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Adhesive recruitment by the viscuous capture threads

Adhesive recruitment by the viscuous capture threads - 553...

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553 Introduction Spider orb-webs represent the confluence of intricate architecture (Craig, 1987a,b; Craig, 2003; Eberhard, 1990) and natural materials that have optimal mechanical properties (Denny, 1976; Guerette et al., 1996; Gosline et al., 1999; Gosline et al., 2002; Hayashi and Lewis, 2000; Craig, 2003; Hayashi et al., 2004; Blackledge and Hayashi, 2006a; Vollrath and Porter, 2006a; Vollrath and Porter, 2006b). The web’s non- sticky, supporting threads effectively absorb the forces of prey strikes and its sticky, spirally arrayed capture threads retain insects long enough for a spider to subdue them (Chacón and Eberhard, 1980). The first modern orb-weaving spiders (Araneoidea clade) appeared in the Cretaceous (Selden, 1989; Peñalver et al., 2006) and have since spun viscous capture threads comprised of fibers covered by regularly spaced, sticky droplets (Fig. · 1). These composite threads are spun from the spigots of two adjacent silk glands (Foelix, 1996): flagelliform glands that produce a pair of supporting axial fibers and aggregate glands that coat these fibers with a complex viscous, aqueous solution (Gosline et al., 1984; Peters, 1986; Peters, 1995; Vollrath et al., 1990). Soon after threads are spun, this solution forms regularly spaced droplets, whose size and spacing are probably determined by the amount of material deposited, the axial fibers’ diameters, and the solution’s viscosity. The size of the glycoprotein granules that coalesce inside each droplet and contribute to thread adhesion may also affect droplet size (Vollrath and Tillinghast, 1991). Hydrophilic compounds in the viscous fluid maintain droplet size by attracting atmospheric moisture and preventing droplets from drying (Townly, 1990; Townly et al., 1991). These viscous droplets replaced the minute, dry protein fibrils that form torus shaped puffs around the axial fibers of cribellar capture threads (Peters, 1984; Peters, 1986; Opell, 1994a; Opell, 1997a), the plesiomorphic capture threads spun by members of the Deinopoidea clade, the sister group of the of the Araneoidea (Coddington, 1986; Coddington, 1989; Griswold et al., 1998; Garb et al., 2006). The Araneoidea now far outnumber the Deinopoidea, comprising over 27% of the 39 · 490 living spider species (Platnick, 2006). Araneoid origin was associated with three changes that enhance orb web performance and may have contributed to the success of this lineage (Bond and Opell, 1998). The web’s ability to intercept prey was enhanced by a transition from horizontal to vertical web orientation (Chacón and Eberhard, 1980; Eberhard, 1989) and by the reduced visibility of viscous capture thread (Craig and Berhard, 1990; Craig et al., 1994; Zschokke, 2002). The web’s ability to retain insects that strike it was improved by the enhanced stickiness of viscous thread (Opell, 1997b; Opell, 1998; Opell, 1999). The sticky prey capture threads of orb-webs are critical to web performance. By retaining insects that strike the web, these spirally arrayed threads allow a spider time to locate and subdue prey. The viscous capture threads spun by modern orb-weaving spiders of the Araneoidea clade replaced the dry, fuzzy cribellar capture threads of the
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