Mites comprise the Acari; the largest group within the arthropod Class Arachnida, with over 48,000 described species. This number is misleading since it is estimated that only between 5-10% of all mite species have been formally described. In contrast with other arachnid groups such as spiders and scorpions, mites are distinctive in both their small size (adult body length ranging from 0.1-30 millimeters), and ecological diversity. Some mites are predators like almost all other arachnids, but mites may also feed on plants, fungi, microorganisms, or as parasites on or in the bodies of other animals. Mites are among the oldest known groups of arthropods, with a fossil record beginning in the Devonian period.
There are a number of instances where mites are important to humans. Many species are serious pests of agricultural crops, either through direct damage or indirectly as vectors of plant pathogens. Other species are parasitic on domestic animals and cause losses in meat, egg, and fiber production. Others, such as the human scabies mite, are direct agents of human disease, or, as in the case of chiggers and ticks, vectors of pathogens. Other mites may affect humans by infesting stored food products. Many species of Astigmata are known as "stored product mites" because they have moved from their ancestral rodent nest habitats into human food stores. Such mites may also cause damage in animal feed by causing allergic reactions in livestock, and are also known to cause skin irritation in humans handling infested materials. A related group of astigmatid mites, also ancestrally nest inhabiting, is the family Pyroglyphidae. These mites have colonized human habitations from bird nests and are the primary source for allergens in house dust. Commonly known as "house dust mites," species particularly in the genus Dermatophagoides produce many proteins that induce allergic responses in sensitive individuals. House dust allergy may take the form of respiratory distress or skin irritation. Mites typically inhabit beds, chairs and carpets in houses, and their shed skins and feces provide the bulk of the allergens in house dust extracts.
On the other hand some mites are beneficial to humans in their role as biological control agents against agricultural pests. Also, the natural role of mites in providing "ecosystem services" in the form of nutrient cycling, cannot be overlooked.