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?:X{ JOURNAL OF MORPHOLOGY22t:ttl-!t9 (1994) FactorsGoverning the Stickinessof CribellarPrey Capture Threadsin theSpider Family Uloboridae 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 spider 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), Araneomorphae (Coddington and Levi, orb-weaving spiders 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.
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