JOURNAL OF MORPHOLOGY22t:ttl-!t9 (1994)

FactorsGoverning the Stickinessof CribellarPrey Capture Threadsin theSpider Family a BRENT D. OPELL I ?:x{":;Ij:f,";:!rf:#::f :t::{,"P,*xr:"?;;'it;,, I ABSTRACT The surface of a cribellar prey capture thread is formed of thousands of fine, looped fibrils, each issuing from one of the spigots on an oval spinning plate termed the cribellum. This plesiomorphic capture thread is retained by members of the family Uloboridae, in which its stickiness differs among genera. An examination of five cribellar thread features in nine uloborid speciesshows that only the number of fibrils that form a thread explains these differences in thread stickiness. Neither the physical features of these fibrils, nor the manner in which they are combined to form threads differs among species. Threads produced by orb-weaving species contain fewer fibrils than those produced by speciesthat build reduced webs. Relative to weight, the number of fibrils that form a cribellar thread is greatest in simple-web speciesof the genus Miagrammopes,less in triangle-web speciesof the genus Hyptiotes, and least in orb-weaving speciesrepresenting five genera. A transfor- mational analysis shows that change in the number of cribellum spigots is directly related to change in the stickiness of cribellar thread. This direct relationship between the material invested in a cribellar thread and its sticki- ness may have been a limiting factor that favored the switch from the dry cribellar threads of uloborids to the adhesive capture threads produced by other orb-weaving families. o 1994Wiley-Liss, Inc.

Two types of prey capture thread are found ship will provide a better understanding of in spider orb-webs, dry cribellar thread, and how cribellar threads operate and what fac- moist adhesive thread. Of the two, cribellar tors may have favored the switch from cribel- thread is more primitive. It is present in the lar to adhesive capture threads. aerial webs produced by the most primitive The family Uloboridae is well suited for members of the largest spider infraorder, the this study, both because it contains the only '91), (Coddington and Levi, orb-weaving that produce cribellar and is retained in the orb-webs spun by mem- threads, and because the stickiness of these bers of the family Uloboridae (Opell, '79). threads differs significantly among its mem- The more speciosesister group of the Ulobo- bers (Table 1; OpelI,'94a). These differences ridae-Deinopidae lineage contains six fami- in stickiness are associated with changes in lies of orb-weavers, the capture threads of web architecture. The family's plesiomorphic which depend on droplets of adhesive for and most common web form is the orb-web '92). their stickiness (Foelix, '82; Vollrath, (Coddington and Levi, '91), although two of This switch from cribellar threads, the sticki- its genera construct reduced webs. Members ness of which appears to be derived from of the genus Hyptiotes construct triangle- '93), mechanical forces (Opell, to adhesive webs that have four radii between which capture threads, the stickiness of which ap- cribellar threads extend, whereas members pears to result from their chemical proper- of the genus Miagramnxopes construct still '90), ties (Vollrath et al., represents a major simpler webs, on whose irregularly branch- evolutionary change in the design of spider ing lines cribellar threads are deposited (Lu- orb-webs.This study examinesthe physical bin, '86; Lubin et al., '78; Opell,'82,'90). As composition of cribellar thread in an effort to these webs becomemore reduced, theircribel- determinewhichfactorsmostaffectitssticki- lar threads become stickier (Table 1-;Opell, '94a). ness.Aclearerunderstandingofthisrelation- This study capitalizes on these differ-

o 1994 WILEY-LISS. INC. LL2 B.D. OPELL encesin stickiness to determine how the fea- also sticks to much smoother surfaces, such tures of a spider's spinning apparatus and of as glass, steel, graphite, and beetle elytra '80; the silk that it produces determine the sticki- (Eberhard, Peters,'86; Opell, '94b). Scan- ness of cribellar thread. ning and transmission electron microscope The cribellar threads of Uloboridae each studies (e.g.,Kullmann and Stern, '81; Opell, consist of three components: 1) a pair of '79,'89,'90; Peters,'84, '86,'92) revealno central, supporting axial lines, each with an adhesive droplets on cribellar fibrils, indicat- average diameter of 273 nrlr;2) a network of ing that unknown forces contribute to the 30-56 smaller paracribellar fibrils that ap- stickiness. Electrostatic charge has been sug- pear to form a superstructure around the gested as a possible force (Peters, '84, '86), axial fibers: and 3) a cloud of thousands of although there are no data to support this very thin, looped fibrils, each with a diameter hypothesis. of about 18 nm, that form the outer sheath of Regardless of the mechanisms involved, (Figs. the cribellar thread la,b; Kovoor and the stickiness of cribellar threads results from Peters, '88; Peters, '83, '84,'86,'92; Peters '80). the interaction of its fibrils with the object and Kovoor, The axial lines are spun that it holds. Using nine uloborid species, from spigots on the posterior spinnerets, the this study tests the hypothesis that the num- paracribellar fibers from spigots on the lead- ber of fibrils that form a thread is the princi- ing edge of the median spinnerets, and the pal factor determining its stickiness. I at- fibrils from spigots on the cribellum, an oval spinning field, located on the ventral surface tempted to falsify this hypothesis by first of the abdomen, just anterior to the spinner- determining if the manner in which cribellar ets (Fig. lc). Each of the cylindrical spinning fibrils were combined to form a thread af- spigots of the cribellum (Fig. ld) contributes fected thread stickiness and then by determin- one fibril to the cribellar thread. Using a setal ing if the physical features of the fibrils them- comb on its fourth legs, a spider draws fibrils selves affected thread stickiness. Finally, I from the cribellum and combines them with determined if fibril number accounted for axial lines to form a composite cribellar prey cribellar thread stickiness. capture thread (Eberhard, '88). Although it is possible to measure the fea- The mechanism by which a cribellar thread tures of cribellar fibrils. their small diam- holds an object is not fully understood (Opell, eters and convoluted arrangement (Fig. 1b) '93). By catching on the setae and surface make it impossible to count the number that irregularities of prey, the coiled fibrils of the form a cribellar thread (Fig. 1a). However, thread appear to act like the soft part of a this number can be determined by counting Velcro fastener as it catches on its counter- the number of spigots on the cribellum that '79). part (Opell, However, cribellar thread producedthe thread (Figs. lc,d).

TABLE 1. Comparison of cribellar thread and cribellar fibril features in nine species of Uloboridael Cribellar thread stickiness: pN/mm contact /p"N/mm contact \ ,et", *"ic[t Thread width | I Node Internode Node \ rnmg I Cribellum width diam. nm diam. nm spacing nm Orb, webs Wait k er a waitq.ke r ens i s 15.5(1.9) 0.43-+ 0.04 (35) 17.4 -+ 1.5 (6) 10.1 -r 0.6 (6) 51.4 * 8.9 (6) Siratoba. referena 11.5(2.8) 0.52* 0.04(23) 17.5 -+ 1.9 (4) 8.7 -+2.4 @) 63.9 * 22.9 (4) Uloborus glontosus 15.4(1.8) 0.34-f 0.08(70) 18.4 -r 1.4 (5) 10.4 -r 1.1 (5) 54.5 * 14.3 (5) Octonobasinensis 17.0(1.5) 0.42-' 0.07(50) 78.8 ! 2.4 (5) 10.7 +-0.9 (5) 87.7 -+ I2.9 (5) P hilop oneI la ari zonic a 15.0(1.2) 0.35-r 0.04(26) Triangle-webs Hyptiotes cauqtus 26.2 (3.4) 0.39* 0.04(51) 19.4 -r 1.0 (5) 10.3* 0.3 (5) 63.1 1 10.0(5) Hyptiotes gertschi 29.8 (3.2) 0.40-r 0.04(33) Simple webs Miag rarnm ope s animotu s 31.5(6.9) 0.43-+- 0.12 (104) 16.2-+ 0.5 (4) 8.6 * 0.7 (4) 42.3 '' 11.1(4) Miagrarnmopes species 24.4 (6.6) 0.40_r 0.09 (24) 18.4 -+ 1.0 (4) 9.5 * 1.0 (4) 52.7 * 73.1(4) rCribellar thread stickiness values are taken from Opell ('94a). For other values: mean + 1 standard deviation (sample size). FACTORS GOVERNING CRIBELI,AR THREAD STICKINESS 113

Fig. 1. Uloborid cribellar thread and cribellum features. a: Cribellar thread of Hyptiotes cauatus. b: Cribellar fibrils of Miagrarnmopes sp. c: Cribellum of Waitlzerq waitakerensis. dz Cribellar spigots of Waitkera waitakerensis.

MATERIALSAND METHODS western Washington; and two simple-web spe- Speciesstudied and study sites cies: Miagranxnxopes animotus (Chickering, '68) I studied the cribellar threads and cribella (X live weight 5.26 mg, SD 2.09), from of adult females (adult male uloborids do not the Luquillo National Forest of Puerto Rico construct capture threads) of nine species, and an undescribed green Miagramnxopes representing seven of the family's 18 genera species(X live weight 3.92 mg, SD 1.36) from (Fig. 2). Included were five orb-weaving spe- the Heredia Province of central Costa Rica. cies; Waitkera waitakerensis (Chamberlain, '46) (X live weight 8.79 mg, SD 2.99), from Crib ell ar t hread featur es New Zealand's North Island; Siratoba refer- Uloborids construct their webs at night or '64) ena (Muma and Gertsch, (X live weight early morning. Therefore, I collected thread 4.31 mg, SD 1.14) andPhiloponella arizonica from webs between 5:00 and 10:00 a.m. to (Gertsch, '36) (X live weight 13.35 mg, SD obtain fresh samples that were not contami- 4.49), from the Chiricahua Mountains of nated or damaged. Threads were collected on southeastern Arizona; Uloborus glomosus microscope slides to which raised, parallel (Walckenaer, 1841) (X live weight 9.39 mg, supports were glued at 4.8 mm intervals. SD 2.66) from southwestern Virginia; and Double-sided tape atop each support securely sinensis (Simon, 1880) (X live anchored the threads and maintained their weight 12.86 mg, SD 4.30), an introduced original tensions. All threads used in this Asian species, collected from free ranging study were examined under a dissecting mi- populations in greenhouses at Virginia Poly- croscope to assure that only intact strands technic Institute and State University. I also were included. studied two triangle-web species: Hyptiotes I measured the features of cribellar fibrils cauatus(Hentz, 1847) (X live weight 7.76 rng, by transferring cribellar threads from micro- SD 3.60), from southwestern Virginia, and scope slide samplers to Formvar-coated cop- '35) Ilyptiotes gertschi (Chamberlin and Ivie, per grids and examining them without fur- (X live weight 9.68 mg, SD 2.95), from north- ther treatment at x 130.000 under a IL4 B.D. OPELL

transmission electron microscope.From en- WAITKERA larged photographic prints, I measured 1-4 fibrils per specimen, including only fibrils that showed no evidence of having been stretched. From each fibril, I measured the SIRATOBA width of three nodes and three internodes and the spacing of nodes in a fibril segment that contained 4-22 (X : 9) nodes (Fig. 1b). On each of the 12 days that I examined HYPTIOTES these threads, I also photographed a grating replica (2,160 lines per mm) at x 130,000 to MIAGRAMMOPES precisely calculate specimen magnification ULOBORUS and determine if magnification was consis- tent from day to day. The standard error of the mean width of one of the replica's 463 nm OCTONOBA wide lines, as measured from photographic PHILOPONELLA negatives,was only 2.7 nm. I evaluated the manner in which fibrils issuingfrom the cribellum are combinedwith axial threads to form a cribellar thread by comparing the cribellum's width with the width of the cribellar thread it produced. To Fig. 2. Cladogram of the Family Uloboridae from determine the diameter of the puffs that form Coddington (1990), showing the phylogenetic positions of these threads, I measured the diameter of the seven genera included in this study. one puff from three regions of a thread and

fg5 Miagr.sp. 24 h F h.'.,1tr5Miagr.animotus 1t4h F

SIMPL E.WEBS { t12 Hypt.cavatus 35,,,,.9 ,, , ,,F f34 Hypt.gertschi34t TRIANGLE. WEBS a,,2.6 Philopone lla f,6e

d0{ Octonoba g4dl e 'e.9 Uloborus 2,9,.e

ba3 S iratoba Q2,,,,b|

a' :(116 Waitkera S6. a ORB-WEBS

8000 6000 4000 2000 0 o soo 1000 lsoo 2000 Spigot Number Weight-specific Spigot Number (Number/mg)

Fig. 3. Comparison of the absolute (left) and weight-specific (right) cribellar spigot numbers of nine uloborid species.Numbers within each box indicate sample size; specieswith different letters have values that differ statisticallv: error bars denote -r-1 standard error. FACTORS GOVERNING CRIBELLAR THREAD STICKINESS 115 used their mean value as a spider's value. are the same as those obtained with a section After a sample of a spider's cribellar thread of a fleshfly (Sarcophagabullata) wing (Opell, '94a) was taken, the spider was collected and its and, therefore, provides a reasonable live weight determined before it was pre- estimate of the performance of a cribellar served in 807oethanol. I removed the cribel- thread. However, different insect surfaces lum of each preserved spider, mounted it in yield significantly different stickiness values water-soluble medium on a microscope slide, (Opell, '94b). and examined it under a compound micro- scope equipped with differential phase con- Relationship of spigot number trast optics. With a computerized digitizing and thread stickiness apparatus, I measured the width (transverse dimension) and surface area of each cribel- As the species included in this study are lum and determined the density of spinning evolutionarily related to different degrees (Fig. spigots in three regions of the cribellum: one 2), their values are not strictly indepen- at its anterior midline, one at its posterior dent. Species that are more closely related lateral margin, and one midway between would be expected to be more similar than these two regions. I then multiplied the mean those that are less closely related, making it spigot density of each cribellum, as deter- inappropriate to analyze the associations of mined from these three density measure- weight, cribellum spigot number, and cribel- ments, by its total surface area to determine lar thread stickiness with traditional regres- (Harvey '91). the total number of spigots on its surface. sion techniques and Pagel, This method of determining the number of Therefore, I employed the method described ('86, '87) fibrils that form a cribellar thread assumes by Huey and Bennett for evaluating that either all or a constant percentage of the the direction and rate of evolution of two cribellum's spigots always operate. continuous variables, the states of which are AII values were tested with a Shapiro-Wilk hypothesized to be functionally linked or co- W-statistic to determine if they were nor- adapted. This method has two steps: 1) the mally distributed (P > 0.05). If they were, inference of a group's ancestral character T-tests (T) were used for pair-wise compari- states from the states of its extant members sons and analysis of variance tests (ANOVA) and 2) the analysis of changes in these char- for multiple comparisons. If one or more acters from these hypothetical ancestors to values being compared were not normally the extant taxa descendedfrom them. If this distributed, Wilcoxon 2-sampletests (W) were analysis shows that changes (both positive used for pair-wise comparisons and Kruskal- and negative) in the two characters are signifi- Wallis K-sample tests (I{\M) for multiple com- cantly correlated, then their states can be parisons. Values were consideredto be signifi- considered to be functionally linked or to cantly different If P < 0.05. have coevolved. I computed the states of three characters in the hypothetical ancestors of the seven Cribellar thread stickiness genera included in this study: spider live The individuals, the cribellar thread and weight, number of cribellar spigots, and features of which were measured in this cribellar thread stickiness. I included weight study, are the same individuals of which the in this analysis to permit evaluation of the cribellar thread stickiness was measured ear- effect of spider size on cribellar thread sticki- lier (Opell,'94a). As describedmore fully in ness. Unless this variable is ruled out, even a that study, stickiness was measured by first significant association between spigot num- pressing with a standard force a 2 mm wide ber and cribellar thread stickiness may not piece of fine sandpaper against a 4 mm long fully account for cribellar thread stickiness. strand of cribellar thread and then pulling I employed the scheme of iterative averag- the sandpaper plate away from the thread at ing described below to determine the state of a slow, constant rate. The force required to these three characters in hypothetical ances- pull the plate from a strand of cribellar thread tors ,{1-.4.6 (Fig. 4). In these equations, the was then divided by the width of the plate value of each genus is represented by the first (measured to the nearest 20 pm) and sticki- initial of its name. All genera but Hyptiotes ness was reported as pNewtons of force per and Miagran.Lnxopesare represented by a mm of thread contact with the sandpaper single species.Therefore, the mean values of plate. Values obtained with a sandpaper plate the two Hyptiotes and the two Miagram- 116 B.D. OPELL nxopesspecies are used in order not to over- RESULTS represent these genera. Cribellar thread width and fibril features Ar: M + A3l2 Table 1 presents the ratios of cribellar thread width to cribellum width and the three A2:W+S+A3/3 features of cribellar fibrils that were mea- A3:(W+S)12+A4+A5l3 sured in the nine species studied. Both the absolute and weight-specific stickiness val- A4: H + M + ((U+ ((O+P)12))12\13 ues of reduced-web species are greater than those of orb-web species(Table 1; Opell, '94a). A5: (((H+ M)12)+ U + ((O+ P)12))13 Therefore, the simplest and most liberal indi- A6:U+O+P/3 cation that differences in the way in which cribellar fibrils are combined to form a thread Next, I computed the change that occurred or that differences in the fibrils themselves between the most recent hypothetical ances- affect cribellar thread stickiness is the demon- tor of each genus and that genus. For ex- stration that one or more of these variables ample, the transition from hypothetical an- differs significantly between threads pro- cestor A2 to Waitkera involved a 1.86 mg duced by reduced-web and orb-web species. increase in weight, the addition of 303 cribel- These differences are not found in the four lum spigots, and an increase in cribellar features that were investigated: 1) cribellar thread stickiness of 0.13 pN per mm of con- thread width to cribellum width, (W, tact. Changes in cribellar thread stickiness P : 0.47), 2) diameter of fibril nodes were then regressed against changes in spi- (ANOVA, P :0.93), 3) diameter of fibril in- der weight and changes in cribellum spigot ternodes (ANOVA, P : 0.24), and 4) spacing number to determine if one or both of these of fibril nodes (ANOVA. P - 0.I2). There- parameters were associated with changes in fore, these features can be eliminated as ma- cribellar thread stickiness. jor determinants of cribellar thread sticki-

WAITKERA 8. 79 Fibril number 390s 15.45 Figure 3 presents the absolute and weight- specific numbers of cribellum spinning spig- SIRATOBA for 4.31 ots each species,numbers that are consid- lBOO ered equivalent to the absolute and weight- 11.46 specific number of fibrils in the threads produced by these species.In contrast HYPTIOTES to the 8.72 four features just discussed, these two in- 7500 dexesdiffer with web type. The absolute num- 28.O1 ber of spigots differ among orb-weavers MIAGRAMMOPES (ANOVA, P :0.0001), but each specieshas 4.59 fewer than the pooled numbers of the two BO72 triangle-web species (W, P : 0.0001 in 27.96 all cases). The number of spigots do not differ ULOBORUS between the two triangle-web species (W, 6.66 P : 0.46) and their mean pooledvalue (7 4717 ,476, 15.38 SD 1510) does not differ from that of the Costa Rican species of Miagrammopes (W, OCTONOBA P : 0.12). This simple-webspecies had fewer 12.86 4098 spigotsthan M. animotus (W, P : 0.0001). 17.O2 Weight-specific spigot number differs among the orb-weaving species (KW, PHILOPONELLA WEIGHT IQ 2A P : 0.0001),but eachhas fewer than Hyptio- SPIGOTS 5110 tes gertschi (T, P : 0.0001 for Philoponella STICKINESS 14.97 arizonica; W, P : 0.0001 for all other spe- cies). Hyptiotes gertschi has fewer spigots Fig. 4. Transformational analysis of spider weight in than Hyptiotes cauatus (W, P: 0.020). Val- mg, cribellum spigot number, and cribellar thread sticki- ness in pN per mm of contact based on the values of ues of the two species of Miagramrnopes do seven genera. not differ (W, P : 0.35) and their combined FACTORS GOVERNING CRIBELI.AR THREAD STICKINESS tL7

mean (I,9I7lmg, SD 647) is greater than der relates to the stickiness of its cribellar that of ^F/.cauatus (W, P : 0.0001). thread. Thus, differences in the number of Regardless of their phylogenetic position, fibrils that form a cribellar thread and not orb-weaving species have both absolute and differences in the fibrils themselves appear to weight-specific cribellar spigot numbers that govern the stickiness of the thread. are more similar to one another than to those Atwo-fold increase in fibril number doubles of either triangle-web or simple-web species. the thread's stickiness (Fig. 5). Consequently, Thus, like changes in cribellar thread sticki- in the Uloboridae. the increased cribellar ness, changes in cribellum spigot number thread stickiness that is associatedwith web appear to reflect changesin web architecture, reduction is achieved at the cost of greater not phylogenetic relationships within the material investment in cribellar threads. family (Fig. 2). This is demonstrated by com- Relative to spider weight, the cribellar threads parisons of the sister orb-web genera Oc- of triangle-web specieshave, on average, 2.3 tonoba and Philoponella and the sister re- times more cribellar fibrils than those spun duced-web genera Hyptiotes and by orb-weavers and the threads of simple- Miagramtnopes. Weight-specific cribellar web species have 2.0 times more cribellar spigot number differs by only 64 spigots/mg fibrils than those of triangle-web species(Fig. between the orb-weavers, whereas it differs 3). Thus, the increased cribellar thread sticki- by an average of 949 spigots/mg between ness that is associatedwith web reduction in Hyptiotes and Miagrammopes. The greater the uloborids is achieved at the cost of in- similarity between the values of Octonoba creasedinvestment in cribellar fibrils. As web andPhiloponella is best explained by the fact architecture becomes simpler, these changes that they produce the same type of web, may serve to reallocate silk from the web's whereas Hyptiotes and Miagranaftiopes pro- non-sticky supporting elements to its sticky duce different web types. prey capture threads. Effect of number on thread stickiness fibril Implications for origin of adhesiue Congruent changesin the number of cribel- capture thread lum spigots thread and cribellar stickiness This study demonstrates the constraints of (Fig. 3, Table 1) support the hypothesis that producing cribellar capture threads: any increased cribellar thread stickiness is change in thread stickiness requires changes achieved by an increase in cribellum spigot in both the spider's spinning apparatus and number. More rigorous support for this func- the amount of silk that it must produce. In tional associationcomes from the transforma- contrast, studies of adhesive threads show tional analysis of spider weight, cribellum that their adhesive droplets have complex spigot number, and cribellar thread sticki- '91: chemical properties (Townley et al., ness.When change in the stickiness of cribel- Vollrath et al., '90) and suggest that changes lar thread is regressedagainst change in spi- in the chemical composition of these droplets der weight, no relationship is found (DF : 5, can affect the thread's stickiness. Unlike t: 0.54, P : 0.62). In contrast, change in cribellar threads, the stickiness of which can cribellar thread stickiness and change in be increased only by quantitative changes in cribellum spigot number produces a signifi- thread composition, both qualitative and cant regression (Fig. 5; DF : 5, t :3.51, quantitative changes probably alter the sticki- P : 0.017). This analysis indicates that the ness of adhesive threads. Additionally, cribellum spigot number and cribellar thread changes in the stickiness of cribellar threads stickiness of Octonoba, Philoponella, and must be achieved by anatomical changes in Waith,erahave changed little from their ances- the cribellum and associated changes in the tral values, whereas the values of Hyptiotes setal comb that cards the fibrils from it. In andMiagrarnnxopes have increasedand those contrast, changes in both the composition of Siratobo and Uloborus have decreasedfrom and amount of adhesivedeposited on a thread their ancestral values. could be achieved with little or no change in the morpholory of a spider's spinning appara- tus. Retatio n s hip &f::rfiilt, number and. The energetic (behavioral) costs of spin- thread stickiness ning cribellar and adhesive threads have not Of the five features examined, only the been compared, although cribellar thread ap- number of spigots on the cribellum of a spi- pears to be more costly to produce. The pro- 118 B.D. OPELL

o o t! z Y o F o OC qg f! c, EO Fo GE trE co E4 o-= = UJ (t p= O.O17 = O.71 z R2 l= 2.64x1O-3x + o.25 o -6 -2000 -1000 0 1000 2000 CHANGEIN CRIBELLUM SPIGOT NUMBER

Fig. 5. Regression ofchanges in the values ofcribel- Figure 4. Letters are the first initials of the genera whose lum spigot number and cribellar thread stickiness from values are plotted: Hyptiotes, Miagrammopes, Octonoba, hypothetical ancestors 42,1'4, A5, and A6 to their dece- Philoponella, S iratoba, Uloborus, and Waitkera. dent genera, as determined from values presented in duction of an adhesive thread requires only for Enerry and Environment Sciences' El that, as axial fibers are spun, they are coated Verde field station in Puerto Rico, the Orga- with adhesive from adjacent spigots on the nization for Tropical Studies' La Selva field same spinnerets. However, when a cribellar station in Costa Rica. and the American Mu- thread is spun, cribellar fibrils must first be seum of Natural History's Southwestern Re- drawn from the cribellum's spigots with suf- search Station, in Arizona. I thank Denis and ficient force to polymerize them and then be Jill Gibbs for their help and hospitality dur- combined with the axial fibers. Increasing ing field studies in New ZeaIand. This mate- the amount of adhesiveadded to axial threads rial is based upon work supported by the doesnot appear to have a very large energetic National Science Foundation under grant cost. However, more enerry is probably re- BSR-8917935. The Government has certain quired to polymerize and manipulate a rights in this material. greater number of cribellar fibrils. Alongwith the material costs of producing more cribel- LITERATURECITED lar fibrils, this energetic cost may have fur- ther constrained increases in cribellar thread Chamberlain, G. (1946) Revision of the Araneae of New Zealand.II. Rec.Auckland Inst. Mus. 3:85-97. stickiness and favored the origin of adhesive Chamberlin, R.V., and W. Ivie (1935) Miscellaneousnew thread. American spiders. Bull. Univ. Utah26:l-79. Chickering, A.M. (1968) The genus Miagrarnmopes (Ara- ACKNOWLEDGMENTS neae, IJloboridae) in Panama and the West Indies. Breviora (Mus. Comp . Zool.) 289: l-28. I thank Robin Andrews, Paula Cushing, Coddington, J.A. (1990) Ontogeny and homolory of the and William Eberhard for comments on ear- male palpus of orb weaving spiders and their potential lier versions of this manuscript. Paula Cush- outgroups, with comments on phylogeny (Araneoclada: ingand Samantha Gassonhelped with sticki- Araneoidea, Deinopoidea). Smithson. Contrib. Zool. ness measurements and Anne Robinson with 496:I-52. Coddington, J.A., and H.W. Levi (1991) Systematics and determinations of cribellum spigot numbers. evolution of spiders (Araneae). Annu. Rev. Ecol. Syst. Field studies were conducted at the Center 22:565-592. FACTORS GOVERNING CRIBELI.AR THREAD STICKINESS 119

Eberhard, W.G. (1980) Persistent stickiness of cribellar Opell, B.D. (1989) Functional associations between the silk. J. Arachnol. 8:283. cribellum spinning plate and capture threads of Mia- Eberhard, W.G. (1988) Combing and sticky silk attach- grammopes animotus (Araneida, Uloboridae) . Zoornor- ment behaviour by cribellate spiders and its taxonomic pholory 108:263-267. implications. Bull. Br. Arachnol. Soc. 7:247-251. Opell, B.D. (1990) The material investment and prey Foelix, R.F. (1982) Biolory of Spiders. Cambridge: Har- capture potential ofreduced spider webs. Behav. Ecol. vard University Press. Sociobiol.26:37 5-381. Gertsch, W.J. (1936) Further diagnosesof new American Opell, B.D. (1993) What forces are responsible for the spiders.Am. Mus. Novitates 852:L-27. stickiness of spider cribellar threads? J. Exp. Zool. Harvey, P.H., and M.D. Pagel (1991) The Comparative 265:469-476. Method in Evolutionary Biology. New York: Oxford Opell, B.D. (1994a) Increased stickiness of prey capture University Press. threads accompanying web reduction in the spider Hentz, N.M. (1847) Descriptions and figures of the Ara- family Uloboridae. Funct. Ecol. 8:85-90. neidesof the United States. Boston J. Nat. Hist. 5;443- Opell, B.D. (1994b) The ability of spider cribellar prey 478. capture thread to hold insects with different surface Huey, R.B., and A.F. Bennett (1986) A comparative ap- features.Funct. Ecol. 8: 145-150. proach to field and laboratory studies in evolutionary Peters, H.M. (1983) Struktur und Herstellung der Fang- biolory. In M.E. Feder and G.V. Lauder (eds):Predator- fiiden cribellater Spinnen (Arachnida: Araneae). Ver. Prey Relationships: Perspectives and Approaches for naturwiss. Ver. Hamburg 26:241-253. the Study of Lower Vertebrates. Chicago: University Peters, H.M. (1984) The spinning apparatus of Ulobori- Chicago Press, pp. 82-96. dae in relation to the structure and construction of Huey, R.B., and A.F. Bennett (1987) Phylogenetic stud- capture threads (Arachnida, Araneida). Zoomorphol- ies of co-adaptation: Preferred temperatures versus og 104:96-104. optimal performance temperatures of lizards. Evolu- Peters, H.M. (1986) Fine structure and function of cap- tion 4/:1098-1115. ture threads. In W. Nentwig (ed): Ecophysiolory of Kovoor, J., and H.M. Peters (1988) The spinning appara- Spiders. New York: Springer-Verlag, pp. 187-202. t;asof Poleneciaproduclo (Araneae, Uloboridae): Struc- Peters, H.M. (1992) On the spinningapparatus and struc- ture and histochemistry. Zoomorpholory 108 : 47-59. ture ofthe capture threads of Deinopis subrufus (Ara- Kullmann, E., and H. Stern (1981) Leben an seidenen neae, Deinopidae). Zoomorphology 112:27 -37 . Faden. Mrinchen: Vindler Verlag. Peters, H.M., and J. Kovoor (1980) Un compl6ment d Lubin, Y.D. (1986) Web building and prey capture in I'appareil s6ricigene des Uloboridae (Araneae): Le parac- Uloboridae. In W.A. Shear (ed): Spiders: Webs, Behav- ribellum et sesglandes. Zoomorpholory 96:91-102. ior, and Evolution. Stanford: Stanford University Press, Simon, E. (1880) Arachnides des environment de P6kin. pp. 132-171. Arrn. Soc. Ent. France 10:97-128. Lubin, Y.D., W.G. Eberhard, and G.G. Montgomery (1978) Townley, M.A., D.T. Bernstein, K.S. Gallanger, and E.K. Webs of Mlagrammopes (Araneae: Uloboridae) in the Tillinghast (1991) Comparativestudyof orb-webhydro- Neotropics. Psyche 85: l-23. scopicity and adhesive spiral compositin in three are- Muma, M.H., and W.J. Gertsch (1964) The spider family neid spiders. J. Exp. Zool.259:L54-I65. Uloboridae in North America north of Mexico. Am. Vollrath, F. (1992) Spiderwebsand silks. Sci. Am. 266:70- Mus. Novitates2l96:I43. 76. Opell, B.D. (1979) Revision of the genera and tropical Vollrath, F., W.J. Fairbrother, R.J.P. Williams, E.K. Till- American speciesof the spider family Uloboridae. Bull. inghast, D.T. Bernstein, K.S. Gallagher, and M.A. Mus. Comp.Zool. 148:433-549. Townley (1990) Componentsin the droplets of the orb Opell, B.D. (1982) Post-hatching development and web spider's viscid spiral. Nature 345:526-528. production of Hyptiotesca.uatus(Hentz) (Araneae: Ulo- Walckenaer, C.A. (1841) Historie Naturelle des Insects. boridae).J. Arachnol. 10;185-191. Apteres,Vol. 2. Paris.

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