22 Trans. Acad. Sci. of St. Louis. 



get on the birds while they are roosting or sitting upon 

 the nest and, finding plenty to eat, remain for long periods 

 of time with the same bird. This might be called acci- 

 dental parasitism, but since it is so frequently met with' 

 I would be inclined to call it semi-parasitism. After 

 several generations, through the action of natural selec- 

 tion, we would have evolved forms that would feed upon 

 little else except the excretions material or dead epi- 

 dermal scales of the birds; and finally a form would ap- 

 pear that would be wholly parasitic. 



The parasites of the third class, those that have arisen 

 from plant feeding ancestors, all belong to a single group, 

 the parasitic Pediculoididae. One of our most common 

 parasitic species is Pediculoides ventricosus Newp. This 

 species is remarkably similar to the plant feeding species 

 of the same genus, so much so that it has been frequently 

 confused with them even by well trained entomologists. 

 This species has recently been extensively studied by 

 Webster'^ who has shown that it is changing its habits 

 of feeding upon insects and now is a serious parasite 

 of man himself. 



But it is among the plant feeding forms themselves 

 that we have seen the beginnings of the development of 

 the parasitic habits. 



Some of these, as Tarsonemus intectus, frequently have 

 attacked man among other animals and have caused him 

 very serious trouble. Here the step in the change of diet 

 is much greater than is usually the case, but the mouth- 

 parts are extremely well adapted for the new life as a 

 parasite. 



The Significance of Distribution According to Host 



Species. 

 Two methods of attack have been employed in study- 

 ing the significance of the distribution of the parasitic 

 Acarina according to host species. The first of these 



•Webster, P. M. A predaceous and supposedly beneficial mite, Pedi- 

 culoides, becomes noxious to man. Ann. Ent. Soc. Amer. 3: 15-39. 1910- 



