STABILIMENTUM VARIATION AND FORAGING SUCCESS IN ARGIOPE AURANTIA AND ARGIOPE TRIFASCIATA (ARANEAE, ARANEIDAE)

Many spiders include conspicuous designs of white silk called stabilim enta in the centre of their orb webs. Stabilimenta are highly variable in their form and frequency and are hypothesized to function in eithe r defence against predators or attraction of prey. These hypotheses ge nerate different predictions about the effect of foraging success on v ariation in the form and frequency of stabilimenta. If stabilimenta se rve as prey attractants, then starved spiders should invest more in th em than well-fed spiders, while the opposite pattern is expected if st abilimenta function as a predator defence mechanism. This study examin es the effect of variation in foraging success of the yellow garden ar giope Argiope aurantia and the banded argiope Argiope trifasciata on v ariation of their stabilimenta and orb webs. Both species built smalle r stabilimenta when fed less, even though a stabilimentum accounts for only 10% of the dry weight of an orb web. Poorly fed A. trifasciata a lso included stabilimenta in their webs less often than did well-fed A . trifasciata. Differences in stabilimenta were independent of differe nces between treatments in the orb webs themselves. These results argu e against the prey attraction hypothesis, but not the predator defence hypothesis, since well-fed spiders invested more in stabilimenta.