Mites are tiny arthropods in the order Acari (or Acarina), which also includes the somewhat larger ticks. The Acari is in the class Arachnida, which also encompasses the spiders and scorpions. Arachnids have four pairs of segmented legs, a body divided into a cephalothorax (that is, a united head and thorax), and abdomen, and they have a simple respiratory apparatus consisting of tracheae and/or book lungs. The Acari are a diverse group of at least 30,000 species, although the group is not yet well studied, and many additional species will undoubtedly be named.
Mites typically have small or minute bodies with roughly oval shapes and little differentiation of the cephalothorax and the unsegmented abdomen. Newly hatched mites are known as larvae, and they have only three pairs of legs. After the first molt the mite has four pairs of legs, and the animal is known as a nymph until the sexually mature adult stage is achieved.
Most species of mites are tiny creatures that live in organic debris of forests and grasslands where they feed on dead organic matter, fungal spores and hyphae, and plant matter. Many other species are predators of other mites and tiny insects such as springtails, or they are parasites of a wide range of animals. Terrestrial mites can be extremely abundant, and their populations may outnumber those of all other species of arthropods in many habitats. A few species of mites are aquatic, including some relatively large and attractive, bright-red or green-colored species known as water-mites commonly in the genus Arrenurus.
Some species of mites are important pests of agricultural plants. Certain species of spider mites or red mites (family Tetranychidae) are important pests of fruit trees grown in orchards, field crops, and greenhouses. Mite infestations may be managed by applying toxic chemicals such as pesticides, by reducing the amount of organic debris that is present in their habitat, or by encouraging species of mites that are predators of the injurious species.
Some mites physically affect domestic animals and humans. The chicken mite (Dermanyssus gallinae) is a serious, highly infectious, blood-sucking parasite of chickens and other fowl. The sheep scab mite (Psoroptes ovis) is a pest of sheep and other livestock. Scabies is an itchy skin condition caused by infestations of various species of itch or scab mites (family Sarcoptidae) that burrow into the skin. Many animals are afflicted by mange, a skin condition caused by species of mange mites (family Psoroptidae). Chiggers or harvest mites (family Trombiculidae) are another group of itch-causing skin parasites which can sometimes be vectors of human diseases in the tropics such as scrub typhus.
House-dust mites in the genus Dermatophagoides (family Acaridae) are common in homes and are an important cause of the allergies that many people develop to house dusts.