Ewing — Significance of ParcLsitism in Acarina. 31 

 MYcoPTES Clap. 



M. tcnax Michael — Arvlcola arvalls (Pall.), Mus sylvaticus L. 



TRicHOECius Can. 



T. brevipes (Can. & Trt.)— Arvicola arvalis (Pall.). Also on rats, 



SCHIZOCAEPUS Trt. 



-S. mingaudi Trt.— Castor fiber L., C. canadensis Kuhl. 



This little group of mammal-infesting parasites is of 

 particular interest to us because of its extremely narrow 

 limits of host distribution, and the factors which cause 

 the same. Up to the present, eighteen species have been 

 described and of these only four are found upon hosts 

 belonging to different genera, twelve are known from only 

 a single host species, and none is found upon hosts be- 

 longing to different families. 



They are very small forms and are all unique in one 

 respect, i. e., in the possession of some specialized ap- 

 paratus for clinging to the hairs of mammals (PL V, 

 Fig. 19). They feed, as do the Analgesidae, upon the 

 oily secretions of the skin and on old epidermal cells. 



What are the factors which cause such very limited 

 host distribution? It can not be on account of their diet 

 for it is the same as that of the Analgesidae, the mem- 

 bers of which may have a very wide distribution among 

 host species. It can not be on account of the habits of 

 the hosts, for they are frequently more or less gregare- 

 ous, and besides have other habits, as that of burrowing 

 and nesting in close proximity to other species, which 

 would offer splendid opportunities for a very wide dis- 

 tribution of the parasites. This point is proved by the 

 fact that these same hosts have other external parasites 

 that do have a wide host distribution. 



As a solution of the question I think that there can 

 be little doubt but that these forms have such very nar- 

 row host limits because of mechanical reasons pertain- 

 ing to their attachment to a hair. The special apparat- 



