i8o FARM FRIENDS AND FARM FOES 



Under ordinary conditions, however, this pest seems to be 

 held in check to a great extent by the tiny fly that develops 

 in enormous numbers at its expense. 



Primary and Secondary Parasites 



The ichneumon flies discussed in the previous pages de- 

 velop as parasites of various plant-feeding caterpillars. 

 They are known zj& primary parasites. 



It very commonly happens, however, that these primary 

 parasite larvae are themselves attacked by other ichneumon 

 fly larvae that attach themselves to the former and suck 

 their lifeblood. Such parasites of parasites are said to be 

 secondary parasites. 



It also happens sometimes that one of these secondary 

 parasites is in turn attacked by a third larva that sucks out 

 its lifeblood. Such parasites of parasites of parasites are 

 said to be tertiary parasites. 



Perhaps you will be able to remember these facts better 

 if I quote a famous old rhyme : — 



The little fleas that do us tease, 



Have other fleas that bite 'em. 

 And these in turn have other fleas 



And so ad infinitum. 



In general these parasites of parasites are spoken of as 

 hyperparasites. 



Attention has recently been called by W. F. Fiske to an- 

 other important phase of parasitism. It very commonly 

 happens that two or even more primary parasites attack 

 the same caterpillar. One of these may devour the avail- 

 able food so rapidly that the other will die. Or they may 

 both become fully developed, but be so dwarfed from the 

 limited food supply that they are unable to lay eggs for an- 



