HYMENOPTERA. 151 



and at the expiration of its pupal stage, intends to gnaw 

 its way out to freedom, air, and sunshine; there to repeat 

 the life cycle of its parents. But the big Thalessa seems 

 to know where to find this fat, juicy, horntail grub, and 

 she stops above where it is lying, deep down under the 

 bark of the tree, throws her long ovipositor in a wide loop 

 over her back, downward into the tree trunk, and begins 



FIG. 59. Oviposition of Thalessa lunator. (Natual size. Folsom, after Riley.} 



boring straight down. How the mother ichneumon 

 knows the horntail grub is under the bark cannot be 

 determined; it is the manifestation of a marvelous power 

 to locate that which cannot be seen. You and I might 

 find the horntail tunnel in the outer bark, but we should 

 be unable to tell which way the larva had turned on its 

 way inward. (Fig. 60.) The writer found in the summer 

 of 1908, four of these horntails unsuspectingly drilling into 

 a box elder trunk ; while around on the other side of the 

 same tree were two of the Thalessas, also engaged in drill- 

 ing (but not so unsuspectingly) , providing for the wants of 



