THE HYMENOPTERA 345 



may often be seen during the summer on trees in which Horn-tail larvae 

 are present. These Ichneumon flies are called the " Long-tailed Thales- 

 sas." The female Thalessa (Fig. 361) crawls about over the trunk of a 

 tree which in some way she discovers is infested by Horn-tails, until a 

 satisfactory place is found, when she settles at that point and begins to 

 force her ovipositor into the bark and wood. The length of the ovipositor 



FIG. 361. Long-tailed Thalessa (Megarhyssa lunator Fab.): a, larva; c, pupa; e, adult 

 female; /, side view of abdomen of adult female, showing attachment of ovipositor; g, adult 

 male. About natural size. (Modified from Felt, N. Y. State Mus. Mem. 8; after Riley.) 



is suggestive of the distance it must be pushed in, in some cases, to reach 

 the tunnel of the Horn-tail larva, and it seems almost impossible for 

 such a slender structure to be forced so far through hard wood. When 

 the tunnel of the Horn-tail is reached, the Thalessa lays an egg in it and 

 then draws its ovipositor out of the tree. Sometimes this process results 

 in the death of the Thalessa, the ovipositor becoming so firmly fixed 



