PROCEEDINGS OF THE NA TIONAL MUSEUM. 



VOL. XXVIII. 



apparent difference between Acarina and other Arachnida. The legs 

 are composed of from five to seven segments; in some forms the 

 apical joints are subdivided, ])ut do not form genuine segments. Tlie 

 lengtli and character of the joints vary in the different families, ])ut 

 usually there are distinguishable the following parts: coxa, trochanter, 

 feuuir, patella, tibia, and tarsus. In some cases the femora are 

 divided into two parts. The legs are provided with hairs and spines, 

 sometimes moditied for some particular function. In several groups 

 there are organs on the anterior legs which appear to have an auditory 

 function. The last joint or tarsus is commonly terminated by from 

 one to three claws or ungues. In some groups there is a difference in 

 this respect between the young and the adult. The claws are not 

 often toothed. In many cases there is a median cup-shaped sucker, 



pulvillus, caroncle, or ambulacrum ])etween 

 the claws or bearing them. 



The reproductive organs, as in other 

 Arachnids, open on the ventral surface of 

 the abdomen near the ])ase. The female 

 aperture (vulva or epigynum) is of various 

 shapes, and sometimes closed by flaps or 

 folding-doors. The male aperture (epian- 

 drum) is usually smaller than that of the 

 female. The body is often provided with 

 hairs, bristles, or scales, which are of char- 

 acteristic nature and arrangement in each 

 species. 



In many of the soft-bodied forms there 

 are chitinous plates, scutte, or shields, some- 

 times so laro-e or so numerous as to almost 



Fig. 2.— Ventr.\l view of a mite. 

 a, beak; h, transverse furrow; 



c. GENITAL aperture; (1, ANAL eomplctcly covcr the mite. The.se shields 



are often sculptured or pitted in a charac- 

 teristic manner. Frequently there are secondary sexual differences 

 both of color and structure, as will l)e 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 of mites is marked l)y great centralization of 

 parts, the various organs being much more crowded together than 

 Avith other Arachnids. The alimentary canal, when fully developed, 

 consists of the pharynx or sucking-organ; the a?sophagus; the stomach 

 or ventriculus, with its cccca; the hind gut or intestine, and the Mal- 

 pighian vessels which enter the latter near the rectum. 



The pluu\ynx is a partially chitinous tube, convex below, concave 

 above; to its upper ^ -irt or roof are attached the muscles, which, upon 

 contracting, elevate the roof. A series of muscles, each moving just 

 after the one in front, produces a steady flow of food to the stomach. 



