Air-breathing (by ordinary or modified tracheæ); the lower forms breathe with the skin. Body at most consists of two chief regions, since head and thorax are always fused together, making up a cephalothorax (Fig. 124); but this may again be united with the always unsegmented abdomen into a single piece (Fig. 127). The last is the case with the mites, in which, therefore, the characters of segmented animals can only be recognized in the limbs. In the true spiders (Fig. 124) the body consists of cephalothorax and abdomen. Arachnids have always four pairs of legs, which, in the true spiders, are attached to the cephalothorax, in the mites to the front part of the unsegmented body. [Illustration: FIG. 124.—A Spider (_Salticus scenicus_). ] The chief Orders belonging here are: (1) true =Spiders= (_Araneida_); (2) =Scorpions= (_Scorpionida_); (3) =Harvestmen= (_Opilionida_); (4) =Mites= (_Acaridea_). Only the last contains species of importance agriculturally. ORDER: Acaridea (MITES). Small arachnids, in which the cephalothorax and abdomen are fused together into one piece (Fig. 127). The just-hatched young have three pairs of legs, the adults, of course, four. Family: =Acaridæ= (_True Mites_). Soft skin. No tracheæ, no eyes. Legs short, often with a sucker at the end. Here belong the =Cheese Mite= (_Acarus siro_), the =Meal Mite= (_Tyroglyphus farinæ_), and several other species living in dead organic substances; also— The =Itch=, or =Mange Mites=. These live as parasites on or in the epidermis, and cause the _itch_ or _mange_ (scabies) in man, as well as in several domestic animals. By means of the structure and habits three genera are distinguished:— 1. Digging, blood-sucking mange-mites living in the skin (_Sarcoptes_); 2. Blood-sucking mange-mites living on the outer surface (_Dermatocoptes_); 3. Mange-mites which merely devour scales of the epidermis (_Dermatophagus_). [Illustration: FIG. 125.—Mange Mite of the Pig (_Sarcoptes scabiei_, var. _suis_), seen from the ventral side. ] It is obvious that sarcoptic scabies is not so easily cured in the same host as the dermatocoptic scabies, since the mites causing the former dig their passages into the epidermis, while those causing the latter at least remain on the surface. Dermatophagic scabies is most easily got rid of, since the mites which cause it not only remain on the outer surface of the skin, but also, instead of holding fast, run about here and there; it is easily understood that this kind of mange can often be removed by simple brushing. [Illustration: FIG. 126.—Mange Mite of the Pig (_Sarcoptes scabiei_, var. _suis_), seen from the dorsal surface. × 200. ]