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Encyclopedia Britannica

MITE

This article appears in Volume V18, Page 619 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: MIC-MOL

MITE , a name applied to an order of small

Arachnida

ARACHNIDA

, with which this article deals, and to a coin of very slight value. The origin of both would appear to be ultimately the same, viz. a root mei-, implying something exceedingly small. It has been suggested that the name for the animal comes from a secondary root of the root mei-, to cut, whence come such words as Goth. maitan, to cut, and Ger. messer, knife. In this case mite would mean " the biter " or " cutter." The coin was originally a Flemish copper coin (Dutch mijt) worth one-third or, according to some authorities, a smaller fraction of the Flemish penning, penny. It has become a common expression in English for a coin of the smallest value, from its use to translate Gr. AenrrOv, two of which make a KOSpavTls, translated " farthing " (Mark xii. 43).
In

zoology

ZOOLOGY (from Gr. Nov, a living thing, and ?byos, theory)

, " mite " is the common name for minute members of the class

Arachnida

ARACHNIDA

(q.v.), which, with the ticks, constitute the order Acari. The word " mite," however, is merely a popular and convenient term for certain groups of Acari, and does not connote a natural assemblage as contrasted with the ticks (q.v.). Mites are either free-living or parasitic throughout their lives or parasitic at certain periods and free-living at others. They are almost universally distributed, and are found wherever terrestrial vegetation, even of the lowliest kind, occurs. They are spread from the arctic to the antarctic hemisphere, and inhabit alike the land, fresh-water streams and ponds, brackish marshes and the sea. The largest species, which occur in the tropics, reach barely half an inch in length; while the smallest, the most diminutive of the Arthropoda, are invisible to the naked eye.
Mites are divided into a considerable number of families. The Bdellidae (Bdella) are free-living forms with long antenniform palpi. The large tropical forms above mentioned belong to the genus Trombidium of the family Trombidiidae. The members of this genus are covered with velvety plush-like hairs, often of an exquisite crimson colour. The legs are adapted for crawling or running, and the palpi are raptorial. They are non-parasitic in the adult stage; but immature individuals of a British species (T. holosericeum) are parasitic upon various animals (see HARVEST BuG). The Tetranychidae are nearly related to the last. A well-known example, Tetranychus telarius, spins webs on the backs of leaves, and is sometimes called the money spider. The fresh-water mites or Hydrachnidae are generally beautifully coloured red or green, and are commonly globular in shape. Their legs are furnished with
long hairs for swimming. The marine mites of the family Halacaridae, on the contrary, are not active swimmers but merely creep on the stems of seaweeds and zoophytes. The Gamasidae are mostly free-living forms with a thick exoskeleton, and are allied to the Ixodidae or ticks (q.v.). A common species is Gamasus coleoptratorum, the females and

young

of which may be found upon the common dung-beetle. The Oribatidae or beetle-mites, so called from their resemblance to minute beetles, are non-parasitic, and often go through remark-able metamorphoses during development. The Sarcoptidae, as stated below, are mostly parasitic forms. Some members of this family, however, live in decaying animal substances, the best known perhaps being the cheese-mite (Tyroglyphus siro) which infests cheese, especially Stilton, in thousands. An allied species (T. entomophagus) often causes

great

GREAT

damage to collections of insects by destroying the dried specimens. They may be easily exterminated by application of benzine, which does not harm the contents of the cabinet.
From the economic standpoint the most important mites are those which are parasitic upon mammals and birds. They belong to the four families, Gamasidae, Trombidiidae, Sarcoptidae and Demodicidae. Most of the Gamasidae are free-living mites. The family, however, contains an aberrant genus, Dermanyssus, of which several species have been described, although they are all perhaps merely varieties of one and the same species commonly known as D. gallinae or D. avium. This species is found in fowl-houses, dovecotes and bird-cages. During the day they lurk in cracks in the floor, walls or perches, and emerge at night to attack the roosting birds. They are a

great

GREAT

pest, and frequently do much damage to birds both by sucking their blood and by depriving them of rest at night. They are sometimes transferred from birds to mammals. The Trombidiidae also are mostly free-living predaceous mites. A few, however, are parasitic upon mammals and birds, the best-known being Trombidium holosericeum, the larva of which attacks human beings, as well as chickens and other birds, sometimes producing considerable mortality amongst them (see HARVEST BUG). Another genus, Cheyletiella, affects rabbits as well as birds. Birds are also attacked by many species of Sarcoptidae, which according to the organs infected are termed plumicolae (Analgesinae), epidermicolae (Epidermoptinae), and cysticolae (Cytoditinae). The Analgesinae (Pterolichus, Analges) live almost wholly upon and between the barbules of the feathers. They are found in nearly every species of bird without apparently affecting the health in any way. The Epidermoptinae (Epidermoptes) occur on diseased fowls and live, as their name indicates, upon the skin at the base of the feathers, where their presence gives rise to an

accumulation

ACCUMULATION (from Lat. accumulare, to heap up)

of yellowish scales. The Cytoditinae (Cytodites), on the other hand, live in the subcutaneous or intermuscular connective

tissue

TISSUE (Fr. tissu, tissue, participle of tisser, Lat. texere, to weave)

round the respiratory organs, or in the air sacs, especially of gallinaceous species. They also penetrate to certain internal organs, and may become encysted and give rise to tubercle-like nodules. Sometimes they exist in such quantities in the air passages as to cause coughing and asphyxia.
The cutaneous mites, mentioned above, and others akin to them, produce no very marked disturbance in the skin of the species they infest. They merely suck the blood or feed upon the feathers, scurf and desquamating epidermis. Hence they are termed " non-psoric " mites. A certain number of species, however, called in contradistinction " psoric " mites, give rise by their bites, by the rapidity of their multiplication, and by the excavation of galleries in the skin, to a highly contagious disease known as scabies or mange, which if not treated in time produces the gravest results. These mites belong exclusively to the Sarcoptidae and Demodicidae. A variety of species are responsible for Sarcoptic mange, Sarcoptes mutans producing it in the feet of gallinaceous and passerine birds by burrowing beneath the scales and giving rise to a crusted exudation which pushes up beneath and between the scales. Feather scabies or depluming scabies of poultry is caused by another species, S. laevis. Three genera of Sarcoptidae, namely Sarcoptes Chorioptes and Psoroptes cause mange or scabies in mammals,the mange produced by Sarcoptes being the most serious form of the disease, because the females of the species which produces it, Sarcoptes scabiei, burrow beneath the skin and are more difficult to reach with acaricides. A considerable number of varieties of this species have been named after the hosts upon which they most commonly and typically occur, such as S. scabiei hominis, equi, bovis, caprae, ovis, cameli, lupi, vulpis, &c.; but they are not restricted to the mammals from which their names have been derived and structural differences between them are often difficult to define and sometimes non-existant. Under favourable conditions the multiplication of this species is very rapid. It has been computed indeed that a single pair may give rise to one million and a half individuals in about three months. Psoroptes lives in the epidermic incrustations to which it gives rise, without, however, excavating subcutaneous burrows. One species, P. communis, is known to affect various domestic animals. Of the genus Chorioptes two species have been described on domestic animals, viz. Ch. symbiotes, which has the same mode of life as Psoroptes communis and Ch. cynotis, which has been detected only in the ears of certain carnivora such as dogs, cats and ferrets. Mange, if taken in time, can be cured by applications of

sulphur

SULPHUR

ointment or of

sulphur

SULPHUR

mixed with an animal or vegetable oil. Mites of the family Demodicidae give rise to a skin disease called " Demodecic or follicular mange," which is often serious and always difficult to cure on account of the deep situation taken up by the parasites. These infest the

hair

HAIR (a word common to Teutonic languages)

follicles and sebaceous glands, and are therefore termed Demodex folliculorum. These mites differ greatly from those previously noticedin the reduction of their legs to short three-jointed tubercles, and in the great elongation of the abdomen to form an annulated flexible postanal area to the

body

BODY

. They live not uncommonly in small numbers in the skin of the human face and their presence may never be detected. They also occur on dogs, pigs and other domesticated animals, as well as on mice and bats, and numerous varieties named after their hosts, hominis, bavis, canis, cati, &c., have been described, but they apparently differ from each other, principally in size.
The mites of the family Eriophyidae or Phytoptidae produce in various plants pathological results analogous to those produced in animals by parasitical Sarcoptidae and by Demodicidae. As in the Demodicidae the abdomen is elongate and annulate, but the Eriophyidae differ from all other mites in having permanently lost the last two pairs of legs. The excrescences and patches they produce on leaves are called "

galls

GALLS

," the best known of which are perhaps the nail-

galls

GALLS

of the lime caused by Eriophyes tiliae. A very large number of species have been described and named after the plants upon which they live. They often inflict very considerable loss upon fruit-growers by destroying the growing buds of the trees. (R. I. P.)

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