FLEAS, FLUKES AND CUCKOOS 



flukes in existence but although their origins are obscure the larvae 

 reveal traces of an independent past. In many groups of animals 

 parasitism has arisen anew several times over, a fact which is confusing 

 and irritating for zoologists as it seriously interferes with their desire for 

 orderly pigeon-holing. 



Parasitism can develop gradually or suddenly. It can be the out- 

 come of a long series of complicated interactions or the result of isolated 

 accidents which occurred a million years ago or only this morning. A 

 long established and widespread habit, such as the wanderlust shown 

 by mites, together with their inclination to creep into cracks and 

 crevices, may be the starting point. On the other hand some unusual 

 occurrence such as the accidental introduction of a number of uni- 

 cellular organisms into a bird's inside, with its food or water, or with 

 the air it breathes, may provide exactly the right conditions and 

 circumstances required to induce them to begin the parasitic mode of life. 



Many animals are saprophagous, that is to say they feed on dead and 

 decaying matter, such as dung or putrefying corpses. It is a short step 

 from a dead nestling to a decomposing flesh wound on a living bird, and 

 the fly-larvae, which occasionally try the latter as an alternative meal, 

 are following one of the well-trodden paths to parasitism. 



Many of the arthropod parasites of birds, such as bugs, mosquitoes, 

 and ticks, were originally suckers of plant juices. As the geological 

 record proves, these groups evolved before birds and mammals and, 

 no doubt, in the past were essentially vegetarians. It is a relatively 

 easy matter for them to pierce the skin of an animal either accidentally, 

 in error, or deliberately if no other food is available, and to extract its 

 body fluids. These accidental and casual drinks of blood which no 

 doubt originally took the form of an occasional meal might easily 

 become a habit, and then a necessity. Blood appears to be a somewhat 

 dangerous beverage, for like alcohol, it can convey certain immediate 

 advantages, yet carries with it the dreaded seeds of dependence. In the 

 case of careless blood drinkers the sins of the parents may be visited 

 upon the children in dramatic fashion — even unto the millionth 

 generation or more. 



Competition for living space is very keen in nature. Even such un- 

 attractive milieus as vinegar, gall and crude petroleum have been 

 successfully populated by certain species of worms and fly-larvae. 

 Sometimes an ecto-parasite finds competition too intense on the surface 

 of the host's body and creeps into a convenient orifice, a step which 



