A test of the insect-attraction function of silk stabilimenta.

Abstract

Stabilimenta are the silk structures on webs of some orb-weaving spiders the function of which has provoked many arguments. I tested insect-attraction as one of the functions of stabilimenta by investigating (1) if stabilimentum-decorated webs intercepted more insects than undecorated webs, (2) the mechanism responsible for a stabilimentum's attractiveness to insects, and (3) how spiders controlled their stabilimentum-building. Insect-interception rates were compared between decorated and undecorated spider webs as well as between decorated and undecorated sticky traps. Presence of stabilimenta significantly increased insect interception rates of Argiope trifasciata webs and sticky traps. The mechanism of stabilimenta's attractiveness was examined by manipulating their ultra violet light (UV) reflectivity. When ultra violet light (UV) was prevented from reflecting from the stabilimenta, the insect-attractiveness of decorated webs spun by A. aurantia reduced significantly. In investigating how spiders regulated stabilimentum-building, the food intake of A. trifasciata was manipulated to determine whether or not spiders controlled their stabilimentum-building according to past foraging success. When spiders were food-satiated, they decreased silk production and web size, increased mesh size, but maintained the same stabilimentum-building pattern. When spiders were food-deprived, they increased silk production and web size, maintained the same mesh size, but slightly increased stabilimentum-building. These results indicated that past foraging success did not seem to significantly affect spiders' stabilimentum-building. More effort is needed to determine how stabilimentum-building of Argiope spiders is controlled.