o PARASITISM 



14S 



producocMn- i^liotosvnthesis. and indeed starved corals expel their 

 guests. The plant coils presumably acquire carbon dioxide and per- 

 haps other end-products from the animals, and it may be an ad- 

 vantage for these to be quickly removed. A clearer example of 

 symbiosis is given bvthe flagellates which live in the gut of termites, 

 and so enable those insects to Uve on wood. For their part, the 

 termites do a preliminary breaking down and softening of the food. 

 Where one partner gets all the benefit, and the other almost 

 inevitably suffers some disadvantage, we have parasitism. It is 

 generally recognised that the line between commensalism and 

 symbiosis on the one hand, and parasitism on the other, is often 

 blurred, but it is not so often reahsed that it is equally difficult 

 to distinguish a parasite from a carnivore. Although we seldom 

 put it into our definitions, the assumption that a parasite is 

 smaller than its host is implicit in our thinking ; we call a flea 

 or a leech a parasite, a spider, which also feeds on the juice of prey 

 which is at first living, a carnivore. Parasites do not in general 

 kill their hosts, or at least not very quickly, but many do, and 

 there is at least one carnivore which does not. Grebes on the Lake 

 of Tiberias feed on the eyes of large fish, which they bite out 

 of the head, while the fish, though blinded, go on living. The 

 association of parasite with host is usually longer in time than 

 that of carnivore with prey, but there is in fact a complete series 

 from parasites which take only an occasional meal to those which 

 cannot five away from the host (or hosts) at all. For the Insecta, 

 for example, Keilin has drawn up the following list : 



1. Aedes, a gnat which needs no blood but will take it if it 

 gets the chance, 



2. Anopheles, a gnat of which the female needs occasional 

 blood meals before it lays eggs. 



3. Stegomyia, a gnat which needs frequent blood meals. 



4. Cimex, the bed bug, which feeds on nothing but blood, drops 

 off the host after a meal, but remains near him. 



5. Piilex, the flea, does not often leave the host, and dies if 

 kept away for long. 



6. Pedicnhts, the louse, remains on the surface but moves about. 



7. Phihirus, the crab-louse, moves very little. 



8. Sarcopsylla, a flea of which the female burrows into the 

 skin and lives permanently in a tumour. 



9. The larvse of Hypoderma, the warble fly, and of some other 

 Diptera, are completely internal. It is impossible to complete 



