220 HALF HOURS WITH INSECTS. I Packard. 



ides will be destroyed. The natural enemies are birds and 

 ground beetles which collect about the rear and van of the 

 moving army. Six species of ichneumon flies prey upon 

 them, among which is a kind of Ophiou (Fig. 170). These 

 parasites either attach an egg to the outside of the body of 

 the caterpillar, or insert it beneath the skin. Mr. "Walsh, 

 who discovered most of these parasites, also found that a 

 Tachina fly, an insect somewhat like the ordinary house fly, 

 was in Illinois so destructive that out of nearly sixty worms 

 all but two had the eggs of these flies stuck in groups of 



Fig. 170. 



Parasite of the Army Worm. 



from one to six on the upper side of the body. From these 

 caterpillars he bred fifty-four Tachinas and only two moths. 

 Such is the effective warfare waged upon one insect by an- 

 other. In this way the balance of nature is preserved. As 

 to their mode of attack he states that "Jeff'erson Russell, an 

 intelligent farmer, had repeatedly, on damp cloudy morn- 

 nings, watched a large bluish green fly, about the size of a 

 blow-fly, attacking the army worm, and depositing its eggs 

 on the shoulders of the victim, as he ascertained by a 



28 



