HYMENOPTERA. 237 



sheaths which, when united, form a tube, being convex 

 on the outer side and concave on the inner. In Fig. 

 185 the three parts are separated from each other, but 

 in Fig. 185, ^, which is a diagrammatic cross-section 

 of the three parts, their relative position is clearly 

 shown. 



Many ichneumon-flies have a short ovipositor (see 

 Fig. 185, X, Eiphosoma), and some of these lay their 

 eggs on the skin outside or within the bodies of cater- 

 pillars. When hatched, the larval flies feed upon these 

 caterpillars. Sometimes the latter pass into the chrys- 



Fig. :85, X. Fig. 185, a. 



alls state, but they cannot escape, and the ichneumon 

 parasites finally feed upon their internal organs. The 

 larvae are footless ; they pass the pupa state within the 

 integument of the caterpillar or chrysahs, and emerge 

 as winged insects. These flies are really very useful in 

 killing harmful insects, such as Pieris rapce, " canker- 

 worms," and the like. Figures of many species of 

 Ichneumonidae are given in Snellen Van Vollenhoven's 

 Pinacographia. 



