Page:The New International Encyclopædia 1st ed. v. 13.djvu/707

* MITCHILL. 629 MITE. lature and in Congit-so, and in 1S04 was elected United States Senator. In 180S lie became o- fessur of natural history and in 1S20 of botany and materia medica in the College of Physicians and Surgeons. He was called the 'Nestor of American Science.' His researches embrace a wide variety of seientifie and philosophical sub- jects, and he published a large luimbcr of papers and several larjjcr works. Consult "ilcmorable Kvents and Occurrences in the Life of Samuel L. -Milchill, of Xew York, from the Year 178(i to 1827," by himself and Dr. Francis, con- tained in Gross's American Medical Biography. MITE (AS. mite, OHG. mixa, mi^^o, Ger. Miiic: probably connected with Goth, maitan, OHG. ntci-an. to cut). Any one of the Acarina, an order in the class Arachnida. They may be distinguished from other arachnids by their small size; by the unsegmented body, without a constriction between the anterior portion or cephalothorax and the posterior portion or ab- domen; and by the lack of median eyes. There are exceptions, however, to all these characteris- tics, and certain forms have been misplaced even by naturalists. The mouth-segments have become luiited to form a beak or rostrum, but this char- acter is not easily recognized. The young mite, on hatching from the egg. is not ])rovidcd with eight legs as are other arachnids, but with only four or .six, except in the case of Ptcroptus. In size mites vary from tinj- creatures, invisible to the naked e.ye, to certain tropical forms fully half an inch long. Typically, mites have four pairs of legs, ar- ranged more or less definitely in two gi'oups. The two hinder pairs are apparently attached to the abdomen, while the anterior pairs are close to the mouth-parts, which consist of mandibles of varied character, palpi, and maxilla>. The man- dibles are tvpically chelate, but in several fami- lies they are reduced to needle-like piercing- organs. In the case of certain ganiasid mites the mandibles are nearly as long as the entire animal, and can be wholly retracted within the body, or suddenly extruded to seize the prey. The palpi are of four kinds. Simple, filiform palpi, which have a tactile function, are found in many fami- lies. In some parasitic forms the palpi are re- duced in size and united to the rostrum. In many predatory mites the palpi are modified for raptorial organs. In some of the water- A MITE OR RED spiRER {Tetrauycbas bimaculatus). mites the palpi have become organs for holding the mite to other objects. The legs of mites are composed of from five to seven segments, and commonly terminate in from one to tlirce claws. In nuiny genera a cup-shaped sucking-disk or ambulacrum is attached to the tarsus or last segment. The reproductive organs, as in other arachnids, open on the imder side of the abdo- men near its base. The body and legs are more or less thickly clothed with bristles, hairs, or scales, which are of characteristic nature and arrangement in each sjiecies. In many of the soft-bodied species there are chitinous jdates or shields, sometimes so large or numerous as al- most completely to cover the mite. In the ticks the body is flat, and of a tough, leathery con- sistency. The sense organs are few and of simple nature. JIanj' mites have no eyes, but in some there are yne or two ocelli-like spots on each side of the cephalothorax. A few families have what are considered organs of hearing. With the tieka this organ is a mendjrane-covcred pit in the anterior tarsi; in the beetle-mites it is a pore on the posterior margin of the cephalothorax, from which arises a bristle. The sense of touch is supposed to reside in some of the hairs of the body or legs. In many mites there is a consider- able difference in appearance between the two sexes, although there is not often much differ- ence in size. Centralization is the peculiar characteristic of the anatomy of mites. The various organs are more crowded together than in other arachnids. The digestive system, when complete, consists of the pharynx, or sucking-organ, the oesophagus, the ventriculus or stomach, with its ca-ca, the hiud- gut, and the Malpighian vessels. The a'sophagus is a long, simple tube extending through the centre of the brain. The stomach is of varied size, ac- cording to food-habits ; in some forms it is very small, while the ca'ca are numerous and long. The hindgut or intestine is a short tube ending in the rectum. The Malpighian vessels, when present, are two in number, and enter the intes- tine near its end. In some mites there is a well- developed dorsal pulsating organ or heart, but in others it is not present. The nervous system consists of one ganglionic mass surrounding the oesophagus, from which all the principal nerves arise. Many mites have an elaborate system of trachea> by which they breathe and which open in various parts of the body, in many common species near the mandibles, but in the ticks and gamasids they open by stigmata near the hind legs. A great number of mites, however, have no internal respiratory system whatever. In these the skin is soft and they absorb oxygen by osmosis. Life History. Nearly all mites deposit eggs, frequently of large size. In a few forms the larvfe issue from the parent, but in some cases it is rather from the egg within the body of the dead mite. In many cases the hard external skin or chorion of the egg sjilits into halves and exposes the lining vitelline membrane; this per- mits the maturing egg to increase in size, which is then called a 'dcutovum.' The young larvse on hatching commonly have six legs, but the gall- mites have but four. During the nymphal stage the mite feeds until it attains adult size. In many cases the nymph moKs directly into the adult mite, but in several families the nymph often transforms into a creature entirely difTer- ent from both the nymph and the adult — the hypopial stage or 'Hypopus,' long supposed to be a distinct genus of mites. On its ventral or under surface is an area of sucking-disks, by which the Hypopus attaches itself to an insect or small mammal, and is transported to some new and suitable locality, where it falls from its carrier, molts into an octopod nymph, begins feeding, and in due time becomes ,an adult mite. The Hypopus is, therefore, not a parasite, but a I