgg qreP.alís :is iL cooperative ? Iqglgq Sociqgå 26: 300-312"

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JANETOS, A"C. ancl B.J" Cole. (1981) Imperfec¡l y oprti.mal aniarnls" Bel-ri¡¡'-t' Ecol.. Sociobiol 9: 203-209" art'hroporl J011NS0t{e D.}f ., B"G" ÄKP.lÌ ancl P. CiìOi'l'l-EY ( 1975 ) " Ì'fodê1i.ng predatÍon : trllas teful killing by darnrselflY naiads. [co] oqY. 56:1081-109J. JUBB, CnA., R.N. lluGIlES and T.A.P. RIIBINÂI,I-,T (198:l). Behavj-our:âl nlechanj-sms of size-selecLion by r:rabs, gql L') feecling on }Iussels, UÉilqe.edrl'l-ìs L' t* *- kql"- 662 81-87.

JULKA, J.l,f, (Lg77). On the possible seasonal fluctuaLiorrs Ín Èhe ' popul-ation of aquatic brrgs in a fÍsh pond' !:i-ç.l!gl i:¡.s-*lq" 11:139-149. influences KAISER,' I'I. (1983)" Small scale spatial heierogencilT preciatiou success in an unexpecterl rvalr : I'lodel exper:inients on (Acarina) Lhe functional response of pr:edalory nites " Oecolosia (iler1in.) 562 249-256" gI.P.- KALI'{IJN, A.J. (1971). The elective sens;es of sharl

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¡{ÁRSIIALL, W.E. and SEVERÏII, }1. (1904)" Some poÍ,r-rts j.n the anaLomy of Ranatra Ilgggr. Itg:." \'/j,Sc* 4ç-u-,1. I4z 4t37 " MARTIN, J"8., N.B. \,JITIIERSPOON and l1.lI.A. KI]ENLEYSIDE (1974). Analvsjs of feeding behaviour in the newt ¡lg!9.P[ç.kt-Ulg:" -v*i¿5þs¡c1¿Þ-' Ca¡e J.- Zgg! 522 277-28I.

MATHAVAN, S", J. ITTIIUKRISIfi'IAN and G.Å. IIELEENAI, (19t10). Stttdi-es on predation on rnosquito larvae bv t-he fish l'þç5gpq5!-ug ggPÊggq' llvclrobiol " 75: 255-258. IÍAYI'IARD-SI'{ITH, J. (1978). Optinizat-ion theory in evoluti.on" Annu. Re-v, Lol*:S.ji.g!- 9z 31-56.

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¡ li ìc:hrn'i ottr of art ( 1970) I1e. Ii-tq].g¡¿ sl* MECÏI, L.D" ' ]ffe flg!;[. -i ãîbtr"ðe¡ cîtvn N"tu n^4u¡-efeÈ Ë-Lgq't-rår- Natural ll#iä"t'v'I" York. genus of }'trepitlacr fr:ont MBl,lKE, A"s. and L.A. ST¿Ì{GE (1.964). A netr on the higher ã-lass;i.fícatioir clf t-ho ,¡\ustral.i.a rvith not-.es '752 ' family" Llogt &* åog,- gLÈ* 67-72' of the APhiri Pa::asi.te MESSENG]IR, P"S. ( 1963) R:i oclitnat- ic studies " L 1rrìctional cxsol-c-:tut'n" 1o Effecl;s of temPeratlÌre on tlie Praan llnLonol , response c¡f fenales to varY i.ng host densj-Lies. Çg,!tt 100: 72.8-7 4I " sensiLiv-it1' ÞlEYlÌR, D"R. ( Ig52). The stabili-ty of huruair.g,usl-atory durin {¡ chanqes i.rr the timä of food deprív¿rtj-on' '&- çSgt!'t- Phvsi ãl . P"i.l'ol . 452 373-376 I (1972) The sal.iva of ljenpitera. -L4-t-* l¡=-e-* Lit:-ç:'qJ*. MTLES, P.\f " " 183-255 " of a c-¡tt the I"1]LlNSITI, luI. and R. HBLLER (1978). Influence Predator optimal for sticklebac!is Gasterost-eus 2'Ì52 642-6i+4. aculeatu.s) " Studi-es combini-ng I'ÍILLER, N.E. ( 1957) Experiments on tnotivation. " pharrnac'ol'ogical techniclties' PsYchological, pirysiol-ogj-cal allrl Science. 126: 127l-7278. j'secl behaviour. :'-n f loc'ks of }IILL]lR P"L" ( 1966) " Synchron diving emiptera : l{ðtoneccirlzre) ' };gS"* .þ A) 4Iz 161-166. response : a test of MILLS, N.J. (1982). Satj.ation and the funcLíonal a ne\'I niodel. l]qgL Er-rt-grno1-* 7z 305-315'

MITTELSTA capture i'n lianti'¿s' - L]- i g.1-oÀ." (Ed" bY 13' ächcer) Pn*rlffi-cations, Oregon' behaviour of Arr!-L+Ê" liza::ds Zt I'f0l1Pù10ND ) T.C. (1931)" Prey-attack " Jis¡p5ychol" 56: 123*136. }lalpigi art Lubul-es; and lul0llAl"lED, U.V" (1977) Pirospo-1-ipi-d content in of" L haemolYmPh I+e. i (lleteroPtera :Ne i 16-19 " (1 Ì'lechani srns of preY selectj-on I I.ÍOLLBS M"C. ancl Iì.0" PIETRUSZKA 983). I of preY morDhology, behaviou¡: J, by predaceous sl-onefl-i es rol.es 25-'31' i and predator hunger. 0ecolop,ia Berli"n 57: ,i" (1903)' Trois nc¡uvelles especes du gener:a 8glg"!Ja l" :t MONT^NDON, A"L" Buli-"- r apparten;nt aux collecLions du ¡fusee civ1qur: cle G¿ireir. Sòc. nnt. Ita1. 352 20*24. I (1983) Foraging patt-erns ancl l-inle bu dgets of t-he crab MORSE, D"ll. val-iar (tìlerl<) spì-11ers X sL-icus enrerton-i Keyserling anrJ l"iisurnenn 1.1: oa o/t 4 ( Arane ae Tl-t omL s l-daeon l' o\/ers " .1. AlachnoI. I I{ORSE, D.ll. an.l P,.S. FRITZ. (1982) Expe::inental and obser:r'¿rti-ona1 stud.ies of ¡-:atch choic.e at djfferent scales b1r ¡¡¿ Crab Sp|dcr' Ecologl' 63 17?-- IB2 IligqUe-*u- -v-{:åq" " : " I'fuRDocil, I^l.l/¡" (1971)" The functj-onal re.sFonsc of predarlors" J" 4¡¡- ICet* 10: 335'-342,

l,fURDOcll, \,J"1,1. and A" oATEi{ (i975). Predatio¡ and populatir:rr stability" 4.4:*-ÐçPL Içs' 9: 1-131. j.n I'ÍURD0CIl, l,l"l,l" ancl A" SIH (1978). Age-depenrlent inLe::ferenc.e a p::edatory insect. ¿" !gi¡¡-liçg]*- /+7: 581-592.

I.{URI)0CK, G.R" and B.S. I'IURDOCI( (1972)" Ter,lacle length and Pr:ey c¿rpture as a function of starvation tine in Jl¡$fj1. Ame::, Zool ,, L2.z 7I9 " IruRTE, A. (i94l+). The r"olves of þlourrt }icKin1ey" !-auna

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NAKAMURA, K. (1982). Prey capture tacLics of a.spJ.de;.' : .An analysj-s basecl on a stinulation moclel for spiclerts grot+th. IlLq-= -P*EU 8..o1" 242 3O2-3I7 .

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PARSOIIS, M.C. (L972a). RespÍ.r'ator:y significance of the thoracic and abdomiial rnorphology of Egþqtg. and nqn-arqg-. -Lt }lgfplfol* Iiçttq. 732 163-194. PARSONS, M.C" (1972b)" Ilor:phology of t.he three antei'j-o:: perlrs of spiracles of Þ-g].qqq-¡E and þ1g!¡g. bl* J.- ZLILI*. 50: 865-876 "

PARSONS, M.C. (1973). IlorPholo gy of the eighuh abdomj-nal spi::acles olì Ranatra 7: 255*265. Be1ostoma and " J* lç!. lll-?.L.

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larva€: : Ä PASTOROK, R.A" (1980). Selcct-ion of prey by -CL¿rq"l¿g¡ig revierv and new evidence for Behavioural Flexibilit y' .[n-: j l,J,c" Evolut og q'rA ISgL-oJ,r Sl" Zry.led!t"-9i1" Çgtry+1tjSU Ed" I K"ffiil- uriïärsi.ti Prés= ltreru Jingland, 1980. Pp. -536*551r, rr I and sj.ze selection by I PASTOIìOK, R.À" (1981). Prey vu1.nr:rability { Chaoborus larvae. Eç:]gg.t. 622 1 311'-1324. The effec-t-' of early st-arr'¿ll,-i-on on --{ PÄUL, i\.J" and J"l'f . PAUL (19S0). . l-al-er feecJing success by King CraL¡ zoeae' J' Fs- l4ar' , []i ol-. 4 Ecol. 44: 247-75I. 1 I ( Competitioti, bodl' 51"u utt4 PE¡,RSON I).L* ard S "l-1. STEINUIIRGER 1980). Lhe rel¿rtive energy balance of aclult Tj-ger Beet'les. Èi19.¡-=- tlidl* .NeL.- 104: 373*311 .

PECI(,ARSKY, B.L. (1980)" PredaLor-prey j-nt-eractions l;eLt'reen Stoneflj-c'-s and I'lnyflies : Behavíou::al observa-tious' k].qgJ:. 61: 932* 943.

PIICKARSKY, B"L. (19S2). Àquatic insect prerlator-prey relat-ions. Þ.i'S'-:- Sç-jgc-g.. 322 26I-266 "

PELLERANO, G"l{, anrl J"}l,de cÄlìL0 (1977). Cornpound eye of \?æ!gg ggfg.qgg. J. Mj.crostrucl-ure of Otnnatj-diun" Pþ:^s+-¡:-.S-çc-' å (llçir¡rS.s ¿.t-æÐ-" 37z I4L"I49" PETTET, R.ll" (i902) " The eggs of tl're \later Scorpion, Ranatra -ls:ss.). Cano ÞL-uqgl" 34: 2I3" Ecoloçlv Ilarper ¿rnd lìor'¡ : IIer'¡ Yorlc, PIitrNKA, E.R. (IC)74)" Eypb!@,.f:" " 2nd Edi-tion" POLIIEI1US, J"T" (1976) " Notes of I'lorth Anericau Nepidae, Ilemipt-era, Ileteroptera. tt"-:|..* En-t-oqq-l* 52: 204-208. POLIlEllUS, JnT. arrd I'J.K. RllISEl'l (1976). Aquatrl c Hemi pter:a, fìeteroPtera of the Ptrilippines. Kg!]g-qgl. 5:259-294. (llemiptera POLLARI) , D.G. (1973). Pla¡rt penetraLi.on by feeding aphids Aphi-doidea) : A revierv. nuU.:- åryÇ-r- þl* 622 63-714. of PO\ÙELL , G.V.N, (7974)" Iixperimental analysis of tire sc¡cía1 value flocking by sl-ar.Liiigs SLU::nus y-U,.:*flg) in reJ at:Lon to predation and foraging . Anim" þl.eJ*_ 22: 501:s05. PRAÌ,lElì, D" (1"964). Nematode-t::apping fungi" lqis:çg" I4tt': 382*388" j'n PUCITKOVA, L.V. (1978). Arlapt-i-ve features of 1eg struc't-ure t-he (iÌng1-ish t::ansl. heteroptera. bt*o_rlgl* ]1*._ 57 : I 89.-196 " " Entornol" Obozr. 572 272-283). PULLIAII, H.R. (1973). 0n the aclvantages of flocl"ing" J--. &g* Þi'q1* 38: 419-422.

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RIECI{ERT, S"E. (i974) The pat-tern of local r¿eb clj-strj.l¡utjo¡r j-n a deset:t--. snider: mechanisms and seasonal- variati.ons. .To /\nin. Ecol . 1+3: 733-7 46. RILEY, C.lì. (I92L). Responses of the large \'later-St::ider, Ggffig Ig&jlg-ig SaJ', to contact and light. Atrlh ll¡:Sqllq.Lt- Sç,ç. Ägg5* I/+223L-289 " RILLING, So, II. I'{IT'IIJLSTAEDT and K"D" llOlìDER (191i9). ft'oy recogniLion in the pl'aying rnantis. Þgþ::i-gg"L" 14 168'"'184" jtr ROBIÌISON, l.I"H" (1975) Tha evoltrliorr of predat:or:y behaviour: " (l-1arenclorr spiders. I+. : iruncti.on and lìv olui:ion of Tìc':havictt:: " I p" 292-3J2-" Press : Oxford " Chap.

ROBll'trSON, M.l{. and II. Ì'ÎIRICK (1971)" The pr eclarLoi:y liehav-i our of the c1¿rrri res (r\raneae : Ar:erneidae) Golden rveb spicler ' !{ePLiJg " lf:tghg. 76: 487*501 "

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