Mites are smallarachnids (eight-leggedarthropods). Mites span two large orders of arachnids, theAcariformes and theParasitiformes, which were historically grouped in the subclassAcari. However, most recent genetic analyses do not recover the two as each other's closest relative within Arachnida, rendering the groupmonophyletic. Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. The small size of most species makes them easily overlooked; some species live in water, many live in soil asdecomposers, others live on plants, sometimes creatinggalls, while others arepredators orparasites. This last type includes the commercially destructiveVarroa parasite ofhoney bees, as well asscabies mites of humans. Most species are harmless to humans, but a few are associated with allergies or may transmit diseases.
The scientific discipline devoted to the study of mites is calledacarology.
The mites are not a defined taxon but are used for two distinct groups of arachnids, theAcariformes and theParasitiformes. Thephylogeny of the Acari has been relatively little studied, but molecular information fromribosomal DNA is being extensively used to understand relationships between groups. The 18 S rRNA gene provides information on relationships amongphyla and superphyla, while theITS2, and the18S ribosomal RNA and28S ribosomal RNA genes, provide clues at deeper levels.[1]
The mite fossil record is sparse due to their small size and low preservation potential.[5] The oldest fossils of acariform mites are from theRhynie Chert, Scotland, which dates to the earlyDevonian, around 410 million years ago[6][5] while the earliest fossils of Parasitiformes are known fromamber specimens dating to the mid-Cretaceous, around 100 million years ago.[5][7] Most fossil acarids are no older than theTertiary (up to 65mya).[8]
Members of the superordersOpilioacariformes andAcariformes (sometimes known as Actinotrichida) are mites, as well as some of theParasitiformes (sometimes known as Anactinotrichida).[9] Recent genetic research has suggested that Acari ispolyphyletic (of multiple origins).[10][11][12][13] A study using molecular data from the mitochondria and nucleus recovered Acariformes assister to theSolifugae (camel spiders) and Parasitiformes as sister to thePseudoscorpionida, with other arachnid orders separating these two groupings on the phylogenetic tree, as shown below.[10]
Several phylogenomic studies have strongly supported the idea that Acari is monophyletic, establishing a sister relationship between Acariformes and Parasitiformes. For example, Lozano-Fernandez et al. (2019) provide evidence for the monophyly of Acari and Arachnida in their research published in *Nature Communications*. Similarly, Howard et al. (2020) add to this perspective, offering morphological, paleontological, and molecular support for a single event of terrestrialization within the Chelicerata in their article in *Arthropod Structure & Development*.
Despite these findings, some researchers have raised questions about their validity, suggesting that the results may be influenced by a long branch attraction artifact.
Mites are tiny members of the classArachnida; most are in the size range 250 to 750 μm (0.01 to 0.03 in) but some are larger and some are no bigger than 100 μm (0.004 in) as adults. The body plan has tworegions, acephalothorax (with no separate head) or prosoma, and anopisthosoma 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.[14]
At the front of the body is thegnathosoma or capitulum. This is not a head and does not contain the eyes or the brain, but is a retractable feeding apparatus consisting of thechelicerae, thepedipalps, and the oral cavity. It is covered above by an extension of the bodycarapace and is connected to the body by a flexible section ofcuticle. Two-segmented chelicerae is the ancestral condition in Acariformes, but in more derived groups, they are single-segmented. And three-segmented chelicerae is the ancestral condition in Parasitiformes, but has been reduced to just two segments in more derived groups.[16] The pedipalps 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 andpharynx.[14]
Most mites have four pairs of legs (two pairs inEriophyoidea[17]), each with six segments, which may be modified for swimming or other purposes. The dorsal surface of the body is clad in hardenedtergites and the ventral surface by hardenedsclerites; sometimes, these form transverse ridges. Thegonopore (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 bearsetae (bristles), which may be simple, flattened, club-shaped, or sensory. Mites are usually some shade of brown, but some species are red, orange, black, green, or some combination of these colours.[14]
Many mites have stigmata (openings used in respiration). In some mites, the stigmata are associated withperitremes: paired, tubular, elaborated extensions of the tracheal system. The higher taxa of mites are defined by these structures:[18][19][20]
Oribatida, formerly known as Cryptostigmata (crypto- = hidden), and Endeostigmata (endeo- = internal) lack primary stigmata and peritremes but may have secondary respiratory systems.[21] For example, oribatids in the suborderBrachypylina have stigmata on the ventral plate of the body that are difficult to see (thus the former name Cryptostigmata).[22]
Astigmata (a- = without) lack stigmata and respire through their cuticle.[23]
Prostigmata (pro- = before/in front) have stigmata at the front of the body, usually on the lateral margins or between the chelicerae. These are associated with peritremes that may be on the prodorsum near the cheliceral bases, or be horn-like and emergent, or form a line or network on the dorsum of the gnathosomal capsule.[19]
Opilioacaridae have four pairs of dorsolateral stigmata that are added sequentially during development.[19]
The other three orders of Parasitiformes, Holothyrida, Ixodida, and Mesostigmata (meso- = middle), have just one pair of stigmata in the region of the fourth pair of legs. They also have peritremes: in Ixodida, these consist of paired encircling plates around the stigmata, while the peritremes in Mesostigmata and Holothyrida are grooves extending from the stigmata anteriorly (sometimes also posteriorly).[20]
Mite digestive systems havesalivary glands that open into the preoral space rather than the foregut. Most species carry two to six pairs of salivary glands that empty at various points into the subcheliceral space.[24] A few mite species lack an anus: they do not defecate during their short lives.[25] The circulatory system consists of a network of sinuses and most mites lacks a heart, movement of fluid being driven by the contraction of body muscles. But ticks, and some of the larger species of mites, have a dorsal, longitudinal heart.[26] Gas exchange is carried out across the body surface, but many species additionally have between one and four pairs oftracheae. The excretory system includes anephridium and one or two pairs ofMalpighian tubules.[27] Several families of mites, such as Tetranychidae, Eriophyidae, Camerobiidae, Cunaxidae, Trombidiidae, Trombiculidae, Erythraeidae and Bdellidae havesilk glands used to produce silk for various purposes. Additionally, water mites (Hydrachnidia) produce long, thin threads that may be silk.[28]
The sexes are separate in mites; males have a pair oftestes in the mid-region of the body, each connected to the gonopore by avas deferens, and in some species there is a chitinouspenis; females have a singleovary connected to the gonopore by anoviduct, as well as aseminal receptacle for the storage ofsperm. In most mites, sperm is transferred to the female indirectly; the male either deposits aspermatophore 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 theAcariformes, insemination is direct using the male's penis.[14] The spermatophora in all mites are aflagellate.[29]
The eggs are laid in the substrate or wherever the mite happens to live. They take up to six weeks to hatch, according to species, then may pass through up to six instars: prelarva, larva, protonymph, deutonymph, tritonymph, and adult. These developmental stages may look different or may be omitted depending on the mite group. All mites have an adult stage.[30] Longevity varies between species, but the lifespan of mites is short compared to many other arachnids.[14]
Mites occupy a wide range ofecological niches. For example,Oribatida mites are importantdecomposers in many habitats. They eat a wide variety of material, including living and dead plants and fungal material, lichens, and carrion; some are predatory, though no oribatid mites are parasitic.[32] Mites are among the most diverse and successful of allinvertebrate groups. They have exploited a wide array ofhabitats and, because of their small size, go largely unnoticed. They are found in freshwater (e.g. the water mites orHydrachnidia[33]) and saltwater (mostHalacaridae[34]), in the soil, in forests, pastures, 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 areparasites of plants and animals.[35] Some 48,200 species of mites have been described,[36] but there may be a million or more species as yet undescribed.[14] The tropical speciesArchegozetes longisetosus is one of the strongest animals in the world, relative to its mass (100 μg): It lifts to 1,182 times its weight, over five times more than would be expected of such a minute animal.[37] A mite also holds a speed record: for its length,Paratarsotomus macropalpis is the fastest animal on Earth.[38]
The mites living in soil consist of a range of taxa. Oribatida and Prostigmata are more numerous in soil than Mesostigmata and have more soil-dwelling species.[39] When soil is affected by an ecological disturbance such as agriculture, most mites (Astigmata, Mesostigmata and Prostigmata) recolonise it within a few months, whereas Oribatida take multiple years.[40]
Many mites areparasitic on plants and animals. One family of mites,Pyroglyphidae, or nest mites, live primarily in the nests of birds and other animals. These mites are largelyparasitic and consumeblood,skin andkeratin.Dust mites, which feed mostly on dead skin and hair shed from humans instead of consuming them from the organism directly, evolved from these parasitic ancestors.[41]Ticks are a prominent group of mites that are parasitic on vertebrates, mostly mammal and birds, feeding on blood with specialised mouthparts.[42]
Parasitic mites sometimes infest insects.Varroa destructor attaches to the body ofhoney bees, andAcarapis woodi (familyTarsonemidae) lives in theirtracheae. Hundreds of species are associated with other bees, mostly poorly described. They attach to bees in a variety of ways. For example,Trigona corvina workers have been found with mites attached to the outer face of their hind tibiae.[43] Some are thought to be parasites, while others are beneficialsymbionts. Mites also parasitize some ant species, such asEciton burchellii.[44] Most larvae ofParasitengona are ectoparasites of arthropods, while later life stages in this group tend to shift to being predators.[45]
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.[48]
Parasitic mites use their hosts to disperse and spread from host to host by direct contact. Another strategy isphoresy; 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 hitchhiker 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.[48]
Public health worker Stefania Lanzia using ascabies mite to publicisescabies, an often overlooked condition especially among the elderly
Mites are tiny, and apart from those that are of economic concern to humans, they are little studied. The majority are beneficial, living in the soil or aqueous environments and assisting in the decomposition of decaying organic material, as part of thecarbon cycle.[35]
Two species live on humans, namely Demodex folliculorum]] and Demodex brevis]]; both are frequently referred to as eyelash mites.
The majority of mite species are harmless to humans anddomestic animals, but a few species can colonize mammals directly, acting asvectors 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 asgamasoidosis,[49]rodent mite dermatitis,[50]grain itch,[51]grocer's itch,[51] andscabies;Sarcoptes scabiei is a parasitic mite responsible for scabies, which is one of the three most common skin disorders in children.[52]Demodex mites, which are common cause ofmange indogs and other domesticated animals,[47] have also been implicated in the human skin diseaserosacea, although the mechanism by whichdemodex contributes to the disease is unclear.[53] Ticks are well known for carrying diseases, such asLyme disease[54] andRocky Mountain spotted fever.[55]
Among domestic animals, sheep are affected by the mitePsoroptes ovis which lives on the skin, causing hypersensitivity and inflammation.[59] Hay mites are a suspected reservoir forscrapie, aprion disease of sheep.[60]
The miteVarroa destructor is a serious pest ofhoney bees, contributing tocolony collapse disorder incommercial hives. This organism 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 spreadRNA viruses includingdeformed wing virus. Heavy infestation causes the death of a colony, generally over the winter. Since 2006, more than 10 million beehives have been lost.[61][62]
Mites were first observed under themicroscope by the English polymathRobert Hooke. In his 1665 bookMicrographia, he stated that far from beingspontaneously generated from dirt, they were "very prettily shap'd Insects".[65] In 1898,Arthur Conan Doyle wrote a satirical poem, "A Parable", with theconceit of some cheese mites disputing the origin of theroundcheddar cheese in which they all lived.[66] The world's first science documentary featuredcheese mites, seen under the microscope; the short film was shown in London's Alhambramusic hall in 1903, causing a boom in the sales of simple microscopes.[65]
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