Acarid: Mites and Ticks - Britannica Encyclopedia

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Acarid
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Acarid, (subclass Acari or Acarida or Acarina), any
member of the subgroup of the arthropod class Arachnida
that includes the mites and ticks.
Introduction
General features
Natural history
General
features
Form and function
Classification
Size range and
diversity of
structure
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diversity of Acari
Some mites are as small as 0.1 mm (0.0039 inch) in
Representative members of subclass
Acari (mites and ticks).
length, while the largest ticks are slightly more than
Encyclopædia Britannica, Inc.
30 mm (1.18 inches) long. Nymphs and adults
generally have four pairs of legs, although newly
hatched larvae have only three pairs. Members of some families have a dense covering of
stiff hairs, or setae, while others are almost nude. Soft-bodied mites and ticks have only a
few hardened plates, or shields, but others are strongly sclerotized with numerous hardened
plates covering the body. Acarids range in shape from minute, soft-bodied, elongated or
circular individuals to large, hard-bodied, spiderlike ones. Some are flattened, but others
assume grotesque body shapes as they become engorged with food or filled with eggs.
Distribution and abundance
Mites and ticks are distributed throughout the world
in almost every conceivable habitat and frequently
occur in large numbers. They are recorded as high as
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acarid
Acarid.
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5,000 metres on the slopes of Mount Everest and as
deep as 5,200 metres in the northern Pacific Ocean.
More than 50 terrestrial species are known from the
Antarctic. A few mites have been found drifting at
high altitudes as atmospheric plankton.
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Acarids have been found in diverse habitats that
include hot springs, caves, harsh deserts, and tundras.
Although many of the approximately 50,000 species
described are free-living, some are parasites of
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Acarid
A scanning electron micrograph of a
peacock mite (Tuckerella; magnified
260x).
Eric Erbe and Christopher
animals. Some taxonomists have estimated that, if all
species were known, the total number of species of
mites and ticks would be more than 500,000.
Importance
Pooley/Agriculture Research Service,
Beltsville Electron Microscopy Unit, U.S.
Department of Agriculture
The Acari are an economically important arachnid
group. Ticks (order Ixodida) surpass all other
arthropods, except mosquitoes, in the number of diseases they transmit to humans. Many
mites also are intermediate hosts of diseases transmissible to humans, domesticated
animals, and crops. Others are pests as a result of their biting or feeding habits or the
damage they cause to food and related products.
Eriophyid and tetranychid mites (suborder
Prostigmata) include many plant-feeding species that
frequently seriously injure or kill the host plant.
Eriophyids are the only phytophagous acarids known
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Holothyrus legendrei
The mite Holothyrus legendrei.
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to transmit plant viruses.
In the mite order Holothyrida (superorder
Parasitiformes), one species of Holothyrus is known
to secrete an irritant substance that is toxic to fowl
and humans. The house-mouse mite (Liponyssoides sanguineus) transmits rickettsialpox to
humans. Widespread species such as the tropical fowl mite (Ornithonyssus bursa), northern
fowl mite (O. sylviarum), and chicken mite (Dermanyssus gallinae) also are pests of
poultry and humans.
The mange, itch, or scab mites (Astigmata) occur on many different animals including
humans. House-dust allergy is caused by species of Dermatophagoides, an extremely
common mite.
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The beetle mites (Oribatida) are among the most
numerous soil arthropods. These mites are important
in the development of soil fertility. Some also act as
intermediate hosts for important tapeworm parasites
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itch mite
Itch mite (Sarcoptes scabiei).
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of domestic animals.
Lyme disease of humans and some animals is caused
by a spirochete transmitted by Ixodes dammini or
other related species. Rocky Mountain spotted fever,
a rickettsial disease that occurs in the United States, is
transmitted to humans by the bite of several species
of hard ticks (Ixodidae), especially the Rocky
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American house dust mite
American house dust mite
(Dermatophagoides farinae).
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Mountain wood tick (Dermacentor andersoni) and
the American dog tick (D. variabilis). Relapsing
fever, an important bacterial disease throughout the
world, is transmitted to humans by certain species of
soft ticks (Argasidae) of the genus Ornithodoros.
Texas cattle fever is a widespread protozoan disease transmitted by cattle ticks (Boophilus).
This disease, no longer prevalent in the United States because the tick has been eliminated,
remains important in many tropical and subtropical countries. Various other diseases
transmitted to animals by ticks include anaplasmosis, tularemia, Q fever, Colorado tick
fever, hemorrhagic fever, and tick-borne encephalitis.
The chiggers (Prostigmata), important pests of humans, also transmit scrub typhus
(tsutsugamushi disease), a rickettsial disease occurring in the Asia-Pacific region.
Natural history
Reproduction and life cycle
The sexes occur separately in acarids; i.e., there are both males and females. Most species
lay eggs (oviparity), but in some parasitic ones the eggs hatch within the female, and the
young are born alive. Many species also can reproduce by parthenogenesis, i.e., by
development of unfertilized eggs.
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Sperm may be transferred either directly or in packets called spermatophores. The male
spermatozoa may be introduced by the male copulatory structure (aedeagus) directly into
the female genital opening or, as in some Astigmata, into a special female copulatory
structure called a bursa copulatrix. The males of species that use the latter method may
have special copulatory structures (e.g., suckers, spurs, or enlarged legs) for grasping the
female. Some males produce a sealed packet containing spermatozoa (spermatophore) that
is transferred to the female genital opening, either directly by the mouthparts of the male or
indirectly by deposition on a surface, after which the female places it in her genital
opening. Eggs begin to develop after fertilization. Although only a few eggs develop
simultaneously in many acarids, large numbers develop at the same time in ticks and some
mites. Eggs are deposited haphazardly on food material by many plant- and grain-feeding
species and are hidden in the soil by predatory soil-inhabiting species. In one predatory
mite, Cheyletus eruditus, females brood a small cluster of eggs and will drive other
arthropods from them.
The primitive life cycle among species that lay eggs has four active immature stages:
hexapod larva, protonymph, deutonymph, and tritonymph. There are many deviations from
this primitive type of life cycle, which is found only in the Oribatida and some Prostigmata.
The hexapod larva, characterized by three pairs of legs, is common to all families of
acarids except Eriophyidae (Prostigmata), whose members have only two pairs of legs in
all active stages. Among the Parasitiformes the Mesostigmata lack the tritonymphal stage.
The Ixodidae may have only one nymphal stage, while the Argasidae may have as many as
eight. Some Prostigmata (Podapolipidae) develop directly from egg to larviform adults,
while others have from one to three nymphal stages. Many Astigmata (superorder
Acariformes) have a nonfeeding (hypopal) stage between the protonymphal and
tritonymphal stages, which frequently occurs during adverse environmental conditions.
Ecology
The free-living acarids include species from all of the
orders and suborders except Ixodida (ticks). The
beetle mites (Oribatida) are largely fungal feeders that
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mite; Opilioacarus texanus
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are extremely numerous in the surface layers of soil.
It has been estimated that as many as six million
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The mite Opilioacarus texanus.
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members of one species can occur in one acre of
pasture soil.
Some families of free-living mites have specialized
styletlike feeding organs (chelicerae), which can
pierce plant cells and are used to suck out their
contents. Feeding by members of the family
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soil mite
The soil mite Diapterobates notatus.
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Eriophyidae (Prostigmata), for example, causes the
formation of galls, dwarfing of shoots, and
malformation of fruits and leaves (witches’-broom).
The family Tetranychidae (Prostigmata) contains the
spider mites, which are foliage feeders. Several species capable of producing silk can spin a
light web over plant leaves.
The families Acaridae and Glycyphagidae (Astigmata), pests of stored grain and cereal
products, have blunt, toothed chelicerae that enable them to scrape and gouge their food
material. Some species of these two families frequently increase to tremendous numbers in
foodstuffs, causing “grocers’ itch” or “copra itch” in humans. There are many free-living
predatory mites that live in soil, humus, other organic matter, or water, and prey on small
arthropods, their eggs, and other small invertebrates.
Many free-living mites utilize insects or other arthropods to disperse themselves, a
nonparasitic association known as phoresy. Adults of the genus Dinogamasus (Laelapidae),
for example, live in a special mite pouch on an abdominal segment of certain carpenter
bees.
Parasitic species are known in all acarid groups except Opilioacariformes, Holothyrina, and
Oribatida. The majority are external parasites, including those most important to humans
and domesticated animals. A few species are internal parasites of animals. One ecological
group contains families found only on the skin surface or feathers of the host. Psoroptidae
(Astigmata), or scab mites, for example, attack the skin surface of mammals and feed on
skin scales. Their continuous abrasion of the skin causes a lesion over which a protective
scab eventually forms. A second ecological group contains several families of mites that
burrow into skin, hair follicles, or quills of the host and feed on fatty secretions, lymph
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material, or, occasionally, blood. The families Sarcoptidae (Astigmata) and Psorergatidae
(Prostigmata), for example, contain species that burrow just beneath the skin of a
mammalian host, causing mange or itch. A third ecological group contains species that
pierce the skin and suck up tissue fluid or blood without actually invading the tissues.
These species may spend either short or long intervals on the host.
The larvae of Trombiculidae (Prostigmata), the
chiggers, parasitize many vertebrates and a few
invertebrates and feed on host tissues (chigger
nymphs and adults, however, are free-living and
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predatory). Species of Dermanyssidae,
fur mite
Macronyssidae, and many Laelapidae (Mesostigmata)
Fur mite (Myobia musculi).
feed on blood or tissue secretions of mammals, birds,
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and reptiles. Some species, which spend much of their
time off the host and in the nest, frequently are referred to as nest parasites, or nidicolous
species. The protonymph of one species of Macronyssidae, found in the mouth of a longnosed bat, causes destruction of tissues; and a species of Spinturnicidae (Mesostigmata) is
found in the anal opening of certain species of cave-dwelling bats. Most members of the
tick families Argasidae and Ixodidae (Ixodida) are obligate blood-sucking parasites of
vertebrates, while most Argasidae nymphs and adults feed on the host for only a few
minutes. Ixodidae in most stages in the life cycle remain attached to the host for several
days.
A few families of mites are found in additional
parasitic associations with vertebrates.
Rhinonyssidae, Entonyssidae, and Halarchnidae
(Mesostigmata), for example, are respiratory parasites
zoom_in
tropical rat mite
Tropical rat mite (Ornithonyssus
bacoti).
Encyclopædia Britannica, Inc.
of birds, snakes, and mammals, respectively. Several
families of mites have established a parasitic
relationship with invertebrates. Larvae of
Trombidiidae and Erythraeidae (Prostigmata) and
other families, for example, are parasitic on insects
but are free living in later stages. In contrast, in the aquatic family Unionicolidae
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(Prostigmata), nymphs and adults parasitize mollusks and sponges, and larvae are freeliving.
Form and function
External features
The subclass Acari is generally distinguished by the lack of body segmentation, although it
is secondarily developed in a few families. This is a characteristic shared only with the
spiders among the arachnids. An anterior region called the gnathosoma contains the mouth,
specialized feeding appendages (chelicerae), and segmented structures called palps, or
pedipalps. The mouth or buccal cavity joins the pharynx internally, and paired salivary
glands may discharge into the mouth or in front of its opening. The chelicerae are basically
three-segmented pincerlike appendages; however, as a result of the diverse feeding habits
of some mites, chelicerae sometimes are modified as piercing organs (stylets). The
pedipalps, which may be simple sensory structures or predatory organs modified for
grasping or piercing, usually have five free segments: trochanter, femur, genu, tibia, tarsus,
and frequently there also is a clawlike apotele (a modified sixth distal segment of the
appendage).
Behind the gnathosoma is a large region (idiosoma)
that bears the legs, the genital and anal openings, and
an assortment of tactile and sensory structures.
Respiratory pores (stigmata) and sclerotized shields
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tick
Environmental scanning electron
microscope image of a tick. Clearly
visible is the underside of the tick's
barbed hypostome (holdfast organ),
which channels the tick's saliva into
the host and transfers the host's blood
into the tick.
© Sarah Swanson
of various shapes and sizes usually are present. The
functions of the idiosoma parallel those of the
abdomen, thorax, and portions of the head of insects.
Although nymphs and adults commonly have four
pairs of legs, some Prostigmata (Eriophyidae,
Podapolipidae, Tenuipalpidae) and Astigmata
(Evansacaridae, Teinocoptidae) have one to three
pairs. Legs have the same basic segmentation as
pedipalps plus a basal coxa. However, fusion or division of segments frequently occurs.
The tarsus may be terminated either by several sensory hairs (setae) or by a clawlike or
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suckerlike apotele. The legs, which frequently bear ridges and spurs, always have tactile
and sensory setae that follow a fixed pattern in position and number. These leg setae are
used in establishing systematic relationships. The first pair of legs usually functions in
locomotion but sometimes is modified as a sensory or predatory structure.
The genital opening, usually located on the underside
between the legs, frequently is protected by one or
more shields or flaps and has two or three pairs of
disks. In some Prostigmata (Cloacaridae,
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black-legged tick
Black-legged, or deer, tick (Ixodes
scapularis).
Encyclopædia Britannica, Inc.
Demodicidae, Myobiidae, Ophioptidae,
Podapolipidae), however, the male genitalia
(aedeagus) is located on the dorsal side. The anal
opening, also generally on the underside, is
surrounded by a shield in the Mesostigmata and is
always closed by a pair of valves. In some Prostigmata (Penthaleidae) and Astigmata
(Chirorhynchobiidae, Knemidocoptidae) the anal opening is located on the dorsal side. As
in all arthropods, the cuticle of acarids is secreted by an outer cell layer called the
epidermis. The cuticle of many acarids absorbs water from the air, enabling them to avoid
desiccation.
There are many different types of sensory receptors, most of which are setal. The setae, of
many shapes and sizes, may be hollow chemoreceptors or solid, tactile structures. Other
specialized setae, known as trichobothria, pseudostigmatic organs, eupathidia, or famuli,
occur only in the Acariformes. A sensory pit called Haller’s organ contains sensory setae
and is found on the tarsal segment of the first pair of legs of all ticks (Ixodida). One to three
pairs of eyes are present on the anterior of the idiosoma in Opilioacariformes, a few
Astigmata, and many Ixodida and Prostigmata. A single median eye also may be present in
some Prostigmata.
Internal features
The digestive system consists of an anterior muscular pharynx, long narrow esophagus,
ventriculus (stomach), short intestine, hindgut, and posterior rectal cavity. The ventriculus
may have several paired gastric cecae (blind sacs), which function partly as food-storage
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organs and enable some acarids to go for long periods without feeding. One or two pairs of
excretory organs (Malpighian tubules), which may open into the hindgut, absorb waste
material from the body cavity and transform it into an organic compound called guanine,
which passes into the hindgut, where it is mixed with other waste products, and eventually
is excreted from the anal orifice. Some Ixodida and Prostigmata have no connection
between the ventriculus and hindgut, and the latter serves as an excretory organ.
The idiosomal body cavity contains various organ
systems bathed in hemolymph (arthropod blood).
Circulation of blood carrying the products of
digestion and excretion may be by movement of
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internal features of a mite
Internal features of a mite.
Encyclopædia Britannica, Inc.
muscles or by a “heart” (or vessel), which is present
in some orders and suborders. The blood of some soft
ticks (Argasidae) differs from that of other acarids
biochemically, although it is similar morphologically.
One to four pairs of respiratory stigmata are present
in the idiosomal region of all acarid orders and
suborders except Astigmata and some Prostigmata
zoom_in
cattle tick
Cattle tick (Boophilus).
E.R. Degginger/Encyclopædia Britannica,
Inc.
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and Oribatida, which respire through the cuticle or
have other types of respiratory organs. Stigmata open
internally into a system of tubes (tracheae) that
radiates throughout the body. Tracheae function in the
exchange of oxygen and carbon dioxide and help to
control water loss in certain species.
A well-developed central nervous system surrounds the esophagus in the idiosoma. A
series of nerves radiating from one part of the brain innervates the legs, digestive system,
musculature, and genitalia. The mouthparts are innervated by another part of the brain.
The male and female reproductive systems vary in structure and position. The gonads may
be paired (primitive condition), united, or multiple (testes in some Prostigmata). There
usually are accessory glands associated with the reproductive systems, especially in males.
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Four types of pheromones, alarm, assembly, attachment, and sex, have been reported in
acarids, with those of ticks (Ixodida) being the best known.
Classification
Distinguishing taxonomic features
Features that distinguish the superorders, orders, and suborders of Acari include the
presence or absence of stigmata—if present, the type, relative position, and structures
associated with the stigmata are important—the position and form of the palpal apotele
(modified distal segment of palpus) when present, the type and location of sensory organs,
the type of mouthparts, and the presence or absence of eyes.
Annotated classification
The process of evolution in mites and ticks has resulted in a degree of specialization and
morphological diversity not found in any other group of arachnids. Acarids, believed to be
derived from two lines of descent, have a long, though meagre, fossil history dating from
the Devonian Period (about 395 million years ago). The acarids may be separated into two
superorders: Parasitiformes and Acariformes (in some classifications the order
Opilioacarida is considered a superorder, Opilioacariformes). Representative orders and
suborders are included beneath superorders.
Subclass Acari (Acarina, Acarida; mites and ticks)
Arachnids usually without visible segmentation; mostly minute in size; larvae usually with 3 pairs of
legs, adults usually with 4 pairs; diverse habitats include plants, soil, animals, stored foods, fresh and
marine water; parasitic forms may transmit diseases; worldwide distribution; about 50,000 described
species.
Superorder Parasitiformes (mites and ticks)
Small to large in size; usually heavily sclerotized mites; 1 to 4 pairs of stigmata on posterior portion of
body; peritremes, or grooves, present or absent; palpal apotele present or absent; tarsi of 1st pair of
legs with sensory organs.
Order Mesostigmata
Generally with a number of sclerotized plates; 0.2–2 mm in size; eyes absent; pair of stigmata between
coxae of 2nd, 3rd, or 4th pair of legs; usually associated with elongated peritremes; palpal apotele
present; tritosternum usually well developed but reduced to absent in some parasitic families; majority
free-living in soil or decaying organic matter; many parasites of vertebrates (except amphibians and
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fishes) and invertebrates; some economically important; cosmopolitan; about 76 families, and 5,050
species.
Order Holothyrida
Heavily sclerotized; 2–7 mm in size; eyes absent; pair of coxal glands opening at base of coxae of 1st
pair of legs; pair of stigmata behind coxae of 3rd and 4th pair of legs, peritremes present; palpal
apotele present; tritosternum absent; terrestrial, under stones and decaying vegetation; recorded from
Indo-Pacific region, southeastern United States; carnivorous; of minor economic importance; 3
families and about 13 species.
Order Ixodida (ticks)
Largest Acari; adults 2–30 mm in size, eyes present or absent; pair of lateral stigmata enclosed in
stigmatal plate (modified peritreme) anterior or behind coxae of 4th pair of legs; palpal apotele and
tritosternum absent; sensory organ on tarsus of 1st pair of legs a pit (Haller’s organ); mouthparts
modified to form hypostome (holdfast organ) with teeth turned backward; active stages mostly external
parasites (some nonfeeders), feeding primarily on blood of vertebrates (except fishes); many members
economically important as disease carriers; cosmopolitan; 3 families and about 825 species.
Order Opilioacarida (mites)
Weakly sclerotized mites superficially resembling members of arachnid subclass Opiliones; 1–2.5 mm
in size; body divided into hairy anterior portion bearing 2 or 3 pairs of simple eyes and nude posterior
portion with over 200 pores, 4 pairs of stigmata, and 12 secondarily developed segments; terminal
palpal apotele a pair of claws; rutellae (hypertrophied setae) present; all coxae of legs movable; 1st 2
pairs of legs with 6 segments, last 2 with divided trochanter, 7 segments; tritosternum paired base
divided; terrestrial, under stones and other debris in semiarid habitats; recorded from southwestern
United States, Puerto Rico, South America, Central Asia, Africa, and the Mediterranean region;
carnivorous and possibly omnivorous; of no economic importance; 1 family.
Superorder Acariformes (mites)
Small to large in size; generally weakly sclerotized mites (except most Oribatida); palpal apotele and
posterior body stigmata absent; anterior portion of body with or without sensory organs that are
specialized setae.
Order Sarcoptiformes
Globular mites; 0.3–1.5 mm in size; chelicerae frequently pincerlike; without stigmata; legs sometimes
terminate in suckers, claws, or some modification thereof; some with hypopus (resting stage) between
first and second nymphal stages; about 230 families and more than 15,000 species.
Suborder Oribatida (oribatid or beetle mites)
Usually strongly sclerotized and slow moving, 0.2–1.5 mm in size; eyes and stigmata absent;
pseudostigmata generally present, palps without claws, 3–5 segments; chelicerae usually chelate;
rutella present; tarsi with 1–3 claws; ventrally with various shields; majority terrestrial in forest humus
and soil, a few aquatic; feed on algae, fungi, or decaying material; of some economic importance;
cosmopolitan; about 145 families and 8,500 species.
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Suborder Astigmata
Homogeneous group includes mange, itch, or scab mites; weakly sclerotized and slow moving; 0.2–1.5
mm in size; eyes rarely present, stigmata absent; palps single segmented (sometimes with 2 false
segments); chelicerae chelate; true claws absent; rodlike sensory setae on tarsus of 1st pair of legs;
wide range of terrestrial habitats; parasitic, predatory, or feed on decaying material; some species
economically important; cosmopolitan; about 65 families and 6,500 species.
Order Trombidiformes
Large group of diverse mites having features that preclude their inclusion in Sarcoptiformes; about 125
families and more than 22,000 species.
Suborder Prostigmata
Heterogeneous suborder includes chiggers; 0.1–16 mm in size; weakly sclerotized with few plates;
eyes present or absent; stigmata, when present, at or between bases of chelicerae or on anterior portion
of body; chelicerae styletlike, chelate (pincerlike), or reduced; diverse in habitat and habits—terrestrial,
aquatic, marine, parasitic, predatory, phytophagous; many species economically important;
cosmopolitan; about 135 families and 14,100 species.
Suborder Sphaerolichina
Globular mites with soft bodies; 1 to 4 pairs of setae; rutella absent; brown, yellow, pinkish red, or
white in colour; economic importance unknown.
Critical appraisal
Acarologists are not in total agreement as to the classification of mites and ticks. Much of
the disagreement concerns the levels at which taxonomic divisions should be made and the
use of ordinal names; consequently, seemingly different classifications are actually more
alike than they may initially appear to be. Knowledge of Acari systematics is so incomplete
that any classification proposed below the ordinal level is difficult and may quickly become
obsolete.
Multiple classifications, including that used above, have been proposed since 1978 for the
mites and ticks. The suborders listed above are treated as orders in other classifications,
with the choice of ordinal names differing in some cases.
Nixon A. Wilson
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Citation Information
Article Title: Acarid
Website Name: Encyclopaedia Britannica
Publisher: Encyclopaedia Britannica, Inc.
Date Published: 27 December 2017
URL: https://www.britannica.com/animal/acarid
Access Date: September 20, 2021
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