INSECTS BENEFICIAL TO AGRICULTURE. 219 



numbers of injurious insects, they often as likely as not eat 

 the beneficial as well as the destructive kinds. Without doubt 

 the lead ing factor in preventing the undue increase of noxious 

 insects are the parasitic kinds belonging to the hymenop- 

 terous families Iclmeumonida?, Braconidas, Chalcididae and 

 Prototrupida?, and the dipterous family Tachinidse. 



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

 the caterpillar or bores under its skin, inserting an egg 

 within the body. Mr. Poulton has carefully watched a 

 Paniscus ovipositing on a caterpillar. It laid fourteen 

 eggs, firmly attaching them to its skin, 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 larvae, none could arrive at 

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

 mon lays a smaller number of eggs on small caterpillars 

 than on large ones, and yet in all cases lays more than can 

 develop. 



The larva of the ichneumon upon hatching works its way 

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

 muscles, nerves, or the vital parts of the caterpillar, but 

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

 ing the blood of its host. 



Many ichneumons are polyphagous, i.e., live on insects 

 of widely different species belonging to different orders; 

 others confine their attacks to a single species. Certain 

 chalcids are secondary parasites, living in the larvae of those 

 parasitic in caterpillars, etc. Most ichneumons have but a 

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

 Ratzeburg observed a brood of Microgaster gloltatus early in 

 May, and another early in August. Though there may be 

 two broods of the hosts, there is as a rule but a single brood 

 of ichneumons. Ratzeburg indeed found that certain ich- 

 neumons parasitic on saw-fly larvge imitated the habit of the 

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

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

 belated cocoons. On the other hand, Pteromalus puparum 



