16 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION^. 



to a single species. Most ichneumons have but a single generation; 

 a few are double-brooded. In Germany, Ratzeburg observed a brood 

 of Microgasfer glohatus 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 

 ichneumons of saw-fly larvie 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 piiparum undergoes an extraordinarily rapid growth; it 

 stings early in June the chrysalids of Vanessa polj/chloros, and by the 

 middle of July the adults appear. Teleas ovulorum requires only four to 

 six weeks to develop; it however flies somewhat later, so as to find the 

 suitable objects on which to lay its eggs. 



Ichneumons rarely develop in adult insects, but certain Braconids 

 infest Cocciuella beetles. The small Chalcids, i. e., Pteromali, mostly 

 inhabit the tender pupie of bark-boring beetles and leaf-rollers. 



Among the smaller ichneumons several females usually inhabit a 

 single host, while from 000 to 700 individuals of Pteromalus puparum 

 may inhabit a single chrysalid, and 1,200 Apanteles a Sphinx larva. 



Most ichneumons develop within their hosts, but mauj' species of 

 Chalcids live on the outside and suck the blood of their host. The 

 ichneumon larvae living within their hosts often undergo the most 

 remarkable transformation of their mouth-parts. In Microgasfer glohatus 

 there are, at first, only the wart-like rudimentary sucking mouth-parts; 

 but after the last molt the larvae acquire ordinary biting mandibles, 

 with which they can gnaw through the skin of their host. However, 

 the food of the ichneumon larvae is wholly fluid, their mouth-parts not 

 allowing them to eat the fat-body of their host. 



Other parasitic insects are the larvae of the Tachina flies, a group 

 closely allied to the common house-fly. Tbe larvae are true maggots, 

 footless, and take their food by suction through the mouth, the mouth- 

 parts being very rudimentary. The Tachina (Senometopia) militaris 

 has been observed by Riley to lay from one to six eggs on the skin of 

 the army- worm, "fa&tening them by an insoluble cement on the upper 

 surface of the two or three first rings of the body." The young mag- 

 gots in hatching penetrate within the body of the caterpillar, and lying 

 among the internal organs absorb the blood of their unwilling host, 

 causing it to weaken and die. 



Other insectivorous insects are the Aphis-lions, the young of the lace- 

 winged flies Ghrysopa and Hemerobius, which are frequently found in 

 trees among plant-lice; also Carabid beetles. 



Artificial breeding of parasitic and predaceous insects. — Among the most 

 important preventive measure against the wholesale ravages of insects 

 is the artificial breeding of parasitic insects. We early advocated this 

 in dealing with the Hessian-fly and wheat midge, suggesting the im- 

 portation of the European parasites of the latter species in straw. Dr. 

 Le Baron has experimented with the parasites of the apple bark-louse. 



