Ewing — Significance of Parasitism in Acarina. 15 



of the blood-sucking members of the family Pentato- 

 midae in the insects. In these cases all that is necessary 

 to establish the parasitic habit is that the preying form 

 shall more frequently attack larger animals than itself 

 and in such a way as not to overpower these or hold 

 them captive while feeding from their substances. 



Another road over which we find the parasitic habit 

 developing is in groups which are scavengers. In these 

 groups the free individuals frequently live upon small 

 bits of dead plant or animal tissues and when they first 

 begin to take on parasitic tendencies they generally con- 

 fine their attention to essentially the same kind of food 

 material, which is found in the dead cutaneous or ex- 

 cretous products of the organism to be parasitized. In 

 these cases, as in most cases of parasitism, the parasites 

 themselves are small and likewise descended from small 

 forms. A good illustration of this method of the origin 

 of the parasitic habit is shown in the origin of para- 

 sitism in the Mallophaga. The Mallophaga are small, 

 flat, apterous insects that live chiefly on the barbules of 

 feathers and on dead cutaneous cells. A study of their 

 development and structure'' has established a very close 

 relationship between them and the Corrodentia, or book- 

 lice. These free-living insects live on small bits of or- 

 ganic matter which they find under logs, bark, etc. That 

 parasitic forms have been evolved from these or from 

 forms of similar habits appears to be evident. Here 

 again we find the change of the kind of food is but slight 

 and also the environmental conditions are entirely satis- 

 factory for those forms which should, perhaps at first 

 by accident, find themselves transferred to the plumage 

 of birds. 



Yet a third way in which zoophagous parasitism may 

 have originated is in forms which live on plant juices. 

 The structure of their mouth-parts is such as to be al- 



* Kellogg, V. L. New Mallophaga, II. Proc. Cal. Acad. Set. II. 6. 

 431-471. 1896. 



