QL ITOES 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 tir. The lack of legs gives a cylindrical simplicity of form to most fly larvae, which hot only makes these insects look lik, e worms, but bas caused many of them to lire the lire of
/ ml? LTpa An
Fro. 171. Structure of a fly larva, or maggot anus; .ISp, anterior spiracle; DTra, dorsal tracheal trunk; LTra, lateral tracheal trunks; mb, mouth hooks; PSp, posterior spiracle
a worm and to adopt the wavs of a worm. In compensa- tion for the loss of legs, the fli" larvae are provided with an intricate system of muscle fibers lying against the inner surface of the body wall, which enables them to stretch and contract and to make all manner of contortionistic twists. At first thought it seems remarkable that a soh-bodied, wormlike creature can stretch itsel( by muscular contrac- tion. It must be remembered, however, that the body of the larva is filled with soif tissues, many of which are but loosely anchored, and that the spaces between the organs are fil'led with a body liquid. The creature is, therefore, capable of pe.rforming movements by making use of its structure as a hvdraulic mechanism; a contraction of the rear part o( the body, for example, drives the body liquid and the soit movable organs forward, and thus extends the anterior parts of the bodv. A contraction of the length- wise muscles then pulls up the rear parts, when the more-
[325]
INSECTS
