42 BUTTERFLIES 



gradually growing larger and larger until at last it reaches 

 a length of possibly an inch. By this time it is likely to 

 have absorbed so large a part of the inside of the cater- 

 pillar that the latter dies. The parasite larva now changes 

 to a pupa, either inside or outside the skin of the cater- 

 pillar, and a little later changes again to an adult Ichneu- 

 mon fly. 



In the case just given, one egg only was deposited within 

 the skin of the caterpillar. In many others, however, a 

 large number of eggs may be so deposited by a single fly. 

 A special group of Ichneumon flies, called the Microgast- 



ers, contains many parasites 

 that have this peculiarity. 

 The Microgaster larvae on 

 coming forth from the cater- 

 pillar have the habit of spin- 

 ning tiny cocoons within 



Tachlnid ParasUe; ^^fly; b. pupartum, which they chaUgC tO pupaC. 



By collecting some cabbage 

 worms which are nearly full grown, and keeping them in 

 a glass jar one can generally get a considerable number 

 of these Microgaster cocoons and rear the flies from 

 them. 



Another group of caterpillar parasites is still more mi- 

 nute. They are called the Chalcid flies. Their life-his- 

 tories are full of interest, and might easily furnish oppor- 

 tunity for a long lifetime of study and experiment. One is 

 likely to get hundreds of these Chalcid flies from a single 

 caterpillar. 



Another interesting group of parasites is that of the 

 two- winged Tachina flies (see cut on this page). The life- 

 story of some of these is comparatively simple: a buzzing 



