10 



REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



minated by from one to three claws or ungues. In some groups there is a difference 

 in this respect between the young and the adult, and frequently one or more pairs 

 of legs are destitute of claws. The claws are not often toothed. In many cases 

 there is a median cup-shaped sucker, pulvillus, caroncle, or ambulacrum between 

 the claws or bearing them. 



The reproductive organs, as in other arachnids, open on the ventral surface of the 

 abdomen near the base. The female aperture (vulva or epigynum) is of various 

 shapes and sometimes closed by flaps or folding doors. The male apertiu-e (epian- 

 drum) is usually smaller than that of the female. The body is often provided with 

 hairs, bristles, or scales, which are of characteristic nature and arrangement in each 

 species. 



In many of the soft-bodied forms there are chitinous plates, scutae, or sliields, 

 sometimes so large or so numerous as almost completely to cover the mite. These 



Fig. 4. — Legs of various mites. (Author's illustration.) 



shields are often sculptured or pitted in a characteristic manner. Frequently there 

 are secondary sexual differences both of color and structure, as will be noticed under 

 each family. The male is often a little smaller than the female, but in many cases 

 there is no apparent difference in size. 



The internal anatomy (fig. 5) of mites is marked by great centralization of parts, 

 the various organs being much more crowded together than with other arachnids. 

 The alimentary canal when fully developed consists of the pharjTix or sucking organ; 

 the oesophagus; the stomach or ventriculus, with its coeca; the hind gut or intestine, 

 and the Malpighian vessels, which enter the latter near the rectum. 



The pharynx is a partially chitinous tube, convex below, concave above; to its 

 upper part or roof are attached the muscles which, upon contracting, elevate the roof. 

 A series of muscles, each moving just after the one in front, jiroduces a steady flow of 

 food to the stomach. The oeso])hagus is a long 8im])le tube; the stomach is of varied 

 size and shape, according to the food liabits. Sometimes there is an enlargement of 

 the oesophagus near its end, thus forming an ingluvies or crop. In some forms the 

 coeca are extremely long or numerous. The Malpighian vessels, when present, are 

 two in number, and enter the short intestine near its end. The latter is sometimes 

 provided with an enlargement, the colon. In many mites tlie digestive system is much 

 8imi)lified. In many, if not all, of the forms allied to Trombidium, and the water 



