TICKS AND MITES, small arthropods usually considered to be an order (Acarida) of the class Arachnida (q.v.). They have the regions of the body the most completely coa lesced of any articulated animals, for not only are the head and thorax more or less united to form a cephalothorax, but the latter is contin uous with the abdomen, often without the slightest indication of the line of union, and except in a few cases the abdomen is entirely. unsegmented. The mouth-parts usually form a piercing and suctorial proboscis composed of the chelicerm and pedipalpi, the latter being made up in part of a jointed, usually tactile palpus, ;while the chelicerm may be stiliform and un jointed or two-jointed and clawed or chelate. Typically the mature forms or imagoes have four pairs of walking legs of from five to eight joints terminated by variously arranged claws, stalked or sessile suckers or pads; but the legs may be variously modified, reduced to mere tubercles, to two pairs or disappear altogether in some parasitic forms. Respiration is ef fected by means of simple or branched trachez opening by from one to three pairs of stigmatm at the bases of the legs, or by means of a pair of air-sacs opening at the base of the proboscis or on the back or, in the absence of all special respiratory organs, may be purely integumen tary. The alimentary canal is commonly a spa cious sac which may be forked or much branched. In most cases there is no blood vascular system. The sexes are separated, the generative ducts open on the base of the abdo men, and fertilization is accomplished through copulation. A few are parthenogenetic and sexual dimorphism is frequent. They are ovi parous, ovo-viviparous or viviparous. The larvx frequently differ greatly from the ima goes in appearance and habits and almost al ways have three pairs of legs. After a few tnolts, usually two, they form nymphs or pupx which also may be very different, and after a single molt metamorphose into the mature form or imago.
Mites and ticks are found in all parts of the world and under every variety of eiwironment. Many are parasitic either temporarily or per manently, and on both animals and plants, a few are commensals in ants' nests, etc., others are predaceous, and seize and suck the juices from weaker animals, some live in decaying vegetable substances, many in damp earth, in moss or under the bark of trees, many are aquatic either in fresh water or the sea, and some of the marine forms descend to great depths. While most of the naembers of this order have no direct relation to human affairs, many affect our interests in important and manifold ways. Some, like the itch-mites, are the cause of annoying diseases of man and ani mals, some like the cattle ticks are the bearers of disease-producing parasites, some are seri ously destructive to our crops or to manufac tured food-products, etc.; and many are bene ficial as destroyers of harmful insects and their eggs. Owing to their great powers of repro duction and their tenacity of life, the harmful species are difficult to combat, preparations of sulphur and lime or powerful insecticides ap plied directly to the colonies being the most effective remedies. The number of known spe cies already amounts to thousands, although the smaller mites have been little studied out side of Europe. They vary in size from the microscopic mites to the large cattle ticks about an inch in length. Differing greatly not only in habits and appearance but also in structure, the Acaricla are divided by systematists into numerous families and subfamilies, of which a few representatives may be mentioned.
The Oribatuice is an extensive family of 20 genera and 200 to 300 species of free-living forms with hard slcins and robust bodies and usually three pairs of stalked stigmatm and simple trachez. They are mostly vegetable feeders, never parasitic and with the exception of a fey,. aquatic forms lives in damp earth under leaves, bark, etc.