78 ANIMAL ECOLOGY 



so of course the animals in a parasite food-chain become 

 gradually smaller and smaller. And just as the carnivores 

 become fewer in numbers, so do the parasites become 

 usually more numerous. Let us take some examples. Fleas 

 are parasitic upon birds and mammals, and many fleas in 

 turn are parasitised by protozoa of the genus Leptomonas.^^^ 

 One squirrel might support a hundred fleas or more, and 

 each flea might support thousands or hundreds of thousands 

 of Leptomonas. The pyramid of numbers in such a case 

 is inverted. Many other examples could be given. Egyptian 

 cattle have ticks (Hyalomma), which in turn carry inside 

 them a protozoan {Crithidia hyalommce)}'^^ Apparently there 

 are never very many stages in such food-chains of para- 

 sites. The reason for this is that the largest parasite is not 

 very big, and any hyperparasite living on or in this must be 

 very much smaller still, so that the fifth or sixth stage in the 

 chain would be something about the size of a molecule of 

 protein ! Actually, bacteria, although they are not animals, 

 may be conveniently included in parasite food-cycles, and in 

 many cases they form the last link in the chain. The plague 

 bacillus lives in the flea of the rat in warm countries (also 

 in the rat itself), the flea lives on the rat, and the rat lives to 

 some extent " parasitically " on mankind. Recent work upon 

 bacteriophages seems to show that there are sometimes still 

 smaller organisms (or something with the power of multipli- 

 cation) which have a controlling effect on the numbers of the 

 bacteria themselves. So possibly the last link in some para- 

 site food-chains may be formed by such bacteriophages. 



10. There has been a good deal of discussion by ento- 

 mologists as to the exact position of the Parasitic Hymenoptera 

 with respect to parasitism. These insects lay their eggs in the 

 eggs, larvae, or later stages of various other insects (and in some 

 other animals too), and these eggs grow into larvae which live 

 as true parasites in the bodies of their hosts, which they 

 eventually destroy, the hymenoptera finally emerging in the 

 form of free-living insects. An animal like an ichneumon, 

 therefore, combines the characteristics of a parasite (in its 

 larval stage) and of a free-living animal (in its adult stage), 



