Spiders Essay

327 WordsApr 16, 20142 Pages
Spiders (order Araneae) are air-breathing arthropods that have eight legs andchelicerae with fangs that inject venom. They are the largest order of arachnidsand rank seventh in total species diversity among all other orders of organisms.[1] Spiders are found worldwide on every continent except forAntarctica, and have become established in nearly every habitat with the exception of air and sea colonization. As of 2008, at least 43,678 spiderspecies,[2] and 109 families have been recorded by taxonomists;[3] however, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900.[4] Anatomically, spiders differ from other arthropods in that the usual bodysegments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. Unlike insects, spiders do not haveantennae. In all except the most primitive group, the Mesothelae, spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulicpressure. Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of silk glands within their abdomen. Spider websvary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-web spiders. Spider-like arachnids with silk-producing spigots appeared in the Devonianperiod about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from318 to 299 million years ago,

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