Ewing — Significance of Parasitism in Acarina. 27 



state that after trying many insects, mostly aquatic 

 forms from the same pool from which the mothers of the 

 larvae were obtained, I was unable to get any of them 

 to attach. In some cases the aquatic insects were con- 

 fined in a small jar in the water of which hundreds of 

 larvae swarmed, yet after days of confinement not a 

 single larva was attached. These results are similar 

 to many others obtained by different workers. 



The reason why these larvae will thrive only upon a 

 single host species or upon a few closely related host spe- 

 cies appears to be physiological differences in the nature 

 of the two diets met with. That they will frequently at- 

 tack many different kinds of animals, vertebrates as well 

 as invertebrates, is well known. Thus man himself is 

 attacked by probably several species. Yet in all these 

 cases the mites do not thrive but soon perish. 



It may be suggested that it is because the larvae do 

 not have a chance to meet with a larger range of host 

 species that they are so limited in their distribution, but 

 I have carefully investigated the habitats and surround- 

 ings of several of our American species, and find that 

 such is not the case. Grass sweepings made in the vi- 

 cinity of Ames, Iowa, where there were thousands of the 

 larvae of Microtromhidium locustarium Walsh present, 

 contained representatives of many orders of insects, but 

 these larvae were only found on a few closely related 

 species of grasshoppers. 



A List of the North American Ixodoidea and Theib 



Hosts. 



Argasidae. 

 AEGAS Latreille. 



A. Irevipes Banks — Cereus giganteus. 

 A. miniatus Koch — Chickens, cattle, quail. 

 A. reflexus Fabricius — Pigeon. 



A. persicus (Oken) — Fowls, ducks, geese, turkeys, ostriches, quail, wild 

 doves, canaries, man, cattle. 



