178 



FARM FRIENDS AND FARM FOES 



fortunate host. In a few weeks the maggots become full- 

 grown, and then each burrows its way out through the skin 



of the victim. Once outside, 

 each maggot spins some sort of 

 silken cocoon within which it 

 changes first to a pupa, and 

 later to an adult Microgaster 

 similar to the one that laid the 



feggs. 



There is much variation in 

 the color, form, and arrange- 

 ment of the cocoons of these 

 Microgaster Flies. Some are 

 white, others yellow. Some are 

 formed singly, others side by 

 side in a compact mass, and still 

 others in a loose mass of fluffy 

 silk. 



There is also great variation 

 in the number of parasites to a single caterpillar. Some- 

 times there is but one parasite to a host, as in the case of 

 certain leaf miners, sometimes there are only a few, but 

 more commonly there are a great many — several hundred 

 in the case of the tomato worm and other sphinx cater- 

 pillars. 



Plant-louse Parasites 



Almost any colony of plant lice will furnish examples of 

 the extremely interesting family of tiny flies that prey 

 especially upon the aphides. These are little four-winged 

 flies so small as to be barely visible to the unaided eye, 

 which develop in their younger stages in the bodies of the 

 plant lice, and cause the death of untold billions of these 

 vexatious pests. 



Caterpillar with Cocoon 

 Mass of Microgaster Para- 

 sites 



