Mites are arachnids, ( that is, spiders and their relatives) of the Order Acari or Acarina, among the smallest members of the spider family.
They are also the most widely distributed, the most diverse in form and habit, and, because they carry disease, the most dangerous to human beings. More than 30,000 species have been described, but many authorities think that there may be as many as 500,000 species as yet to be discovered. They have been recorded as high as 5,000 meters on the slopes of Mount Everest and as deep as 5,200 meters in the northern Pacific Ocean. More than 50 terrestrial species are known from the Antarctic. A few mites have been found drifting high in the atmosphere. Most species are free-living, that is, not parasitic on other life forms.
Everyone knows what they look like: the undivided and unsegmented body, with eight legs and sucking mouthparts. All are fluid feeders. Some suck animal or plant tissue fluids; others liquefy solid food by injecting enzymes. There are separate males and females, though some species can reproduce by parthenogenesis; most hatch from eggs. Most have one or more immature stages, during which they may have six or even four legs.
The ones we have problems with are the parasitic species, especially the ticks, relatively large mites (2 to 6 mm) which suck blood from vertebrates, including ourselves. Besides being unbelievably disgusting (finding a huge, blood-gorged tick with its head embedded in one's own body has few parallels for sheer ugliness) these mites carry diseases: viruses (encephalitis, Colorado tick fever), rickettsias (Rocky Mountain spotted fever, scrub typhus, Q fever), spirochetes (relapsing fever and Lyme disease) and sporozoans (babesiasis). Cattle, sheep and horses can also be infected with diseases by ticks, who may also carry tapeworms. Mites have also decimated domestic honey bees.
They attack our plants too. No indoor gardener is ignorant of the dreaded spider mite, and food crops are also attacked by various mite species. Further, there are grain mites, sugar mites, and others, which attack our stored foodstuffs.
On and on. Mites in household dust are a major cause of dust allergies. The "chiggers" of the southern United States are immature stages of mites. The skin disease scabies is caused by a mite which completes its entire life cycle burrowed into the skin, and is passed from person to person by contact. Mange on domestic animals is caused in a similar fashion.
The other side of the story
Not to be too self-centered, we have to remember that the vast majority of mite species have little to do with us or our plant or animal dependants. Some are beneficial.
- Enormous mite populations are present in leaf litter and soil, and play an important role in that ecology. Mites are the most numerous arthropods in soil. They feed on plant and animal material, and are probably the most important single animal (except for the microfauna) in the breakdown of organic material. This function alone undoubtedly outweighs all the harm mites do us otherwise.
- Cheese mites are common in stored food, damp flour, old honeycombs, and insect collections. Mite-infested cheese will be more or less covered with a grey powder. There are some European cheeses into which a culture of cheese mites is deliberately introduced. Altenburger cheese is one. The mites are said to impart a characteristic "piquant" taste. When the cheese is covered with the tell-tale greyish powder, consisting of enormous numbers of living and dead mites, cast skins, and feces, it is then ripe and particularly valuable.
PHYLUM: Arthropoda, SUBPHYLUM: Chelicerata, CLASS: Arachinida, ORDER Acarina
Pearse, Buchsbaum, Living Invertibrates, Blackwell Scientific Publications, Boston, Massachusetts 1987;
Wonderful color pictures by David Walter, University of Queensland, at http://www.uq.edu.au/entomology/mite/mitetxt.html (November, 2002)