STRUCTURE AND CLASSIFICATION. 



27 



CHAPTER IV. 



MUSCLES, DIGESTION, AND CIRCULATION. 



Insects have no internal skeleton or bony structure, the body 

 wall serving for the attachment of muscles and to protect the 

 inner organs. 



Cutting transversely through the thorax of an insect at its mid- 

 dle, we find the greatest portion of the body cavity occupied by 

 muscles, which are arranged in bundles attached to the entire 

 inner surface of the body wall, proceeding to all the appendages, 

 and thus providing the power that moves the animal. Centrally, 

 the alimentary canal occupies a part of the body cavity, its size 



Fig. 12. 



Ideal section through an insect.— a, alimentary canal; A, heart; «, nerve-cord; s, stig- 

 mata ; t, tracheal tubes ; /, legs ; w, wings. 



depending considerably upon the kind of insect in hand. Just 

 below the dorsal surface is a small tubular structure, at once the 

 heart and only blood-vessel, while just above the ventral surface 

 is a white, knotted cord, — the nervous system. Laterally, there 

 is an opening on each side, from which a ringed tube leads into 

 the body, dividing and subdividing almost immediately into 

 innumerable branches, and this is the respiratory or breathing 

 system. Frequently we find around the alimentary canal a con- 

 siderable fatty mass, while everywhere through the insect body 



