Adhesive recruitment by the viscous capture threads of araneoid orb-weaving spiders

Abstract

SUMMARY 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 Deinopoidea and feature regularly spaced moist, adhesive droplets. The stickiness of a cribellar thread is limited by its tendency to peel from a surface after the adhesion generated at the edges of contact is exceeded. In this study we test the hypothesis that viscous thread overcomes this limitation by implementing a suspension bridge mechanism (SBM) that recruits the adhesion of multiple thread droplets. We do so by using contact plates of four widths to measure the stickiness of six species' viscous threads whose profiles range from small, closely spaced droplets to large, widely spaced droplets. The increased stickiness registered by an increased number of thread droplets supports the operation of a SBM. However, the accompanying decrease in mean per droplet adhesion shows that droplets interior to the edges of thread contact contribute successively less adhesion. Models developed from these data suggest that the suspension bridge mechanism is limited to a span of approximately 12 droplets.

Persistent stickiness of viscous capture threads produced by araneoid orb‐weaving spiders

“…We measured thread stickiness with an instrument composed of an interchangeable contact plate attached to the lever arm of a jeweled escapement, which transferred force to a load cell machined to increase its sensitivity (Opell and Hendricks, 2007). A linear actuator pressed the thread against a contact plate at a speed of 0.06 mm/sec until a force of 25 mN was generated, at which time the direction of travel was immediately reversed.…”

Viscoelastic solids explain spider web stickiness

“…Many attempts to understand the nature of these adhesives and to mimic their properties emphasize on determining the ligands in the glycoproteins responsible for adhesion, but ignore critical aspects of the structure itself. Th is is partly because of an inability to probe the glycoproteins through the viscous coat and partly because of the diffi culty in separating the contribution of the glue droplets from the highly extensible axial silk for the force required to peel a single thread off a surface 5 .…”

Parasitoid suppression and life-history modifications in a wolf spider following infection by larvae of an acrocerid fly

“…Like most adhesive surfaces, cribellate silk resists detaching primarily along the edge of contact with a smooth substrate such as the wing of an insect. Thus, adhesive forces are determined not by the total area of contact but rather by the surface energy along the edge at which detachment occurs and the total number of cribellate fibrils (Opell 1994;Opell & Hendricks 2007).…”

“…Viscid capture threads overcome this constraint on adhesion through a highly effective suspension bridge mechanism that is enabled by the high elasticity of both the flagelliform axial fibers and the viscous glue droplets themselves (Opell & Hendricks 2007;Opell et al 2008;Sahni et al 2010). The viscid glue droplets have their own hierarchical structuring and consist of a core of cross-linked fibrous glycoproteins embedded in a liquid matrix (Opell & Hendricks 2010).…”

“…As a viscid thread begins to pull away from a surface, individual glue droplets extend greatly before they detach (Sahni et al 2010;Opell et al 2011). This allows multiple glue droplets to simultaneously resist pull-off, generating significantly more adhesion (Opell & Hendricks 2007). Furthermore, up to 50% of the total work required to pull a viscid thread free from a surface comes not from the glue, but instead from the extension of the axial fibers themselves (Sahni et al 2010).…”