PARASITES 



79 



the latter being carnivorous, or herbivorous, or in some cases 

 not feeding at all. For this reason the Parasitic Hymenoptera 

 have often been referred to as " parasitoids." There are 

 numerous other kinds of animals which have alternate free- 

 living and parasitic stages in their life-history, but not many 

 of them in which the adult is free-living as it is in these 

 hymenoptera. It is usually the larva which undertakes the 

 task of finding a new host, where this is done by active 

 migration. 



As Richards ^^^ has pointed out, insects like ichneumons, 

 braconids, and chalcids, do not have directly a very big effect 

 upon the food-cycle in a community, since they are merely 

 turning the tissues of their host into hymenopteran tissue, 

 and very often the enemies of the host eat the adult hymeno- 

 ptera as readily as they would eat the original host if it had 

 survived. Of course it does make a certain amount of differ- 

 ence ; for one thing, there is naturally a great loss of energy 

 in the process of turning host into parasite, and therefore 

 the activity of the hymenoptera reduces the available food- 

 supply to some extent. 



II. It has been necessary in this chapter, as well as in the 

 last, to speak continually of food-chains as if they commonly 

 consisted of simple series of species, without taking into 

 account any of the complications found in actual practice. 

 This is of course far from the truth, because the food-relations 

 of animals are extremely complicated and form a very closely 

 and intricately woven fabric — so elaborate that it is usually 

 quite impossible to predict the precise effects of twitching 

 one thread in the fabric. Simple treatment of the subject 

 makes it possible to obtain a glimmering of the principles 

 which underlie the superficial complication, although it must 

 be clearly recognised that we know at present remarkably 

 little about the whole matter. One of the most important of 

 these principles is that the sizes, and in particular the relative 

 sizes, of the various animals which live together in a com- 

 munity, play a great part in their lives, and partly determine 

 the effects which the various species will have upon one another. 

 One important effect we have mentioned — that the food- 



