Sz.Ç^- "S\ \¿/ Dafoel\'Oü9' Alan Campbell Juiy 1969

$Sz.Ç^- "S\ \¿/ Dafoel\'Oü9' Alan Campbell Juiy 1969$

THE PREDATORY BEHAVTOUR OF THE LARVAE OF COLYMBETES

SCULPTTLIS (HANNTS) AIO GRAP}IODERUS OCCTDENTALIS

HORN (COLEOPTEIRA: DYTISCTDAE)

A -LNESIS Presented to The Department of EntomologY the Facutty of Graduate Stutdies and Research University of Manitoba

In Partial Fulfillment of the Requirements for the Degree Master of Science

./, \-- L.'y Sz.ç^- "s\ \¿/ DAFoEL\'oü9' Alan Campbell Juiy 1969

c Afan Campbell L969 ABSTRACT

The purpose of this thesis was to sttrd.y and compare the predatory behaviour of the larvae of Colymbetes sculptilis (Harris) and Graphoderus occidentalis Horn, and to show how factors su.ch as prey size and movements affected Prey capture. Laboratory observaticns \üere conducted on the developmental rate, locomotor h,ehaviour, PreY detection. prey capturre techniques and feeding behaviour of these two species. The complete life cycle through 99g, three larval instars, pupa to adult, requires 38-40 days for both species r'eared at 20oc t zoc. Although Colynt]cetes larvae can swim by moving the legs they are usually found to cra.wl over a substrate, such as aquatic vegetation. Graphoderus larvae almost continuously swim around in the water by moving the l-egs. The larvae of both species must thrrust the head forward and capture prey with the mandibles. Although the Colymbetes larvae must thrust the whole body at prey hanging onto a su-bstrate the Graphoderus larvae can capture prey by a rapid thrust of the thorax and head while swimming. The Graphoderus thírd instar larva can strike at prey at a greater distance, rnore rapidly and are more successful in thej-r attacks on mosquito larvae than the Colyrc¡betes thiro ii l- l- l- instar larvae. The size of prey affected. the ca.pture success and handling times of both species. The larger insta.rs required a shorter handling time than the smaller instars when feeding on the same size of prey. Some of the components of the functional response of a predator to prey density, such as hunger, described by Holling (1966), are discussed. ACKNOWLEDGEMENTS

Grateful acknowledgement is extended to my advisor, Dr. A. J. Thorsteinson, Professor, Department Head, Depart- ment of Entomology, University of Manitoba for his con-' tinual interest and most valuable. guidance throughout the study anci in the preparation of this thesis. The writer is also. grateful to Dr. R. Brust, Associate Professor' Department of Entomology, University of Manitoba for his i,-nterest and valued counsel-. The teaching staffr. graduate stu.dents and technicians of the Department of Entomology, University of }îanitoba víere very helpful throughout the course of this study. Sincere appreciation is extencl-ed to Mrs . J. J. Cameron f or the typing of this thesis. Acknowledgment for confirmilg the writerrs ic.entifi- cation of the ai'ult d.ytiscids. goes to Vf. J. Brown, Canada Department of Agriculture, Entomology Institute, Ottawa, Ontario. This study was made possible by a grant from the National Research Council of Canaoa and a University Fellcw- ship awarded to the writer by the University of Mranitoba.

IV TABLE OF CONTENTS

CHAPTER P¡!GE

Ï TNTRODUCTTON 1

ÏÏ LITERATURE REVIEW. 2

A. The Dytiscidae 2

1. Systematics. 2 2. Evolution. 2 3. General Biology. 5 4. Behaviour. 6

B. Tnsect Fredation B

TÏT MATERTALS AND METHODS. . L2

A. The Predators. . T2

B. The prey 13

C. General Procedure. 16

D. Measurements 1B

E. Photography. L9

IV LIFE HTSTORY AND DEVELOPMENT 20

A" The Adults 20

B. The Eggs 20

C. The Larvae 25

D. The Pupal Cell and pupa. 27

V LOCOMOTOR BEHAVTOUR. 2B

VÏ PREDATORY BEHAVTOUR 34

A. Introduction 34

B. Prey Detection 4A

V vt CHAPTER PAGE l. Morphological f'eatures . 40 2. Methods'. . 42 3. Results and Discussion . 44 4. Conclusion 52 C. PreY CaPture 53 1. General Prey Capture Behaviour 53 2. Details of Strite. 54 3. Position of Predator and PreY at Time of PreY CaPture. . 66 4. Strike Success 69 5. Discussion B0 D. Handlíng of PreY 81 E. The Non-feeoing ancL Total Tinie 90 VII DISCUSSÏON A}]D CONCLUS]ON. . 94 VI]T SUIVMARY. . 97 APPENDIX I - List of SYnJrols 101 FEFERENCES CTTED 103 LTST OF TABLES

TABLE PAGE I. The relative sizes of all predators and prey used in the predation experiments. . 15 II. Egigs 1a.id by four Graphoderus occidênt'alis females over a period of 5 daYs 26 ITI. Some measure¡n.ents of parts of head and body length of three Colymbetes anci three Graphod.erus third instar larvae,

in niillimeters. . 4L ïv. Response of Colymbetes third instar larvae starved 12 hours to artificial stimulation- 45 V. Appendectomy treatments of Colymbetes and Graphoderus third instar larvae used in prey detection experiments at 20oC ! 2oC. 46 VI. The numbers of A. sticticus (Ys4) fourth inst¿Lr larvae eaten by append.ectomized colymbetes (Rc3) third j-nstar larvae in the light or dark d.uring 12 hours 47 VTI. The numbers of A. sticticus (Ys4) fourthr instar larvae eaten by Graphoderus (ng¡) third instar larvae in light or dark during L2 hours 49

v'aa viii

TABLE PAGE (SS), (Th) VIII. The strike success handling time ' and non-feeding time (tnt¡, toLal time (TT) of appendectomized Colyimbetes (Rc3) thiro instar larvae feeding on A. sticticus (Ys4)

fourth instar larvae. . 51 ïx. Tiie strike r¡-easurements of ColynLl¡etes sculptilis (Rc3) third instar larvae

from pictures taken witti a Bolex 16 mm

camera at 64 frar'¡es per second. . 58 X. The strike measurements of Graphod.erus occidentalis (ng3) thirci instar larvae from pictures taken with a Bo1ex L6 inm camera at 64 frames per second 63 XI. The posítion of the preoator, Colymbeleq (Rc3) and. Graphoderus (ng3) thiro instar larvae, and the region of PreY, A. aegypti (Ya4) fourth instar' larvae, pierced by the mandibles immediately after

a strike. . 6B (SS), (Th) XII. The strike success handling time ' non-feeding time (Tnf), and total time (TT) of Colymbêtes sculPtilis (nci) feeding on A. aegypti (Yai). 72 ix

TABLE PAGE (Th), XTÏÏ. The strike success (SS) ' handling time non-feeding time (Tnf ) , and total tin'e (TT) of Graphod.erus öceidentalis (ng3) feecling on å. aegypti (Yai) . . 73 (Th), xïv. The strike success (SS) ' handling time non-feeding time (Tnf), and total time (TT) of Graphoderus (ng3) third instar larvae feeding on â. vexans (Yv4), corixids (YC), (YD), Lêstes sP. (YL) Daphnia sp. ' Mayfly (YM) 7B XV. Colymbetes (Rc3) third. instar larvae feeding on 25 L. aegypti (Ya4) fourth instar larvae B6 XVT. Graphoderus (ngS¡ thiro instar larvae feeding on 18 A. vexans (Yv4) fourth instar larvae. B8 LTST OF FTGURES

FIGUR.E PAGE 1. A. The stages of Colymbêtês 'sculptilis found in Winnipeg pools during 1968.. 2l B. The stages of Graphoderus occid.entalis found in Winnipgg pools during 1968 2l 2. A. The development of ColYmbetes scülptilis reared at 20oc ! zoc. 22 B. The development of Graphoderus occidêntalis reared at 20oC ! 2oC. 22 3. A. The functional response of golymbgtee third instar larvae feeding on different densities of å. aegypti (Ya4) fourth instar larvae. . . 39 B. The feeding rate of Colymbetes third instar on å. aegypti (Ya4) fourth instar la-rvae. . . 39

4. A. Cross section of container (70x50 mm crystalizing dish). . 67 B. The regions of a mosquito larva that rüere attackeci by the predators . 67 5. A. The strike success (SS) of thre three larval instars of Colynìbetes sculptilis (Rci) capturing the different gges of five immature stages of A. aegyptí (Yai) 7L

x xi FTGURE PAGE B. The strike success (SS) of the three larva]- instars of Graphoderus 'occidêntãlis (ngi) capturing the different 39es of five immature stages of å. aêgYpti (Yai). . 7L 6. The strike success (SS¡ of Colyn'rl¡etes (ncS¡ and Graphoderus (ng¡) third instar larvae capturing the four larval stages of å. aegypti (Yai), and the fourth instar larvae of A. vexans (Yv4) ' A. sticticus (Ys4) , and ê.. fitchii (Yf4) 76 7. The strike success (SS) of Graphoderus (ng3) third instar larvae feeding on â. aegyÞti (Ya4) and å. vêxans (Yv4) fourth instar larvae, Daphnia sp. (YD) adults, corixid (YC) mature nymphs, mayf,Iy (YM)mature nymphs, and Lestes sp. (YL) young nymph 79

B. A. The handlilg time (Th) of the three Col)rmbetes (Rci) larval stages feeding on the five immature steiges of â. aegypti (Yai) 83 B. The handling time (Th) of the three Graphoderus (Rgi) larval stages feeding on the five immature stages of A. aegypti (Yai) B3 9. Some individual observations of Graphoderus (Rg3) third instar larvae feeding on the x].l

FIGURE PAGE four larval stages of A. aegypti (Yai). . B4 10. The handling time (Th), non-feeding time (rnf), and total time (TT), of Co1Vmbêtes (Rc3) third instar larvae feeding on 25 \. aegypti (Ya4) fourth instar larvae. . B7 11. handling time (Th), non-feeding time (Tnf), and total time (TT), of Graphoderus (Rg3) third instar la-rvae feeding or: 18 A. vexans (Yv4) fourth instar J.arvae. B9 LTST OF PI,ATES

PIê..TE PAGE I Some of the apparatus used in the pred.ation

experiments. 14

II The stages of Colymbetês sculptilis (uarris) 23 III The stages of Graphocierus occidentalis Horn. . . 24 fV Swimming movement of d.ytiscid larvae 29 V The avoidance reaction of a Graphoderus occidentalis third instar larva when

disturbed. 31 Vf The dorsal, lateral, ventral views of the ïieads of Colymbetes and Graphoderus third

instar larvae 32 VII The strike of a Colymbetes sculptilis third instar larva (lateral view). 56 VIIT The strike of a Colymbetes sculptilis third instar larva (lateral view). . 57 IX The strike of a Graphoderurs occj-dentalis third instar larva (lateraI view). . 61 X The strike of a Graphoderus occidentalis third instar larva (lateral vj-ew) . . 62 XI The strike of a Dytiscus sp. third instar

larva (lateral viev¡) 65 XII The immature stages of Graphoderus occidentalis (R1, 2, 3) and A. aegypti xiii xiv

PLATE PAGE (Y1' 2, 3, 4, P) 70 XIII Some natural or pctential prey of Graphoderus larvae collected from Winnipeg pools. . 77 XIV A Colyrnbetes (Rc3) third instar larva removing the remains of prey (mosquito larva) from mandibles with forelegs. . 9l CHAPTER I

INTRODUCTTON

The larvae of the Dyt.iscid.ae are perhaps the most voraciol-ts predators in the animal kingdom and among the few animals which have preoral digestion. These larvae have grooved or hollow mandibles which they use to capture and inject glandular secretions from. the. gut into their prey. These secretions kilI arrd ciissolve the prey which is t''hen. sucked back into the larr¡a. Although the. general predatory behaviour arrd dytiscid larval feeding mechanism has been known for many years (wilson, 1923¡ Usinger, L963) , this information is simply anecdotal and detailed studies on the prey capture behaviour of any one species of dytiscid larvae are sorely lacking. The purposê of this thesis is to stud'y and. compare the predatory behaviour of the larvae of two dytiscid species,¡, Colymbetes sculptilis (Harris) and GgepTleq us occidentalis Horn, anc1. to show how factors such as prey movements anO size affect prey capture and predator-prey relationships. CHAPTER ÏÏ

LTTERATURE REVIEVI

A. The Dytiscidae 1. Systematics The systematics of the adults of the family Dytiscidae has been well studied. Some of the author's of keys and descriptions of the adult dytiscid species ãrre as follows: Wallis (1939), Wil-son (1923) , Balfour-Browne (1950), Hatch (1953), Pennak (1953), Young and Severin (1956) ' Usinger (1963) , Gordon arrd Post (1965) . Some authors (Leech and Chandler in Usinger, !963, and others) have descrj-beci the dytiscid larvae to genera. However, there is practically no information on the species of the dytiscid larvae except for a few descriptions (Leech and Chandler in Usi-nger, 1963, Ga1ewski, L963) and a thorough systematic study of the larvae of this family is needed. There are no adequate descriptions of the larvae of Graphoderus sp. and Colymbetes sp. and the only waY, at present, to recognize the larvae to species is to trace the animal's development to the adult stage and ioentify the Iatter. 2. Evolution Galewski (1963) studieci the taxonomy and ecology of

2 the immature stages of central European species of Rhantus Dejean, and found. ecological aciaptations between species and within the three larval stages of any one species. He considers there i-s an evolutionary trend: (1) increased numbers of setae on the locomotory organs, and (2) longer palpi and antennae. He found larvae that hunt swift moving crustacea in deep water to have Jonger palpi and antennae. while larvae in small water bodies, which catch less agile prey (e.9. benthic orga.nj-sms such as larvae of Tendiped.id.ae or Oligochaeta)r âs a ru1e, have shorter sensory organs. Considering the ecological adaptations of the Rhantus larval stages Galewski (1963) found the three larval stages of any one species to have morphological differences. For example, the first instar larva shows primitive characters conmon with larvae of Agabus Leach (which is consid.erecr to be a possible ancestor of the genus Rhantus) such as chaetotaxy of the cerci and legs, and. are poor, clumsy swimmers strongly bound to the bottom and aquatic vegetation. On the other hand, the third instar larvae are the most advanced in their adaptivity to aquatic life and are the best swimmers least dependent on any support. Galewski (1963) found that the first instar larvae of all the species of Rhantus to differ considerably morpholcAically, while the se.cond and third irrstar larvae showed greater uniformity in their morphological features. He explains the progressing convergierice in morphology with the fact of the gradual "stepping into the water" by the Iarval stages as they evolve. The first stage larvae are edgewater d.wellers depending largely on plant support and seem to inhabit the most diversifiea sites so they under- went the greatest morphological changes. The 2nd and 3rd stages went progressively further into the water gradually freeing themselves from the need of sup¡;ort. The deeper and more homogenous conditions of the water body could naturally produce fewer morphological differences between the species. Balfour-Browne (1950) separates the British Vüater Beetles into three main groups (tribes) . He considers the most primitive to be the Hydroporines (Hydroporus, Hygrotus, Bidessus and others); the Colymbetines (Copelatus, Agabus. Platambus, Ilybius, Col].mbetes, Rhantus) are more specialized and the Dytiscines (oytiscus, Cybister, Hydaticus, Acilius, Graphoderus, Thermonectus) the most specialized.. He considered the intermediate position of the Colymbetines indicate not an intermediate stage ín a phylogenetic 1ine, but an evolutionary trend along which the Dytiscines have already advanced farther while the majority of the Hyd.roporines have lagged behind. Not only dies he consider morphological features but physiological needs in comparison of these three. groups. For example; "the Hydroporines obtain air from that dissolved in water, ât least until 5 their final stages, the colymbetines are only subaquatic for the first few days after they hatch from the egg. About half way through that stage they begin to come occasionally to surface for air. The larvae of the Dytiscini, on the other hand, depend mainly upon atmospheric air from within a s¡r-ort time of emerging from the egg" (Balfour-Browne, 1950, p. 28). 3. General Biology The strong-swinrming Dytiscidae are the dominant water beetles in most parts of the world. The voracity of both adult and larvae of these beetles is wel_l known; the dytiscid larvae will attack and feed on practically all aquatic organisms from small crustacearis, mosquito larvae, tadpoles, and fish to the larvae of their own kind. There are numerous reports on the general life history and. ecology of the Dytiscidae, and some of the authors are listed below;

Wallis (1915), Wilson (L9231 , Mozley (1932) , Hinman (1934) , Lairo (l-947) , Dineen (1953) , Hodgson (1953) , pennak (1953), Zimmerman (1960), Macan (1961), James (1961, 1964, !965,

L966, L967) -í Galewski (1963) , Usinger (1963) . The life history of dytiscid beetles is well dccu- mented in Leech and Chand.ler (in Usinger LgG3) and pen.nak (1es3). The dytiscids, considered in this thesis, are found in lentic habitats especially eutrophic and distrophic lakes, ponds and pools. Indeed, this type of water bcdy is 6 usually found tc be relatively smaller, shallcwer ancl more shelte.red than the larger lakes, ârrd it has a high concerntre- ticn of nutri.ents supportilrg a rich ana well defined biotic community (Wil-son, 1923; Leech and Chandler in tïsinger 1963). 4. Behaviour The locomotory abjlities of dytiscid larvae vary, bu.t, in gerrera'l , they are eith.er e:;cellent crawiers anð,/or swimmers. Most larvae use their legs (usurally frin.ged with hairs) for either cr'awling o\¡er vegetaitiori or sv,'inrrninE tïi.rough water. Wilson (1923) found the larvae of Î;accophilus sp., in an aqllarium, to "crawl about over the plants very slov;ly, feeling about with each foot befor:e putting it forward arrd keeping the abdomen elevateci at an angle of [,5a.' Wj-lsorr (1923) considerec'i larvae of Pyt'iscus sp., Thermonectus sIi. anc.l. Acilius sp. to be active swj-mmer's. WiLson found the larva of a Thermonectus sp. "swims with an even and. rapid motion, propellilg itself by short strokes of thLe legs always keeping the ak;domen curved upward, ofte,n at 45o when disturh'ed, it snaps its body-, often to quite a di.starr.ce, by- a suclden contraction of the l.ongitudinal muscles of the aT..¡dominal segÍnents . " Metapneusti-c respiration is commori in the dytiscid l-arvae, especially those of Colymbetes sp. arrd Graphocierus sp., that j.s only the terrLinal s¡,iracles at the erid of the abdomen function for air jntake and. exh:l-lation (Vüiggteswcrth, 1965). Colymbetes larvae breathe by raising the tip of tlLe 7 abdomen to the surface of the water while stí1l standingr on the plants. However, in clear water they are obJ_iged to swim to the surface in order to breathe. Wilson (1923) writes that a Thermonectus sp. larva "thrusts its tail freguently to the surface for breathing a-nd seems unable to remain long under water. vühile at the surface it holds itself (and hangs) in place by depressing the cerci down upon the surface fi1m. " T'he pred.atory and feeding behaviour of dytiscid larvae has been described by various authors (Blunk, L923i Wilson, 1923; Lorenz | 1952; Lee, Lg61) , but not at. great depth. There are no descriptions of the pred.atory loehaviour of the larvae of colym.betes sp. but wilsorr (1923) does describe the attack on prey made by Thêrmonêctes sp. (similar to Graphoderus sp. larvae). He says, ',the larva can flex the body sucl-dsnfy at the first abdominal segment and. straighten it at. great rapidity . the result is a snap.rr The Thermonectes larva hiere found to rsnap' at, capture and feed on prey such as mayfly larvae, Tropist'ernus larvae, notonectid and belostomatid nymphs. Wilson writes "after seizing its prey it swims about, holding the prey i-n its mandibles, and jerking its head and prothorax from side to side this movement assists it in thrusting its mandibles into the hard skin of its prey. After the rnandibles are fixed satisfactorily it remains quietly suspended from the surface film and sucks the juices out of its prey. I !ühen j-t wishes to obtain a new hold, it ?gain swims about anc shakes its prey and then quiets down, repeating this until all the juices harre been extracLed and nothing but the sliell of the body is left. " Wilson found Thermonectes larvae first selected the Tropisternus larvae as prey before feeding on any of the other prey. Schöne (1951) found larvae of Acilius (similar to Graphoaerus sp) and Dytiscus to possess a dermal light sense in the tergites which enables thero to retain dorsal light response after blinding.

B. Insect Predation There ar'e a number of revielus and papers of field and laboratory e>',periments that are relevant, in varic;us ways, to the theme of this t-hesis i.e., the bekravioural aspect of insect predation. These articles are: Elton (1927) , Pcpham (194C, 1942), Hoogson (1953), Holling (1955, 1958, L959a, b, 1961, L964, 1965, 1966), Mittelstaedt (1957), Riling et al (1959), Roeder (1959, 1960), Edwaris (1963), Ocium (1963), Morris (1963) , Pritchard (1964, 7965) , Haynes and S-isojevic (.1966) | Humphries ano Dríver (1967) , Lee (L967), Roberts et al (1967), Young (1966, L967) , Ellis and Bord.en (1968), Slobodkin (1968) , Mukerji and Le RorLx (1969) . In a review Edwards (1963) discusses the modes of predation by arthropod predators; there are two broad groups (1) those that. go forth and hunt their prey, anci (2) those that sit and wait. In the first. grciup, there: are varying 9 grades of predation, from the ranäom blundering intc' Prey of predatory mites and. some insect larvae to the delicately controlled hunting flights of the dragonfly. The second group also varies from the web spider to antlion la.rvae with their pits. Ed.wards considered that i-n both active and inactive predators vision is the principal means of locating prey although tactile and olfactory sense may also be important-. Holling (196f) considered the major factors affecting predation were: " (a) Prey density, (b) predator density', (c) characteristics of environment (e.9. number ano variety of alternate foods), (d) characteristics of prey (e.9., defence meclianisms) , (e) chara.cteristics of the pred,ator (e..g. attack techniques)." He believes the two variables, prey and predator oensity, are universal variables i.e., the basic components which are always present in predation. (c) (d) (e) present Th.e last three f actors, , , ' c¿in be or absent and are considered subsidiary components which are present in some situations and not in others. Holling (f966) analyzed the first factor, i.e. the functional response of invertebrate predators to prey density, by breaking it into four comPonents with oiscrete subcomponents (symbols are shown) a.s follows: 1. Rate of successful search (TS): (a) Lhe reactive distance of the predator for prey, i.e. the maximum ciistance at which a predator will react 10 by attacking prey (D),

(b) speed of movement of the precì.ator (VD) .

(c) speed of niovement of the. prey (VV1 , (d) the capture success, i.e. the proportion of prey, coming close enough to be attacked, that are successfully captured (SS). 2. The time prey are exposed to predators Tr: (a) the time spent in activities not related. to feeding, ê..g. nonfeeding periods (Tn) , (b) the time sperrt in activities related to feeding

(ra) .

3. Time spent handling each prey (T") : (a) time spent pursuing and subduing each prey (TP), (b) time spent eating each prey (TE), (c) time spent in a 'digestive pauser after a prey is eaten and during which the predator is not hungry enough to attack (TD).

4. Hunger (H) .

Holj.ing (1963) found that D, TP and T'D to vary with hunger v¡hile he considereó V, S, TN, TA, TE, to remain constant. Holling (1961) considered the subsidiary factors could change the magnitucle of the functional, response of the pred.ator to prey density (or vice versa) but the form of this response v¿ould remain constant. Exam.ples of these three factors that affect predation are: (c) characteristics 1_1 of environment: temperature change, palatability of prey. (e) characteristics of predator: food preference, ability to dete.ct, capture and kill prey, efficiency anC speed, of attack, efficiency of sensory receptor, i.e. visual, olfactory, ta-ctile sense. (d) ckraracteristics of prey: type of distribution of prey, s¡;eed of evasiveness of prey, inconspicuous habits, i.e. colouration, mimi-cry of unpalat- able prey, which modify strength of stimulus used by predator:s in locating prey. Both the movement and size of the prey affect the pred.atory behaviour of the predator. Rilling et al (1959), Holling (L964, 1966) working with praying mantids, Pritcharci (1965) with dragorrfly larvae, Lee (1967) and Young (1967) with dytiscid larvae, found that the optJ-mal movement and size. of the prey was a prime factor in elicitilg a predatory response and obtaining a su.ccessful capture. The factors that affect pred.ation vary in slrer! situation, but there are basj-c coÍrponents which are always present in predationr âs alreaciy discusseci. Some c:f the above factors will be considerec'T- further in the thesis. CHAPTER ÏÏÏ

MATERÏALS AND METHODS

A. The Predators The three larval instars of the two species of dytiscid beetles, ColYmbeteq sculptilis (Harris) anci Graphoderus occidentalis, Iiorn, v¡ere used in the predation experiments. The larvae of bot:h these species \,vere oþtained by two methods: (f) The adult Graphoderus females, collected from Winnipeg ponds, laid eggs in the labor:atory aquaria. The larvae that emerged from these eggs \^¡€r:re used in both developmental and predation studies. (2) All three larval j-nstars were col-lected f rom Vfinnipeg ponds, identi- fied to genus in the laboratory, and used in the experiments. The dyti-sc:id larvae are comparatively simple to ioentify to. genera, but ident-ification of the Manitoba species is difficult as proper descriptions and keys are lacking. Holúever, both C. sc:utptitis and 9. occidentalj-s are the predomina-nt species in the Winnipeg environs. Thus it was not considered necessary tci try to identify the larvae to species as there was a small probability of using species other than sculptilis and occidentalis in the pre- d.ation stud.ies.

L2 13 The dytiscid. larvae were stored ín white plastic pans (30x20x10 cÍr, see Plate I) with pond water and aquatic plants. These pans were placed in a wooden trough with cold running tap water (5oC to 13oC) to reduce the activity and development of the larvae. The size of each of the three l_arval instars used in the experiments i-s listed in Table I and v¡as measured. by larval- length. To standardize thiese three sizes, for each of the two predator species, the largest size that a larva could attain in each irrstar was used. The symbols useci to describe these predators are shown in Tabl-e f and Appendix r. The predators \,vere preconditioned before an experiment by being; (1) allowed to feed continually so that there would be normal feeding behaviour in the experiment, (2) a-r"lowed to feed normally, but starveci twelve hours before the experiment so they would be initially hungry. The predation experiments could be carrieci out only when the dytiscid larvae were available i.e. from mid- April to late August 1968 and L969 (see Figure 1).

B. The Prey The sizes, stages ano s¡.''ecies of prey used in the predation experiments are listed in Table f. A permanent laboratory crrlture of Aedes aêgypti (Linn) lvas continually maintained so that the immature stages were always available. â. vexans (Meigen) gggs v,7ere obtained from wilo adult females arrd hatched when the PLATE f

Some of the apparaturs used in the predat.ion experiments. The apparatus shown are: Stop watch; pen; data sheet; the v,¡híte plastic pan is 30x20x10 cm; the containers are pyrex crystalizíng disties 70x50 mm. ,.::l 1 ,./.1 ::| ,t /¿ 'ü. tl¿.

iq. ,_T¡i -.i w%,,"

?(+.'w

./ ,l¡ ,#' ilN â c l¿ YJ

þe- iÆ

%,,'l' 't. 'l:'¡

{îr 15

TABLE T. The relative sizes of all pred.ators and. prey used in the predation experiments. Average of 20 irrdividuals. Symbols are âIso shov¡n.

Length Vüeight Name Stage Synbol fltm mgm

Colymbetes sculptilis Ll. RC1 6 .97 il ll L2 RC2 73.L4 ¡t It L3 RC3 21. 50 Graphoderus occidentalis L1 Fgl 9 .43 tf T1 JJL Rg2 15. B9 tf L3 Rg3 20.97 Aedes aegypti L1 Yal 0.0076 I il L2 Ya2 0.0s40 t1 It L3 Ya3 0.2600 lf tr L4, Ya4 B. BO 0 .9 500 il il Pupa YaP 0 .9600

Aedes vexans L4 Yv4 7 .03 0.6360

Aedes sticticus L4 Ys4 7 .20 0.8920

Aed.es fitchii L4 Yf4 9.90 2 - 4600

Lestes sp. Y.nym ]aL i .20

May fly M. nym YM 8.00

Corixid M. nym YC 5 .10

Fairy shrimp Adult YFS 12 .10

Daphnia sp. Adult YD 2.5

Abbreviations: Y.nym Young nymph

M. nym Mature nymph I6 mosquito larvae were required. A. fitchii (Felt and Young) and. A. sticticus (Meigen) fourth instar larvae wer€: collected from Vüinnipeg ponds in May, identifieo and used in the experiments. All mosquito larvae \,vere cultured. in white plastic pans (30x20x10 cm) with distilled water and crushed ciog food at 20oc t 2oC. The largest and most uniform size of any one larval instar was used. The la.rvaI inst--ar was determined kry head capsule measurements, siphon and. dorsal saddle si.ze. The prey, other than mosquito larvae, such as Fairy slrrimps, Daphnia sp. corj-xids etc. (see Table I), were collected from Winnipeg ponds arrd stored separately in white plastic pans until required for the experiments. The symbols useci to describe these prey are shown in Table I and Appendix I.

C. General Procedure The. general prccedures for all the experiments will be described. here, but any particular variations will be revealed when the results are discussed. The laboratory pred.ation experiments entailed the use of Pyrex crystallizing dishes (70x50 mm) (see Plate I) which will be calleci I containers t f or the purposie of brevity. A 4 cm distilled water depth in the containers and. a corrstant temperature of 20oC ! 2oC (Iab. te:mp.) was maintained throughout the experiments. A suLrstrate of L7 nylon screen (L2.5 mesh/cm2), for use as a foothold by the 'crawlingr colymbetes larvaef was piaced. on the síoes arid

bottom of the containers. No suLrstrate \^7ðìs needed for the 'swimmingr Graphode.rus larvae. Normal daylight. and laboratory lights were used for the experiments in the light. For the experimerrts ccnducted in the dark a large cardboard box in a ciarl< room \^/as useci to exclucle light from the feeding animals. Only one predator (one species arid stage) uas placed in each container. The types and. d.ensity of prey used varj-ed and are specified later. Tn most experiments a constant density of 30 mosquito larvae was maintainect by replacing a mosquito larva when. it was killed. or fed upon. The prey tha.t were more than half eaten were considered. as being fed on. Any prey l.ess than half eaten was recorded as only being partially fed on. Two major types of experiments r¡¡ere conducted: (1) A number of prey (i.e. 5t 10 or 50) was added to a container, with a predator, and after a period of tíme (i.e. I or 12 hours) the number of prey eaten rn¡as counted and rec:orded. (2) I^lith a constant number (30) and. one predator, in a con- tainerr ârr observer record.ed the number of strikes made to capture one prey, the time taken to handle (Th) a prey, the non-feeding period (Tnf) (i.e. the time when a predator is not feed-ing on a prey) , and the position of prey urhen f irst caught by the predator's marrd.ibl-es . A control 1B container with the correct density of prey, without a predator, was maintained during the e>"periment. A combination ì^/as considered as the tYpe of predator feeding on a s:pecific type of prey (e.9. first instar Colymbetes larva feeding on third instar A. aegypti larvae). An observation was considered as the actual recording of a predator capturing and feecl.ing on one preY. At l.east ten observations (or more) were lf:âdê for any one combination. A replicate was considered. as being crie combinatiort with at least ten or more observations. In other worCs, a replicate was considered as a type of predator feeoing on x number of any cne type of prey (x > 10 prey). At least three replicates \^Iere made on any one cor.rlcination.

D. Measurements The weights of the mosquito larvae !üere measured on a Mettler Analytical Balance (Hf 0 ¡ in m-illigrams . Three sets of 10 mosquito larvae, of any particular stage, were placed on small aluminum pans, vacuum dried at 60oC for 12 hours and then weighed An olympus binocular microscope and- eyepiece micro- meter was used to measure the predators in millimeters (see Table I for prey and. predator measurements). A mercury thermometer was used to record the temperature of water in the control container during each experiment. The laboratory temperature was found to be 20oC + 2oC- L9 The tj_me was measured with a Gralab micrrotimer and¡or a stop watch.

E. Photography

A Bolex (H16 Reflex) 16 mm camera, with ¿L yvar lz2.g, f.75 mm lens and a bellows unit, ât f. 4.0-2.8 and 64 frames Per second, Plus x film (100 A.s.A.), was used to record the predatory sLrike and swimming beliaviour of the dytiscid larvae. A plexiglass compartment was usec. to contain the subjects to be filmed.

Th.e dytiscid larvae vrere starved for IZ hours and then placed into the compartment and flood lights turned on. when th-e predator was in the correct position a mosquito larva was placed in front of it. The preoator striking and feeding on the mosquito larva was su.bsequently f ilmed. CHAPTER TV

LTFE HISTORY AI\D DEVELOPT{ENT

Vfeekly collections of dytiscid.s were made through- cut 1968 and the spring of 1969 from 10 ponds (wherr free from ice) in the Wírrnipeg enviror.s. Figures 1A anci B, show the occurence of stages of ColyrLlbetes sculptilis and Graphoderus occidentalis found in the Wi.nnipeg pools in 196,8. The complete life cycle through gg9, three larval instars, pupa to ad.ult, lasted about 38-40 days for both species, ât 20oC t 2oC (see I'igures 2A and B).

A. The Adults The adults of both species \,vere collected throughotrt the year (1968) from the porrds. Although collections during the winter were not attempted, d.rie to heavy ice cover, it is presumed that both species overwinter in the adult stage under the icer cover of permanent ponds. Adults were collected, in some ponds, just before freeze up and after ice breakup. See Plate fTC and IIIE for an illustra- tion of the ad.ults.

B. The Eggs Acirr.lt females were collected, during early spring ano sunìmer, and kept in laboratory aquaria in the hope they would fay fertile gggs. The Graphoderus females laid an 2A ¿/

FIGURE 1

A. The stages of Colymbetes sculptilis found in Winnipeg p.ools during 1968. N.B. Eggs were not found, but it is presumed this is the eEgi 1ayil9 period. B. The stages of Graphoderuq occidentalis found in Winnipeg pools during 1968. Fis. I o

under ice eggloying-l under ice Adult ¡!!ll!l¡llll¡ltllrr!rr¡rrrrr¡t!!!r!tr!!rIrl!¡l!lllll¡lllttr!lttt!rtttM!l!tt!!¡r!!rit!!!r!t!l!ll.'- KEY L-lorvol instor 1,2,or3 L1 lllll!!¡llllllllll trrr-Stâges observed in pools L2 r¡¡r¡rlrrr¡i¡rrrlrr¡

L3 !ttt!tt!ttttrrrlf!Il

trtrrttrltll JFMAMJJASOND

MO N THS

Fig. I b

under ice þ eggloying under ice Adult ,llrl!!!l¡llttl!!lt!!ttt!t!!!rrlrrt!rtrl!!rrrrrr!!!r!rt!rr!!tr¡trt!rr!t!r!t¡¡!!rr!rr!t!!t!!¡ttr!t-l

L2 ltl!rIrrrltrt!¡!!!!t

L3 lt!¡!!¡i¡¡lt!!!t!t¡!!t

I lllr¡tttrll J FMAMJJASOND

MONTHS '2 ')

FIGURE 2

A. The development of Colymbetes sculptilis reared at 20oC t zoc. Egg stage not observed, so g9g incubation period esti- mated. (Average of 20 individuals, vertical lines demonstrate the ranges). B. The development of Graphoclerus occidehtalis reared at 20oC ! zoc. (Average of 20 individuals, vertical lines demonstrate the ranges). E GG Ll Lz PUPAL PUPA E E 30 sIJ x=) ¡d og-s20 É, J ot¡- lO

- F- (9 z. UJ J to 2A 30 T IME IN DAYS

EGG L. LÐ L? PUPAL PUPA Ë rrrrv¡CELLlt¿ E 30 LLI >, :) X ïzo Bä É. J ll o 00

t-- (Ð z. hJ J to ao 30 T!ME IN DAYS ,.2 "1 J

PLATE II

The stages of Colymbetes sculptilj-s (Harris) A. Dorsal view of a third instar larva.

Average length 21.5 mm. B. Ventral view of pupa. C. Dorsal view of a male adtLlt. Average

lengtli 14.00 mm. 3

v JXI

PLATE fII The stages of Gr¿iphoderus occidentalis Florrr. A. Lateral view of an egg. Average length 2.24 mm. Note 6 la-teral ocelli well

developed (top rigLrt sicle) . B. Dorsal view of a third insta-r larva. Average length 2t.56 Írm. C " Lateral view of a rersting larva in pupal cell made of moss. D. Ventral view of a- pupa in pupal cell. E. Dorsal view of an adult. Averagie length

L2.60 mm. g 25 average of 6.75 eggs/day (S.n. 0.2487r 4 replicates). (See Table TI). The gggs v¡ere stuck onto the surface of aquatic plants and aquarium glass, by the females, with an adhesive material (see Plate TIIA). Figure 1A shows the Graphoderus egg laying period i.e. mid-May to l-ate Ju1y, 1968. The eggs took from 6-7 days to hatch when subjecLed to 20oc t 2oc. Fggs v¡ere not obtained from the Colymbetes adults. Tt is presumed the Colymbetes females lay eggs in early spring as many temporary ponds, present only during the spring, were colonized by ColYmbetes larvae.

C. The Larvae Tï:.e larvae of Colymbetes were found from early April until late June, 1-968 and L969 (see Figure 1A) . The larvae of Graphoderu-s were found from early June until late August, 1968, but were collected earlier in L969, starting mici--May (see Figure 18). The larvae of both species were reared in a laboratory incubator at 20oC t zoc. Twenty newly ha.tchecl larvae of both species were placed into individual containers full of pond water and fed a variety of prey including daphnids, chironomids, mosquito larvae, etc. The time taken for each stadium was recorded. and the length of larval exuvial were measured. in millimeters (see Figures 2A and. B). The third stadium lasted the longest while the second stadiu¡r was 26

TABLE II. Fggs laid by four Graphooerus occidentalis females over a perlõä-õF-5-Ey*

Female no. No. of eggs laio Date

6 /June/68 B B 10 7 o 7 6 5 6 O

B 7 7 9 6

9 B 6 4 5

10 7 7 7 4

Total 36 33 36 30 135

Average eggs/ 7 .2 6.6 7-2 6.0 6.75 female/day

Note: Of 135 eggs laid only 114 hat-checÌ. The proportíon of

fertile eggs !üas 84.42. 27 the shortest for both species.

D. The Pupal Cell and Pupa When the third instar larvae !-v'€rê ready to pupate they began moving around restlessly. Each restless lan'a \^ias transferred to a container with moist moss in which. the Iarva formed a cell or chan-dcer in which they eventually pupated. Plate ITTC shows a Graphrocierus larva in the pre- pupal stage in an opened pupal cell. Both species took about 6-10 days to form the pupal cell ano complete pupation. Plates IIB and IIfD show the pupae of Colymbetes sculptilis and Graphoderus occidentalis respectively, ín a pupal cell. The tr)upal stage lasted from 7 Lo B days before the ad.ults emerged when kept at 20oC E 2oC. CHAPTER V

LOCOMOTOR BEHAVIOUR

Balfour-Browne (1950), and Galewski (1963), found that the morphological features of rocor¡.otor organs (e.g. more setae on legs and abdominal segments to increase propulsion) indicated the swimming ability of the d.ytiscícì, l-arva.

Tn comparing the morphological features and swirnming ability of the larvae of colymbetes and. Grapl-ioderus the differences in their morphology and swimming ability are evident. Both colymbetes and Graphoderus larvae use their three pairs of legs for locomotion (see plate T\¡A and B). However, colymbetes larvae have thicker legs compared to the more slender strearn-lined legs of Graphoderus rarvae. The colymbetes larvae can swim efficient,ly by moving their legs but are usually found to crav"'l over a substrate such as aquatic algae and plants. Nachtigall (in Rockstein 1965) found the larvae of the small d.]'tiscids "sho\n¡ intermediate stages between a crawlilrg and swimming mode of life: Rhantus'sp., colyrnlcetes sp., ancl rlybius fenestratus have smooth iegs and crawl in their first two instar.s, in the third instar they have swimn:-ng hair-equipped legs and they sh¡im. t' fn contrast, all three instar larvae of Graphoderus 2B PLATE IV

Swimming movement of dytiscid larvae. A. Action of legs of COlymbetes sctLlptilis third instar larva moving forward.. In ten

frames the larva travelled 10.58 mm in 156 mi11íseconds. B. Action of legs of Grqrphoderus occidentalis thi-rd instar larva mor¡ing f orward. In terr frames the larva travelleð. 3.25 mm in 156 milliseconds.

N.B. Each diagram represents a picture taken

wit-h a converrtional Bolex 16 mm camera at 64 f.p.s. Each interval between frames is 15.6 milliseconds. o 30 possess long swimming hairs on their slender legs anct continuously swim around. The last two abdominal segments possess a fringe of hairs on the lateral margins (see Plate IIIB) : these are lacking in all- Colymbetes larr,rae (see P1ate IIA). The Graphoderus larvae are perfectly built and adapted to a swimming existence arrd. can move steadily and rapidly through water using their 1egs. They have large lateral tracrr-eal trunks filled with air and unlike the Colymbetes larvae they are able to remain alrp.ost stationary at any 1eve1 in the water (ealfour-Browne, 1950). If ois- turbed, the larvae can dart quickly from danger by flexi-ng the body suCdenly at the first a.bdomina.l segment and straightening it at. great rapidity, resulting in rapid propulsion through the water (see Plate V). Schöne (1950) studied the light orienLation of two dytiscid larvae; $giliug sulcatus and Dytiscus marginalis. He has shown that the structure and arrangiement of the twel-ve lateral ocelli that the d.ytiscid larvae possess (see Plate VI) are especially suitable to mediate optical orienta- tion in the water by sensing the d.:.stribution of the k'rightness of light in visual space. Schgne (1951) found that the larvae of Acilius and Dytiscr,rs possess a derm.al light sense in the tergites which enables them to retain the dorsal light reaction after total blinding. Aithough, detailed experirnents were not performed on ')

PLATE V

The avoidance reaction of a Graph'oderus occidentalis third inst-.ar larva v;,hen disturbed. The larva flexs the body suddenly at the first abdominal segment and straightens it at great rapidity, resulting in rapid propulsion through the water. This sequence of pictures show a larva, having captured a mosquito larva, successfully escaping v¡ith its catch.

N. B. Each interval betir¡een frames is 15 . 6 milliseconds and each side of the squares in the background is 5.29 mÍr. .\ å tr

'_ þ.: -w i ' i*ô118!_

:+i .* ,.* '#

r3 -frd t7 *-l-"*\\* '\" :* | 'il¡-:$Rs,

, .r:ì!Sr sr.ìrï

**t'

ìl '')'1

PL¡,TE VT

A. Dorsal vielu' of the head. of a Colymbetes third instar larv¿1. B. Dorsal view of the heaci of a Gra-phoderus third instar larva. C. L¿rteral view of the head of a Col-ymbetes third instar larva. D. Lateral view of the head. of a Graphoderus third instar larva. E. Ventral view of the head of a Colymbetes thirc instar larva. F. Ventral view of the heao of a G¡ep4eqerus third instar larva

Colymbetes and Graphoderus the dorsal light rea.ction was observed. in the thiro instar larvae of both spec:ies. By placing a 40 watt bulb 20 cnr underneath a container, in a darkeneci room, the larva would. ro11 over and swim with its dorsal side facing the light source. When the brilb was moved to 20 cnì. above the container the larva would ro11 over and swim with the dorsal sioe facing the light scurce. This behavi-our \^ras observed for three larvae of each of the above species. I'rom this simple experiment it can be assumed that both Colymbetes and Graphoderus third instar larvae or.ientate to a light source by use of liglrt sensitíve organs which are probably the lateral ocelli and the dermal light sensitive tergites, as Sctröne (1960 and 1961) has shoi,'¡n in similar dytiscid beetle species; Acilius and Dytiscus. CI{APTER VT

PREDATORY BEHAVTOUR

A. Introduction Holling (1963) consio'ered the characteristics of th:e predator to be a component or factor, amongst four others, which could affect predation. The differences in predator characteristics, such as their ability to detect, capture, and kill prey, could change the magnitud.e of preciation. The predator's food preference, speed. and efficiency of attack, sensory receptors used to locate and ident.ify prey (visual, auditory, tactile and olfactory senses), feecing patterns, are all important characteristics some of which will be analysed in this chapter. Rilling, et al (1959), studying prey recognition in the praying mantis, described the normal pattern of mantis feeding behaviour j-n a sequence of separate actions: mantis- in-ambush, visual following, strike, catch, raising to mouth, eating. The action may be broken off at any point, d.epenci- ing apparently upon the strength of the hunger drive ana ttie stimulus-value of the prey. Holling (1966) analyseci the 'functional response' of invertebrate pred.ators to prey density and broke down the sequence of actions of a predator in attackíng its prey into discrete components and

34 35 subcomponents. only the main components will be listed here: (1) successful search, (2) the time prey are exposed to predators, (3) the time spent handling each prey. Hunger, another component, is consid-ered to affect the above components in one way or other.

rn studying the predatory behaviour of colymbetes anci Graphoderus larvae there \^/ere c.,nly three major seciuences of events that could be visually studied. These were: Tnf = non-feeding time (shown in minutes)

SS = strike (or capture) success (shown as Z) No. of prey captured x 100 No. of strikes nrade Th= handling time (shown in minutes) TT= Tnf * Th = Total Lime or the interval between capture of one prey and the rrêxt capture The only subcomponent of Hollilg's (L966) re,te of successful search (Ts) that was studied. here was the strike success (SS) . Holling (1966) def ines SS ês, ',The proportion of prey, coming close enorr_gh to be attacked, that are successfully capturedr'. The other subcomponents D, vD, and VY (see Literature Review, page 10) were not studied. The non-feeding time (Tnf) refers to the perioo between the time a predator finishes feeding to the time it successfully captures another prey. During the Tnf a pre:dator may do a variety of things from swimming around searching for prey or restilg or breathing at the surface of the water. 36 The time spent in a tdigestive pauset after a prey was eaten and during which the preda_tor was not hungry enough to attack and the actual rate of searching for prey could not be visually determined by merely observi-ng the Cytiscid larvae. Thus both latter factors are includeci irr the non- feeding time in the following experiments. Thre handling (fn¡ represents the period dtrring which the prey is captured, subdued and eaten. The sum of the handlilg and non-feeding time (Th + Tnf = TT) is equal to the total time the prey is exposecl, to the predator. The TT can be referred to as the time period between the time of capturing one prey to the time of capturing another. If the total time for all the prey eaten by one predator, in an experiment, is required a sum of all the individual TT is made. For example, in most experiments only ten prey are eat.en by one pred.ator and the average TT is shown. If the I of TT is required. the average TT is multiplied by a factor of ten. For the purpose of brevity symbols \^/ere assigned to the pred.ators and prey, âs follows: q = predator Y = prey Rc, Fg =. genus of predator e..9. Colymbêtes and Graphoderus respectively Ya, Yv, Yf, Ys = species of prey; mosquito larvae YL = T,estes sp. 37 YM = mayfly nyrnph YC = corixids YD = DaÞhhia sp. i = stage = I, 2, 3, 4 larval instars p = puPa Appendix I and Table I give a complete 1ist of the symbols used for prey and predatcrs. The two major factors that affect pred.ation are prey density and the hunger drive of the predator. Both of the.se factors have been dealt with by Holling (1966). How- ever, rittle was known how ttiese factors would affect col-Yrnbetes and Graphodêrus larvae. scrne prelirnínary experiments v¡ere performed only on cqÅymbêtes (Rc3) thiro instar larvae. There were two experiments performed. (1) Measu,re- ment of the functiorral response of ColymbeLes (Rc3) to mosquito larval density'. Method: Twenty one Rc3 were starved for 12 hours and placed in 8.5 cm diam. containers wj.th 200 ml. of water at 20oC t 2oC. Varying ctensities from one to seven Ya4 were pla-ced in each container. There

\^iere three replicates of eac:h density of ya4. whenever a Ya4 was eaten another was repl ¿gq¿ to keep the cierisity constant. Observations on the number of yaL were taken every 15 minutes for eight hours . (2) The rate of feeciirrg of Colymbetes (Rc3) orr mosquito larvae (Ya4) after Rc3 had been starved for 12 hours. Method: Ten Rc3 were starv'ed. 72 hours before the beginning of the experiment and. then 38 placed in 6.5 cnr diameter containers wj-th a ciensity of 30 Ya4 at 20oc t zoc. Tl:.e number of Ya4 eaten every 15 minutes were recorded ancl replaced to keep the density constant. The results of the abcve two experiments are shown in I'ígure 3 A and B. The ma>.,imum number of prey eaten in eight hours was about 40 Ya4 at densities over A.025 YaA/mL. At lower than 0.025 YaL/ml densities there wer'e fewer Ya4 eaten (Figure 3 A). Figure 3 B shows that Colymbêtes (nc3) starved for 12 hours ate an average of 7.0 YaL/lnour for about three hours after which there was a drop of the feeding rate fluctuating around. an averaige of 4.L \aL/hour. The major point of these two experiments was to observe the. general trends orr how prey density and hunger of the predator would affect the feeding rates. Although the affect of high densities of prey were not sti:rdied it was ciecid.ed that the prey densities would be starr.dardizeo to (1) thirty mosquito larvae for the direct observation experiments and (2) fifty m.osquito larvae for experimental results that v¡ere observed at twelve hour intervals. These prey densities were considered to be the optimum densities on consid.ering various factors such as avail- ability of prey and maximum feeding rate. The hunger levels were standardized to (1) 0 hours starved so the predator would har¡e a normal feeding rate, (2) LZ hours starved so the predator would be hungry and feed readily. The prey d.ensity and the hunger level of predator ') '/t -< "/

FTGURE 3

A. The functional response of Colymbetes third instar larvae feeding on different densities of â. aegypti (Yaa) fourth instar larvae. The dots d-enote an average of 3 replicates and the vertical linesrepresenttls.E. B. The feedi-ng rate of Colyr,ibetes third instar larvae on â. aegypti (Ya4) fourth in.star larvae. The dots denote an average of 10 replicates and vertical linesrepresent!1S.E. E NO. OF Yd,4 EATEN IN B HOURS Þ NO. OF Yó4 EATEN .Þo)@õ

o-s C Ð z u) ol I o Tl o lo o o À 1 o (¡N) 4A were standardized and the physical factors such as tem¡:era- Lure, light, and clarity of water v""ere carefully controlled so that other factors that affect the predatory behaviour of Colymbetes and Graphoderus larvae could be observed. The abitity to detect, eapture, and kill prey (important preciator characteristics) were studieci and are discusseC in the following sections.

B. Prey Detection This section is devoted to an attempt to describe the mechanisms by which the Colymbetes arnd Graphode.rus larvae detect potential prey. 1. Morphological Features Pl-ate VI shows the major sensory organs on the heads of Colymbetes and Graphoderus third. instar larvae. Table III. gives some measurements of the head appendages indicating the relative sizes of the third instar larvae of the two species. The head of a Colymbetes third instar larva is wider than that of a 9seqhgcaIüa third instar larva (2.97 mm compared to 1.77 mm, respectively). The antennae, labial palps, and. maxill-ary palps are longer in the Colymbetes larvae compared to Graphoderus larvae (see Table III). Although both Colymbetes and Graphoderus larvae have twelve stemmata the two dorsal pairs on the Graphoderus la-rvae are well developed compared to those of the Colymbetes larvae (Plate Vl). The relative sizes and lengths of these TABLE TTT. Some measurements of parts of the head and body length of three agly*¡SlSg and three Graphoderr-is third inbtar larvae, in millimeters.

Body len<¡th Head width .i:iåti" Stemmata A.ntenna Labial Maxil.lary di am. * iength F'aip pa]¡r

Colymbetes

I 2r.6 2.96 T.L2 O.LLz 2.L2 L.12 L.25 2 2L.6 2.99 I.L2 O.TT2 2.12 I.I2 L.25 3 2,I.3 2.96 L.T2 0.712 2.12 T.I2 t.25 q'l Average 2L.5 ) L.I2 0.112* 2.12 L.L2 3-.25

Gra.phoderus t 20.48 L"76 0.75 0.225 0.875 0.875 4.625 2 20.48 r.7 6 0.75 0.2?,5 0.875 0. B7s 0.625 3 20. B0 L.79 0.75 0.225 0.875 0.875 4.625

Average 20.56 \.77 0.75 0 .22,5x 0.875 0.875 0.625

Diameter of on-1-y the dorsal pair of lateral ocell i are: given. Eachr larva had 6 stemmata on eâch lateral side of he-ad. È ts 42 orgaris seem i:o inciicate that both species are well adaptecl for mechano- anC. cliemore.ception, while Grai:hod'e-rus larvae are better equ-ipped for vistial orientat.ion, because of their larger ster,rmata, than arer Co'lVmbêtes la-rvae. Both spec-j.es have nr-Ìmercius pr'ojections or flattenei hairs an.ci forraations which occui: on the frontal marg'in of the l.arval head. These projections sLrall be called t:he anteri.or process hairs. Wilson (1923) illust.rated thie anterior process hairs of several d.ytiscid species but d.id not st:ate their function. Blunk (1923) described the ar¡terior' proc.:ess hairs of DlztiscUs sp. anci judgeci by thej-r formation tha-t th,ey fu¡rcti.oned as mechanical- sensors. The legs of Colymbetes larvae possesis short thickened krairs that prokrably function as touch receptors. Gra-phoäeruEr larvae cio not possess such large stout hairs on the lecs.

2. Methocis

(a) Ten ColymÏ:e-Eee (Rc3) larvae, starved. for L2 hours, were placeci in irrdividual- containers with a srnall (slightly moving) black dumri,y 10 cm in frcnt of th.e head. A strike (+) \'ras recorded when the dummy was attacked, but an avoidance (-) reaction was recorded v¡hen the Rc3 moved. av¡ay from the dummy. No reacticn was recc¡rded as (0).

Three presentations were: made to ea-ch Rc3. The sanre procedtire was rrsed with a srr1all stream of air bubbles. Both the dununy arrd air bubbles, ât different times, hiere 43 placed. in a. glass tube (diam . 12.6 mm) , open at both encls, arrd again preserìted to the ten llc3. The glass tube actec1. as a partition betr,r'een the Rc3 and artificial prey sc that orily vistial stimuii were alloweci to reach Rc3. Immec'iiately after the p::esentation of artificj.al prey with the 91ass partition the latter wzì.s rêI-,LoveC and. only the a::tj-ficial prey (dun',my or air bubbles) v;ere presentecì. (b) To test which organs are usei to detect prey some appendectomj-es were perf ormed. on the aritennae, mar:illary and lak¡ial palps arid. atnterior process hairs. To reduce movement and facilitate handli-n.g the larvae were wrapped. gerrtly in a damp cottorr. gauze witJ- only heads e>

3. Results ¿rnd Di-scussiorr (a) rh" 9olytb"te* (Rc3) larvae \^7ere found to atta.ck the mcving k,lack d.ummy anC, stream of air bubbJ-es vigorously but with a. glass partiti.on between there v¡ere no attacks (Tabl-e IV). The moving dr.immy and air bubbles evidently created a disturbance in the water. Although haviî9 no chem:Lca1 resernblance to natuira.l ¡:rey the artificial- prey werÉr attacked by the lic3. Although visual stimuli were preserrt when the glass partition v¡as useci no attacks were observed. Pres;umably no perceptible mechanical vibratioris v¡ere transmitteci through the water to the Rc3. These resuits tend to irrdicate that the ColYmbetes larvae detect Frey mainiy with mechanoreceptor:s. It is cf interest that- mature ae:shnj.d dragonfly larvae, whi-ch are considered to cietect prey visuaLlly (Prit.chard, 1966) , were okrserveö to attempt to capture the black dummy a-nCr stream of air bubbl-es throu.gh the. glass partition. The Graphodems (ng¡) larvae !üere founci not to attack the b-l-ack dumn1, or ai-r bulrbles readily; thus it was not considereci worthv¡hile using them in this e:xperiment. (b) Table VI shows the results obtained from the appendectomize-d Rc3 feeding on Ys4. Ncrmal Rc3 fed equally 45

TA.BLE IV. Resporrse of Colymbetes third instar larvae starveil 12 hours to artificial stimulation.

Stimuius WithouL glass With glass Viithout glass (+) (0) (-) (+) (o) (-) (+) (0) (-)

Airbubbles 29 0 1 0 30 0 30 0 0

Blackdururry 28 0 2 0 3Cl 0 29 0 I

Signs: (+¡ = strike (0) = no reaction (-) = avoidance reaction 46

T¿IBLE V. Appendectom¡r treatments of Co1lznrbetes and Graphoderus thi.rd insta7-fã?Çae used-Tñ-@ylããtection experiments at 200c t 20c.

Treatments 1234 Appendages 'LorD f,orD IorD LorD

Antennae l- +

M.axillary Palps + +

Labial Palps +

Anterior Process liairs + +

Key: * = appendage intact - = appendage cut or cauterized L - Light þ = Dark 47

TABLE VI. The numbers of A. sticticus (Ys4) fourt-h instar i-ãrvae eaEãñ-by a.ppendectomized Col!"mbetes (Rc3) third instar larva.e in the light or dark durirrg 12 hours

Treatment 112234 ReplicatesLDLDLL

1 41.03 42.60 38.00 45.00 2,9 (K) 0, 2(K)

2 40.30 42.00 40.00 40.00 0,0 (K) 2,L7(K)

3 40.00 43.00 44.00 43.00 1,5 (K) 1,12 (K)

Average 40 .4a. 4L.63 40 . 60 42.60 i,3 .6 (K) 1' 10 (K)

S "8. 0 .249 6 0 .5623 L.43 1.18

Notes: (K) = kj.lleci but not eaten. Initial density of 50 Ys4. Rc3 sta-rved L2 hours. Tabl-e V describes treatments I to 4, Lt D. 4B in the liglrt or dark (treatment lL and. lD) (t = 0 .3159, not significant at o = 0. 1) . Similarly, Rc3 v¡ith anterrnae and pa-lpi cut off (treatment 2L and 2D), but with anterior proc€ìss hairs i ntact ate as many Ys4 as the nornral Rc3 (treatment lL ano lD) . These r'esults suggest strongly th.at neither the stemrnata, antenrrae rì.or palpi are thems;elves importanL in. the d.etection and subsequent capture of prey. Deprived of any visual stimuli and their antennae and palpi (treatment 2D) the Rc3 ate practicaLly the sanìe number of Ys4 as the normal Rc3 (treatment lL). It can be inferred that there are other organs orL the hociy of Colymbetes Iarva that are important in prey detection. The results from treatmerrt 3L ancr 4L (Table VI) indicate that the anterior process Liairs are important in prey capture and probably prey dete.ction. Scme of the Ys4 were ca,ptured and killed, but not fed upon. Indeeci, onr'-y an average of one Ys4 was eaten in 12 hours. Further discussion is niade later in section C, page 50. Treatments 2, 3, and 4 were- not conducted on Graphoderus (ng3) larvae as there were not enough Rg3 and Ys4 at the required stages and at the same time. However, treatments lL and. 1D were performed and the resttlts are shown in T'ak;le VII. Significantly fewer Ys4 were eaten on average in the da.rk (22.3 Ys4) than j-n the light (33.7

Ys.4) at cr, = 0 .01 (t = 4.3169 ) . This inäicates tlrat probak;ly Graphoderus larvae use vision to some ext-ent in 49

TABLE VII. The numbers of Ä. stícticus (ys4) fourth instar 1ãrvaeEêilny Graphoderus (Rg3) third instar larvae in light or dark-. duri ng 1-2 hours.

Treatment Replicate ,1L 1D

1 37.33 24.30

) 29.6A 19.60

3 34.30 23.00

Average 33.7 4 22.33

S. E. 1. B3 l.14

Notes: Initial density of 50 Ys4. Rg3 starveci 12 hours " Table V descr-i-bes treatments 1 L and. 1 D. 50 prey detection. The lack cf vistr-a} stimuli in treatment lD flây, however, affect the. general activity of Graphoderrus Ie.rvae. Thus the lowering of' the nuniber cif prey eaten may be due to a reduction of GraphOderus larval activity and not a lack of a visual stimul-us produced by a potential prey. (c) The results of the inclividual observations on Rc3 feeding on 10 Ys4 are shown on Table VIII. The Rc3 with lL and. 2L treatments hai. normal hanoling times (Th) cf 7.2g ancl 7.40 minutes ano strike successes (SS) of 79.0e¿ and 66.62 respectively. There was a drop in both the Th and SS for Rc3, but an increase in the Tnf, with 3L and 4L treatments. The Rc3 would strike at the Ys4 bu.t the only Ys4 captured were those that were trapped between the legs of Rc3. No feeding was cbserved foy arry captured Ys4 were seldom helo by the mandibles for long (average 0.05 mins. ) . These results indic¿rte that anterior process hairs ar.e important in prey caPture and hand.ling. Vlith the anterior process hairs, antennae, and palpi removed the Rc3 cannot feed on prey, however, the Rc3 were still able to detect prey. Because Rc3 were able to strike at and capture prey which were close to their legs it is believed that the legs possess mechanoreceptors i.e. stout sensory ]rairs. Not enough R93 v¡ere available, at the correct time, to complete the latter experiment. However' some casual observations on 1 Rg-l with 4L treatment were macie. The 51

vrrr' time 'ABLE iåf;, :"äå5'":iï::årii'li*3"T*lËii total time (TT), of appendectomized Colyrnbetes (Rc3) thiro instar larvae EëêE-in-grcn A. sticticus (Ys4) fotLrth instai larvae

SS Th Tnf TT Treatment Z S.E. mins. rn-ins. mins.

1L i9.0 s.9 7.29 L.72 9.01

2L 66.6 2.6 7.4A 2.L5 9.55

3L 2.6 1.3 0 .05 7 .52 7 .57x

4L 2,.9 0 .2 0.05 6.58 6.63*

Note' * = partial feeding (In th-i.s case, oo feeding !ûas obsertred, for the captured Ys4 were not

held k,y the mancl.ibles for long i.e. Th = 0.05 rnins). Constant density of 30 Ys4. Rc3 starved 12 hcurs. Each value is an average of 3 replicates. 52 Rg3 would strike at a Ys4 but after hariing captured it the Rg3 would- almost immediately open its mandibles and allow the Ys4 to escape. These obs'ervatiorrs conf irni the f act that the anterior hairs are important in handling of prey.

4 - Conclusion on the krasis of the above data it is concluded that the eyes, anterrnae ano palpi of Colymbetes thirC' instar larvae are not important in prey detectj-on although they probably d6 help slightly. The anterior process Tiairs are important in aiding the larva to hand.le its prey, probably by functioning a-s a mechanoreceptor. It is believed. tkrat the sensory hairs on the legs of Colymbetes larvae aid considerably in prey detectionr hov¡ever no conclusive data were obtaineC so the last statement is merely speculative. No adequate data were obtained to show which sense organs of Graphoderus third instar larvae are useci for prey detection. It is probable, however, that the 2 pairs of dorsal stemmata are important.

C. Prey Capture 1. q"treggl- F-re]' Capture Eeha\¡iour Vfhile Colymbetes la.rvae. generally crawl arnongist veEetation Graphoderus larvae swim freely in the operi water. The larvae cf both species must be relatively hungry to attack a prey for extremely well ted larvae: rarely pay attentj.on to potential prery and may even ntove 53 away frcm i t. When a larva has been wj-thout fooii f or several days, however, it may strike at a pas;sing prey even thotrgh the mandibles are already holding food. Aithough the larvae of both species must capture their prey b1z rapid movement of their mandibles they employ slightly different prey capture technigues. Ttre Colymbetes larva hangs on a substrate with jaws oF)en or closed waitin.g for prey to come close. When the prey is within striking distance the larva rnoves its L'ody f orward at a ra¡:id rate, still hanging onto the substrate, and. snaps its mandibles at the victim. Occasionally the prey may swim between the legs of th.e larva and become trapped. The larva v,'i11 d.etach its legs from the substrate and cling onto the prey while bending the head and thorax so the mandibles can pi-erce the victj-m. Once the ¡:rey is secure on the mandibles, the larva returns to clínging to a substre:te. fn contrast, the Gsgpgccêrus larvae, because of their swimrning habj,t, must propel the v,rhole body towards the prey supported only by their bouyancy in water. When the larva has located a potential prey it swims towards the prey and orientates its median axis with head facing towards the prey. As the larva comes within striking distance it bends its thoracic segments at the irrtersegmental areäs between the pro-, meso-, metathorax and first abdomin¿rl segment. The body takes on an S-shaped appearance. Vühen the larva is correctly orierLted toward,s thie pre! the 54 mandibles open wioe and the sLrike usually follows. With the unbending of the t¡r-oracic s;egment, straightening of the last two abdominal segiments, and thrusting of the legs away from the thorax, the head and mandibles are propelled, tTr-rough the water, towards the prey at a rapid rate. The manciibles snap around the prey ar¡d the larva resumes a normal j-shaped swimming posture. In both species, when the larvae are correctly oriented towards the prey the nrandibles open wide and the strike usually follows. The mandibles rray, howe\rer, open and close (and Graphooerul larva may straighten its thorax) without a strike showing that up to this point the behar¡iour is still plastic. Once the strike has been released, however, its path cannot be modified. The momentum of th.e strike carries the head sor,twhat further than the poirrt of contact with th.e prey. fndeed the head travels almost the same distance again as the original distance betrteen the head and prey at the beginning of the strike beforer the larva can regai-n complete locomotor control. At this time the prey is firmly held between the anterior process and mandibles. 2. Details of the S'trike The predatory strike of a dytiscid larva is \¡ery rapioì. and, to study its details and timing, high-speerd motion picture te.chniques were employec. Attempts to record the strike wi.th a niovie camera are diffici:tt for 55 several reasons. The f irst is tl:.at a high i-ight intensity is required. and this and the heat tha.t accompanies it are both unfavourable for the normal behaviotrr of the d).tiscid larva " The exact time of the strike is often difficult to predict and much film is wasted. In this study these prc- blems were parfially solved b1' reducing the light intensity and. opening the lens aperature to f.4-2.8. Opening the aperature, however, produced a sma1l de.pth of field so that continual adjustment was necessar!' to keep the image cf the subject sharply focusec1. The following analys5.s of the dlztlscid larvalstrike will follow closely the method of analysi.s Pritchard. (1965) useci while studying the strike of aeshnid ciragonf Iy nymph-s. In this study, pictures suitable for arralysis were obtained at 64 frames per second, using a conventional Bolex 16 mm cine canìera. The information obtaineci is limited, but a few interesting details of the strike of dytiscid larvae have been discovered. (a) The Colymbetee larval-strike The data for Colyml:etes sculptilis have been obtairred from a. study of five strikes by five third instar larvae at mosquito larvae that were suspended in front of them. Two such strikes are shown in Plates VIT and VIII. The mosquito l-arvae were struck at wTren they v"'ere 1.05 to 3.49 mm from the front edge. of the anterior process, the average distance being 2.16 nrr, ts.n. 0.37) (See Table IX) . ') lu

PLATE VÏI

The strike of a Colymbetes sculptilis third instar larva (lateral view). The interval between pictures is 15.6 milliseconds. The tength of each side of the squares in the background. is 5.29 mm.

L..,'1

PLATE VTTT

The strike 'of a Colymbêtês sculptilis third instar larva (lateral view). The larva missed the mosquito larva. The interval between pictures is 15.6 milliseconds. The length of each side of thre large squares in the background is 6.5 mm. ..::. î.' ;:L:;ilzl.fi:.lþlþ"1i/aj TABLE TX. The strike measurements of Colymbetes sculptilis (nc:¡ third instar larvae from pictui:eê-Tffieñ v.ffi a eolex 16mm camera at 64 frames per seconci.

Time for forward Distance from Distance Ant. Time for Mbls* thrust and Time for arresting Ant. Proc.* to Proc. * travels Strike to open wide closing of Mbls forward thrust in Yi* just before during strike No. in millisecs. in millisecs. millisecs. strike in mm. an mm.

1 46 .8 15.6 3L.2 1.05 4.69

2 109 .3 3L.2 3L.2 1. B0 3.64

171.8 62.4 ls .6 t. B0 4.56

15 .6 62 .4 46.8 3 .49 6.L7 3L.2 15.6 3L.2 2.64 4.4L

Average 7 4.9 3i .3 3L.2 2.16 4.70 cEl 25.8 9.5 ¿,L 0 .37 0 .46

Notes: *Mbls. = Mandibles; Ant. Proc. = Anterior processi Yi = Prey or mosquito farva. One frame irrterval = 15.6 milliseconds and the squares in the picture background

have sides of 5.29 n:tm. tn co 59 The avererge distance the hea-d moved drie to the legs thrusting the whole body forward, measured. from the irritial position of the anterior process before the strike (see plate VII, Picture No. 4 and. Plate VIII, Picture No. 4) to the farthest distance it travels after the strike, \,vas an average of

4.70 nìm (S.e. 0.46) with a range of 3.6tt to 6.17 mm. The Colymbetes third instar larval strike can be divio.ed into f our stages: the opening wide c;f the manciibles, the forward thrust of the whole body, the closing of the mandibles, and arrest of the f orr¡'ard body momentum or

thrust. Coiy$þglgg_ larvae can hold Lheir mandibles c)pen for over 15 minutes waiting for prey to come into reach. However, when the strike is aloout to Lre made the mandibles (whether open or closed) open wide. This first action ta.kes from 31.2 to 171.8 millisecondsl viith an average of 74.g milliseconds (S.g. 25.8) (Table IX) . When the mandibl-es are fully open the body is thrust for.ward. As the pictures were taken at a slow speed it is difficult to obtain an accurate time for the second and. third stage, but the time for the forward thrust and closing of the mandibles was found to be an averagie of 37.3 millisecond.s (S.8. 9.5) with a range of 15.6 to 62.4 milLise.conds. The slowing down of the forward thrust after prey capture took an average of

lrh" times in this stucl.y are based on one frame interval = 15.6 millisec:onds. 60 3L.2 milliseconds (S.u. 4.4). Thus the complete operation from opening the mandibl-es to completed closure of the forward thrust took, on the average, 143.4 milliseconds.

(b) The Graphoderus larval strike The data for Graphoderus occidentalis have been obtained from a study of eight strikes by eight third instar larvae at mosquito larvae that were suspended in front of them. Two such strikes are shown in Plates IX and X. At an average of 6.31 mm (S.E. 0.49)- | measured from the anterior process to the prey, the larva begins to bend its thorax. The mosquito larvae were struck at when they \,üere 2.64 tc 5.82 mm from the front edge of the anterior process, the average distance being 4.03 mm (S.e. 0.33) (rable x¡. The average distance the head travels due to the straighten- ilg of the bent thorax, thrust of the abdomen and legs, measured from the initial position of the anterior process before the strike (see Plate IX, Picture Nos. 5 to 9, and Plate X. Picture Nos. 6 to 9) to its farthest position after the strike was 7.26 ilun (S.n. 0.72) with a range of 4.23 to 9.69 mm (rable x). The Graphoderus larvaI strike can be divided into five stages: archir-rg of the thorax, oF)ening of the mandibles (during first stage), thrusting forward of head, closing of mandibles, and arresting fonrard thrust. The bending or arching of the thorax takes an average of 283.6 milliseconds i,t

PLATE TX

The strike of a Graphoderus occid.entalis thirc', instar larva (lateral view). The interval between pictures is 15.6 milliseconds. The length of each si.de of the squares in the background is 5.29 fltm.

,r1

PLATE X

The strike of a Graphoderus occidentalis thiro instar larva (lateral view). The larva missed capturing the mosquito larva. The interval between pictures is 15.6 milliseconds. The length of each sicre of the squares in the background is 5.29 mm.

TABLE X. The stril

Time for Distance in mm Distance in mm Distance in arching of Time for Mbls Time for from Ant. Prcc. from Ant. Proc. nm Ant. Proc. Strike thorai in to open wide strike in to Yi, when to Yi just travels during No. millisecs. in millisecs. nii1li-secs. archi.ng begins before strike strike

1 328.0 15 .6 15 .6 5.82 4.23 9 69

2 343.6 3r.2 15 .6 6.L4 3.70 9 69

3 359.2 15 .6 46.8 4.76 4.23 9 32

A = 28I.t 3r.2 15.6 5 .19 3 .97 6 7B

5 296.7 46 .8 15 .6 5.29 2.64 4 23

6 2L8.6 62.4 46 .8 3 .44 2 "BB 4 50

249.9 15.2 15.6 6.L4 4.76 7 52

I87.4 3L.2 3L.2 7.94 5 .82 35

Average 283. b 3L.2 25.3 6 .31 4.03 7 .26 caa 20.I 5.5 4.72 0 .49 0 .33 0.72

Àlotes: Mbls . = Mandibles; Ant. Proc. = Anterior process; Yi = Prey. One frame interval = 15 .6 mill-iseconds anci the sqlLares in the picture background. have sides of 5.29 mm. (,or 64 (S.n. 20.L) with a range from 187.4 to 359.2 milliseconds (Table x). The average time for the mandibles to open wide is 31 .2 milliseconds (S .8 . 5 . 5 ) . \¡lhen the mandibles ö-re fuIly open the thorax straightens ano the head is thrust forward at a rapid rate. Because of the slow speed at which these pictures \^iere taken it is difficult to oh'tain an accurate timing of the third, fourth and fifth stages, but nearly all strikes \^rere completed within two frames with an average of 25.3 millisecorrds (S.8. 4.72) . Thus the complete operation from the arching of the t¡r-orax to the final stopping of the forward thrust tookr olt the averâge, 308.9 milliseconds. (c) The Dytiscus larval strike Data for Dytiscus sp. have been obtained from the filming of only one strike. Plate XI shows a Dytiscus sp. third instar larva strike and miss a tadpole. Although tliere is little data to support the followinE discussion Plate XI and casual observations suggest that Dytiscus larvae possess an intermediate prey capture method between that of Colymbetes and Graphoderus larvae. Unlike Colymbetes larvae Dytiscus larvae can capture prey while swimming. Dytiscus larvae usually use the whole body in a dorsal thrust of the head towards the prey. The abdomen. is "whipped" downward or ventrally and the head is thrust dorsally. The opened mandibles are closed about the prey and the thrust is arrested. Lr

PLATE XI

The strike of a Dytiscus sp. third instar

1 arva (lateral view) . The larva misseð, capturing the tadpole. The interval between pictures is 15.6 millisecond.s. The length of each side of the squares in the background is 5.29 Inm. 8i ..t^

':-11

iÊ ì ' -' " i*".-t1J - *"f*"í-ìì t.,3

I 66 3. Position of 'Predator and Prey at Time of Prey Capture I¡Ihile comparing the predatory beharriour of çelymbetes (Rc3) and Graphoderus (ng3¡ larvae observations were macie on the position of the larvae in the container and the bociy region of prey pierced by the mandibles at the time of prey capture. The container hTas dj-vided into fir¡e areas (Eigure 4 A) three of which described the side of the container for the 'crawlingr Colymjretes larvae and two of which describeci the areas of water for the 'swimmingt graphoclerus larvae. The prey or mosquito larvae Ya4 were divided into four regions (Figure 4 B), the head, trr-orax, abdomen and tail. The results are shown in Table XI. At no time was Rc3 found to strike at the Ya4 while swimming. Only when the Rc3 were on the substrate did they strike at and capture Ya4. There was no signific¿Lnt difference (F = C.04, q = 0.05) between Ts, Ms, and Bs, for Rc3. On the other hand, Rg3 was found rrot to strike at prey whil-e hanging from the substrate and captured prey only when swi-mming in Tm or Mm. The l-evels in the container at whrch Rg3 caught Ya4 differed significarrtly (t = 24.8¡ o = 0.005); Rg3. generally struck at Ya4 hanging from the water surface. Aithough Rg3 are capable of capturing Prey swimming in the water the reason for the last result may be e>lplained by: (1) the majority of captured Ya4 tended to hang from the surf ace , (2) Rg3 usually strike at prey that ¿ire above i ,.-l l-' t

FIGURE 4

A. Cross section of container (70x50 mm

crystalizíng dj.sh) . Note: Divisions show areas in water used to describe positiorr of predator.

Ts = Top side, Ms = Middle side, Bs = Eottom side, Tm = Top middle, Mm = Midcile middle. Scale of dlagram is 1:1. B. The regions of a mosquito larva that were attacked by the preciators.

Note: Hd = Headr Th = Thoraxr Abd = Abdomen, T = Tail. A

T_=-a_Abd __T_Th _r H d _t llttt rtt¡t

B TABLE XT. The position of the predator, colymbetes (ncr¡ and Graphoderus (ngs) third instar larvae, andTñêEÇio-n of prey, Ã:-èg)õF (Ya4) fourth instar larvae, pierced by the mandibles immediately after a strike.

Position of Ya4 wheri first Position of Ri in Replicate caught hy Rr mandl-þJ-es container No. Predator Prey Hci Th.. Abd. Ta.il Ts Ms Bs Ttr, Mm

1 Rc3 Ya4 I 2 6 T 532A0 ) Rc3 Ya4 2 0 5 3 52300 3 Rc3 Ya4 0 4 4 2 24400

Average Rc3 Ya4

I Rg3 Ya4 2 B 0 0 00082 2 Rg3 Ya4 0 7 3 0 00091 3 Fg¡ Ya4 0 7 3 0 00082

Average Rg3 \7-IAT A 0.66 7.33 8.33 I.66

Notes: Hd = Head; Th = Thcrax; Abd-. = Abdomen; Ts = Top side; Ms = Itdiddle sioe; Bs = Bottom sid.e; Tm = Top middle; Mm = Middle mjddle. See explanation in Figure 4 A and B. One replicate represents one Ri feeding on t,en Ya4. Each Ri was starved LZ hours .

Or oo 69 their heads. The ltc3 pierced the majority of the Ya4 in the abdomerr while Fg3 pierced. the majority of Ya4 in the thorax. The different methods of prey capLure explain this dj.ffer- ence. The Rc3 lay in ¡amJ¡ush' waiting for the Ya4 to randomly pass by; thus the majority of Ya4 successfully captured \,ìrere caught by the central part of the boc'i-y. On the cther hand, because the Ya4 hang from the water strrface at a 70o angle the most vulnerable and conspicuous part for the Rg3 to attack would be the thorax.

4. Strike Success The only methoci used, in this work, to measure the strike accuracli of Colymj¡etes and GraphOcierus la.rvae was by the observation of the nurnber of strikes compared to strccessful captures made or SS = No. of captures x i-00. NÐ . oF sErlEes Ail three la-rval instars of Colymbetes and Graphoderus (starved for 12 hours) were observed to capture the five immature stages of å. aegypti. Plate XII illu- strates the immature stages of Grapho4eruq anci â. aegypti showing their relative sizes. There was a total of 15 combinations observed. Each combirrat-ion was replicateci three times. Each predator was allowed to capture 10 Yai. The number of strikes and. captures were recorded. and the r:esults ar'e shown irr Figure 5 A and B. The average SS and S.E. are shown on Table XII for Rci and Table XIII for Rgi. 'iú

PLATE XII

The immature stages of Graphoderus occidentalis (R1, 2, 3) and â. aegypti (yl, Z, 3, 4t p). Stages preserved in 75e" alcohol. --^ i j't-' -ft

FTGURE 5

A" The strike success (SS) of the three larval instars of Colymbetes sculptilis (Rci) capturing the Cifferent Sges of five immature stages of â. aegypti (vai). Þ The strike success (SS) of the three larval instars of Graphoderus occidentalis (ngi) capturing the different ages of five immature stages of â. aegypti (vai).

# = Averêìges are computed from less than 10 captures.

N. B. The larval stages, average weights anci pges of A. aegypti, reared at 20oC ! zoc are as follows:

Stage Wêight (mgm) Age (Hours )

Ya1 0.0076 24

Ya2 0.0540 66

Ya-? 0.2800 LL2

Ya4 0.9s00 LiB

Yap 0.9600 204 qt Þ STRIKE SUCCESS O/O STRIKE SUCCESS O/O òo@

.T.3 3(l -N

Þ 6) V c, rn m \¡ -¡ oN oN -n Tl o(o o(o -'(t) -'o) z \ z o8I- Oo¿N c c Ð / v U) .J' 5 s; I ò I

/ TABLE XïI. The strike success (SS) , handling time (rh). non-feeding time (Tnf ) and total tíme (TT), of Colymbetes sculptílis (nci) feeciing on å. aegypti (Yai)

Predator Prey SS Tir Tnf TT Rci Yai S. E. Min. S. E. M-i-n. Min.

RcI Ya1 54.9 7.85 3.26 0.22 4.55 7.BL Rc1 Ya2 59. B 4.77 4.64 0.08 5.20 9.84 Rcl Ya3 53.5 3.30 7 ,14 (18 full feedings 5.96 13. 10 # # Rcl Ya4 69.2 (e captures ) 18.75 (4 fulI feed.ings ¡t J[* Rcl Yap 25.0 (7 captures ) 16.11 (4 full feediirgs 1T

P.c2 Yal 37 .6 2 61 L.25 0.14 6 .86 8.11 F'c2 Ya2 53.3 3 76 2.46 0.2c1 3. 84 6.30 Rc2 Ya3 52.5 3 B2 3.64 0.16 2.93 6.57 Rc2 Ya4 76.9 3 77 6.54 (16 full feedings) #rf Rc2 Yap 36.7 3 02 L2.I3 (9 full feedíngs) #*

R.c3 Yal 26.6 0 B3 0.75 0.02 6.98 7.73 Rc3 Ya2 53.5 4 04 L.44 0.14 t.72 3. 16 Rc3 Ya3 61.8 5 93 2.40 0 .09 0 .66 3.06 Rc3 Ya4 83.3 0 00 5.25 0.03 0 .46 7 .25 Rc3 Yap 73.4 2 BO 4 .82 0.24 1. t5 5 .97

Notes: # l,ess than 10 observations made in i rep'liç¿¡s. * Some partial feeding observed; Min. = minutes. The three larval stages of Colymbetes sp. arrd 5 immaturre stages of aegypti were used. Each VãTüê-f-sar'aVerageof3rep1icateswitLi-10observationsirleach replicate (i.e. I Rci fed on 10 Yaj.). Ea.ch Rci was starved 12 hours. { N) TABLE XIII. The strike success (SS¡, hanoling time (Th) , non-feed.ing time (Tnf), and total time (TT), of Graphoderus occidentalis (ngi) feeding on A. aegypti (vai)

Predator Prey SS Th Rqi Yai ClIa Mín. S.E. Min. Min.

Rg1 Ya1 L4.27 1. B0 0.14 1. 89 3.69 72.r * Fgr Ya2 7 4.2 5 .02 7.3C 0.2L 20.43 27.73 Rgl Ya3 72.6 3.68 L2.20 I.7I t9 .20 31.40 * t I Rgl Ya4 # # 63.00 +i xlt Fgr Yap # 1f #ti

R-g2 Yal 77 .2 2. B0 0.63 0.06 0.66 I.29 Rg2 Ya2 7 r.6 ) a,t 1.63 0.28 0.33 3L.96 Rg2 Ya3 83.3 0.00 7 .37 0. 85 9.46 16.89 I Fg2 Ya4 tt' 26.00 (4 full feedings) # s0.40 #*. II +* Rg2 YaP if 24.I0 (4 ful1 feeaings) # ø4.00 11

7B F-93 Ya1 63.1 3 75 0.34 0 00 0 430 Rg3 YaZ 73.2 1 49 0. B1 0 05 0 21 1 02 Rg3 Ya3 86.7 5 62 2.98 0 36 0 483 46 Rg3 Ya4 81.5 3 B1 L2.36 0 92 1 10 13 4'i Rg3 Yap 70.0 2 BO 11.61 t .50 1 22 12 47

Notes: # Less than 10 observations ma.de in l replicate; * Some partial feed.ing observed; Min. = minutes. The 3 larval stages of Graphoderus sp. and 5 imri,ature stages of aegypti \^iere useci. Eãõ vãfFis an average of 3 replicates wEñ--Tõ- observations in each replicate (i.e. 1 Rgi fed on 10 Yai). Each Rgi was starved L2 hours {(, 74 The Th arid TT are also stiown on these latter- tables, but the results will be discussed in the following sections. The total average SS for Rci capturing all Yai (excludilrg Rcl capturing YaA and Yap) was 54.5U (F = 3.30) however there was a significant difference in Lhe mean values at q = 0.025 level. The total average SS for Rgi capturing all Yai (excluCilg Rgl + 2 capturing Ya4 and Yap), on the other hand, was 75.02 (F = 1.008) with no significant differences in the mean values at the o = C.05 level. Both the analysis of variance and Figure 5 A anct B show the SS of Rgi varies considerably compared to the SS of Rgi while capturing Yai. These results indicate that the overall method of prey capture by Rgi is superior to th¿rt of Rci when feeciing on immature stages of mosquito â. aegypti in confined laboratory containers. The different sizes of Yai do not affect the SS of Rgi as much as they do Rci (Figure 4 A and B) . Both Rcl and Rc2 have a higlier SS than Rc3. Because the Rcl and Rc2 are smal-ler than Rc3 they are adapteci to capture a small prey such as Yal. However, as the prey becomes larger the smaller RcI and Rc2 cannot successfully capture the larger: prey. The SS of Rc3, on the other hand, increases with the larger prey. A similar trend is seen with the SS of Rg3. The highest SS is reached at 83.3? (S.8. 0.00) for Rc3 feeding on Ya4 while the SS for Rg3 was 86.7U (S.E. 5.62) feeding on Ya3. The Yap have a different swimming T;ehaviour and 75 anatomy and \^iere harder to capture (Figures 5 A and B) compared to Ya4. Using the same procedure as above the Rc3 and Rg3 were also observed. striking at A. vexans (Yv4), A. sticticurs (Ys4), â. fitchii (vf¿) and å. aêgypti (Yai) . The results obtained are shown in Fignrre 6. The weight of the mosquíto larvae. gives an inciication of their size. The maximum SS of ; (1) Rc3 was 83.32 (S.8. 0.00) capturing Ya4 , (2) Rg3 was 86.72 (S.U. 5.62) capturing Ya3. Agaín Rg3 is shown to have a higher overall- and less variable average SS of 7B.Lz (s.e. 2.9) than the SS of Rc3 which is 59.9% (S.8.

7.3). There is a major lowering of SS when the F.c3 capture mosquito larvae weighing lower than 0.5 mgm or higher than 2.0 mgm. The optimal size of mosquito larvae for the highest SS (i.e. over 702) lie between 0.5 mgm and 2.0 mgm for Rc3 and about 0.1 to 2.5 mgm for Rg3.

Individ.ual observations were made of Rg3 feeding orr natural prey such as A. vexans (Yv4), Daphnia sp. (YD), Corixids (YC) , mayf lies (YM) , and Lestes sp. (YL) (see Plate XIII and Table I for their relative sizes), collected from Winnipeg ponds. The same procedure as in the last two SS experj-ments was followed. The results are shov¡n in Table XIV and Figure 7. The average SS for Rg3 capturing Ya4 is also shown in Figure 7 for urse as a comparison. No SS observations \{ere made on gelyrlsles larvae capturing these prey. The Ya4 and Yv4 were the most easily captured; ':! iYt

FIGURE 6

The strike st-iccess (SS) of gclymÞStêa (Rc3) and Graphooerus (Rg3) third instar larvae capturing the four larval stages of â. aegyÞti (Yai), and the fourth instar larvae of â. vexans (Yv4), A. sticticus (Ys4) , and A. fitchii (Yf4)

The vertical lines indicate ! 1 S.E. (for 3 replicates).

N.B. The averagie weighLs of the prey are as follows: 4"Y Weight irr mgm Ya1 0 .0076

Ya2 0.0540

Ya3 0.2800

Ya4 0 .9500 Yv4 0.6360 Ys4 0.8924

Yf4 2 .4640 KEY x - Rc3 o - Rg3 90

ò=g 80 cn To Ø LrJ ()()60 ) Ø 50 td Y tr40 t- Ø

o.5 t.o t.5 2.A 2.5 WEIGHT OF Yi mgm l- "'1

PLATE XII]

Some natural or potentj.al prey of Graphodêrus la.rvae coll-ected from Win.nipeg ponds. Prey preserved in 752 alcohol..

A. Mayfly (Vlt¡ Mature nymph

B. Lestes sp. (YL) young nlumph C. Corixid (YC) Adult D. Paphni"_sp. (YD) Acrults

TABLE XIV. The strike success (ss), harrdling time (rrr¡, non-feeding time (Tnf) and Total time (TT) of Graphoderus (Rg3) third inãtar larvae feedíng on å. vexans (FA]|l corLxids (yC) , Daphnia sp. (YD), Lestes sp. (Yi,), ãFfV (yM).

Replicate Combination SS Th Range Tnf Range TT nc. Pred. Pre o, mins. Mrax. Min. mins. Max. Min. mins. 1 Rg3 Yv4 90 9 6.65 B 14 6 1. 89 9.5 . 01- 8.54 2 Rg3 Yv4 B3 3 7 .Aot 9 06 0 0.L7 0.9 .01 7 .66 3 Rg3 Yv4 66 6 7 .09 11 04 5 LL) 18.0 .03 11.51 A J Yv4 100 0 7.94 I2 05 0 0.66 6.0 .01 8.60 Average Rg3 Yv4 85.2 7 .29 I.7 B 9 .07 1 Rg3 YC 45.4 5.19 8.5 1.0 0.36 5.55 ¿ Rg3 YC 24.39 5.64 L2.0 2.5 3.29 8.93 3 Rg3 YC Over 10 min. period Rg3 caught Y but could not hold onto YC 4 3 YC 27 .0 s.2c 7 .3 2.3 r.62 6 .82 Averagie Rg3 YC 32.2 5.34 L.7 5 7.r I Rg3 YD 52 6 1 13 2.0 16 0.31 93 .03 1 44 2 F'93 YD 66 6 0 BO 1.5 66 0.10 33 .03 0 90 3 Rg3 YD 50 0 1 T2 1.5 B3 0.2I 50 .03 1 33 4 Rq3 YD 62 5 2 20 ¿. t 0 0.19 50 .03 2 39 Average Rg3 YD 57 .9 1. 31 4.20 1.51 Rgr3 YL 5B.B 24.6 35 1.98 26.58 3 YL 66.6 13.9 32 4.44 18.35 Average Rg3 YL 62.7 L9.2 3.22 22.46 1 Rg3 YM 4I.66 10.6 35 3 .4I 23 .33 14.01 2 Rs3 YM 45 .40 9.1 13 3.15 15 .75 T2.28 Average Rg3 YM 43.5 9.8 3.28 13. 14 Notes: Constant density of 20 Yi. Rg3 starved for L2 hotrrs. Each \¡ replicate = 10 oþservati.ons. See Table I for sizes and stages co of prey. .7rl

FÏGURE 7

The strike success (SS¡ of Graphoderus (Rg3) third. instar la.rvae feeding on å. aegypti (Ya4) and A. vexans (Yv ) fourth instar larvae, Daphnia sp. (YD) adults, corixid (YC) mature nym.phs, mayf 1y (YM) mature nymphs , and Lestes sp. (YL) young nymph. K e y.

tl - A. AEGPYTT ñ -A.VEXAIUS Ël - DAPFü|NtA SP. ffi - coRtx tDs H - MAVF',[-tES ffi - LESTES SP.

BO Ø (n ()IJ () 60 :f Ø 40 IJ \¿ Ëro U)

TYPES OF PREY BO the average ss was 83.3% arrd 85.2% respectivety. There was a lower SS f or YD (57 .92) than f or ya4. The reciuction of ss was probabry due to Yn being a smaller animal cr 'targetl to capture. Due to the slow movement of the tcrawling' yL the SS of Rg3 was relatively high at 62.72. YC and YM \^¡ere more difficult to catch than the mosquito larvae (ya4 or Yv4) because they were larger and faster and their hard.er cuticle made it difficult for the mandibles of Rg3 to take hold. Thus Rg3 were successful- in only 32.2? and 43.5% of the attacks on YC and YM respectively.

5. Discussion The predatory strike behaviour diffei:s from the locomotor behaviour of the dytiscio larvae studied. While the Cc¡lymbetes larva. generally crawls orr a substrate and strikes forward at passilg prey the Graphoderus larva is free-swimmi-ng and must strj-ke dorsally at prey above its head. The free-swimming Graphoderus larvae have cor.re to possess a faster and more accurate prey capture techriique compared to that of the 'crawlingr Colymbetes larvae. On average, the Graphoderus larva will strike at a mosquito larva further from the anterior process (4.03 mm, S.E. 0.33 of Graphoderus compared Lo 2.L6 rnÍl, S.E. 0.37 of Colymbetes), at a faster rate (25.3 milliseconds of Graphoderus compared. to 68.5 milliseconds of Colymbetes) ancl at a higher overall accuracy than a CollznrJcetes larva (75.0çà of 81

Graphoderus compared to 54.12 of colyr'betes) . The proportion of strikes that result in a catch is affected to some extent by the activity of the prey and its size. When a prey is moving, errors in the 'judgmentr of both distance and direction of the prey occur. The strike Successes of Colynìbetes third instar larvae are affected b1' a smaller range of mosquito larvaI size than are Graphod-erus third instar larvae. That is, the strike success fa1ls below 70? when (1) Colynrlc.etes third instar larvae attempt capture of mosquito larvae below 0.5 mgm and above 2.0 il9ffi, (2) when Graphoderus thiro instar larvae attempL capture of mosquito larvae below 0.1 mgm and above about 2.5 mgm. The activity of the prey was not studied so a direct com-parison between prey activity and the capture success could not be made. However, casual observations showed that prey such as corixid and mayfly nymphs \.úere stronger swimmers than mosquito larvae. Thus, Grap4oderus third instar larvae \^/ere about 80% successful in capturing mosquito fourth instar larvae but only 32.2% to 43.52 successfui in their attacks on corixid and mayfly mature nymphs.

D. Handling of Prey The handling time (Th) , recorded in these experiments, was measured from the time of prey capture to the removal of the undigested. contents. This period includes the subdui-ng, eating and final discarding of prey and any other swinrming B2 activity during this feeding.

Observatiorrs were ma,cie. on the hancrling ti.mes of all three larval stages of Rci arrd Rgi (starved 72 hours) feed.ing on teri of each immature stage c,f Yai. Whenever a Yai was eaten another Yai was replaced so that a constant density of 30 Yai was maintained. Each combination was replicated three times. The results ar:e shown in Tables Xrïr XIIT arid Figure I A and. B. The handlilg time was less for the larger predator stages (e.9. Rc3 or Rg3) than for the s¡raller (e..9. Rc2 anci Fg2) (Figure B A and B). Rcl and 2 and Rg 1 and 2 could. not subdue Ya4 and Yap as effectively as YaI, 2, and 3 because of the larger size of Ya4 and Yap compared to Ya1,

2, 3. Each predertor instar took a lcnger time to feed orr progressively larger Yai stages. Platting the Yai weights versusi handling times resulted in linear curves. That is, the weight of tLie prey versus the ha-ndling time of the predator vras directly proportional. Figure 9. gives an inciication as to how the individual Rg3 feeding rates on different Yai stages oc:curred. The Th, Tnf , and number of strikes nrade is includ.ed. The general trend. of Rg3 feeding on Yal, 2 more rapidly than Ya3, 4 can be observed. Holling (1966) found hunger not to affect the handling time of praying mantis adults feeding on flies. Similarly, the Th of bot-h Rc3 and Rg3 were not affected by f

FIGURE B

A. The handling time (fn¡ of the three ColYmbêtes (nci) larval stages feeding on the five inrnature stages of â. aêgypti (Yai) . B. The handling time (Th) of the three Graphoderus (ngi) larval stages feedinE on the five immature stages of A. aegypti (Yai).

Note: The average weights of the Yai are as follows: â. aegypti stages Weights in mgm Yal 0.0076

Ya2 0.0540

\7^? I d--' 0.2800

Ya4 0.9500

Yap 0.9600

* = partial feed.ing observed A B

20 20 (n Ø z z >16 = t6

= = =z t' Ft2

t¡l l¡J = =.l-8 F8 (9 (9 z z J Åo zô4 z - - o.2 0.4 0.6 0.8 o.2 0.4 o.6 0.8 WEIGHT OF lYoi IN mgm. WEIGHT OF lYoi lN mgm. 84

FTGUF.E 9

Scr¡.e individual observations of gl=lfreqg (ng3) third instar larvae feeding on the four larval stages of â. aegypt.i (Yai) .

Note: These observations v¡ere chosen asi they \¡iere the closest to the averages shown in Ta.ble XIf I. The number of strikes made for a successful- capture, the Tiandling time (th = straight line), and non-feeding time ftnf = dotted line) are shown. F="'* 2 I

2 l t I l z OF STRIKES MADE l¡J F-6 rI l-rj 'õ5 t l.L^ t- o I E l¡J 3 (D zfo

30 40 50 60 70 80 90 TIME IN MINUTES. B5 hunger. This was strown by starving the pred.ator.s for L2 hours and observing their feecting rate over several hours. The Rc3 were obsen¡ed to feed on 25 YaA (Table XV and Figure 10) and the Rg3 were observed to feed on 18 Yv4 (Tabl-e XVf and Figure 11) . As both Rc3 and R.g3 ate more mosquito larvae presumably they became less Ïiungry, however, the Th of both Rc3 and Rg3 remained relatively constant at an average of 5 .2 minutes (S. p. 0 . 145 ) arrd 6 .7 4 miriutes (S. u. 0.206) respectively (Figu-res 10 and 11) . Any variation that did occur \^/as probably d.ue to the slight differences in weight of the mosquito larvae. Further disctrssion of Figures 10 and 11 will be made irrlÞE:-fo*ing section. At the beginning of the experiments the Lrungry larvae wouId. occasionally strike at and. capture another mosquito larva while the remains of the former mosquito larva was still attached to the mandibles. Only after both the victims were manipulated by the mandibles and the bc;dy conterrts dissolved and. suckeci out would the skins be dis- carded. The individual Th reaciings, in this case, were recorded as the total Th for the two feedings divided by two. The Tnf was recorded as 0.00 mi-nutes. The method of discardilrg the remains of the uneaten part of the prey varies slightly between Colymbetes and larvae. The ColYmbêtes larva opens its man- =GggÉgg"r* d-ibles, bends its head downward wh:lle the f orelegs are brought above the head and on the downv¡ard thrust the B6 TABLE XV. Colymbetes (Rc3) third instar larvae Eeãdjn-g c,n 25 A. aegypti (Ya4) fourth instar larvae.-

No. Th Range Tnf Range TT Ya-4 mean mr-n. max. me¿rn mtn. ma>:. mean

1 5.2 4.3 7.0 0.1 0.0 0.3 5.3 2 5.0 4.3 tr2 0.2 0.0 0.3 5.2 3 4.0 3.6 4.5 0.1 0.0 0.3 4.L 4 5.2 3.3 7.8 0.0 0.0 0.0 5.2 5 4.5 4.0 s.1 0.6 0.0 1.6 5.1 6 6.7 4.I 8.1 0.3 0.0 0.8 7.0 7 5.6 3.6 8.1 0.1 0.0 0.1 5.7 oô 5.9 5"3 6.6 0.8 0.0 2.1 6.7 9 4.6 4.r 5.0 2.0 0.6 2.8 6.6 10 5.1 3.6 8.0 0.1 0.1 0.1 5.2 11 6.0 3.6 9.L 2.I 1.5 3.0 8.1 T2 L) 3.6 5.5 r.9 1.5 3.8 6.L 13 5.0 4.3 5.6 5.1 0.8 11. 3 10 .1 T4 6.3 4.0 8.0 3.7 0.3 6.0 10 .0 15 5.5 3.0 6.0 1.5 0.5 3.0 7.0 16 4.3 3.5 5.0 4.7 0.8 11. 1 9.0 L7 5.0 4.3 5.4 5.6 1.0 6.2 10 .6 1B 4.6 4.L 5.4 8.1 2.0 20.6 12.7 19 4.6 4.0 5.6 6.5 7.2 7.0 11. 1 20 5.5 2.9 7.0 5.5 c.B 9.3 11.0 2L È:tr 4.0 6.1 15.8 5.0 20 .8 2r.0 22 5.2 4.2 6.5 11.0 11.0 li.0 16.2 23 6.5 4.0 8.3 20.3 12.6 30.3 26.8 24 6.1 4.0 8.1 L4.7 9.0 25.0 20.8 25 6.1 L2 7.8 2L.L tB.0 24.3 27 .2

Notes: Values show average of three replicates and t-he ranges in minrrtes. Rc3 starved for L2 hours. Tnf = non-feeding period, Th = handling time, TT = totãl time. í'

FIGURE 10

The handling time (Th) , non-feeding ti.me (Tnf ) , and total time (TT), of gglytulee (nc3¡ third instar larvae feeding on 25 A. aêgypti (ya4) fourth instar larvae. Values are an average of three replicates. 28

KEY (.D 20 o - T T, TOTAL l¿l T|ME x - t h, HANoLING Tt ME l- 18 o - :) T d, NON-FEED|NG TtME 16 =

=t4 bl o_o /t -J l-

r=u\rr'={\/:ì ,ì:){ft,r ,t-*-, V .'a-.- /t-^ ,z\ ¡-'/

68tOt2t4t6t8 20 22 ?4 26 NO. OF Yo4 EATEN BB

TABLE XVI . Graphoclerus (FgS ) third instar larvae feeding on 18 A. vexans (Vv+¡ fou.rth instai larvae.

No. Th R-ange Tnf Ranqie TT Yr¡4 mean mrn. max. mean ml-n. max. mean

1 7 .06 4.75 9.0 0.16 0 0 7 .22 2 7. 81 6.08 9.0 0.11 0.08 0.25 'Ì .92 3 8.39 6.58 I0 .5 0.03 c 0 8.42 4 8.2t 6 .00 12 .0 0.06 0 .01 0.16 8.27 5 7.7r 6.00 10 .0 0.78 0.01 3.0 8.49 6 7 ¿"É, 5 .16 11.0 0.30 0.01 1.0 7.75 7 6.20 s.00 6.9 0.57 0 .06 2.0 6 .77 B 6.62 4.66 8.3 0.73 0.02 1.0 7 .35 9 7.00 6.00 8.0 s.54 0 .16 8.0 L2.54 10 6.69 6.00 8.1 2.64 0.02 9.5 11. 33 11 6.75 4.50 B.B 3 .00 0.5 8.0 9.75 L2 6.66 4.00 8.5 3. 87 0.5 7.5 10.53 13 5. Bl 4 .16 8.5 6 .87 0 .06 13 .0 L2.68 I4 5 "20 3.33 7.L 2.69 0.02 7.s 7 .89 15 s. 83 0 6.5 9.25 5 .00 t5 .0 15.08 16 5.58 4.33 7.0 9.00 5.0 L2.0 14.58 17 6 .06 5.7s 6.3 9.50 6.0 13 .0 14.56 1B 6 .46 4.50 8.0 8.25 0.02 L2.0 14-70

Notes: Values show averege of three replicates and thre ranges in minutes. Rg3 starved 12 hours. Tnf = non-feeding period, Th = handling time, TT = total. time. í'

I¡TGURE 11

The hanciling time (Th) , non-feeding time (Tnf ) ,

and total time (TT') , of .GgaphoAesgg (ng3¡ third instar larvae feeding cn 16 A. vexans (Yv4) fourth instar larvae. Values are an average three replicates. KEY

O-TTTTOTALTIME X - Th T HANDLING TIME o - Tnf , NON-FEED|NG TIME t6

t4 CN bJ È12 f EE=lo z. b l¿J Å. ;2

46 Btotzt4 NO. OF Yv4 EATEN 90 forelegs reÍiove the prey remains from the mandibles (Plate XIV). The Graphoderus larva discards the prey remains by opening the mandibles and by moving the head in a dorsal and ventral direction once or- twice; the remains usually float free. No photographs were obtained for the discarding action by a Graphooerus larva.

E. The Non-feeding and Total Time The non-feeding time (Tnf) measured for both Colymbetes (Rci) and Graphooerus (Rgi) larvae included a variety of factors. Two of these factors were: (1) The rdigestive pauser after a prey ís eaten and during which the predator is not hungry enough to attack, (2) The rate of sea.rching ar waiting for a prey. Both these factors could not be separated by mere observation of the behaviour of either Colymbetes and Graphoderus larvae as ìt was difficult tcr tell when the rdigestive pauser stopped and searching for prey began The total time (TT) is the sum of both Lhe Th arrd Tnf and can be considered. as the time when the predator fj.nishes one prey, through a Tnf and Th, to the finishíng of the next meal. The T1' can also be considered as the time the prey are exposed to the predators i.e. the sum of all TT in that combination. Th.e observations on the Tnf anci TT \¿vere recorded from the sanie experiments as the SS and Th; thus the same procedure r¡/as used. The results are shown on Tables XIf , ,t.1

PLATE XlV

A Colymbetes (ncS¡ thiro instar larva remcr,'ing the remains of prey (mosquito larva) frorn mandibles with forelegs.

Note: The interval between pictures is 15.6 milliseconds. The length of each síde of the large squares in thre background is 6 .5 mm. 'l::;

ú.lJç' t i.- 92 XIII, XrV. XV and XVI. Hower¡er, only the results to be discussed are shown in Tables XV and XVI and figures 10 and 11. Both Figures 10 and 11 show curves that have a s-'i-mi1ar trend. At first, when Rc3 and Rg3 were hungry (they were starved. for 12 hours) the Tnf was s¡.rall- so the TT follol,¡ed the Th curve closely. After E mosquito larvae had been eaten the Tnf became larger, although the Th curve remained relatively consLant, the TT curve followeo the Tnf fluctuations. As the Rc3 and Rg3 became satiated. the Tnf and TT became larger and tended to level off for Rg3 (Figure 11). How- ever, the TT and Tnf in Figure 10 tended to become larg'er for Rc3. Although observations were not continuerd for a longer period it is expected that the TT arrd. Tnf of Rc3 would have lowered and fluctuated ahout a mean of probably 14.6 minutes (see Figure 3 B where normal feeding rate fluctuated about arr average of 4.lI Ya4 eaten/hour/L Rc3 or 1 Ya4 eaLen/I4.6 rninutes). These results pgree with hoiling's (1966) finciings that the hunger of a starved predartor is lowered with every feeding. However, hunger rises again after every feeding until the next meal. Because the density of prey was high enough the hunger of the predator gradually declined (this was manifested by an increased non-feeding time) to a stable level. Figure 10 shows an .r=-ncreasilg but f luctu- ating non-feeding time. These fluctuations may be e:

DTSCUSSTON AND CONCLUSTON

A considerable part of the life of d.ytisc;j_d beetle larvae is associated with the detection and capture of prey. Not only do the characteristics of the predator, such as the ability to detect and capture prey, but alão the characteristics of the prey, such as size and movement, determine prey capture. Indeed, the speed of attack of the predator on the prey is import.ant. Roeder (1959) has shown the imporLance of these times is not reflected by their magnitude, but by the relative times of attack by the predator as compared with the startle time of tlie prey. In Roederrs words: "A millisecond or so v¡ithin the nervous system must often mark the difference between the quick and. the d.ead" (Roeder, 1959, p. 290) . Roeder (1963) further explains that the success of both parties depends upon a short response time. Thus the preyrs evasive beliaviour and the predatorrs attack or efficiency are complementary to one another. Balfour-Browne (1950) and Galewski (1963) consiaered the swimming ability ano morphology of the larvae of the Dytiscid.ae to show an evolutionary trend of a graciuerl return or rstepping into the water'. This speculation for example, is manifested by the swimrning ability of the 94 95 dytiscid larvae i.e. the Hydroporines tend. to be dependent on vegetation as a support, the Colymbetines less sor while the Dytiscines are the least dependent and swim freely in open water. The prey capture methods seem to have developed along with the swimrrring abilities of the dytiscid larvae. The 'crawling' Colymþêtês (a Colymbetine) larvae, detect prey with mechanorece¡:tors and capture prey by thrusting the body forwards while still hanging onto a substrate. The 'swímming' Graphoder_us (a Dytiscine) larvae, on the other hand, probably detect prey to a large extent with their stemrLrata and capture prey by 'snapping' the head dorsally while swimming, free from any substrate. The Colymbêtes larvaer oñ the whole, are slower ano less accurate in prey capture than are the Graphoder'us larvae. The Colymbetes larvae, because of their crawling habit, generally capture slow nloving animals, such as Corethra, ceratopogonids, and chironomids that are found crawling in algae (Wilson, 1932) . The Graphoderus larvae, on Lhe other hand, with their free swimming existence and prey capture methoO are better suited than Colymh,etes larvae in capturing fast moving free-swimming animals such as mosquito larvae, crustaceans and corixids that are found in open water. The data obtained from the Dytiscus larval strike suggest that Dytiscus larvae possess an intermediate prey capture method between that of Colymbetes and Graphoderus 96 larvae. DYtiscus larvae usually use the whole body in a dorsal thrust of the head towards the prey. Unlike Colymbetes larvae Dysticus larvae can capture prey while swimming. Although Dytiscus larvae can thrust the head dorsally at a rapid rate the whole body is used in the thrust and not in the specialized way of bending the thorax as Graphoderus larvae. Thus the results from this thesis inclicate that Graphoclerus larvae have probably developed more specialized swimming and prey capture techniques than Colymbetes larvae (and possibly Dytiscus larvae), in their evolutionary trend to adapt to an aquatic existence. CHAPTER VITI

SUMMARY

1. Both adult Colymbetes sculptilis (Harris) and Graphoderus occide.ntalis Horn can be collected through.out the year from ponds around vüinnipeg. The two species overwinter under ice of permanent ponds in the ad.ult stage. colYmbetes starts breeding in early April and larvae are found until late June. Graptroderus starts breeding in mio- May and larvae are found until late August. 2. The complete life cycle through eggr three larval instars, pupâ to adtilt, requires 3B-4C days for both species reared at 20oC t 2oC. 3. Colymbetes larvae possess thick strong legs and can swím efficientry by moving their legs, but are usually found to crawl over a substrate, such as aquatic algae and plants. 4. Graphoderus lan¡ae possess long swi.mming hairs on their slender legs and almost continuously swim around in the water. They have large lateral tracheal trunks filled with air and unlike colymbetes larvae they are able to remain almcst stationary at an.y level in the water. without leg movement. 5. Both Cofymbetes and Graphoderus larvae possess

97 98 a dermal light sense and manifest a dorsal light reaction. 6. Colymbêtes third instar larvae possess wider heads and longer antennae, labial palps and maxillary palps than Graphoderus third instar larvae, but .C..pfrod.ru= ha= larger stemmata.

7 . The anterior process hairs;, located on the anterior margj-n of the headr possesseci by both species, function as mechanical sensors and are used in the hanciling arrd probably the detection of prey. B. g!1yn'ìJcglg¡_iarvae detect Flrey mainly with me.chano- receptors p::obably with the stout sensory setae orr the legs. Graphoderus larvae probably use vision (especially the two dorsal pairs of stemmata) to some extent in prey detection. 9. CoJyn,l¡etes larvae capture prey by a rapid thrust forward of the body, while still hanging onto the substrate. The strike is d.ivided into four stages; opening wide of mandibles, the forward thrust of whole body, closing manC- ibles and arrest of the forward momentum or thrust. Although the complete operation takes an average of I43.4 milliseconds the forward thrust takes âri average of 68.5 milliseconds (including stages 2, 3 , 4) . The Colymbetês Iarva will strike at prey wlien the anterior process is an average of 2.L6 mm from prey. Graphoderus larvae capture prey by a rapid. dorsal thrust of thorax and head while swimming. The strike is divided into five stages; arching of thorax, opening of 99 mandible (during first stage) , thrusting head forr,.:ard, closing of mandibles, and arrest-ing forward thrust. Although, the complete operation takes an averagie of 308.9 milliseconds, the thrust forward of head only takes 25.3 milliseconds (including stages 3, 4, 5) . fhe Graphoclergq larva, will strike at prey when the anterior process is an average of 4.03 mm from prey. 10. Colynrbetes la.rvae captured the nrajority of mosquito larvae by piercing the abdominal region while Graphoderus larvae captured the majority'of mosquito larvae by the thorax. 11. The average strike success was 54.52 for all Colymbetes Iarva,l instars ani. 75.0% for all Graphoderus larval instars when feeding on all immature stages of A. aegypti. L2. The strike si-lccess of Colymbetes larvae was found to be more affected by prey size than that of

Graphoderus larvae. The strike success falls below 70% for Colymbetes third- instar larvae feed.ing on mosquito larvae below 0.5 mgm anCr above 2.0 mgm while for 9fgpþgqCrug larvae it is below 0.1mgm and. above 2.5 mgm. 13. Corixids and nnayf 1y nyrnphs are harder to catch than nrosqu-ito larvae because they are stronger swim¡rers and have a harder cuticle for the mandibles cf Gra.phode.rus larvae to take hold. J.4. Plotting the weight of the prey versus the 100 handling time of the predator results in linear curves and so are directlY ProPortional. 15. The larger instars have a shorter hanciling time than the smaller instars when feeding on the same size of prey. 16. The handling times of the third instar larvae of Colymb,etes and Graphoderus were not a"ffected by hrunger when f eeding on mosciuito larvae. L7. The non-feeding time varies with the hunger of the predator. The non-feeding time is short when the predator is hungry, but becomes longer as the predator becomes satiated and eventually with normal feed'ing the non-feeding time fluctuates about a mean. 18. It is plausible to regard the Predatory hehaviour of Graphoderus larvae to be more specialized than that of Cclymbetes larvae. Graphoderus larvae are well adapted to capturing free swimming aquatic animals while Colymbetes larvae are better adapted to a crawling existence and capturing slow moving and crawling Prey. APPENDIX T

LIST OF SYMBOLS

Es Bottom side of container

D Dark

F Analysis of Varj-ance Statistic i Stage: I, 2, 3, 4 instar larvae or p = pupa

K Kil1ed pre1z, but did not eat L Light LI First instar larva

T,2 Second instar la.rva L3 Third insta.r larva

Mm Middle rniddle part of container

Ms Middle sid.e of container

R Preclator Rcl Coly-mbeLes first instar larva

Rc2 Col)rmbetes second instar larva

F.c3 Colynbetes third instar larva Rgl Graphoderus first irrstar larva

Rg2 Graphoderus second instar larva

Rg3 Graphoderus third instar larva qtrl Stanciard. Error

SS The strike success or the success of capturi-ng a

prey once a strike is mad.e TT The tctal time or the peri oa predator is exposed. to prey 101 r02 Th - The handling time or time spent subduing, eating prey

TM Top middle part of container Tnf The non-feeding time or period between feedings m^ J_5 Top side of container t Student t test statistic x - A number Y - Prey

YC Corixid nature nymph

YD Daphnia sp. adult YL - Lestes sp. young nymph

YM - Mayfly rnature nymph Ya1 - ê. aegypti first insta.r larva Ya2 - â. aegypti second instar larva Ya3 - â. aegypti third instar larva Ya4 - â. aegypti fourth instar larva YaP - å. aegypti pupa Yf.4 - å. fitchii fourth instar larva Ys4 - A. sticticus fourth instar larva Yv4 - A. vexans fourth instar larva REFERENCES CITED

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Lorenz , K. Z. 1952. King Solomonrs Ring. Meth-uen & Co - Ltd. (f968 edition - Univ. Paperbacks). Macan, T. T. L96I. Ecology of Aquatic Insects. Ann. Rev. Ent., 7, 26L-288. Mittelstaedt, Horst. 1957. Prey Capture in iviantids. Recent Advances in Invert. Physiology. Ed. Scheer B- T- U. of Oregon Ptr-blications. 51-71. Morris, R. F. 1963. The effect of pred-ator gge anci prey defense on the functional response of Podisus macr-rliven'tris Say to the density of liy_phantr:'-a cunea @nt. 95: 1009-1020. Mozley, A. 1932. A biological study of a temporary ponci in western Canada. Amer. Na.t. 662 235-249. Mukerji, M. K. and E. J. LeRoux. L969. The effect of preciator age on the functional response of Pod'ius maculiventiis to prey size of Gallària melrõñêTIã. õail-EãE:-E z t|q-zzt . Nachtigall , Vü. 1965. Locomotion : Swimn-LJ-ng (hydrociYnamics ) of-aquatic insects. From the Physiology of Insecta, Vot. 11. Ed. Morris Rockstein. pp. 255-28L. Odum, E. P. 1963. Ecology. Holt, Rinehart and Vlinston, N. Y. Pennak, R. W. 1953. Fresh-water invertebrates of the United States. Ronald- Press Co., New York. Popham. E. J. I94A. The variation in the colour of certain - species of Arctocorisa (Hemiptera, Corj-xidae) and its siEnifica-ncõl--Proc. Zoo. Soc. Lond. 4I (3) : L35-I72 - . 1942. Further e)