MOSQUITOES AND FLIES
are represented by internal buds, which, when they enter the period of growth during the early stage of metamorphosis, 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, 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 compensation for the loss of legs, the fly 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 soft-bodied, wormlike creature can stretch itself by muscular contraction. 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 hydraulic 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 lengthwise muscles then pulls up the rear parts, when the move-
[ 325 ]
