MOSQUITOES AND FLIES 



are represented by internal buds, which, when they enter 

 the period of growth during the early stage of metamor- 

 phosis, are turned inside out to form the legs of the adult 



fly. 



The lack of legs gives a cylindrical simplicity of form 

 to most fly larvae, which not only makes these insects look 

 like worms, but has caused many of them to live the life of 



Fig. 171. Structure of a fly larva, or maggot 



An, anus; ASp y anterior spiracle; DTra, dorsal tracheal trunk; LTra, lateral 



tracheal trunks; mh, mouth hooks; PSp, posterior spiracle 



a worm and to adopt the ways of a worm. In compensa- 

 tion for the loss of legs, the fly larvae are provided with an 

 intricate system of muscle fibers lying against the inner 

 surface of the bodv wall, which enables them to stretch 

 and contract and to make all manner of contortionistic 

 twists. 



At first thought it seems remarkable that a soft-bodied, 

 wormlike creature can stretch itself by muscular contrac- 

 tion. It must be remembered, however, that the body of 

 the larva is filled with soft tissues, many of which are but 

 loosely anchored, and that the spaces between the organs 

 are filled with a body liquid. The creature is, therefore, 

 capable of performing movements by making use of its 

 structure as a hvdraulic mechanism; a contraction of the 

 rear part of the body, for example, drives the body liquid 

 and the soft movable organs forward, and thus extends the 

 anterior parts of the body. A contraction of the length- 

 wise muscles then pulls up the rear parts, when the move- 



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