vi] LARVAE AND THEIR ADAPTATIONS 73 



brown plate, traversed by three slits. And as we 

 watch this curious degraded larva thrusting its nar- 

 row head-end into the depths of its ofttimes loathsome 

 food-supply, we understand the advantage of access 

 to the air-tube system being mainly confined to the 

 hinder end of the body. 



Maggots, differing from that of the Bluebottle 

 only in minor details, are the larval forms of a vast 

 multitude of allied species and display great varia- 

 tion in the nature of their food. Most, however, hide 

 their soft defenceless bodies in some substance which 

 affords shelter as well as food. The Bluebottle mag- 

 got burrows into flesh, that of the House-fly into 

 horse-dung or vegetable refuse. The maggot of the 

 Cabbage-fly eats its way into the roots of cruciferous 

 plants, that of the Mangel-fly works out a broad 

 blister between the two skins of a leaf, into which 

 the newly-hatched larva crawls directly from the egg. 

 A large number of species, forming an entire sub- 

 family (the Tachininae) have larvae that feed as 

 parasites within the bodies of other insects. 



The habit of parasitism by maggots in back-boned 

 animals has led to some remarkable modifications of 

 the larva and to curious adventures in the course of 

 the life-story. The Bot-fly of the Horse (Gastrophilus 

 equi) and the Warble-fly of the Ox (Hypoderma 

 bovis, fig. 22) lay eggs attached to the hairs of 

 grazing animals, which, at least in the case of 



