Water mites are rarely more than a few millimeters long, but their bright colours and rapid movement often bring them to our attention. Mites are truly ubiquitous. They have successfully colonized nearly every known terrestrial, marine, and fresh water habitat including polar and alpine extremes, tropical lowlands and desert barrens, surface and mineral soils to depths of 10 meters, cold and thermal surface springs and subterranean waters with temperatures as high as 50C, all types of streams, ponds and lakes, and sea waters of continental shelves and deep sea trenches to depths of 5000 meters. Some idea of mite abundance and diversity can be gained from analysis of one square meter of mixed temperate hardwood or boreal coniferous litter, which may harbour upwards from one million mites representing 200 species in at least 50 families. Within this complex matrix of decomposing plant matter, mites help to regulate 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 on which they live.
Plants, including crops and the canopies of tropical rainforests, are inhabited by myriads of mite species feeding on mosses, ferns, leaves, stems, flowers, fruit, lichens, microbes, other arthropods and each other. Insects, especially those that build nests, live in semipermanent habitats like decaying wood, or use more ephemeral habitats like bracket fungi and dung, are hosts to a cornucopia of mite commensals, parasites and mutualists. None of these mites exceed a centimeter in length, and the vast majority grow to less than a millimeter, yet they often have a major impact on their hosts. Additionally, eight families of very early derivative acariform mites are lumped into the Endeostigmata, usually considered a suborder of the Prostigmata, but clearly containing taxa that belong to both major acariform lineages. The Mesostigmata contains in excess of 65 families and 10,000 described species, the other two parasitiform orders each comprise three families.