Eiving — Significance of Parasitism in Acarina. 57 



act like a vise upon the partially encircled hair. In 

 Myocoptes it is the posterior group of legs which is mod- 

 ified into clasping organs. Here the apical joints, which 

 are broad and flat, can fold upon the basilar ones like, 

 the blade of a knife into its handle. 



Perhaps one of the most significant things that ap- 

 pears in connection with the evolution of the parasitic 

 Acarina is the development of marked sexual dimor- 

 phism. In the free forms from which the parasitic ones 

 arose there was evidently little or no sexual dimorphism, 

 yet in some of the Analgesidae, for example, it is very 

 marked as will be seen by looking at PL IV, Figs. 15 

 and 16, and PI. VI, Figs. 24 and 25. In most of these 

 cases the dimorphism consists of an increased size of the 

 male over the female, and in his possession of special 

 organs for holding her during pairing. In the genus 

 Analges these modifications for clasping reach such an 

 enormous size as to cause some conjecture as to the 

 factors influencing their development. They consist sim- 

 ply of an enormous enlargement of the third pair of legs, 

 but to such an extent that the two legs taken together 

 may be equal to the total size of the body of the mite 

 (See PI. VI, Fig. 25). How such enormous structures 

 if used alone for holding the female during coition could 

 have been developed through natural selection is a puz- 

 zle. It may be that the members of this genus are the 

 descendents of a primitive ancestor which was a mutant. 

 If this is true this initial variation was out of all reason- 

 able limits in its magnitude. Again it may happen that 

 these organs are used by rival males in striving with 

 each other for the possession of the females, but their 

 structure hardly suggests it, and besides I am not aware 

 that in the Acarina any one has ever noticed any rivalry 

 between the males over the females. I am inclined to 

 think that these structures have an important, and per- 

 haps their chief function, as organs for holding on to the 

 feathers of the host. As the male is the one that hunts 

 out the female, and is thus much the more active, his 



