Orb-web visibility: the influence of insect flight behaviour and visual physiology on the evolution of web designs within the Araneoidea

Ecological differentiation among spiders that build orb-webs depends on the ability of webs both to absorb the kinetic energy of prey impact and to intercept prey (web visibility). Analyses of Diptera flying near webs show that Drosophila melanogaster Meigen avoids high-fibre-density orb-webs sooner than they avoid low-fibre-density webs. From analyses of the flight potential and visual capabilities of D. melanogaster, Trichoporosopon digitatum (Rondani) and Culex quinquesfasciatus Say, it seems surprising that orb-webs capture any prey at all. The thin diameter and refractive properties of web silks give spider webs a translucent quality that greatly reduces visibility. Web suspension in low- and diffuse-light environments, as well as against visually confused backgrounds, decreases one of the most important visual cues for prey, contrast between the web and its background. Among spiders that capture prey which are characterized by specialized visual and flight capabilities, oscillating webs and three-dimensional webs may enhance web capture effectiveness. Web oscillation and the static distortion of the plane of some orb-webs allow webs to ‘escape’ from the range of visual resolution of potential prey.