Eimng — Significance of Parasitism in Acarina. 51 



Adaptive Convergence. 



Although the Parasitic Acarina have descended from 

 many widely separated free-living groups, yet when ex- 

 amined many of these forms show a remarkable super- 

 ficial resemblance which at first sight suggests a very 

 close phylogenetic relationship (See PL III, Figs. 10 and 

 11). This superficial resemblance is a direct response 

 of these originally widely separated groups to the same 

 environmental conditions. 



The flattened form of the body has been assumed 

 by many of the Acarina of parasitic habits, and it is in- 

 teresting to note that we find a duplication of the same 

 process in some of the parasitic insects. It is obvious 

 that a compressed form of the body is an advantage to 

 those parasites that inhabit fur-bearing animals, as it 

 enables the parasite to pass easily between the hairs of 

 its host. Examples of such parasites possessing this 

 form of body are the fleas, among insects, and the fur- 

 inhabiting Listrophoridae among the mites. 



On the other hand a depressed form of body is an ad- 

 vantage to those parasites that live upon birds and need 

 to crawl between the superimposed feathers; this form 

 offering less resistance to movements and affording bet- 

 ter footing for the locomotor organs. Familiar examples 

 of parasites having this form of body are the Mallophaga 

 among insects and the Analgesidae among mites. 



We find the need and the development of the locomotor 

 organs becoming less and less as we pass from the free 

 to the occasional, to the semiparasitic, and finally to the 

 permanent parasitic epizoa; likewise we find a gradual 

 development of clinging organs. These organs may be 

 of several kinds, but of special importance are the hooks 

 or claws. Many of the parasitic groups have these 

 structures developed for adhering to their hosts. These 

 may be developed on any or all of the legs as is the case 

 in the Ixodidae, Argasidae, Analgesidae, Listrophoridae 



