THEIR RELATION TO EACH OTHER in 



cells are cuckoos in habit and are effective in limiting 

 bee increase. As parasites they are not of so much 

 importance, for while they do their share in limiting 

 numbers under normal conditions, they do not readily 

 increase in abundance when the host becomes un- 

 usually plentiful. Of much greater importance are 

 those species that depend upon grasshoppers; for 

 these do really aid in effective control, the larvae in- 

 creasing in relative numbers as the egg-pods themselves 



FIG. 52. A bee-fly and its pupa. 



become plentiful. There are other predatory larvae 

 among the Empidce or dance flies and similar small 

 families; but none of any great importance, until we 

 reach the Syrphidce or flower flies. Among these we 

 find a large number that are specific feeders on plant 

 lice and among their important checks. 



In this connection it is interesting to note that in 

 every order in which there are predatory insects at 

 all, there are some that feed largely or exclusively on 

 plant lice. We will also find, in another connection, 

 that these same plant lice are also seriously influenced 



