ANATOMY. 57 



the mouth and directed forward; which indeed is the 

 usual type in the Acarina. It may be seen well in the 

 drawing of Tyroglyphus siro (vol. ii), in Lentungula 

 algworans (PL I, fig. 8), Glycyphagus platygaster 

 (PL XIV, fig. 6), etc., li. 



THE LEGS. 



These organs in the Tyroglyphidas are ordinarily 

 more chitinized than the body, often forming a striking- 

 contrast which reminds the observer of the nymphs of 

 many Oribatidse ; a good example of this is Histiogaster 

 cortwalis (vol. ii). 



The leg consists of five free joints; which is the 

 typical number all through the Oribatidse, the Sarcop- 

 tidae and some of the smaller groups of Acarina. The 

 primitive form of each joint, or rather of the integument 

 which covers it, may be considered to be cylindrical ; 

 or that of a truncated cone as it usually slightly 

 diminishes in diameter toward the distal end. The 

 form is greatly modified in various ways in different 

 joints and in numerous species ; in the Coxa the cylin- 

 drical shape is usually almost lost ; in the Tarsus the 

 distal end is almost closed, and sometimes nearly 

 pointed. Apophyses for holding purposes may arise, 

 as in the male Aleurobim faring but these instances 

 are uncommon. Respecting the holding leg of the 

 male Rhizoylypliux, which is the third leg, a question 

 might possibly be raised as to whether it really is a leg 

 of five joints or only of four. The answer to this 

 question depends upon whether the great fixed claw- 

 like process which terminates this joint, and is almost 

 the whole of the joint, is to be considered as the 

 homologue of the true claw, or unguis, of the other legs ; 

 or as the homologue of the whole tarsus, the claw being 

 absent. Had the solution of this problem depended 

 upon Rhizoglyphus alone it might probably have been 

 a difficult one, but the existence of Lentungula seems 

 to me to simplify it, as in that genus the distal or fifth 



