58 FLEAS, FLUKES AND CUCKOOS 



in Other words, to protrude as little as possible above the surface of the 

 host. 



Fleas are, generally speaking, much more active insects than lice. 

 They have more need to be. The feather lice can run, but they generally 

 confine themselves to sudden short rushes which enable them to move 

 quickly out of sight if they are momentarily uncovered by the bird's 

 preening. 



Species restricted to the head or neck, out of reach of the host's 

 beak, can sacrifice speed and become more closely adapted to the 

 feathers. Fleas, except for one or two rare exceptions such as the 

 stick-tight flea of poultry (see p. 62), not only have to move rapidly 

 on the host, but they need to jump on and off at very short notice. 

 Hence they are provided with long powerful legs, whereas those of the 

 feather lice are short and weak. 



It is perhaps unnecessary to stress the fact that the feather lice bite 

 and chew their food, while the fleas suck it up in liquid form through 

 tube-like mouth-parts (Plate XII). Expressed differently, the latter 

 cannot eat, they can only drink. 



It is when we come to regard these two groups of insects as a whole 

 that we realise how great the contrast is between the two. 



Ornithologists calculate that there are approximately 8,500 species 

 of birds in the world to-day. Louse experts estimate that there are more 

 than three times as many feather lice, making a total of approximately 

 25,500 species. Of bird fleas about 60 are known and named. Possibly 

 the total is somewhere around a hundred. 



The feather lice show great diversity in form and structure, whereas 

 the fleas, at least to the naked eye, present a very homogeneous 

 appearance. Again, each order of birds has its own characteristic 

 feather lice — -just as it has its own Cyclophyllid tapeworms — whereas 

 the bird fleas show no such restriction to a group of hosts. Moreover, 

 many species of feather lice are host-specific, that is, confined to a single 

 species of bird, whereas most of the fleas are not. Finally many feather 

 lice have specialised habitats upon the bird's body — some are confined 

 to the head, others to the wing feathers, others again live inside the 

 quills. Except for the two or three species in which the females are 

 sedentary and therefore obliged to congregate on the head (see p. 76), 

 fleas have no special location on the host's body. 



Thus an immense gap divides these two orders. The feather lice, 

 one can deduce, are a very ancient group. They are also a highly 



