A subclass of small arthropods belonging to the class Arachnida and comprising the mites and ticks. There are about 50 000 described species, with perhaps 20 times this number still unknown, distributed worldwide in a wide variety of terrestrial and aquatic habitats. Many are free-living in soil or on vegetation, feeding on organic matter or preying on other small arthropods, while a significant number are parasites of plants and animals, including domesticated animals and humans. The adult body is generally globular or ovoid, with four pairs of legs. Unlike spiders, there is no ‘waist’, the abdomen being fused to the more anterior prosoma. At the front of the body the capitulum bears the mouthparts, variously adapted for cutting, crushing, or piercing. The eggs hatch into a three-legged larva, which subsequently moults to a nymph resembling the adult. Ticks (up to 3 cm long) are ectoparasites of vertebrates, feeding on blood drawn through the skin of their host. They transmit a wide range of diseases, including certain forms of encephalitis and Lyme disease. Mites are much smaller (up to 4 mm long) and are parasitic or free-living. They tend to feed on feathers, hair, skin secretions, or skin debris, causing, for example, scabies in humans and mange in domesticated animals. The house-dust mite (Dermatophagoides sp.) can provoke allergies or dermatitis. Spider mites are damaging parasites of plants and may infest some arable and greenhouse crops.

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