Mites

Mites are small s belonging to the class a and the subclass. Mites are not a as they span two different groups of arachnids: the  are sister to the s, while the  are sister to the s; also, they exclude the s, order, although ticks and mites are closely related. Mites are distantly related to s and s.

The body is in two sections, the or prosoma (there is no separate head), and an. The scientific discipline devoted to the study of ticks and mites is called.

Most mites are tiny, less than 1 mm in length, and have a simple, unsegmented body plan. Their small size makes them easily overlooked; some species live in water, many live in soil as s, others live on plants, sometimes creating s, while others again are or. This last type includes the commercially important  parasite of s, as well as the mite of humans. Most species are harmless to humans but a few are associated with allergies or may transmit diseases.

Evolution and taxonomy
The mites are not an exact taxon, but the name is used for members of several groups in the subclass. The of the Acari has been relatively little studied, but molecular information from  is being extensively used to understand relationships between groups. The 18 S rRNA gene provides information on relationships among and superphyla, while the, and the  and  genes, provide clues at deeper levels.

Taxonomy
The third edition (2009) of the standard textbook A Manual of Acarology uses a system of six, grouped into three superorders:
 * Superorder  – a small order of large mites that superficially resemble, hence their name
 * Superorder  – ticks and a variety of mites
 * - predatory mites from the southern hemisphere
 * – hard ticks and soft ticks
 * – bird mites, phytoseiid mites
 * - large, diverse order
 * - diverse order of parasitic and predatory mites
 * Superorder  – the most diverse group of mites
 * – plant parasitic mites (spider mites, peacock mites, gall mites, red-legged earth mites, etc.), snout mites, chiggers, hair follicle mites, velvet mites, water mites, etc.
 * - small order of mites containing two families
 * - large order of sucking mites
 * – oribatid mites, beetle mites, armored mites (also cryptostigmata)
 * – stored product, fur, feather, dust, and human itch mites, etc.
 * – stored product, fur, feather, dust, and human itch mites, etc.

Fossil record
Most fossil acarids are no older than the (up to 65 ). Earlier fossils are too few to enable mite phylogeny to be reconstructed from palaeontological evidence, but in 2002 an oribatid mite from the  (c. 480 mya) was found in, Sweden. The first find of Parasitiformes from the Mesozoic was of an argasid tick larva in amber (90–94 mya) from New Jersey. Other fossils including the first opilioacariform mite are preserved in of  age (44 mya).

Phylogeny
Members of the superorders and  (sometimes known as Actinotrichida) are mites, as well as some of the   (sometimes known as Anactinotrichida). Recent genetic research has caused a change in the naming scheme, however, and recent publications have changed the superorder to an order. Other recent research has suggested that Acari is (of multiple origins), with ticks and spiders more closely related than ticks and mites. The cladogram is based on Dabert et al. 2010, which used molecular data. It shows the Acariformes to the  (camel spiders), while the Parasitiformes are sister to the.

External
Mites are tiny members of the class a; most are in the size range 250 to 750 µm but some are larger and some are no bigger than 100 µm as adults. The body plan is similar to that of ticks in having two, a (with no separate head) or prosoma, and an  or abdomen. Segmentation has almost entirely been lost and the prosoma and opisthosoma are fused, only the positioning of the limbs indicating the location of the segments.

At the front of the body is the or capitulum. This is not a head and does not contain the eyes or the brain, but is a retractable feeding apparatus consisting of the, the s and the oral cavity. It is covered above by an extension of the body and is connected to the body by a flexible section of. The mouthparts differ between taxa depending on diet; in some species the appendages resemble legs while in others they are modified into chelicerae-like structures. The oral cavity connects posteriorly to the mouth and pharynx.

Most mites have four pairs of legs, each with six segments, which may be modified for swimming or other purposes. The dorsal surface of the body is clad in hardened and the ventral surface by hardened s; sometimes these form transverse ridges. The (genital opening) is located on the ventral surface between the fourth pair of legs. Some species have one to five median or lateral eyes but many species are blind, and slit and pit sense organs are common. Both body and limbs bear e (bristles) which may be simple, flattened, club-shaped or sensory. Mites are usually some shade of brown, but some species are red, orange, black or green, or some combination of these colours.

Internal
Mites have a typical arachnid digestive system, although some species lack an anus: they do not defecate during their short lives. The circulatory system consists of a network of sinuses and lacks a heart, movement of fluid being driven by the contraction of body muscles. Gas exchange is carried out across the body surface, but many species additionally have between one and four pairs of e, the s being located in the front half of the body. The excretory system includes a and one or two pairs of s.

Reproduction and life cycle
The sexes are separate in mites; males have a pair of in the mid-region of the body, each connected to the gonopore by a, and in some species there is a chitinous ; females have a single  connected to the gonopore by an , as well as a  for the storage of. In most mites, sperm is transferred to the female indirectly; the male either deposits a on a surface from which it is picked up by the female, or he uses his chelicerae or third pair of legs to insert it into the female's gonopore. In some of the, insemination is direct using the male's penis.

The eggs are laid in the, or wherever the mite happens to live. They take from two to six weeks to hatch, and the first stage larvae have six legs. After three moults, the larvae become nymphs, with eight legs, and after a further three moults, they become adults. Longevity varies between species, but the lifespan of mites is short as compared to many other arachnids.

Niches
Mites occupy a wide range of. For example, mites are important  in many habitats. They eat a wide variety of material including living and dead plant and fungal material, lichens and carrion; some are, though no oribatid mites are. Mites are among the most diverse and successful of all groups. They have exploited a wide array of s, and because of their small size go largely unnoticed. They are found in fresh and salt water, in the soil, in forests, pastures, agricultural crops, ornamental plants, thermal springs and caves. They inhabit organic debris of all kinds and are extremely numerous in leaf litter. They feed on animals, plants and fungi and some are of plants and animals. Some 48,200 species of mites have been described, but there may be a million or more species as yet undescribed. The tropical species  is one of the strongest animals in the world, relative to its mass (100 μg): It lifts up to 1,182 times its own weight, over five times more than would be expected of such a minute animal. A mite also holds a speed record: for its length,  is the fastest animal on Earth.

Parasitism
Many mites are on plants and animals. One family of mites,, or nest mites, live primarily in the nests of birds and animals. These mites are largely and consume,  and. , which feed mostly on dead skin and hair shed from humans instead of consuming them from the organism directly, evolved from these parasitic ancestors.

Parasitic mites sometimes infest insects. ' attaches to the body of s, and ' (family ) lives in their e. Hundreds of species are associated with other bees, mostly poorly described. They attach to bees in a variety of ways. For example,  workers have been found with mites attached to the outer face of their hind tibiae. Some are thought to be parasites, while others are beneficial. Mites also parasitize some ant species, such as .

Plant pests include the so-called s (family ), s (family ), and the s (family ). Among the species that attack animals are members of the mites (family ), which burrow under the skin. s (family ) are parasites that live in or near the s of s, including humans.

Dispersal
Being unable to fly, mites need some other means of dispersal. On a small scale, walking is used to access other suitable locations in the immediate vicinity. Some species mount to a high point and adopt a dispersal posture and get carried away by the wind, while others waft a thread of silk aloft to balloon to a new position.

Parasitic mites use their hosts to disperse, and spread from host to host by direct contact. Another strategy is ; the mite, often equipped with suitable claspers or suckers, grips onto an insect or other animal, and gets transported to another place. A phoretic mite is just a hitch-hiker and does not feed during the time it is carried by its temporary host. These travelling mites are mostly species that reproduce rapidly and are quick to colonise new habitats.

Relationship with humans
Mites are tiny, almost invisible, and apart from those that are of economic concern to humans, little studied. The majority are beneficial, living in the soil or aqueous environments and assisting in the decomposition of decaying organic material, or consuming fungi, plant or animal matter, as part of the.

Medical significance
The majority of mite species are harmless to humans and s, but a few species can colonize mammals directly, acting as s for disease transmission, and causing or contributing to allergenic diseases. Mites which colonize human skin are the cause of several types of itchy skin rashes, such as, , , , and ;  is a parasitic mite responsible for scabies, which is one of the three most common skin disorders in children.  mites, which are common cause of in s and other domesticated animals, have also been implicated in the human skin disease, although the mechanism by which demodex contributes to the disease is unclear.

are known primarily for their, but they can also spread disease in some limited circumstances, such as. The is the only known vector of the disease. s, found in warm and humid places such as beds, cause several forms of allergic diseases, including, and , and are known to aggravate.

, sheep are affected by the mite Psoroptes ovis which lives on the skin, causing hypersensitivity and inflammation.

In beekeeping
The mite Varroa destructor is a serious pest of s, contributing to in. The mite is an obligate external parasite, able to reproduce only in bee colonies. It directly weakens its host by sucking up the bee's fat, and can spread es including. Heavy infestation causes the death of a colony, generally over the winter. Since 2006, over 10 million beehives have been lost.

In culture
Mites were first observed under the by the English polymath. In his 1665 book , he stated that far from being from dirt, they were "very prettily shap'd Insects". The world's first science documentary featured s, seen under the microscope; the short film was shown in London's Alhambra in 1903, causing a boom in the sales of simple microscopes. A few years later, wrote a satirical poem, Parable, with the  of some cheese mites disputing the origin of the   in which they all lived.