Ewing — Significance of Parasitism in Acarina. 47 



When one species is found upon more than one host 

 it is interesting to note that the malformations produced 

 may be quite different on the different host species, al- 

 though frequently they may be very similar. 



Since the species of mites in this group have such nar- 

 row limits in their host distribution it is interesting to 

 study the causes of it. In the first place is there a ready 

 means of transference from one kind of a host to an- 

 other? I think that there can be no doubt but that these 

 species are easily transferred from the leaves of one 

 kind of a plant to those of another. In many forests we 

 find the branches of one tree mingled with those of an- 

 other or superimposed over those of another, so that dur- 

 ing a high wind they would be frequently rubbed to- 

 gether. 



Again a tree or bough when falling frequently would 

 lodge in another, and the mites which it carried would 

 be transferred to the latter. Since the gall inhabiting 

 forms always inhabit open galls and since all of the 

 Eriophyidae travel very well, though they have but four 

 legs, they would thus be enabled to pass easily from one 

 host to another when these hosts are in actual contact. 



I am inclined to attribute this very limited distribution 

 of the different species of this group to physiological 

 reasons. The forms are very small and have a simple 

 and somewhat degenerate digestive system, hence too 

 great a change in the nature of the diet could not be en- 

 dured. In this regard it may be interesting to note that 

 some of the members of the phytophagous Tetranychi- 

 dae are well nigh omnivorous. They are hardy forms 

 and do not have a delicate digestive system. 



