Ewing — Significance of Parasitism in Acarina. 19 



practically all the other free-liviBg Acarina, they move 

 horizontally. Thus it can be seen how these organs alone 

 are excellently adapted for clinging to a host; as a 

 tnatter of fact they are perfectly typical of the adhering 

 forms of palpi as we find them in the parasitic trombi- 

 doidean larvae (PI. V, Fig. 20). Next, we actually know 

 of some species of the genus Cheyletiella which live upon 

 the backs of fur bearing animals. Here they apparently 

 feed only upon the other parasites present, but the con- 

 ditions are perfect for the development of the parasitic 

 habit. All that is necessary is that the normal food sup- 

 ply be cut short; then, according to the law of the sur- 

 vival of the fittest, only those individuals will live that 

 are able to sustain themselves for a while, at least, upon 

 another diet. Since they are already accustomed to the 

 animal juices of smaller forms, it is but a slight step for 

 them to take in order to be able to digest the juices of 

 mammals. Perhaps those individuals which first partook 

 of these juices only did so for a very short time and 

 perhaps from some wound upon the animal. 



In the case of the parasitic Peritremata (the poultry 

 mite, blood-sucking bird mites, ticks, etc.), not only can 

 we trace the anatomical homologies of the different 

 groups step by step as we pass from the free-living to the 

 parasitic groups, but we can also trace in a parallel man- 

 ner the development of the parasitic habit. 



Among the free-living Gamasidae it is a frequent oc- 

 currence to find species which live in association with 

 other animals. Mention here will only be made of a few 

 cases. Some of these forms use their pseudohost only 

 for the purpose of transportation. Many species, as 

 Macrocheles moestus Banks, live in association with 

 other animals, acting somewhat as scavengers. I have 

 frequently observed such cases in the rearing of insects 

 and also in my experiments upon inoculations with para- 

 sites upon some of the higher animals. In some cases, 

 as with species of Antennopliorus, the mites come in as 



