Functional values of stabilimenta in a wasp spider, Argiope bruennichi: support for the prey-attraction hypothesis
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Many orb-weaving spiders decorate their webs with conspicuous ultraviolet (UV)-reflective stabilimenta. The prey-attraction hypothesis suggests that stabilimenta are visually attractive to prey and thus may increase the spiders’ foraging success. However, previous studies on the function of stabilimenta have produced conflicting results in Argiope species. Using a combination of field and laboratory studies, we examined whether the linear stabilimentum of Argiope bruennichi contributes to prey interception. We recorded prey interceptions in 53 webs with stabilimenta and 37 equally-sized webs without stabilimenta, classifying captured prey according to their taxonomical group and size. On average, 6.2 ± 4.7 prey items were intercepted in webs with stabilimenta, while 3.2 ± 2.9 items were intercepted in webs without stabilimenta. The effects of stabilimenta on foraging success appear to be due to increased interception of UV-sensitive insect pollinators, including 20 families of Diptera, Hymenoptera, Coleoptera, and Lepidoptera. The mean number of UV-sensitive prey was 4.4 ± 3.6 in webs with stabilimenta compared with 1.8 ± 2.1 in webs without stabilimenta. Webs with and without stabilimenta did not differ in the mean number of UV-nonsensitive prey captured. The linear stabilimentum showed strong positive effects on the interception of large prey: webs with stabilimenta captured more than twice as many large prey (≥5 mm) than webs without stabilimenta, whereas there was only a slight difference in the interception rates for small prey (<5 mm). Comparisons among different Argiope species suggest that the stabilimentum may have different adaptive functions in different species or ecological contexts.