User:Abyssal/Portal:Cenozoic/Natural world articles/29

Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids. Spiders are found nearly worldwide in nearly every habitat with the exception of air and sea. Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, and joined by a small, cylindrical pedicel. Spiders generally have very centralized nervous systems for arthropods. 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 webs vary widely in size, shape and the amount of sticky thread used. Spiders' guts are too narrow to take solids, and they liquidize their food by flooding it with digestive enzymes and grinding it with the bases of their pedipalps.

Spider-like arachnids with silk-producing spigots appeared in the Devonian period about, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from, and are very similar to the most primitive surviving order, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before. (see more...)