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__NOTOC__ The evolution of spiders has been going on for at least 400 million years, since the first true spiders (thin-waisted arachnids) evolved from crab-like chelicerate ancestors. Today, there are 42,473 described spider species within the diverse phylum of arthropods. Major developments in spider evolution include the development of spinnerets and silk secretion. ==Early spider-like arachnids== Among the oldest known land arthropods are Trigonotarbids, members of an extinct order of spider-like arachnids. Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration through book lungs, and walked on eight legs, with a pair of leg-like pedipalps near the mouth and mouth parts. Arguments still remain open as to whether they possessed the ability to create silk. This had been a popular thought for quite some time, until an unpublished fossil was described with distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk. Regardless, trigonotarbids are not considered true spiders, but are instead accepted as an independent group of arachnids not directly ancestral to modern spiders. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of spiders」の詳細全文を読む スポンサード リンク
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