3026 INTRODUCTION Capture thread is the sticky, spiral component of a spider’s orb-web that is supported by the web’s non-sticky radial lines. By lengthening prey retention time, these sticky threads give a spider more time to locate, run to and subdue prey before they escape from the web (Chacón and Eberhard, 1980), a capability that is particularly important for the capture of large, profitable prey (Blackledge and Eliason, 2007). Two sub-clades of orb-weaving spiders comprise the Araneoidea clade, the Deinopoidea and the Araneoidea (Coddington and Levi, 1991; Griswold et al., 1998; Griswold et al., 2005). Like their non-orb-weaving ancestors, the Deinopoidea produce dry, cribellar capture threads formed of thousands of fine protein fibrils that are supported by a pair of axial lines (Eberhard and Pereira, 1993; Opell, 1994; Opell, 1999; Peters, 1984; Peters, 1986; Peters, 1992). In contrast, the orb-weaving members of the more diverse Araneoidea produce viscous capture threads composed of regularly spaced aqueous droplets supported by a pair of axial fibrils (Peters, 1986; Tillinghast et al., 1993; Vollrath, 1992; Vollrath et al., 1990; Vollrath and Tillinghast, 1991). At the center of each droplet is a glycoprotein granule that is thought to confer thread stickiness (Tillinghast et al., 1993; Vollrath and Tillinghast, 1991). The fluid that covers these granules and surrounds the axial fibers contains hydrophilic compounds, which attract atmospheric moisture, maintaining the droplet volume (Townley et al., 1991; Vollrath et al., 1990). Each of a spider’s paired median spinnerets bears a single flagelliform gland spigot and two aggregate gland spigots. The flagelliform glands produce axial fibers and the aggregate glands produce viscous aqueous material. Aggregate gland material initially forms a continuous cylinder around the fibers, but quickly condenses into a series of regularly spaced droplets. In some species these droplets have a pattern of larger primary droplets with smaller secondary droplets between them (Fig. 1). Just as a thread’s axial fibers and viscous droplets are physically linked, so too are they functionally linked (Agnarsson and Blackledge, 2009). Thread adhesion is generated when droplets contact a surface and, as the thread is pulled from this surface, the adhesion of multiple droplets is recruited by the axial fibers in what has been termed a suspension bridge mechanism (Opell and Hendricks, 2007). This is demonstrated by the observation that when the stickiness of viscous threads is measured with contact plates of increasing width, thread stickiness increases, something that is not observed when this procedure is used to measure the stickiness of cribellar threads (Opell and Schwend, 2008a). This mechanism operates imperfectly, with each successive pair of droplets interior to the edge of thread contact contributing progressively less adhesion until a limiting number of droplets contacts a surface, after which no additional thread stickiness is achieved (Opell and Hendricks, 2007). The contribution of axial fiber extensibility can be documented by stretching threads to reduce their extensibility and then measuring their stickiness with contact plates whose widths are increased in proportion to thread elongation, thereby maintaining the number of droplets that contribute to a thread’s stickiness (Opell et al., 2008). This procedure shows that the per droplet stickiness of stretched threads is less than that of threads at their native tensions and indicates that axial fiber extensibility accounts for roughly one- third of a viscous thread’s stickiness. In this study we confirm the hypotheses supported by these earlier studies and examine other features that affect the stickiness of viscous threads by studying threads produced by 16 araneoid species The Journal of Experimental Biology 212, 3026-3034 Published by The Company of Biologists 2009 doi:10.1242/jeb.030064 The adhesive delivery system of viscous capture threads spun by orb-weaving spiders Brent D. Opell* and Mary L. Hendricks Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA *Author for correspondence (bopell@vt.edu) Accepted 11 June 2009 SUMMARY The sticky viscous capture threads in araneoid orb-webs are responsible for retaining insects that strike these webs. We used features of 16 species’ threads and the stickiness that they expressed on contact plates of four widths to model their adhesive delivery systems. Our results confirm that droplets at the edges of thread contact contribute the greatest adhesion, with each successively interior droplet contributing only 0.70 as much adhesion. Thus, regardless of the size and spacing of a thread’s large primary droplets, little adhesion accrues beyond a span of 20 droplets. From this pattern we computed effective droplet number (EDN), an index that describes the total droplet equivalents that contribute to the stickiness of thread spans. EDN makes the greatest positive contribution to thread stickiness, followed by an index of the shape and size of primary droplets, and the volume of small secondary droplets. The proportion of water in droplets makes the single greatest negative contribution to thread stickiness, followed by a thread’s extensibility, and the area of flattened droplets. Although highly significant, this six-variable model failed to convincingly describe the stickiness of six species, a problem resolved when species were assigned to three groups and a separate model was constructed for each. These models place different weights on the variables and, in some cases, reverse or exclude the contribution of a variable. Differences in threads may adapt them to particular habitats, web architectures or prey types, or they may be shaped by a species’ phylogeny or metabolic capabilities. Key words: adhesive system, capture thread, orb-web, prey capture, viscous thread. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY