The functions and evolution of the web decorations built by Argiope spiders.
Many predators convey inaccuracy signals to mislead the prey to make unfavorable decision. For example, the larvae of the New Zealand glowworm produce bioluminescence to attract prey, and the bolas spider deceit male moths by emitting sex pheromones of female moths. Some orb-weaving spiders build conspicuous structures called decorations on their webs to mislead their prey or predator. There are several hypotheses about the functions of decorations. They can be classified into vision related and non-vision related functions, and the former receives more supports. The vision-related functions include attracting prey, defense against or decreasing web damage. Among them prey attraction function had received much support but empirical evidence of predator-defense function was controversial. In this study I first investigated the effects of spider coloration and decoration on insect interception rates of webs, and whether presence of decorations may decrease predation risk of spiders. The results of field manipulative studies showed that decorations function to attract prey, but at the cost of increasing visibility of spiders to predators. Most genera of orb-weaving spiders have linear form decorations, but the cruciate form can only be found in the genus Argiope. In the second part of my study I tested whether cruciate decorations were derived from linear form by a molecular phylogenetic approach. I used two molecular makers (mitochondrial COI and COII partial sequences) to construct the molecular phylogeny of the genus Argiope and to reconstruct ancestral character states on the tree. Tree topology showed that the linear form was the ancestral state in Argiope and the cruciate form evolved at least twice. I then compared the prey interception performance of webs with cruciate or linear decorations to assess the driving force for such evolutionary pattern. Result of field manipulative study showed that the former intercepted significantly more prey than the latter. Higher attractiveness of cruciate decorations may be related with the form vision of flying insects. In honey bees, there are three types of orientation-sensitivity cells which allow them to distinguish different orientation patterns. However, whether the better foraging performance of cruciate form decorations results from higher number or arrangement pattern of silk bands awaits further study.
Updated: Mar-10-2014 01:22:51 (Taiwan, GMT+08:00).