180 PHYSIOLOGY OP INSECTS. 



do not employ their wings for flight, are probably not ab- 

 sent, but repose in a rudimental state ; while the muscles, 

 whose active uses are required in the neighbouring seg- 

 ments, are increased at their expense. 



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. The muscles 

 which serve to raise a leg and lift it forwards, and all simi- 

 lar simple movements, are linear or cylindrical when at 

 rest, but in contraction become fusiform, the extremities 

 attenuating, the centre incrassating, and the whole being 

 abbreviated. The muscles which serve to perform the 

 wriggling undulating motion, so common in larvae, espe- 

 cially those which are without legs, are triangular, and in 

 motion contract alternately each side of the triangle, so 

 that each angle becomes acute and obtuse in turn. 



Muscles in insects more frequently cross and intersect 

 each other than in vertebrated animals : in some instances, 

 where connected with the organs of respiration and circu- 

 lation, they are excessively minute, and appear actually 

 interwoven, crossing in every direction, like multitudes of 

 slender threads. It is in such situations that dominant mo- 

 tion exists ; motion altogether independent of, and unin- 

 fluenced by, the will. 



The principal muscles in insects are generally in pairs ; 

 that is, each principal muscle has an antagonist muscle, 

 which, after any given movement, has the power to restore 

 the original position. The muscles destined for the govern- 

 ment of the limbs are all of the linear or cylindrical form, 

 and all in pairs, a flexor and extensor muscle being found 

 in each joint of the legs, &c. 



Nerves. — In vertebrated animals the brain is situated in 

 the head. Brain is supposed to be the seat of mind : all 



