THE TWO-WINGED PARASITES 189 



Variations in Life Histories 



Until recently the life histories of the other Tachina 

 Flies have been supposed to be similar to the development 

 of that preying upon the army worm. But the careful 

 studies of Dr. C. H. T. Townsend upon the parasites of the 

 gypsy and the brown-tail moths show a marvelous variation 

 in the life histories of these flies, and establish their great 

 value as enemies of hairy caterpillars as well as of those 

 with smooth skius. Dr. Townsend found that at least one 

 of the flies that preys upon the brown-tail moth deposits 

 eggs upon the young caterpillars, the eggs hatching and 

 penetrating the skins of the caterpillars, much as in the 

 case of the parasite that kills the army worm. But the 

 other Tachinid parasites had surprisingly different life his- 

 tories, the discovery of which required months of patient 

 observation. 



Several species of these flies deposit their small eggs upon 

 the leaves where the caterpillars are feeding, the eggs be- 

 ing often fastened to the freshly eaten edge of the leaf. 

 The eggs are swallowed by the caterpillar with the leaf 

 particles, generally escaping injury from the jaws because 

 they are so minute. Soon after being swallowed, the eggs 

 hatch into tiny maggots that burrow through the wall of 

 the alimentary canal into the general body cavity of the 

 host. Here they absorb food and grow, finally changing 

 to pupae and again to adult Tachina Flies — meanwhile 

 causing the death of the caterpillar. 



These observations upon the leaf laying of the Tachinid 

 eggs indicated an unsuspected ability on the part of the 

 flies to overcome difficulties. Evidently the hairy clothing 

 of the caterpillars would prove of no value as a protection 

 from parasites if the very food is laden with the eggs of 



