176 



FARM FRIENDS AND FARM FOES 



tory of one of these in brief is this : The adult fly de- 

 posits its egg within the body of a caterpillar, especially the 

 caterpillars of the large silkworm moths, like the polyphe- 

 mus moth or the cecropia moth. This egg shortly hatches 

 into a small white footless maggot that absorbs the body 

 juices of the host, as the victim is often called, but at first 

 does not seriously interfere with its development. The 

 caterpillar with its enemy inside continues to feed and grow 

 for some time. It is commonly able to spin its cocoon and 

 perhaps change to a pupa within the cocoon before the 

 larva of the parasite becomes so large as to kill the host. 

 It does kill it, however, finally, and then within the protect- 

 ing shelter of the host's cocoon, 

 the larva of the Ophion Fly 

 changes to a chrysalis. The fol- 

 lowing spring it willchange again 

 to an adult fly, like the one that 

 laid the egg in the caterpillar so 

 many months before. This fly 

 is provided with sharp jaws by 

 means of which it gnaws its way 

 out of the cocoon and flies freely 

 about. It has thus completed 

 the cycle of a parasite's life. 



Sometimes the egg of the par- 

 asite is not laid within the cater- 

 pillar until after it has spun its 

 cocoon. This is often the case 

 with certain medium-sized Ich- 

 neumon Flies of the genus 

 Pimpla. These may sometimes be seen standing upon the 

 cocoons of the common apple-tree tent caterpillar, with the 

 ovipositor thrust down through the cocoon in the- manner 



Ichneumon Fly and Butter- 

 fly Chrysalis in which it 

 developed 



