Prey attraction as a possible function of discoid stabilimenta of juvenile orb-spinning spiders

Abstract

Stabilimenta, the conspicuous, ultraviolet (UV)-reflecting, white silk structures around the hub of orb webs, are thought to increase a spider's foraging success by attracting prey (prey attraction hypothesis). No studies, however, have yet investigated the function of nonlinear and noncruciform stabilimenta and the stabilimenta of juvenile spiders. We examined whether the discoid stabilimenta spun by juvenile Argiope versicolor, an orb-spinning spider, function to attract prey. When we measured the UV reflectance of the web silks of A. versicolor juveniles, we found that the discoid stabilimentum silk, but not spiral silk or radial silk or the joint where the two types of silk met, reflected UV light. We then carried out two series of choice experiments in the laboratory to determine whether the webs with discoid stabilimenta attract more fruit flies than undecorated webs when UV+ white light (300–700 nm) is present. We also examined whether the discoid stabilimenta still attract insects when the UV section of the spectrum (300–400 nm) is eliminated (UV−). Webs with discoid stabilimenta attracted more fruit flies than the webs without stabilimenta in UV+ white light. However, in UV− light, fewer fruit flies flew to the webs, even when they were decorated with discoid stabilimenta. In a field experiment, significantly more insects were intercepted by webs with discoid stabilimenta than by webs without stabilimenta. We suggest that the discoid stabilimenta of A. versicolor juveniles act as a visual signal that attracts insects, supporting the prey attraction hypothesis.