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Mites (Acari or Acarina) are themost diverse and abundant of all arachnids, but because of their small size(usually less than a millimeter in length) we rarely see them. Theticks are an exception, in that they are usually bigenough to see, especially when they are filled with blood. Red velvetmites are also among the giants of the Acari (to 10 mm), and can often beseen hunting on the ground or on tree trunks. Water mites are rarely morethan a few millimeters long, but their bright colours and rapid movementoften bring them to our attention. At the smaller end of the mite sizerange are species like the human follicle mite or the honeybee trachealmite - small enough to raise a family within a human hair follicle orwithin a bee's respiratory tube, and too small (ca. 0.1 mm) to see withouta microscope.
Mites are also among the oldest of all terrestrial animals, with fossilsknown from the early Devonian, nearly 400 million years ago (Norton et al.1988, Kethley et al. 1989). Three major lineages are currently recognised:Opilioacariformes, Acariformes andParasitiformes (Krantz 1978, Johnston 1982, Evans 1992).About 45,000 species of mites have been described - a small fraction(perhaps 5%) of the number of species estimated to be alive today.
Mites are truly ubiquitous. They have successfully colonized nearly everyknown terrestrial, marine, and fresh water habitat including polar andalpine extremes, tropical lowlands and desert barrens, surface and mineralsoils to depths of 10 meters, cold and thermal surface springs andsubterranean waters with temperatures as high as 50C, all types ofstreams, ponds and lakes, and sea waters of continental shelves and deepsea trenches to depths of 5000 meters. Some idea of mite abundance anddiversity can be gained from analysis of one square meter of mixedtemperate hardwood or boreal coniferous litter, which may harbour upwardsfrom one million mites representing 200 species in at least 50 families.Within this complex matrix of decomposing plant matter, mites help toregulate microbial processes directly by feeding on detritus and microbes,and indirectly by predation on other microfauna.
Many mites have complex symbiotic associations with the larger organisms onwhich they live. Plants, including crops and the canopies of tropicalrainforests, are inhabited by myriads of mite species feeding on mosses,ferns, leaves, stems, flowers, fruit, lichens, microbes, other arthropodsand each other. Many mites found on agricultural crops are major economicpests (e.g. spider mites) or useful biocontrol agents (e.g. phytoseiidmites) of those pests. Mammals and birds are hosts to innumerable speciesof parasitic mites (e.g. scabies and mange mites), as are many reptiles andsome amphibians. Insects, especially those that build nests, live insemipermanent habitats like decaying wood, or use more ephemeral habitatslike bracket fungi and dung, are hosts to a cornucopia of mite commensals,parasites and mutualists. None of these mites exceed a centimeter inlength, and the vast majority grow to less than a millimeter, yet theyoften have a major impact on their hosts.