58 Trans. Acad. Sci. of St. Louis. 



liability of becoming detached from the host is much 

 greater than hers. In this regard it is interesting to 

 note that he has many more enormous bristles than the 

 female which is in harmony with the suggestion just 

 given (since I have already shown that such long bristles 

 in the epizoa frequently keep them from becoming de- 

 tached from their hosts). 



Generalizations Regarding Various Aspects of Para- 

 sitism IN THE ACARINA. 



As a whole, the order Acarina has not been very ex- 

 tensively studied in the different parts of the world, but 

 this statement can hardly hold true in considering the 

 parasitic forms, for it is concerning these that we know 

 the most, at least in regard to geographical distribution. 

 Our knowledge of the ticks and the ''Bird Mites" in- 

 cludes forms from almost every part of the globe. How- 

 ever, much is yet to be learned in connection with the 

 distribution and life histories of most of the smaller 

 groups. 



In our study of parasitism in the Acarina it is inter- 

 esting to note the tremendous range of its various as- 

 pects. The parasitic forms have originated independ- 

 ently from several rather distantly separated free an- 

 cestors. The process of evolution in each separate group 

 has been going on for vast periods of time, upon hosts 

 which belong to different classes and even subkingdoms. 

 These hosts in turn have become evolved, they have as- 

 sumed different habits and environments. The processes 

 of degeneration and specialization have wrought great 

 structural changes, and with them vast differences in 

 their relation to each other and their hosts. When we 

 consider all of these things it is not so much to be won- 

 dered at that in this group we probably find a larger 

 variation in the different aspects of parasitism than is 

 presented by any other group of the Arthropoda. 



The following classification of the parasitic Acarina is 

 given, based upon an analytical study of their structure, 

 habits, food relations, etc. This is simply an arplica- 



