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

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.