BEES, WASPS, ANTS, ETC. 



513 



sites. The female ichueuiuou-fly lays her eggs either in or upon the body of the insect 

 on wliicli her young are to feed. In the former case, the body wall of the victim is 

 pierced by the ovipositor; in the latter case, the egg is fastened to the outer surface 

 by a viscid substance. In some instances this viscid substance is drawn out in the 

 form of a thread, and the egg attached to the distal end of it. These stalked eggs are 

 usually attached to the head of the infested larva in such a position that the larva 

 cannot rid itself of them. When the eggs hatch, the young ichneumons soon penetrate 

 the body wall of their hosts. Here they feed upon the fatty portion of their victim, 

 so that it continues to live until they become fully grown. Many caterpillars infested 

 by these parasites live until after they have spun their cocoons and then perish. It is 

 an interesting fact that, even in these cases where the ichneumons are protected by 

 the cocoon of their host, each one spins for itself a cocoon before transforming to a 

 pupa. Lepidopterous cocoons are often found which are packed full of the cocoons 

 of ichneumons. These facts seem to indicate that these species of parasites have 

 descended from forms whose habits were such that they needed cocoons, and that 

 the cocoon-making habit has been retaiued when it is obviously of little or no use 

 to the species. 



In the case of caterpillars which die before spinning their cocoons, the ichneumon 

 I.-u-viS, when full grown, leave the body of the caterpillar and attach their cocoons to 

 the outside of it or to some neighboring object. This emergence from the caterpillar 

 frequently takes place before the host dies, and thus a feeble catei-pillar may often be 

 seen crawling about with the cocoons of its parasite attached to it. Although the 

 majority of ichneumon-flies infest the larva3 of other insects, some are egg parasites, 

 and a few infest pupfe. It should be borne in mind that each species of these para- 

 sites infests one or more particular species of insects. Thus, from a particular 

 species of insect we may expect to obtain only certain 

 species of parasites. The importance of this group of in- 

 sects, from an economic standpoint, is very great. The 

 undue increase of insects injurious to vegetation is pre- 

 vented by these parasites, perhaps more than by any other 

 agency. 



The above generalizations refer to the entire group of 

 ichneumon-flies, the Ichneumonidaj of certain authors. 

 This group is now divided into two families. One is 

 called the Braconidse, and for the other the name of 

 Ichneumonidae is retained. The best character for sepa- 

 rating these two families is by the nature of the venation 

 of the front wings. In the Braconidse there is only one 

 recurrent vein, while in the Ichneumonidse there are two. 

 This is illustrated by Fig. 640. A and B represent the 

 wings of Agathis and Bracon, two Braconid genera ; and 

 C and D the wings of Ophion and Metopius, two Tchneu- 

 monid genera. The letter a in each wing indicates the 

 margiuo-discoidal vein, and b the recurrent veins. In the 

 Bi-aconidjB the abdomen is often composed of only three or four segments; whilst in 

 the Ichneumonidae it has always at least five segments. 



As illustrating the Braconidie, we will cite only three genera, JSracov, 3Iicrogas- 

 ter and Aphidius. Fig. 641 is Br aeon palpebrator, greatly enlarged. Fig. 642 repre- 



FiG. 640. — Wings of Braoonuise and 

 Iclineumonidse. A, Agathis ; B^ 

 Bracon,- C, Ophion; D, Metopius. 



VOL. II. 



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