8 



REPORT 108^ U. S. DEPARTMENT OF AGRICULTURE. 



Fig. 1.— Dorsal view of a mite: a, 

 Mandibles; 6, palpi; c, legl; (f, eyes; 

 e, leg II; /, cephalothorax; g, abdo- 

 men; Ji, leg, III; i, leg IV. (Au- 

 thor's illustration.) 



the mites, and Tomogastra for the other arachnids. Thorell pointed out the relation 



of certain eupodid mites to the Solpugida. and this is, I think, the correct solution of 



the position of the Acari. 



Although quite easily recognized at sight, it is 

 not easy to give definite characters whereby to dis- 

 tinguish a mite from other arachnids. The abdo- 

 men (fig. 1) and cephalothorax are broadly united 

 to each other, and often there is no distinction 

 between these parts. Usually there is no trace of 

 segmentation, but in some forms it is quite distinct. 

 Eyes are often present, but rarely only a median 

 pair, as we find in phalangids and solpugids. The 

 mouth segments have become united to . form a 

 beak, rostrum, or capitulum. However, this is not 

 easily made out in some forms. Commonly the 

 larva at birth has but three pairs of legs, and obtains 

 the fourth pair only after a molt and metamorphosis. 

 In the Eriophyidse, however, there are but two 

 pairs of legs in both adult and young, and in Pterop- 

 tus the young have eight legs at birth. The adult 

 mite has (except in the Eriophyida?) four pairs 

 of legs; often arranged in two groups, the hinder 

 pairs apparently arising from the abdomen. How- 

 ever, it is not probable that such is the case; 

 rather the coalescence of the abdomen and cepha- 

 lothorax has effaced the true outhnes of these por- 

 tions. It is probable that the abdomen of mites 



is more than -the abdomen of other arachnids. If one examines aSoIpuga, he sees 



that the cephalothorax is di\dded into several portions, and it may be that the 



cephalothorax of mites represents only the anterior 



of these, while the abdomen of mites represents the 



abdomen of Solpuga plus the two posterior divisions 



of the cephalothorax. In Tarsonemus and some 



other genera the abdomen shows on the dorsum 



distinct traces of segmentation. On the venter 



(fig. 2) there is still less distinction between parts, 



and in several groups, as Ixodidae and some Para- 



eitidse, the genital segment is pushed forward so 



far that the genital aperture is close to the mouth. 



In other forms the genital opening is at the 



extreme tip of the body, and the anus is upon the 



dorsum. 

 The cephalothorax, prosoma, or anterior part of 



the body commonly has one or more pairs of simple 



ocelli-Uke eyes. They are usually sessile, but some- 

 times elevated on pedicels. 

 The mouth segments form typically a truncate 



cone or beak. Sometimes it is partially or com- 

 pletely retracted into the body. The mouth parts 



are the mandibles and palpi. Frequently there are 



other parts, as a hypopharynx, a lip, or definitely 



separated maxillae, as will be mentioned under each family. Several investigators 



have Claimed that there are three or four mouth appendages, and there are struc- 



tiures in some forms that indicate three. The mandibles generally are of two joints. 



Fig. 2.— Ventral view of a mite: a, 

 Beak; b, transverse furrow; c, geni- 

 tal aperture; d, anal aperture. (Au- 

 thor's illustration.) 



\ 



