PHYSIOLOGY OF INSECTS. 135 



leaves the ground ; wings, therefore, would be an 

 incumbrance to it. We find that its pro- meso- and 

 metathorax are very small and insignificant seg- 

 ments, while the mesothorax of the productive 

 female is the largest segment in her body ; but it 

 is one of her duties to perform a long flight, and 

 to use the wings which that segment bears. 



503. The muscles in those wing-bearing seg- 

 ments which do not employ their wings for flight, 

 are probably not absent, but repose in a rudi- 

 mental state ; while the muscles, whose active uses 

 are required in the neighbouring segments, are in- 

 creased at their expense. 



504. The form of muscles in insects is as various 

 as their size, and depends on the motion required 

 of them : every muscle is precisely of the shape, 

 as well as size, best adapted for the office it has 

 to perform. 



505. The muscles which serve to raise a leg 

 and lift it forwards, and all similar simple move- 

 ments, are linear or cylindrical when at rest, but in 

 contraction become fusiform, the extremities at- 

 tenuating, the centre incrassating, and the whole 

 being abbreviated. 



506. The muscles which serve to perform the 

 wriggling undulating motion, so common in larvae, 

 especially those which are without legs, are tri- 

 angular, and in motion contract alternately each 

 side of the triangle, so that each angle becomes 

 acute and obtuse in turn. 



