192 



FARM FRIENDS AND FARM FOES 



ished. If you will look at a nest of our native tent cater- 

 pillars in spring, you will see silken webs leading from the 

 tents in all directions. These are made by the individual 

 caterpillars on their daily journeys in and out. 



It is evident that the Tachina Fly knows about this trail 

 of the silk-spinning caterpillar. And apparently she can 

 tell whether it is a fresh trail on which the caterpillar has 

 gone out and not returned, or an old one which would be 

 comparatively useless for her purpose. " Doubtless, ' ' writes 

 Dr. Townsend, "the flies larviposit only on freshly laid 

 strands which have not lost the odor of the caterpillar." 

 Upon this fresh strand of silk she deposits a living larva 

 that is wonderfully adapted to making its way in the world. 

 On the hind end of its body there is a curious sucker-like, 

 membranous, cup-shaped case, by means of which the tiny 

 maggot is fastened to the leaf on or near the silken thread. 

 On the front end of its body is a slightly sickle-shaped hook, 

 admirably designed for rending the skin of the caterpillar. 

 Thus it is anchored in the path of its prospective victim 

 with its stiletto ready for business at a moment's notice. 

 As long as all is quiet, it lies lazily down, but on any dis- 

 turbance, such as would be made by the homeward return 

 of the caterpillar, our young pirate becomes active and 

 ready to fasten upon the expected caterpillar. 



If you will examine a good-sized web-worm or gypsy 

 moth caterpillar you will find that it is well protected by its 

 hairy covering along its back and sides. But on the lower 

 surface, the skin is more exposed, and it is obvious that this 

 Tachinid larva lying in wait with its stiletto will find the 

 caterpillar as it crawls above it an easy victim for its hook. 

 When the mouth hook is thrust through the skin, the pull 

 of the caterpillar loosens the hold of the parasite upon the 

 membranous case at the hind end of its body and the par- 



