Ewing — Significance of Parasitism in Acarina. 41 



ing and either fall off or would be incapable of loco- 

 motion. 



Among the birds themselves it is by far the more usual 

 thing to find a species of Analgesidae upon closely related 

 forms, but such is not always the case. For example 

 Proctophyllodes ampelidis (Buchh.) is found on members 

 of five different families, four of which are not closely 

 related; they are the Ampelidae, Mniotiltidae, Fringilli- 

 dae, Laniidae, Corvidae. Megninia cuhitalis (Megn.) is 

 found on three different families, Phasianidae, Anatidae 

 and Corvidae, each of which belongs to a different order. 

 Other cases of the same kind exist so that we are forced 

 to the conclusions that under favorable conditions a spe- 

 cies may have a very wide range of hosts. 



Since quite a number of these species, representing 

 most of the important genera, are found on widely sep- 

 arated host species, it may be regarded as an indication 

 that the reason why other species have not as great a 

 distribution is simply because they have not had the 

 chance to be transferred to other hosts. However, of 

 the two attempts which I made at transference of these 

 mites from their normal to a foreign host, neither was 

 successful. 



When the members of this family are detached from 

 their hosts they walk freely about and at a moderate 

 rate. I tested their vitality under these conditions and 

 found that frequently they would live for three days. 

 These two facts make the transference from one host to 

 another much easier and more probable and doubtless 

 are great factors in the explanation of the distribution of 

 many of the species. 



Of special interest in regard to their host distribution 

 are the forms which are found upon the raptorial birds. 

 Of the ten species of Analgesidae found on the Fal- 

 conidae, four are known to occur on more than one 

 host species. Although these four species are distributed 

 among several hosts none of them are on any birds outside 



