FEATHER LICE 129 



feather lice at any rate, Addison's remark is justified : "A very ordin- 

 ary Microscope shows us, that a louse is itself a very lousy Creature." 



Origin — Evolution — Classification 



If the photographs of Mallophaga in this book are shown to an 

 expert he can say after a superficial glance from which order of birds each 

 specimen was collected. Similarly if you were to show the expert a 

 louse and say : "I took this off a snipe," he might reply : " Yes, but 

 that day you also shot a partridge and put it in your game bag with the 

 snipe." This is possible because groups of related birds — say the game- 

 birds, the waders, the hawks — each have their distinctive types of lice. 



Close correlation between bird and parasite can be explained 

 by the theory that birds were parasitised at an early stage of their 

 evolution, before they had diverged greatly from the generalised 

 ancestral type. As the birds evolved and became adapted to different 

 environments and ways of life, there were modifications and changes in 

 their body form, in the physical structure of the feathers, and in the 

 temperature and secretions of the body. The Mallophaga, 

 closely associated as they are with the surface of the body and the 

 feathers, had to become adapted to these changes. Each step, therefore, 

 that took a group of birds further away from the ancestral type and 

 from other evolving groups, was followed by the Mallophaga living on 

 it. The morphological changes in the feather lice, however, were slower 

 and less drastic than in their hosts, and the differences between any 

 of the Mallophaga are now less than those between say a penguin and 

 a peacock. The environment of the Mallophaga is formed mainly by 

 the external characters of the bird — the feathers and skin texture — 

 together with the temperature and secretions of the body. The changes 

 in this environment were probably smaller than those in the external 

 environment of the bird, and the resulting modifications in the structure 

 of the louse are, therefore, less. It is also possible that the Mallophaga 

 after an initial evolutionary spurt became more stable, in the evolution- 

 ary sense, than their hosts, and thus remained more constant in form. 



This slower rate of evolution in the Mallophaga is the reason why 

 they have changed less than their hosts, and have retained more 

 characters which show their relationship to each other. The curlew and 

 the oyster-catcher, both waders (Charadrii), belong to different families 



