FLEAS 93 



became modified into Ornithopsylla laetitiae. It would thus repre- 

 sent its direct ancestor. A detailed study of the morphology of 

 these fleas, however, shows that this is extremely unlikely and the 

 attractive theory has to be abandoned. Both, no doubt, originated from 

 North American rabbit flea stock [Hoplopsyllus) ^ but are not themselves 

 very closely related. Curiously enough little or no collecting has been 

 done from nests of sea birds on the eastern coast of North America but 

 it is highly probable that some of these, too, harbour species descended 

 from rabbit fleas. If and when such fleas turn up, they may give 

 us a more direct clue to the immediate ancestry of Ornithopsylla 

 laetitiae. 



Along with the house-sparrow, the common hen flea of Europe was 

 also introduced into the United States, where it has spread on to 

 numerous wild birds as well as domestic poultry. Up to date there is no 

 parallel case of a modern introduction of a bird flea from North 

 America into Europe, but occasionally a rather weird flea, the South 

 American parrot stick-tight flea, is taken ofif a variety of captive birds 

 at the Zoo including tame pigeons. There is always a possibility that 

 this species might spread to native birds via the ubiquitous sparrow 

 which hops in and out of the aviaries at the Zoo. In South America it 

 has a wide range of hosts, but was first recorded from a parrot. This flea 

 is a relative of the jigger ( Tunga penetrans) which burrows beneath the 

 skin and heavily infests the feet of the natives in South America and 

 Africa. It causes great irritation and abscesses develop at the spot 

 where it is embedded as the result of secondary infections. Like the 

 jigger the female of the parrot stick- tight flea is permanently fixed to the 

 host but it remains attached to the surface of the skin. The modifications 

 arising from the sedentary habit (see p. 63) can be studied in this and 

 allied species of fleas. 



It has already been pointed out (p. 68) that bird fleas — at least 

 C. gallinae — can breed on mammals as well as on their true host. A 

 relatively loose bond with the host was probably one of the character- 

 istics necessary to allow the change from mammal to bird to occur in 

 the first place. Consequently bird fleas are frequently found on mam- 

 mals. When a cat catches a sparrow for example, it generally carries 

 it about for a while and allows it to get cool. The fleas soon leave 

 the bird, and at times change on to the cat and so in a small way 

 avenge the death of their host. How long they survive and whether, 

 outside the laboratory, they do in fact breed on a mammahan host is 



