44 FLEAS, FLUKES AND CUCKOOS 



part in guiding the parasite along certain well defined lines of develop- 

 ment. The permanent ecto-parasite becomes adapted to the external 

 covering of the host — the feathers or hair and the skin texture. Such 

 characters affect the mouth-parts of the parasite, its integument, and 

 claws. Certain feather lice in which adaptation has become very close 

 cannot lay their eggs on feathers of a different structure from that of 

 their normal hosts. In other cases ovulation and hatching can only take 

 place within very narrow ranges of temperature. The fertility of the 

 eggs may also depend on whether the parasite has fed on the right host. 

 Many ecto-parasites seem to be closely adapted to the chemical com- 

 position of the blood and feathers of their host. In a carefully controlled 

 experiment Wilson has shown that one of the chicken lice would feed 

 on feathers of an American heron, but the nymphs did not complete 

 their moults and the adults died within three to sixteen days. Some 

 bloodsuckers placed on an abnormal host will often refuse to feed, 

 others attempt to do so without enthusiasm or success, while others 

 again will imbibe the strange blood but die shortly afterwards. As 

 Lucretius remarked, " What is food to one man may be fierce poison 

 to others." 



Most lice are strongly host-specific, but a notable exception is the 

 species from pig and man. The human louse will feed and breed on 

 swine, and the pig louse is equally at home on man. There is an obvious 

 resemblance between the near naked skin of the domesticated pig and 

 man, and again, the chemical composition of their blood must have a 

 lot in common — at any rate neither proves lethal to the lice in question. 

 There are other parasites which normally feed on these two hosts : 

 the human flea [Pulex irritans) and the jigger {Tunga penetrans)', a tick 

 [Ornithodoros moubata); certain of the floor maggots {Aucheromyia) , a 

 nematode worm {Ascaris lumbricoides) and the influenza virus. 



In some cases, when a parasite appears to live normally on a strange 

 host some part of the biological cycle is nevertheless disturbed. The 

 human louse breeding on the pig produces an abnormally high 

 proportion of females — a factor which could lead ultimately to the 

 extinction of the race. 



Where endo-parasites are concerned the chemical composition of 

 the various body fluids is probably of much importance, and also the 

 composition of the gut contents, and the physical structure of all 

 the internal surfaces which the parasite pierces or clings to during the 

 course of its life-cycle. The rapidity with which food passes through an 



