GROWTH AND TRANSFORMATION 183 



of the highly nutritious food of these grubs and their pro- 

 longed period of residence in a crowded nest or hive. 

 Similarly the muscoid maggot, with its front region tapering 

 towards the head armed with strong mouth-hooks, is excel- 

 lently adapted for burrowing into the mass of its foodstuff — 

 the bluebottle's larva, for example, into soft flesh, the house- 

 fly's into horse-dung or garden refuse, the cabbage-root 

 fly's or the mangel fly's into its appropriate plant-tissue ; 

 and in many cases it is easy to recognise the advantage of 

 restricting the functional spiracles to a pair of large ones 

 at the tail-end which remains nearest to the free surface of 

 the food-mass within which most of the maggot's body is 

 buried. In the larvae of several groups of flies such as the 

 gnats (Culicidae) and the drone-flies (Eristalis) these tail 

 spiracles are found at the end of a very short or elongate 

 hinder outgrowth of the body, enabling the grubs, which 

 live under water, to obtain contact with the atmosphere 

 through the surface-film and thus breathe the upper air 

 while they feed in a ditch or puddle which is possibly most 

 foul. 



The study of such a series of insect-larvae as we have 

 rapidly passed in review brings out clearly the striking 

 adaptation of each to its own manner of life during the 

 period of immaturity and growth. It also suggests that the 

 adaptations have been brought about by the divergence in a 

 less or greater degree of each larva from the form and con- 

 ditions of its parent. Those young insects such as grass- 

 hoppers, cockroaches, and bugs, which resemble their 

 parents closely in form, usually live in the same surroundings 

 as the adult and on the same kind of food. The fact of 

 larval adaptation to special life- conditions in conjunction 

 with the fact that an insect-larva's structure is comparable 

 with that of an adult rather than with that of an embryo, 

 suggests most strongly that the creatures during the early 

 stages of their life-history have diverged from the primitive 

 parental type, in many cases by degeneration, while the 

 adults have diverged by specialisation and elaboration. 

 This view is confirmed by consideration of our series of 



