334 



Beneficial Insects. 



tiny flies Platygaster tipidcc and Macroglenes penetrans, which 

 infest and destroy the wheat-midge (Cecidomyta ti'itici) and 

 Pachymerus calcitrator, a parasite of the corn saw fly [Cephus 

 pygmceus). 



In some seasons these beneficial flies are so numerous as to 

 completely check the destructive progress of the insects they 

 parasitise, and it may almost be said that the increase or 

 decrease of the latter, and their dangerous effects upon crops, 

 depend upon the number of their enemies. 



