INSECTS /:. !M TO AGBIOULTUR 219 



numbers of injurious insects, thej often as likely as not 

 the beneficial as well as the destructive kinds. Without doubt 

 the leading factor in preventing the undue increase of noxious 

 insects are the parasitic kind- belonging to the hymenop- 

 terous families [chneumonidae, Braconidaa, Chalcididae and 

 Prototrupidae, and the dipterous family Tachinidae. 



An ichneumon-flj lays its eggs either on the outside of 

 the caterpillar or bores under its skin, inserting an 

 within the body. Mr. Poulton has carefully watched ;i 

 Paniscus ovipositing on a caterpillar. It laid fourl 



. firmly attaching them to its .-kin. most of them in 

 the sutures between the segments, and on the sides of the 

 body. An excess of eggs are laid, since some do not 

 develop; for if all gave out larva-, none could arrive at 

 maturity within the body of the future Lost. The ichneu- 

 mon lays a smaller number <>!' eggs ou .-mall caterpillars 

 than on larg -. ami yei in all cases lav- more than can 



develop. 



The larva of the ichneumon upon batching works its way 

 into the interior of its host. Here it does not injure the 

 muscles, nerves, or the vital part.- of the caterpillar, hut 

 apparently simply lies motionless in the body-cavity, absorb- 

 ing the blood of it- host. 



Many ichneumons an- polyphagous, i.e., live mi in-. 

 of widely different species belonging to different orders; 

 others confine their attacks to a single spec • -. Certain 

 chalcids are secondary parasite-, living in the larvae of those 

 parasitic in caterpillars, etc. Most ichneumons have but a 

 single generation; a few are double- brooded. In Germany 

 B seburg observed a brood of Microgasti - earlyin 



May, and another early in August Though there may be 

 two broods of the hosts, there is as a rule but .1 single bi 

 of ichneumons. Ratzeburg indeed found t!. lin ich- 



neumons parasitic on saw-fly larva' imitated the habit <.f the 

 latter of living more than a year. Le., they did not develop 

 until the greater number of saw-flies had I from the 



belated cocoons. On the other hand, PA 



