Mites comprise the Acari, which are the largest group within the arthropod class Arachnida, with over 48,000 described species. This number is misleading because it is estimated that only between 5% and 10% of all mite species have been formally described. In contrast with other arachnid groups such as spiders and scorpions, mites are distinctive in both their small size (adult body length ranging from 0.1 to 30 mm) and their ecological diversity. Some mites are predators, like almost all other arachnids, but mites may also feed on plants, fungi, or microorganisms or as parasites on or in the bodies of other animals. Mites are among the oldest known groups of arthropods, with a fossil record beginning in the Devonian period.
Unlike insects, with bodies divided into head, thorax, and abdomen, the arachnid body is ancestrally divided into two functional units, the prosoma (the first six body segments) and the opisthosoma (the remaining segments). The body of a mite is further modified in that these original units are fused. A secondary subdivision separates the first two body segments into a structure termed the gnathosoma, specialized for feeding, and the remainder of the body, termed the idiosoma, containing organs of locomotion, digestion, and reproduction. Most mites show no evidence of external body segmentation, other than the serial appendages. The gnathosoma bears the first two pairs of appendages, the chelicerae, which may retain the ancestral chelate, or pincer-like form, or may be highly modified as stylets for piercing and sucking; the pedipalps, which may be almost leglike, are strongly modified for grasping prey or attaching to a host or highly reduced. The anterior idiosoma typically bears four pairs of walking legs, the first pair of which may be modified as antenna-like, sensory structures. Legs may also be modified for attaching to a host.
Occasionally legs of males are modified for grasping a female during mating or for intraspecific combat.
The mite’s body cuticle may be entirely soft, divided into a number of hard, sclerotized plates, or almost entirely sclerotized. In a few mites, crystalline, mineral salts also strengthen the cuticle. Such modifications balance the needs for flexibility in movement and protection from predators. The body surface bears setae, typically hair-like sensory organs, arranged in characteristic patterns in different subgroups of mites. Setae are primarily hair-like, but may take on an incredible variety of shapes, from thick spines, to flat plates, to highly branched, feather-like forms. The pedipalps and legs also bear tactile setae as well as chemosensory structures termed solenidia, which are organs of smell and taste, and other specialized sensilla that are sensitive to infrared radiation. Simple eyes, or ocelli, may be present on the anterior idiosoma, and specialized sensory organs, the trichobothria, on the anterior idiosoma or legs may detect vibrations or electric fields.
Like other arthropods, the inside of a mite’s body is a hollow cavity, the hemocoel, in which the internal organs are surrounded by fluid, the hemolymph. Hemolymph distributes food materials and waste products and contains hemocytes, which are the cells that serve as the mite’s immune system, but it does not contain oxygen-binding proteins as are found in the blood of vertebrates and some other arthropods. The mite’s digestive system is divided into the three parts typical of arthropods: foregut, midgut, and hindgut. The midgut may be divided into diverticulae for food storage, particularly in parasitic mites. Some mites lack a connection between the midgut and the hindgut; these mites feed only on fluids and do not defecate. The hindgut in these mites is transformed into an excretory organ for elimination of nitrogenous wastes. Other mites, with entire guts, may have Malpighian tubules, like insects, extending from the junction of the midgut and hindgut as excretory organs. The internal reproductive system typically consists of a single ovary (paired in the Astigmata) in the female and paired testes in the male. Females typically possess a spermatheca for sperm storage after insemination, and both sexes have various accessory glands and ducts to the exterior as part of the system. Tracheal systems for respiration have evolved independently a number of times in the Acari. These open at spiracles, or stigmata, on various parts of the body in different groups. Other mites lack any respiratory system, and gas exchange occurs through the cuticle in these groups.
CLASSIFICATION OF MITES
The classifications of mites used by various authors vary considerably in the number of higher categories recognized and the hierarchical ranking of the various groups. The simplest system, used by Walter and Proctor (1999), recognizes three orders within Acari: Opik’oacariformes, Parasitiformes, and Acariformes. The Opilioacariformes, comprising a single family with about 20 species, is considered the most primitive. These mites are relatively large (2-3 mm) and resemble small opilionids in their general form, having a leathery cuticle that retains traces of external segmentation. These mites resemble the Parasitiformes in having a tracheal system opening laterally on the body, but they have four pairs of stigmatal openings in contrast to the single opening of the Parasitiformes. Opilioacarids resemble some Acariformes in feeding on solid food particles and bearing a pair of rutella, which are sclerotized food-processing structures located near the ventral apex of the gnathosoma.
The order Parasitiformes is a diverse group comprising 76 families divided among three suborders: Gamasida (or Mesostigmata), Ixodida, and Holothyrida. Compared with the Acariformes, this order is morphologically relatively conservative, with most species retaining the same basic body plan. The Holothyrida includes 3 families and around 30 species of heavily sclerotized, predatory or scavenging mites of tropical regions. The Ixodida, or ticks, includes 3 families and around 850 species exclusively parasitic on vertebrate hosts. The vast majority of parasitiform mites are included in the Gamasida, with 70 families. Most gamasid mites retain the ancestral predatory lifestyle, but the group includes a number of parasites of vertebrates and other arthropods, a few mites which feed on pollen or fungi, and one small group of detritivores capable of feeding on solid food particles.
The order Acariformes is the largest and most diverse group of mites, in terms of both its morphology and its ecological diversity. Hundreds of families are recognized, and over 30,000 species are included. Acariform mites are characterized by the internalization of the basal leg segment, the coxa, leaving the next segment, the tro-chanter, as the first functional leg segment. Most acariform mites also possess structures termed genital papillae. While associated with the genital region in the postlarval instars, these structures are actually osmoregulatory organs.
The order Acariformes is conveniently divided into two suborders, Trombidiformes (largely equivalent to the Prostigmata of some authors) and Sarcoptiformes (including the Oribatida and Astigmata of some authors). Most trombidiform mites have tracheal systems opening on or near the gnathosoma. Many have strongly modified chelicerae adapted for piercing animal prey, plant tissue, or the skin of a host animal. Sarcoptiform mites ancestrally feed on solid food and have gnathosomal rutella, like the Opilioacariformes. Tracheal systems opening at the leg bases or anterior dorsal idiosoma have evolved independently several times in this group. Sarcoptiform mites are most diverse in soil habitats, but many have adapted to patchy habitats and have developed commensal or parasitic associations with vertebrates and other arthropods (Fig. 1 ).
FIGURE 1 (A) Tropical rat mite, Ornithonyssus bacoti. a vertebrate parasite. (B) Peacock mite, Tuckerella sp., a plant-feeding mite. (C) Cosmochthonius sp., a soil mite. (D) Heteromorphic deutonymphs, phoretic on a predatory mite.
LIFE CYCLES, DEVELOPMENT,
Most mites exhibit a fixed developmental pattern, passing through the same number of instars regardless of how much food is available. The most complete pattern consists of egg, prelarva, larva,protonymph, deutonymph, tritonymph, and adult. The prelarva and larva are distinguished by having only three pairs of legs; the fourth pair is added at the protonymphal molt. Other immature stages are distinguished from each other by a characteristic pattern of additions of leg and body setae. The prelarva is typically a short-lived stage, either passed completely within the egg or, if it actually hatches, having a highly regressive morphology. The few active prelarvae known do not feed and typically begin the molt to the larval stage within hours after hatching from the egg. This life cycle is found in the Opilioacariformes and ancestrally in the Acariformes. Reductions from this number of instars appear in other groups of mites. Within the Parasitiformes, the prelarva is not observed, and the tritonymph is retained only in some Holothyrida. In one family of Ixodida, the Argasidae, the number of nymphal instars is not fixed. Molts take place after each blood meal in these ticks, but the adult morphology develops only when the mite reaches a minimum body size. In many trombidiform mites, the last nymphal instar is suppressed, and in some extreme cases, all immature stages are suppressed and passed within the body of the female mite. After an extreme form of engorgement termed physogastry on fungal food or host-insect hemolymph, these females give birth to fully developed adults. Another developmental pattern observed in the large trombidiform subgroup, the Parasitengona, involves alternation of active and inactive instars. Active stages in this life cycle include the larva, deu-tonymph, and adult, while the prelarva, protonymph, and tritonymph are morphologically regressive, inactive stages.
Mites exhibit a variety of reproductive strategies and modes of sperm transfer. Ancestrally, mites appear to practice indirect sperm transfer, with males producing and depositing a package of sperm, termed a spermatophore, on the substrate. Females then take the spermatophores into their reproductive tract. This type of reproduction is found in most acariform subgroups, and individuals of the two sexes may or may not be in close contact at the moment of insemination. In known parasitiform groups, males typically use their cheli-cerae to assist in directly inserting a spermatophore into the female’s primary genital opening (as in Ixodida and primitive Gamasida), or the male chelicerae bear an organ termed the spermatodactyl which is used to transfer sperm from the male’s genital opening into secondarily developed sperm induction pores near the bases of the female’s legs. These paired openings lead to a median spermatheca which is connected directly to the ovary, where fertilization takes place. Direct insemination involving the development of an intromit-tent organ, the aedeagus in the male, has appeared independently in several groups of acariform mites. Secondary sexual dimorphism typically accompanies direct mating, with males often having modified appendages for holding the female during mating. Many such males also practice precopulatory guarding of immature females, either merely waiting near a juvenile female about to molt or actively attaching to her. This last behavior is taken to an extreme in the sar-coptiform family Chirodiscidae, species of which live on the hairs of mammals, in which immature females are legless and unable to move. They must be found upon hatching by an adult male who uses an elaborate clasping organ to attach to, and carry about, the juvenile female until her legs appear at adult eclosion.
Sex determination mechanisms and reproductive modes also vary widely throughout the Acari. Some mites are diploid in both sexes, with males having either a Y chromosome or no sex chromosome. Other mites are arrhenotokus, in that females are diploid and males haploid. Such males develop from unfertilized eggs. An unusual reproductive mode, termed parahaploidy, is found in some Gamasida. In these mites, fertilization is necessary for egg development, but in males, the paternal genome is inactivated shortly after the first embryonic cell divisions, and adult males are functionally haploid. Finally, thelytoky, or all-female parthenogenesis, is found in many groups of mites. Such mites reproduce clonally, with diploid eggs developing directly into females without fertilization.
Mites exhibit a breadth of ecological interactions unmatched in any other arthropod group. Mites may be found in all geographic provinces, from tropical rainforests to arctic tundra and rocky outcrops in Antarctica and from desert habitats to the deep ocean trenches. They dominate the microarthropod fauna of the soil where they may be found several meters deep or even in groundwa-ter. They occur in all types of aquatic habitats, including freshwater lakes, streams, seepage areas, and even hot springs. Unlike insects, mites are also quite diverse in marine habitats, ranging from the intertidal zone to the deep trenches.
A single square meter of temperate forest soil may contain upward of 250,000 mites, belonging to a hundred different families. In the litter and upper layers of organic soil, mites play many roles in food webs based on decaying plant materials. Gamasid and some trombidiform mites are the dominant predators in such systems, feeding on nematodes, small annelids, collembolans, other mites, and the eggs of insects. These predatory mites have developed several strategies for prey detection and capture, from active, foraging species in the gamasid families Laelapidae and Parasitidae, and the trombidiform families Raphignathidae and Cunaxidae, to more sedentary species in the trombidiform families Caeculidae and Cheyletidae, with palps or forelegs modified as traps for unwary prey. Most soil-inhabiting mites, however, are detritivores or fungivores, feeding directly on decaying organic materials or on fungi or microorganisms growing upon them. The greatest diversity of detri-tivores belongs to the sarcoptiform subgroups collectively known as ori-batid mites. These mites are typically slow moving and may take up to 3 years to complete the life cycle. Adults tend to be well sclerotized as a defense against predators, while soft-bodied juveniles may burrow into substrates to avoid predation. Oribatid mites are primarily detritivores, feeding directly on particulate organic material. Others preferentially scrape decaying leaves for their microbial or fungal floras. Despite their numbers, compared with earthworms or other larger soil invertebrates, mites actually process a relatively smaller amount of organic material and are thus of less importance in converting biomass to nutrients again available to plants. However, in terms of the cycling of particular nutrients, notably calcium, mites play an essential role. Mites are also extremely important in the dispersal of bacterial and fungal agents of organic decomposition. Mites feeding on such substrates ingest bacteria or spores that can often pass undigested through the mites’ guts. The movement of the mites through the soil, with the associated deposition of fecal pellets containing decomposer propagules, provides a much more efficient dispersal of these organisms than simple physical processes.
Specialized soil types have specialized mite faunas. Dry, sandy, and nutrient-poor soils typically harbor a fauna of primitive acariform mites that show little morphological change from their Devonian fossil ancestors. This entire community may consist of such living fossils, with this type of nutrient-poor soil likely similar to the original terrestrial environment at the time of the first land-colonizing animals. Another highly specialized fauna of mites inhabits the deeper layers of mineral soils. Because there is little organic material that filters down to these layers, many of the mites feed directly on the sparse microbial flora or are predators on nematodes that are able to extract nutrients from the limited resources. Deep soil mites tend to be quite small and soft-bodied and may be elongated to allow for movement through very tight spaces between mineral soil particles. Most are effectively aquatic because the deep soils are often saturated, with the interstitial spaces filled with water.
Mites in Patchy Habitats
Mites living in large, continuous habitats such as the soil and litter layers generally have limited dispersal capability. Being small and lacking wings, mites would seem to be limited to such habitats. However, mites also form major components of the microarthropod communities associated with patchy habitats, which are those separated by distances greater than mites’ ability to walk. Such mite communities occur in habitats such as decaying logs, dung and manure, carrion, fungal fruiting bodies, nests of insects and vertebrates, and other concentrations of organic matter such as treeholes, sap flows, and other specialized habitats associated with plants.
Common in patchy habitats are specialized gamasid mites in the families Parasitidae, Macrochelidae, Laelapidae, Digamasellidae, and Uropodidae. These species are typically predators on insect eggs and larvae, particularly those of the Diptera that also frequent patchy habitats, and nematodes. Among the Trombidiformes, species in the subgroup called Heterostigmata are largely associated with patchy habitats, feeding primarily on fungi. The most diverse group of mites in patchy habitats is the sarcoptiform subgroup Astigmata, which appears to have had its origin in such associations.
All of these groups, and some others as well, are able to exploit these habitats, which are generally unavailable to most mites, through a specialized dispersal mode termed phoresy. Phoresy involves one organism utilizing another, larger organism to facilitate its dispersal. In all of the mentioned groups of mites, one life stage is typically specialized for phoretic dispersal on an insect, myriapod, crustacean, or mammal host. Gamasid mites disperse either as inseminated females or as deutonymphs, which is the final juvenile stage in this group. Female laelapid and macrochelid mites typically attach to insect carriers by grasping host setae or other structures with their chelicerae. Parasitid and digamasellid mites disperse as deutonymphs, often in the space under the elytra of beetles, and may roam freely over the insect’s body. In the Uropodidae, the deutonymph is often specialized for dispersal and may attach to the host by secreting a sticky substance from posterior ventral glands. This material is drawn into a stalk that hardens in air and connects the mite to its host.
Heterostigmatid mites disperse as adult females, with many species exhibiting a polymorphism in this stage. Nondispersing females have normally developed anterior legs and are not attracted to insect or mammal hosts. Dispersing females, or phoretomorphs, have very enlarged forelegs with a grasping claw that allows attachment onto insect setae or mammalian hair. In the Astigmata, the deutonymph is highly specialized for dispersal. These deutonymphs look nothing like the preceding or following instars, having no mouth or mouth-parts, but bearing suckers or claspers at the posterior end of the body for attaching to a host. They are typically heavily sclerotized and able to withstand major fluctuations in environmental conditions. Many astigmatid mites inhabit naturally occurring patches of organic matter such as decaying wood or mushrooms and disperse on any insects that frequent the habitat. Others have developed closer associations with particular insects, notably nest-building bees, wasps, ants, and termites, and depend on these insects not only for dispersal but also for creation of the habitat in which the mites live. Still other astig-matid mites have adapted to the nests of vertebrates, with many species inhabiting mammal nests specialized for phoretic dispersal on the mammalian host itself. Species living in birds’ nests still disperse on nest-inhabiting insects such as beetles and fleas.
A few of these phoretic associations between mites and insects have become mutualistic, with mites providing either ” cleaning services ” or “pest control” for their hosts. Some species of Old World carpenter bees may carry several species of mites on their bodies. The astigmatid mites in these communities are kleptoparasites, feeding on the provisions intended for the bee’s offspring. A female bee may also carry trombidiform mites in the genus Cheletophyes (family Cheyletidae) in specialized pouches, termed acarinaria, on the thorax. These mites are obligate predators of the astigmatid mites. The same female bee may also have a large acarinarium in the anterior part of the abdomen, carrying large (2-3 mm) gamasid mites in the genus Dinogamasus (family Laelapidae). These mites have modified chelicerae that scrape the cuticle of the bee larva and remove potentially pathogenic microorganisms and fungal spores as well as cuticular exudates. Some astig-matid mites in the family Histiostomatidae are mutualists in the nests of sweat bees (family Halictidae). Feeding stages of the mites have highly modified chelicerae for filter feeding. These mites wander over the nectar and pollen provisions, straining potentially harmful microorganisms. Deutonymphal mites ride in a rudimentary acarinarium on the propodeum or anterior gaster of the female bees.
A number of different groups of mites have successfully colonized and diversified in aquatic habitats. The most diverse of these, with over 40 families, and 5000 species, is a group within the trom-bidiform subgroup Parasitengona that is termed Hydracarina or, more simply, water mites. This lineage is characterized by the enlargement or multiplication of the genital papillae (often termed acetabula in this group), acariform organs of osmoregulation that allow these mites to maintain ionic balance in hypoosmotic freshwater environments. The parasitengone life cycle is unusual, with its alternation of active and inactive stages, and in most terrestrial and aquatic species, the larva is parasitic, typically on an adult, flying insect. This parasitic larva not only acquires nutrients by feeding on its host, but also is able to disperse over some distance while on the host. The deutonymph and adult stages are typically active predators on other arthropods or their eggs. Water mites are primarily inhabitants of freshwater habitats including temporary ponds, permanent ponds and lakes, streams and rivers, and interstitial waters. One family of water mites, the Pontarachnidae, has invaded marine, intertidal waters and has lost the genital papillae, while another, the Thermacaridae, is restricted to hot springs and capable of surviving temperatures close to 50°C. Mites in standing waters may crawl about on the substrate or aquatic vegetation, but many species have morphological adaptations for active swimming. These include anterior displacement of the leg bases and long setae, termed swimming hairs, on the legs. These mites actively seek and capture aquatic crustaceans and small insect larvae. Mites inhabiting running waters are typically smaller, with flattened bodies, robust legs, and often sclero-tized plates on the body. These mites crawl on and in the substrate, feeding on the eggs of aquatic insects and other microinvertebrates. Some of these mites are specialized predators of the eggs of the same insect species used as hosts by their larvae. Some stream-inhabiting species live deep in the interstitial waters, often having quite elongate bodies for squeezing through the spaces between rock and sand particles. Many water mites are brightly colored, either retaining the red color common among the ancestral, terrestrial Parasitengona or becoming a cryptic blue or green. Some water mites have modified the ancestral parasitengone life cycle by producing fewer, larger eggs. Larvae hatching from these eggs transform to deutonymphs without feeding or dispersing on a host.
Another relatively large group of aquatic mites forms a separate trombidiform lineage, the family Halacaridae. These mites are most diverse in marine habitats, with most species found in intertidal waters. Some halacarids, however, have been collected in abyssal depths up to 7000 m. Feeding ecology of halacarids varies, with some species retaining the ancestral predatory behavior, while others feed on algae or as parasites on crustaceans, echinoderms, or cnidarians. Some halacarids have reinvaded freshwater habitats, presumably via groundwater connections. Such mites are often collected from well water, and a number of species are restricted to freshwater habitats.
Other groups of mites contain aquatic taxa, but none has diversified to the extent seen in the water mites and Halacaridae. Mites in the oribatid family Hydrozetidae are often collected on aquatic vegetation, while those in the family Trimalaconothridae occur in the substrates of ponds and streams. The sarcoptiform group Astigmata includes the family Hyadesiidae, all species of which live in marine, intertidal habitats. These mites are unusual among the Astigmata in living in more or less continuous habitats, and they have lost the dispersing deutonymph from the life cycle. Some species in the family Algophagidae live in brackish waters, and one is known from a fast-flowing river. Other Astigmata live in temporary aquatic habitats, such as water-filled treeholes and other phytotelmata, or water-filled plant cavities, such as pitcher plants, bromeliads, the leaf axils of aroids, and the flower bracts of heliconias and related plants. These species still retain the phoretic deutonymph that disperses on an insect host. Relatively few gamasid mites have become aquatic, but some species in the family Ascidae live in phytotelm habitats or regularly flooded swamp or flood-plain soils. Some of these have a modified cuticle around their respiratory openings that functions as a plastron, holding a bubble of air against the spiracle when the mite is submerged.
Mites on Plants
Unlike all other arachnid groups, several groups of mites have evolved the ability to feed on living plant tissue. Most species belong to one of several lineages of Trombidiformes, each of which has independently evolved this capability, but all share the modification of the chelicerae into piercing stylets. One lineage, the superfamily Tetranychoidea, contains the spider mites and their relatives. Spider mites (family Tetranychidae) are so named because some species utilize silk in constructing webbing on leaves or pads for oviposition and also for dispersal via ballooning much in the manner of some spiders. Silk production is not unique to this group, however, because it is also found in related trombidiform groups not associated with plants. Tetranychoid mites have elongate cheliceral stylets that pierce leaf or root tissue and feed on cell contents or on interstitial fluids. Most species are relatively host specific and do little damage, but some, such as the twospotted spider mite, Tetranychus urticae, are polyphagous and are serious pests of agricultural crops, particularly herbaceous annuals, such as beans, and fruit trees. Another tetranychoid group, the false spider mites, or flat mites (family Tenuipalpidae), also includes serious agricultural pests.
A second lineage of plant-feeding mites, the Eriophyoidea, contains extremely tiny species with a highly modified body form. These elongate, worm-like mites have only two pairs of legs at the anterior end of the body, but possess a sucker at the posterior end and move inchworm fashion over plant surfaces. All are obligate plant feeders, using their short stylets to pierce individual cells. Most species are highly host specific, and a single plant species may harbor many species in this group, most of which simply wander over the leaf surfaces. Large populations of such mites may cause loss of color in leaves, leading to the common name rust mites. Another common name, gall mite, refers to the ability of some species to induce characteristic galls on leaves, buds, stems, flowers, or fruits of their host plants. Salivary chemicals mimic certain plant growth hormones and induce the formation of galls in which the mites live. Simple erineum galls form when epidermal cells produce elongate hair-like growths upon which the mites feed. Pouch galls are like erinea but actually form into elongate cavities within which the mites live. Mite-induced proliferation of woody tissue causes “witches’ brooms” on trees. Although rusting and gall formation are often unsightly and may affect fruit set in orchard crops, the most important effects of eriophyoid mites on agricultural systems are as vectors of viral pathogens such as wheat streak mosaic virus. On the other hand, other, highly host-specific, eriophyoid mites have been used as virus vectors in the biological control of weeds.
Other plant-feeding mites occur in the families Penthleidae, including the redlegged earth mite, Halotydeus destructor, a serious pest of grasses and herbaceous plants in the Southern Hemisphere, and Tarsonemidae. This last family includes such serious agricultural pests as the broad mite, Polyphagotarsonemus latus, which, true to its scientific name, is a polyphagous pest of many agricultural crops.
Other mites living on plants are beneficial as predators of phytophagous mites. Chief among these are species in the gamasid family Phytoseiidae. These mites range from generalist to specialist predators, often attacking economically important species of spider mites, flat mites, and eriophyoids. Several species of Phytoseiidae are commercially marketed for biological control of these pests.
A great many lineages of mites contain parasites of vertebrate and invertebrate animals, some of which are of importance in human and veterinary medicine. Most important of these is the Ixodida, the ticks, but the Gamasida contains a diversity of parasites of reptiles, birds, mammals, insects, myriapods, and crustaceans, most of which belong to the mite superfamily Dermanyssoidea. Among the vertebrate parasites, several different types of parasitism occur. The simplest of these is facultative parasitism, in which typically nest-inhabiting predators may feed opportunistically from a wound on a bird or small mammal host. Other nest-inhabiting mites are obligate parasites, but get on the host only to feed. Notable among these are the northern fowl mite, Ornithonyssus sylviarum (family Macronyssidae), and the chicken mite, Dermanyssus gallinae (family Dermanyssidae), both of which parasitize a variety of wild birds and domestic poultry and will bite people. Many of these mites have chelicerae modified for piercing and sucking blood or tissue fluid. Finally, some gamasids have become permanent parasites, spending all their time on the host’s body. These may have enlarged claws or spurs on the body for holding onto the host. Several different groups of dermanyssoid mites have become endoparasites, living in the respiratory tract of snakes, birds, and some mammals, notably dogs and seals. Some species in the family Rhinonyssidae can cause respiratory distress in cage birds. Other endoparasites inhabit the ear canals of ungulates such as cattle and goats. Some parasitic gamasids act as vectors of bacterial, viral, and protozoan pathogens to their normal hosts, but only one, the derman-yssid Liponyssoides sanguineus, acts as a vector for a bacterial pathogen from mice to humans, causing the disease known as rickettsialpox.
Other dermanyssoid mites are parasitic on arthropods, with the most important being the honey bee parasite, Varroa destructor (family Laelapidae). This mite is responsible for the worldwide decline in populations of the European honey bee, Apis mellifera. The mites feed on hemolymph of bee larvae, causing the adult bee that develops to have aborted wings that prevent the bee from foraging. Buildup of mites in a bee colony causes its destruction over time. This mite became a pest after colonizing A. mellifera from its ancestral host, the Asian honey bee, A. cerana. In the normal host, this mite is not pathogenic to the colony because populations do not reach damaging levels.
Several groups of trombidiform mites have become parasitic, the largest of which is the Parasitengona. This group includes the water mites discussed above, but also a number of terrestrial groups. Larvae of most species parasitize insects, in which they may reduce fecundity or longevity. Larvae of one family, the Trombiculidae, or chiggers, parasitize vertebrate hosts. All groups of terrestrial vertebrates may serve as hosts for this very large group of parasites. Most of the over 5000 described species are known only from their parasitic larval stage. Chiggers feed on tissue fluid and lysed host tissue. On most hosts, they appear not to affect the host negatively, but species biting humans induce an immune reaction that not only causes the death of the chig-ger, but also causes a relatively long-lasting, itchy lesion. Different species of chiggers have achieved “pest” status in various parts of the world. Most are merely irritants to human hosts, but some species in the genus Leptotrombidium, ranging from Japan and Korea, west to Pakistan, and south to northern Australia, act as vectors of a serious bacterial pathogen from rats to humans. The disease, termed scrub typhus or tsutsuga-mushi disease, can be fatal if untreated. A chigger is able to vector the pathogen, the bacterium, Orientia tsutsugamushi, despite parasitizing only a single host in its lifetime, because the pathogen remains in the mite’s body and enters the eggs of the female mite. Thus, larval chig-gers are capable of transmitting the pathogen at hatching.
Another diverse group of trombidiform parasites is included in the superfamily Cheyletoidea. Different families in this group parasitize reptiles, birds, and mammals, with the family Demodicidae containing two species specifically parasitic on humans. Demodex folliculorum, an elongate, worm-like mite, inhabits the hair follicles of the face and occasionally other body regions, whereas D. brevis lives in the sebaceous, or sweat, glands in the skin. Although heavy infestations have been linked to acne rosacea, most people harbor these mites with no discernable effect. Other demodicids can be more pathogenic in their normal hosts, such as D. canis in dogs and D. bovis in cattle. The former can cause a mange condition, with hair loss and irritated skin, especially in puppies, while the latter causes large nodules full of mites to form in the skin. Species in other cheyletoid families parasitize birds, living on the skin, in feather follicles, or inside feather quills. One interesting group in the family Cheyletidae lives within feather quills but feeds as predators on other quill-inhabiting mites.
The trombidiform lineage Heterostigmata includes many parasites of insects. The honey bee tracheal mite, Acarapis woodi (family Tarsonemidae), is of considerable economic importance as a pest in the respiratory system of honey bees. Other parasitic Tarsonemidae inhabit the defensive glands of coreid Hemiptera, one of the most unusual habitats known, even among mites!
Among the Sarcoptiformes, the Astigmata includes a great diversity of parasitic species, the hosts of which include mammals, birds, and insects. Certain nest-inhabiting astigmatid mites that ancestrally dispersed via phoretic deutonymphs have modified the nature of the association. Instead of merely attaching to the hair or skin of the host and simply dispersing, deutonymphs in several groups in the super-family Glycyphagoidea associated with small mammal hosts, and species in the family Hypoderatidae with bird hosts, enter either the hair follicles or the subcutaneous tissue of their host. Despite lacking a mouth and functional gut, these deutonymphs engorge, with some Hypoderatidae increasing their body volume up to 1000-fold. The mode of nutrient acquisition in these parasites is unknown, but some are able to complete the remaining, free-living part of the life cycle in the host’s nest without additional food.
Other groups of astigmatid mites have become permanent parasites of birds or mammals, eliminating the deutonymph from the life cycle. Among mammal hosts, these mites are most diverse on marsupials, rodents, insectivores, primates, and bats, with relatively few occurring on carnivores or ungulates. Most are relatively nonpatho-genic, feeding primarily on sebaceous materials on the hair shafts. Others, however, can cause problems for their hosts.
Species in the family Psoroptidae live on the host’s skin or in the ears and feed by abrading the skin with their chelicerae and imbibing tissue fluids. These mites irritate the skin and cause itching. Several species of psoroptid mites occur on domestic animals, notably the carnivore ear mite, Otodectes cynotis, common in cats and dogs, and the scab mites in the genera Psoroptes and Chorioptes in horses, cattle, sheep, and others. The sheep scab mite, Psoroptes ovis, particularly causes economic damage by causing loss of wool.
Probably most important among parasitic astigmatid mites are species in the family Sarcoptidae. Commonly known as mange mites, species in several genera can parasitize humans and domestic animals. Naturally most diverse on marsupials, bats, primates, and rodents, several species have been able to colonize new hosts. Sarcoptes scabiei is ancestrally a parasite of humans, causing the skin disease scabies. Like other sarcoptids, these mites burrow into the superficial layers of the skin. In healthy humans, this disease is an itchy annoyance, but in immune-compromised individuals, a serious condition known as crusted scabies can develop in which the patient may harbor millions of mites in large, crusty lesions all over the body. S. scabiei has also been able to colonize many domestic animals, notably dogs, pigs, cattle, camels, and others, in which the disease known as sarcoptic mange can be fatal due to the large mite populations and secondary bacterial infections.
Many other astigmatid mites parasitize birds, in which, again, most do not cause harm. Feather mites may be very diverse on an individual bird, with one parrot species known to harbor almost 40 species. Like their fur mite counterparts on mammals, these mites feed on skin oils and do not harm the host. Others, however, may parasitize the feather follicles, skin, or respiratory tract. Skin-inhabiting species in the family Knemidokoptidae can be quite pathogenic in domestic poultry, cage birds, and wild species. Endoparasitic species in the family Cytoditidae live in the air sacs and can cause respiratory distress in poultry.
IMPORTANCE OF MITES
As indicated above, there are a number of instances in which mites are important to humans. Many species are serious pests of agricultural crops, either through direct damage or indirectly as vectors of plant pathogens. Other species are parasitic on domestic animals and cause losses in meat, egg, and fiber production. Others, such as the human scabies mite, are direct agents of human disease or, as in the case of chiggers and ticks, vectors of pathogens. Other mites may affect humans by infesting stored food products. Many species of Astigmata are known as stored-product mites because they have moved from their ancestral rodent nest habitats into human food stores. Such mites may also cause damage in animal feed by causing allergic reactions in livestock and are also known to cause skin irritation in humans handling infested materials. A related group of astigmatid mites, also ancestrally nest inhabiting, is the family Pyroglyphidae. These mites have colonized human habitations from bird nests and are the primary source for allergens in house dust. Commonly known as house dust mites, species particularly in the genus Dermatophagoides produce many proteins that induce allergic responses in sensitive individuals. House dust allergy may take the form of respiratory distress or skin irritation. Mites typically inhabit beds, chairs, and carpets in houses, and their shed skins and feces provide the bulk of the allergens in house dust extracts.
On the other hand, as indicated above, some mites are beneficial to humans in their role as biological control agents against agricultural pests. Also, the natural role of mites in providing ” ecosystem services” in the form of nutrient cycling cannot be overlooked.