USEFUL HYMENOPTERA. 185 



many joints. The veins of the wings, when a submarginal 

 vein exists, form distinctly closed cells, but it may be absent 

 and the system reduced to one or two veins. 



Trochanters 2-ringed, tarsi generally 5-jointed. 



Abdomen frequently stalked, and in the female provided with 

 a long ovipositor, formed of a slender borer and two lateral 

 sheaths. 



Larrae soft and tapering at both ends, generally white, and 

 without hair or legs. 



Pupae with the limbs free, soft, and white. 



The season for the flight of these extremely useful insects 

 falls between May and August. The $ lay their eggs either on 

 or in other insects (Lepidoptera, beetles, and Hymenoptera), 

 which they pierce with their ovipositors, generally attacking 

 the larvae, less commonly the pupae, and seldom the perfect 

 insects. Certain minute species attack the eggs. Only the 

 larger larvae are attacked as a rule. An ichneumon will rarely 

 attack an insect which has already been pierced, 



The larvae appear soon after the eggs have been laid, and 

 may pass the winter in the pupae of the host. 



They pupate in cocoons, sometimes outside the host, some- 

 times enclosed in its own pupal skin ; the species of Pteromalus 

 alone form exceptions to this rule. The ichneumon-wasps cut 

 out a round piece from the cocoon to emerge, passing the 

 winter under moss, in stumps, etc. 



The whole series of transformations generally requires 

 8 6 weeks, and the generation is usually single, but sometimes 

 double. 



Ichneumon-wasps are shy, and run and fly rapidly ; they do 

 not, however, go far from their birthplace ; they may appear 

 in great numbers, and are constantly quivering their wings. 



Most of the larvae are parasitic within their hosts, whose 

 juices they suck, but some remain outside them (many species 

 of Pteromalidae and Chalcididae). Infested larvae continue 

 living, and eat ravenously in order to supply their parasitic 

 guests as well as themselves ; they do not, however, reach 

 maturity, but die either as larvae or pupae. As a rule, 

 ichneumon-wasps increase more than proportionally to the 

 injurious insects, Thus, in the valleys of the Rhine and 



