64 Trans. Acad. Sci. of St. Louis. 



SUMMARY. 



As TO THE OkIGIN OF PaKASITISM IN THE ACARINA. 



We have very strong evidence indicating that the para- 

 sitic habit has originated independently at least eleven 

 times in the phylogeny of the Acarina. 



Among the zoophagous parasites the parasitic habit 

 has been developed from three different types of free- 

 living Acarina; (a) predaceous forms, (b) scavengers, 

 (c) forms living upon the juices of plants. 



The chief reason for so frequent occurrence of parasi- 

 tism in the Acarina is because of their minute size; and 

 the predaceous, scavenger, or plant-sucking habits of the 

 free-living forms. 



Among our living forms we can to-day trace out all the 

 stages of advancing parasitism including the occasional 

 or erratic parasitism, semiparasitism, facultative parasi- 

 tism, even to the fixed and permanent type, and finally to 

 endoparasitism. 



As is usually the case with other parasites, we generally 

 find here a gradual increase in the state of degeneration 

 as we follow the advancing stages of parasitism from its 

 origin among free types. In the Analgesidae however 

 this is not true as they are in several respects highly 

 specialized and have undergone little or no degeneration. 



We find in the Acarina a process of degeneration which 

 in its completeness is seldom obtained in the animal king- 

 dom. We must remember that the Arachnida are in 

 many ways a highly evolved group, certainly much more 

 BO than the Crustacea, yet here we find degeneration 

 almost equal to that of the classical example of Sacculina. 



As TO THE Distribution According to Host Species and 

 ITS Significance. 



The parasitic Acarina may be arranged into three 

 classes depending upon their distribution according to 

 host species : — 



