Ewing — Significance of Parasitism in Acarina. 17 



usually are blind, without prehensile organs, atracheate, 

 and are more or less sluggish. To this class belong: 



A few of the Gamasidae. 



The Orlbatidae, Xothridae and Hoplodermidae. (Many of the species 

 of these families live partially on small vegetable organisms, as fungi, 

 but on the whole they are to be regarded as scavengers.) 



Practically all the Tyroglyphidae. 



III. The free-living phytophaga. The phytophaga, as 

 a rule, have piercing mouth-parts. They possess the spe- 

 cial senses developed to a moderate degree, while the 

 palpi have become reduced. In their movements they 

 are not as agile as the predaceous forms. To this class 

 belong : 



Some of the Tetranychidae. 



A few of the Oribatidae and Nothridae. 



Some of the Pedlculoldidae and Tarsonemidae. 



From these three classes of free-living forms, or from 

 their ancestral types of very similar habits and of some- 

 what similar structure, there arose the various groups of 

 our present day, living, parasitic Acarina. To show how 

 this process probably took place, and how many and 

 what kind of lines of descent there are in the parasitic 

 forms, will be the next object for our consideration. 



Standing in close relation to the free-living forms of 

 the first class, either as genera of the same families or 

 as closely related families, or groups of genera or fam- 

 ilies, we have the following apparently natural groups : 



The parasitic Cheyletidae. 



Families of the trombidoidean type with parasitic larvae (Rhyncholo- 

 phidae, Trombidiidae, Hydrachnidae, Halacarldae [mostly]). 



Ixodldae, Argasidae, Dermanyssidae and some genera of the Gamasi- 

 dae. 



A few parasitic forms of Uropodidae. 



In a similar manner the following groups of parasitic 

 Acarina are allied to the free-living forms of the second 

 group, the scavengers : 



Some of the Gamasidae (?). 

 Canestrinidae and Sarcoptidae. 

 Listrophorldae (in part). 

 Analgesidae. 



