114 INSECTS 



worms on the march, almost every one of them with 

 the seeds of death conspicuously placed upon them. 

 Out of hundreds of pupae gathered where armies had 

 been feeding, only single examples of moths were ob- 

 tained. 



And in this connection we might note that a check 

 to the further increase of any plant-feeding species 

 does not necessarily mean an immediate cessation of 

 injury. Indeed in the armies of parasitized caterpillars, 

 every one fed until it was fully grown, and so far as 

 injury to the crop was concerned, it made not an ounce 

 of difference to the owner of the grain. And so we 

 must realize clearly that all these natural checks are 

 not imposed to prevent the plant-feeding insects from 

 injuring the farmer's crop, but simply to prevent undue 

 increase in relation to the surroundings and to preserve 

 the balance of nature. 



Yet these Tachinid flies are among the most effec- 

 tive engines of destruction to the species which they 

 infest. They develop quickly, the females lay a large 

 number of eggs and they are themselves not seriously 

 affected by secondary parasites. They are therefore 

 able to maintain their relative proportion to their host 

 no matter how rapidly that multiplies because of the 

 removal of other checks. 



Not all Tachinids lay their eggs directly on the 

 body of the host. It has recently been demonstrated 

 by a series of most interesting observations made in 

 the course of the gypsy moth work in Massachusetts, 

 that some species lay very small eggs on the leaves of 

 infested trees and that these eggs, eaten by the cater- 

 pillar with the leaf tissue, hatch when they get into 

 the stomach and bore through the walls into the body 

 cavity. Yet other forms lay their eggs on leaves, on 

 which caterpillars are feeding, fastening each to the 



