Insect attraction to ultraviolet-reflecting spider webs and web decorations

The foraging performance of any predator is dependent on its ability to locate prey. All spiders produce silks and many locate insects by producing silk traps. We measured the reflective properties of silk produced by primitive, non-web-weaving spiders and derived aerial web spinners. We found that primitive spiders produce silks that reflect ultraviolet (UV) light and primitive aerial web weavers spin UV-reflecting catching silks that attract Drosophila. Derived, web-spinning spiders in the genus Argiope, however, produce catching silks that exhibit low reflectivity in the UV and, in fact, reflect little light at all. Nevertheless, Argiope decorate their webs with bright, UV-reflecting bars and crosses that attract prey. We found that more insects were intercepted per hour by decorated webs with spiders than by undercorated webs from which the spider had been removed. In addition, within-web analyses showed that when only half of a web was decorated, more insects were intercepted by the decorated halves than the undecorated web halves. We propose that UV-reflecting decorative silks, together with the UV-reflecting body surfaces of A. argentata, act as a visual display that attracts prey.