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A subclass of Arachnida, the mites and ticks; also called Acarina. All are small (0.004–1.2 in. or 0.1–30 mm, most less than 0.08 in. or 2 mm in length), have lost most traces of external body segmentation, and have the mouthparts borne on a discrete body region, the gnathosoma. They are apparently most closely related to Opiliones and Ricinulei.

The chelicerae may be chelate or needlelike. Pedipalps are generally smaller than the walking legs, and are simple or sometimes weakly chelate. Legs may be modified by projections, enlarged claws, heavy spines, or bladelike rows of long setae for crawling, clinging to hosts, transferring spermatophores, or swimming. Mites frequently have one or two simple eyes (ocelli) anterolaterally on the idiosoma (occasionally one anteromedially as well). The ganglia of the central nervous system are coalesced into a single “brain” lying around the esophagus. A simple dorsal heart is present in a few larger forms.

Mites are ubiquitous, occurring from oceanic trenches below 13,200 ft (4000 m) to over 20,800 ft (6300 m) in the Himalayas and suspended above 3300 ft (1000 m) in the atmosphere; there are species in Antarctica. Soil mites show the least specialized adaptations to habitat and are frequently well-sclerotized predators. Inhabitants of stored grains, cheese, and house dust are also relatively unspecialized. The dominant forms inhabiting mosses are heavily sclerotized beetle mites (Oribatidae). Flowering plant associates include spider mites (Tetranychidae) and gall mites (Eriophyidae). Fungivores are usually weakly sclerotized inhabitants of moist or semiaquatic habitats, while the characteristic fresh-water mites include sluggish crawlers, rapid swimmers, and planktonic drifters. Mites associated with invertebrate animals include internal, external, and social parasites as well as inactive phoretic stages on Insecta, Crustacea, Myriapoda, Chelicerata, Mollusca, and Parazoa. A similar diversity of species are parasites or commensals of vertebrates. Some parasitic mites are disease vectors. Over 30,000 species have been described, and it is estimated that as many as 500,000 may exist.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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