Stabilimentum variation and foraging success in Argiope aurantia and Argiope trifasciata (Araneae: Araneidae)

Many spiders include conspicuous designs of white silk called stabilimenta in the centre of their orb webs. Stabilimenta are highly variable in their form and frequency and are hypothesized to function in either defence against predators or attraction of prey. These hypotheses generate different predictions about the effect of foraging success on variation in the form and frequency of stabilimenta. If stabilimenta serve as prey attractants, then starved spiders should invest more in them than well-fed spiders, while the opposite pattern is expected if stabilimenta function as a predator defence mechanism. This study examines the effect of variation in foraging success of the yellow garden argiope Argiope aurantia and the banded argiope Argiope trifasciata on variation of their stabilimenta and orb webs. Both species built smaller stabilimenta when fed less, even though a stabilimentum accounts for only 10% of the dry weight of an orb web. Poorly fed A. trifasciata also included stabilimenta in their webs less often than did well-fed A. trifasciata. Differences in stabilimenta were independent of differences between treatments in the orb webs themselves. These results argue against the prey attraction hypothesis, but not the predator defence hypothesis, since well-fed spiders invested more in stabilimenta.