GMAT 考满分题库

     In semi-aquatic ecosystems, bodies of water present peculiar difficulties for the hunting strategies of spiders. Ponds, lakes, and rivers can provide unnavigable escape vectors for prey organisms, making it likely that predators will lose out on valuable nutritional resources. The water spider Dolomedes, in particular, demonstrates adaptations that allow it to take advantage of waterborne food sources.

     The difficulty that members of the species Dolomedes face is best evidenced by the typical hunting strategies of terrestrial spiders. All spiders produce silk and arboreal species generally spin their silk into fine, latticework structures that they suspend in the environment to trap passing arthropods, which entangle themselves due to the exigencies of the forested ecosystem. In environs lacking tree coverage or comparable large structures, however, web-hunting becomes inefficient, and other strategies of predation have to be pursued. That many spider species persist in environments lacking significant tree coverage suggests that certain behavioral adaptations enable them to locate food effectively in a variety of environmental circumstances.

     One such adaptation is the proactive use of water bodies by Dolomedes, who eschew silk in favor of water tension, which they use to monitor the movements of prey animals. Organisms fall into a pond, lake, or river, and send waves vibrating from the point of impact across the surface of the pool. Dolomedes uses these vibrations to locate and capture the fallen organism. For arboreal spiders, who can only monitor the vibrations caused by insects caught in the webs they've spun, this is useless information. Such a hunting strategy does not require that the arboreal spiders pay attention to extraneous prey when other prey is acquired in abundance by other adaptations. In comparison, Dolomedes acquires its prey by yet another aquatic adaptation, using a coat of tiny, hydrophobic hairs that allow the spider to submerge itself in the liquid environment. By causing a pocket of air to gather and surround the surface of the spider's body, these hairs allow Dolomedes to submerge itself in water, thus giving it access to sources of prey closed off to other species.