Testing the functions of egg sac/prey pellet decorations made by Cyclosa mulmeinensis (Araneae: Araneidae).
Most functional studies on decorations focus on spiders of the genus Argiope, which build decorations composed entirely of silk. The abundance and diversity of Cyclosa spiders are much higher than those of Argoipe, however, the functions of detritus decorations build by them have received little attention. Previous studies suggest that detritus decorations may serve to camouflage the spider, attract prey, stabilize the web or as anti-predation device. However, currently most of them did not have direct evidence generated from field manipulative studies. In this study, I performed field manipulations to test whether the decorations of Cyclosa mulmeinensis exhibit the aforementioned functions. The results showed that the size of prey pellet correlated well with the spider body length. A comparison of color signals of spiders and decorations showed that the main predators, wasps, could not visually distinguish the body coloration of spiders from that of egg sacs and prey pellets. Thus, C. mulmeinensis seemed to manipulate size and color of decorations to match themselves. In a field experiment, I categorized webs into different groups according to number of prey pellet/egg sacs on webs, and attack events on these spiders were monitored using video cameras. The results showed that spiders incorporating more egg sacs or prey pellets on webs experienced more attacks. However, when there were more items on the webs, most of the wasp attacks were directed toward prey pellet or egg sacs. These results indicated that decoration might increase visibility but could dramatically decrease mortality of spiders. While webs decorated with prey pellets or egg sacs were visited by wasps more frequently, they also intercepted higher number of prey. Moreover, the strength of webs with egg strings was significantly higher than those without, indicating that egg sac decorations could function to strengthen the webs. Therefore, decorations built by C. mulmeinensis not only serve as conspicuous anti-predation device, but also function to attract prey and to strengthen the webs.
Updated: Mar-10-2014 01:22:51 (Taiwan, GMT+08:00).