CHAPTER III 



MOVEMENT 



One of the facts most easily to be noticed by the observer 

 of living insects is that the vast majority of these creatures 

 are constantly moving. According to its kind, its inborn 

 habits, or its impressions received from the outside world, 

 an insect walks or runs, creeps or swims, jumps or flies. 

 These actions if carefully watched are seen to result from 

 the beautifully co-ordinated movements of various parts of 

 the exoskeleton. For example, when a beetle walks three 

 of its six legs — the front and hind legs of one side with the 

 intermediate leg of the other — are lifted and carried forward ; 

 then the feet of these three limbs press the ground, and the 

 other three legs are lifted and moved forward in their turn. 

 If the movements of a single leg are watched it will be seen 

 that the long stout segment or thigh is moved in relation to 

 the body and that the adjoining more slender segment or 

 shin moves in relation to the thigh, the angle between thigh 

 and shin being alternately acute (when the knee joint is bent 

 or flexed) and obtuse (when the knee joint is straightened 

 or extended). 



Such visible movements on the part of an insect are 

 due to the longitudinal contraction of the specialised tissue 

 that forms the muscles which, situated inside the legs or 

 other parts of the exoskeleton, are so attached to these that 

 by their power of longitudinal contractiHty they can move 

 the parts in relation to each other. The fibrous structure 

 of insect-muscles is readily seen on examination with a 

 hand-lens, and a small fragment of such muscle, suitably 

 treated, shows that it is composed of an enormous number of 



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