144 FLEAS, FLUKES AND CUCKOOS 



It may well be that there are certain superficial resemblances between 

 the external characters of gulls and petrels due to adaptation to life at 

 sea, which has favoured a certain limited interchange of lice. The 

 possibility of such secondary infestations — whether ancient or relatively 

 recent in origin — must always be borne in mind when considering the 

 distribution of a genus of feather lice in relation to host affinities. 



Convergent and parallel evolution. The classification of birds, which is 

 intended to reflect their true relationship, is based mainly on the 

 characters of the muscles and skeleton — structures which do not directly 

 affect the parasite. If two unrelated groups of birds, perhaps in response 

 to the same environment, developed similar external characters, then 

 the adaptations to these characters forced on their Mallophaga might 

 produce in the latter a superficial resemblance to one another even 

 though they were really not closely related. This type of convergent 

 evolution in the parasites can suggest false relationships between the 

 hosts. For example, Pkilopterus, a head louse genus parasitising the 

 Passeriformes, is mostly very uniform in character. Certain species, 

 however, have developed a line of thickening on the front margin of the 

 head, and have a different arrangement of head sutures and of the 

 struts supporting the mouth-parts. Lice with this type of head have 

 been found on eleven species of Passeriformes belonging to nine different 

 families and also on the family Momotidae (the motmots) usually placed 

 in the order Coraciiformes. Does the similarity of these Mallophaga 

 suggest that the families of birds on which they are found are more 

 closely related to each other than to any other of the families of passer- 

 ines ? There is no evidence of this from ornithological sources, but it 

 may be that the bird species all have some character in common : the 

 head feathers of some, at least, of the hosts are hard and shiny, showing 

 iridescence. The species of the genus Philopterus are sedentary and highly 

 adapted to the particular feathers on which they move and feed. Any 

 change in feather structure will tend to affect the front of the head 

 which is used to push through the plumage, and also the mouth-parts 

 and their supporting structures used in grasping and feeding. In the 

 case of these species from the motmots and some of the passerines, 

 therefore, it seems possible that they have responded with similar 

 modifications to a similarity of the feathers, possibly a hardening of the 

 surfaces. The resemblance between these lice is, therefore, due to con- 

 vergence in response to a similar environment. In such cases it is easy 



