388 



AN ECONOMIC ENTOMOLOGY. 



the various families heretofore mentioned constitute one of 

 nature's means of keeping- witliin bounds the insects of other 

 orders, and, indeed, the various parasites themselves. Not only 

 are the little creatures parasites upon almost all other insects, 

 but as a matter of fact they are parasitic even upon each other, 

 or hyperparasitic, so that it is a clear case of "dog eat dog." 



Fig. 447- 



Ceiaphron lyilicitm, parasitic in wheat plant-louse. 



They undoubtedly check injurious species, but also each other ; 

 in this way preserving a balance from year to year which keeps 

 all the forms at about the same relative level. Of course the 

 " hyperparasites," as the forms infesting the other parasitic 

 species are termed, are distinctly injurious from the agriculturist's 

 stand-point ; perhaps even more so than the forms actually feeding 

 on vegetation. 



The question of making economic use of parasites belonging 

 to this order has been frequently discussed, but no practical 

 results have yet been obtained. Occasionally several species 

 attack a single host, and yet, withal, they seem to produce no 

 effect upon it in the long run. Even though we may breed out 

 of a lot of caterpillars more parasites than butterflies or moths, in 

 the season following the caterpillars will be just as abundant as 

 they were the year before. Just where the checks to parasite 



