EFFECT OF PARASITISM ON THE PARASITE 43 



host is added or one cut out. The life-cycle of tapeworms is charac- 

 terised by the absence of free-swimming larvae but in many cases up to 

 four different intermediate hosts are used. Roundworms also frequently 

 depend on intermediate and transport hosts, and the filarias are taken 

 up from one host and put back on another one by blood-sucking insects. 



Complicated life-cycles are unquestionably characteristic of internal 

 parasites. On the other hand extremely simple life-cycles are met with 

 among external parasites, particularly those like feather lice, mites, and 

 the sheep ked, which pass generation after generation on the same 

 individual animal. Parasites, it is true, develop many features in 

 common but free-living organisms become adapted to a specialised 

 environment in the same way, and we find for instance that cave- 

 dwelling animals all over the world are often characterised by blind- 

 ness and pallor. Parasitism merely provides a particular habitat and 

 mode of life which calls forth certain equally distinctive adaptations. 



Some parasites are able to live on a wide range of hosts belonging 

 to different orders or even classes of animals, but it is more usual for a 

 parasite species to be confined to a relatively small group of hosts. 

 These may embrace a whole order such as the ducks, geese and swans 

 (Anseriformes) or two or three related species like the swallow and house- 

 martin, or even a single species or even subspecies of bird Parasites 

 which are confined to one particular host, or to a group of related 

 hosts, are said to be host specific. 



Host specificity is the result of the parasite adapting itself to life in a 

 certain environment, and if the adaptation is very close it is unlikely 

 that it will be able to survive on, or in, any other host. Many free- 

 living animals have become adapted to particular environments or highly 

 specialised diets, and are therefore unable to live elsewhere. The 

 crested tit in Scotland is restricted to areas where there are old rotten 

 pine stumps in which it nests. The marsh fritillary butterfly {Euphydryas 

 aurinia) in Britain is confined to an environment where the devil's-bit 

 scabious is found — the only plant upon which the female will lay her 

 eggs, although the larva will feed on honeysuckle, snowberry and 

 certain other leaves. The koala bear can only survive on a diet of fresh 

 eucalyptus shoots and is therefore restricted to places where the plant 

 grows. Such examples could be multiplied indefinitely. It may 

 originally be a single attribute which links a parasite to one particular 

 animal but once the association has begun all the characteristics of the 

 host, morphological and physiological, as well as biological, play their 



