Chou, I-Chia

Chou, I-Chia

Title

Master student

Project

A test of prey attraction and predator defense functions of detritus decorations built by Cyclosa spider.

Abstrct

In some orb-weaving spiders, in addition to regular components of web, they also construct extra structures on webs called decorations. In many spiders, decorations are made entirely of silk, but in some species such as those in the genus Cyclosa, carcasses, debris, leaf, and even egg sacs are also used. So far there is no direct test of the functions of detritus decorations built by Cyclosa spiders. In this study, I first tested the prey-attraction function by manipulating the presence of detritus decorations in Cyclosa confusa, then examined this treatment's effect on spiders' foraging performance. Results from field studies conducted in Orchid Island showed that the insect interception rate of the experimental group (decoration removed) was not significantly different from that of the control group (decoration remained). This result suggests that detritus decoration did not serve as prey attractant and their presence may even reduce the foraging success of Cyclosa spiders. In this study, I also tested the predator-defense function by manipulating presence of decoration then observing the predator's response. In addition, how visual signals of detritus decorations were perceived by predators were examined by calculating the colour contrasts of the decorations and spiders against various backgrounds. The videotaped observations made in the study sites revealed that paper wasp repeatedly directed their attacks on decorations in most of the recorded attacks, rather than on the spiders. The colour contrasts of spiders and decorations against various types of vegetation backgrounds differed, which indicats that the visibility of decorations varied when viewed in front of different vegetations. The colour contrast of C. confusa against the decoration did not exceed the threshold value, indicating that the hymenopteran predators could not distinguish the spider from the decorations. All the results suggest that decorations seemed to be able to prevent paper wasps from accurately attacking the spiders, thus might enhance the survival of spiders. Various groups of yeasts were found on webs, decorations, spiders, and plants in the study sites. Totally 74 yeast strains were separated and 37 morphospecies were identified. Twenty-eight yeast strains were analyzed with biolog and eight species were identified. Ribosomal DNA ITS regions were amplified and digested by endonucleases HinfI, HaeIII, and HhaI and 74 yeast strains could be separated into 31 groups. The distribution of most of yeast species did not show specificity. However, a widely distributed species, Aureobasidium pullulans, showed a specificity with C. mulmeinensis by occurring only in the bodies, webs, decorations of such spiders and the plants (pineapple and screwpine) on which the webs were built. Whether this specificity simply resulted from chance events or involved certain ecological interactions awaits further study.

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Updated: Mar-10-2014 01:22:51 (Taiwan, GMT+08:00).