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Acari (or Acarina) are a taxon of arachnids that contains mites and ticks. The diversity of the Acari is extraordinary and its fossil history goes back to at least the early Devonian period. As a result, acarologists (the people who study mites and ticks) have proposed a complex set of taxonomic ranks to classify mites. In most modern treatments, the Acari is considered a subclass of Arachnida and is composed of two or three superorders or orders: Acariformes (or Actinotrichida), Parasitiformes (or Anactinotrichida), and Opilioacariformes; the latter is often considered a subgroup within the Parasitiformes. The monophyly of the Acari is open to debate, and the relationships of the acarines to other arachnids is not at all clear. In older treatments, the subgroups of the Acarina were placed at order rank, but as their own subdivisions have become better-understood, it is more usual to treat them at superorder rank. Most acarines are minute to small (e.g., ), but the largest Acari (some ticks and red velvet mites) may reach lengths of . Over 50,000 species have been described (as of 1999) and it is estimated that a million or more species may exist. The study of mites and ticks is called acarology (from Greek /, ', a type of mite; and , ''-logia''), and the leading scientific journals for acarology include ''Acarologia'', ''Experimental and Applied Acarology'' and the ''International Journal of Acarology''. ==Morphology== Mites are arachnids and, as such, evolved from a segmented body with the segments organised into two tagmata: a prosoma (cephalothorax) and an opisthosoma (abdomen). However, only the faintest traces of primary segmentation remain in mites; the prosoma and opisthosoma are insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the capitulum or gnathosoma and, according to some workers, is also found in Ricinulei. The remainder of the body is called the idiosoma and is unique to mites. Most adult mites have four pairs of legs, like other arachnids, but some have fewer. For example, gall mites like ''Phyllocoptes variabilis'' (family Eriophyidae) have a worm-like body with only two pairs of legs; some parasitic mites have only one or three pairs of legs in the adult stage. Larval and prelarval stages have a maximum of three pairs of legs; adult mites with only three pairs of legs may be called 'larviform'. Also members of the Nematalycidae within Endeostigmata, who lives between sand grains, have often wormlike and elongated bodies with reduced legs.〔(Of Knots & Worms Not: Gordialycus )〕 The mouth parts of mites may be adapted for biting, stinging, sawing or sucking. They breathe through tracheae, stigmata (small openings of the skin), intestines and the skin itself. Species hunting for other mites have very acute senses, but many mites are eyeless. The central eyes of arachnids are always missing, or they are fused into a single eye. Thus, any eye number from none to five may occur. 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「Acari」の詳細全文を読む スポンサード リンク
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