Ewing — Significance of Parasitism in Acarina. 21 



enger. The members of the Tyroglyphidae live on refuse 

 matter of various sorts, but especially on decaying ani- 

 mal matter and upon sores of animals, etc. The species 

 just mentioned differs from most the members of the 

 family in that it is always associated with a certain 

 species of insect, in this case the Oyster Shell Scale. 

 Here it feeds, not on the live scale insects, as was once 

 supposed, but only on the dead insects and the old egg 

 shells. However, there is a form present in association 

 with the same scale insect, and which has been much 

 confused with the mite just mentioned, which does live on 

 the fresh eggs of the scale, and I have observed it occa- 

 sionally attacking the scale insects themselves. This is 

 Hemisarcoptes malus (Shimer), (PI. HI, Fig. 5); which, 

 as the name indicates, may be regarded as the connecting 

 link between the true Sarcoptidae and the scavenger free 

 forms. 



Among the scavenger types the first steps toward para- 

 sitism are perhaps the constant association of the mite 

 with some other particular species of animal, as is the 

 case of Moniesiella entomopJiagus Laboulbene. The next 

 step in the case of the origin of parasitism among the 

 Canestrinidae and Sarcoptidae was perhaps the change 

 of diet from dead tissues to that of live tissues as found 

 in fresh eggs. Then began a partial life on some fresh 

 animal food and finally a complete parasitic form was 

 evolved. 



In the Analgesidae, of course, the change of diet by 

 forms taking up the parasitic habit would be very slight. 

 Here the ancestral type was probably the same as that 

 of the Canestrinidae, that is, was similar to our present 

 day Tyroglyphidae. In this connection, I may mention 

 that I have discovered a species of Tyroglyphidae that is 

 repeatedly found upon the plumage of birds. Mr. W. R- 

 Thompson, a specialist in the group, has also confirmed 

 my observations. In the case of this species, Glycypha- 

 gus spinipes Koch, there is but little doubt that these mites 



