THE PAKASITES OF THE CABBAGE WOKMS. 



17 



work their way out of the skin of their host^ as the unfortunate 

 cabbage worm in which they develop is called. After the 

 parasites have emerged, the caterpillar becomes merely a dead 

 and shapeless mass of skin. 



Fig. 17. — I'arasitic Fly and it8 Cocoon, much magnified. 



The maggots so strangely free from the caterpillar whose 

 life-blood they have absorbed crowd together on the leaf. 

 Each spins by means of a liquid silk a sheltering cocoon. 

 In these 'Cocoons the maggots change to pupa?. • 



If such a mass of cocoons is placed in a bottle, or a covered 

 tumbler, a week or two later you are likely to find in the vessel 

 a number of small black flies. Each cocoon will have a tiny 

 cap partly open at one end, showing where the flies have come 

 out. 



These flies are the parasites full-grown. They are similar to 

 the fly which laid the Q^£f in the cabbage worm some weeks 

 before. Set them at liberty and they will go forth in search of 

 other cabbage worms, to repeat the tragic cycle of insect life. 



