1^0 FLEAS, FLUKES AND CUCKOOS 



niche on hosts belonging to one order are generally very similar in 

 character, and fall naturally into distinct genera. Thus as a rule all the 

 head lice parasitising one order of birds such as the raptores (Falconi- 

 formes) belong to one genus {Craspedorhynchus) and all the wing lice 

 to another {Falcolipeurus) . 



As the evolution of the birds lost its initial momentum and slowed 

 down, the character differences which affected their Mallophaga must 

 have been gready reduced. The waders, gulls and auks, which together 

 comprise the order Charadriiformes, show great diversity in habits and 

 general body form, such as legs, beak, and size, but the environment 

 they provide for the parasite — for example the physical and chemical 

 composition of the feathers and blood— is probably constant throughout 

 the order. Even if lice become isolated on one species of bird, or group 

 of species, within this order they are not subjected to any violent 

 change. This results in the development of only small constant distinc- 

 dons, in other words, specific diflferences. A number of these species 

 makes up the genus distributed throughout the order. Even when a 

 group of closely related hosts appear to provide the parasites with an 

 exactly similar environment, the lice on each host may be disdnct 

 species, differing in non-adaptive characters — often the male genitaUa 

 — which have developed as the result of isolation and time. 



The isolation of lice within an order of birds has occurred much later 

 than the isoladon between orders — hence the lice of gulls and plovers 

 differ from each other less than the Hce of ducks and plovers. 



Many birds are parasitised by four or five species each belonging to 

 a different genus, and in addidon may harbour two or even three 

 species of one of these genera. These latter species may differ in small 

 ways only, such as the arrangement of hairs on the abdomen, or by 

 some character of the male genitalia, or the presence or absence of en- 

 larged antennae in the male. 



This is, of course, a highly over-simplified picture of the process 

 of evolution in the group. In reahty it has become modified and the 

 Hnes obscured by various causes which will be further discussed on 

 pages 142-145, but until we have more information on the distribu- 

 don, morphology, biology and genetics of the group no definite 

 conclusions can be reached. However, these tentative suggesdons may 

 serve some useful purpose in demonstrating the complexity of the many 

 factors which have influenced the evolution of the Mallophaga. " By 

 reason of their subtilitv, intricacy, crossing, and interfering with one 



