266 A BOOK OF INSECTS 



near allies, they once laid their eggs in carrion or refuse, 

 and that the parasitic mode has been adopted in compara- 

 tively recent times. This view is supported by the addi- 

 tional fact that Tachinids usually lay their eggs upon any 

 caterpillar that they chance to encounter ; whereas a given 

 species of ichneumon commonly confines itself to one 

 kind of host. 



The maternal instincts of bot-flies (OEstridcc) lead to 

 parasitism of a peculiarly revolting kind. The eggs of 

 the warble-fly {Hypoderma), laid by the parent insect 

 upon the hairs of a cow or an ox, are licked off and swal- 

 lowed by the unsuspecting beast. The newly-hatched 

 larvze penetrate the wall of its oesophagus, and burrow 

 through the tissues until they arrive at points along the 

 back, just under the skin, where they complete their 

 growth. They eventually fall to the ground in order to 

 assume the pupa state. The presence of these " bots " 

 beneath the skin gives rise to more or less serious sores, 

 while the hides of badly infested animals are rendered 

 almost valueless on account of the holes made by the 

 parasites. The larvas of other bot-flies develop in the 

 stomach of their host, or in the sinuses of its head which 

 they reach through the nostrils. These habits appear very 

 remarkable when we remember that the adult insects 

 resort to their victims solely for the purpose of egg-laying. 

 They cannot suck blood or avail themselves of any other 

 kind of food, for their mouths are reduced to mere 

 functionless pores. Their sole office in life is to prepare 

 the way for a debased but pre-eminently safe existence on 

 the part of their progeny. 



It is a relief to turn from these sordid details to the 

 nesting habits of the higher Hymenoptera- True, among 

 wasps, we find much callous butchering ; but this is always 

 accomplished in such a straightforward manner that criti- 



