OVIPOSITOR IN ACTION 23 



clears the sheath, passing, however, through 

 the extreme tip, which holds it in place, hori- 

 zontally into the tree. This explanation is 

 necessary because the casual observer would 

 be unlikely to notice any happenings from the 

 time the fly first took up her position — the 

 ovipositor, to all appearance, being just as it 

 was ; however, it is now an empty sheath. 



That such an operation is possible is due to 

 the extraordinary strength and flexibihty of 

 the ovipositor, which is not only a drill but a 

 tube, and so sensitive that at the moment it 

 comes in contact with the larva, the ichneu- 

 mon is prompted to place her egg in the skin 

 of the creature. When this is accomplished 

 the ovipositor is withdrawn ; coming out in 

 the same loop-form, it springs back into the 

 sheath and the fly goes off in search of another 

 victim. 



Now is the Sirex doomed, though not to 

 immediate destruction, for it must hve to 

 nurture the parasite during the latter's larval 

 period. Thus, both grow together, the Sirex 

 hving on the wood of the tree and the parasite 

 deriving nourishment from the fatty tissues 

 underlpng the skin of its host. When both 

 are full fed and the tunneUing operations have 

 ceased, the Sirex larva pupates, including the 

 parasite within its chrysahs. Whereupon the 



