130 THE AMERICAN NATURALIST [Vol. XL VII 



species with hosts, and the catalogue of host species with 

 parasites, this group of ectoparasitic insects should be 

 accessible to a wider attention and a more thoroughgoing 

 study from entomologists than it has hitherto had. In- 

 deed the group is deserving of some special attention 

 from general naturalists because of the interesting sig- 

 nificance of the conditions of distribution and species 

 forming which obtain in it. It is to the setting out of a 

 few facts and some of the significance concerning distri- 

 bution and species forming among these insects that the 

 present paper is devoted. 



II 



The Mallophaga compose a small fairly homogeneous 

 group of about fifteen hundred insect species (as so far 

 known), which pass their whole lives on the bodies of birds 

 and mammals. Those species found on birds are never 

 found on mammals, and those on mammals never occur 

 on birds. Indeed, with exceptions in only two genera, 

 the Mallophaga of birds are of distinct families from 

 those of mammals, the two groups being distinguished 

 structurally by the loss in the mammal-infesting kinds 

 of one of the two tarsal claws, an adaptation connected 

 with the difference between the feathers and hair of the 

 hosts as habitat. Of the 1,500 known Mallophagan spe- 

 cies less than one hundred are mammal-infesting, the 

 others all occurring exclusively on birds. 



As to the relations of the Mallophaga to other insects 

 I am convinced that they should be treated as a distinct 

 order, finding their nearest affinities in the wingless 

 Psocidae, such as Atropos, the familiar book louse. This 

 relationship is shown not only by similarities in external 

 structure, some of which, however, are only examples of 

 parallelism or adaptive convergence, but by certain more 

 important common characters of internal anatomy, not- 

 ably a curious pharyngeal sclerite (perhaps the greatly 

 modified hypopharynx), found in both these groups and 

 nowhere else among insects. The habitat and food 



