mite, small, often microscopic chelicerate that, along with the tick, makes up the order Acarina; they are related to the spiders. The unsegmented mite body is typically oval and compact, although a few, mostly parasites, are elongated and wormlike. There are four pairs of legs. The movable head is attached to the body by a hinge. There are four stages in the life cycle: egg, larva, nymph, and adult.
The thousands of different mite species are worldwide in distribution and occupy diverse habitats, including plant galls, mosses, other animals, and surface litter or upper layers of the soil. One group, the water mites, has returned to an aquatic environment, both fresh- and saltwater. Mites eat plant or animal substances, decaying organisms, and humus, and also infest stored food products such as cheese, meat, grains, and flour. The spider mite, or red spider, which is a mite and not a spider, feeds on plants and is destructive to crops. Many mites are parasitic on other arthropods, on mollusks, or on vertebrates. Mange and scabies mites lay their eggs in the skin and cause irritation in humans and fur-bearing animals. Other species are parasitic on the skin of birds and reptiles, and some live in the respiratory channels of birds and mammals. Chiggers, the larvae of harvest mites, transmit the organism that causes scrub typhus. Fowl mites feed on the blood of poultry. House dust mites, which thrive in moist environments and eat flakes of human skin and other minute organic material, produce digestive enzymes that are excreted in their feces and produce allergic reactions in many people.
The larger members of the order Acarina, the ticks, are all parasitic in at least one developmental stage; most parasitize mammals and birds although some have reptilian and amphibian hosts. Tick-borne diseases of livestock (e.g., babesiosis, anaplasmosis) are of great economic significance. An anchoring structure in the tick's mouth enables it to embed its entire head under the skin of the host, where it sucks the host's blood. If a tick is pulled off the host, the head usually remains embedded in the skin. Members of the family (Argasidae) of soft ticks, with a membranous outer covering, hide in crevices and come out at night to suck blood. Hard ticks (family Ixodidae), which have thickened outer plates made of chitin, remain attached to the host for long periods.
Ticks transmit Rocky Mountain spotted fever, tularemia, Lyme disease, equine encephalitis, several forms of ehrlichiosis, and other diseases. Each species needs three different hosts to complete its life cycle. Typically the larval stage will feed on small reptiles, birds, or mammals; the nymph stage will parasitize larger vertebrates; and adults will parasitize large herbivores and livestock. The adult of the ixodid species Ixodes scapularis, the vector of Lyme disease and babesiosis in the E United States and Canada, usually chooses deer as its host ( I. scapularis of all stages will feed on humans). The closely related I. pacificus, which transmits Lyme disease and Rocky Mountain spotted fever in the western states, prefers livestock in the adult stage. Ticks can sometimes harbor more than one disease organism at a time.
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