ANATOMY AND PHYSIOLOGY 



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9. RESPIRATORY SYSTEM 



In insects, as contrasted with vertebrates, the air itself is 

 conveyed to the remotest tissues by means of an elaborate sys- 

 tem of branching air-tubes, or tracheae, which receive air 

 through paired segmentally-arranged spiracles. Each spiracle 

 is commonly the mouth of a short tube which opens into a 

 main tracheal trunk (Fig. 167) extending along the side of 



Diagrammatic cross section of the thorax of an insect, a, alimentary canal; d, 

 dorsal vessel; g, ganglion; s, spiracle; w, wing; i, dorsal tracheal branch; 2, visceral 

 branch; 3, ventral branch. 



the body. From the two main trunks branches are sent which 

 divide and subdivide until they become extremely delicate 

 tubes, which penetrate even between muscle fibers, between the 

 ommatidia of the compound eyes and possibly enter cells. In 

 most cases each main longitudinal trunk gives off in each seg- 

 ment (Fig. 1 68) three large branches: (i) an upper, or dor- 

 sal, branch, which goes to the dorsal muscles; (2) a middle, 

 or visceral, branch, which supplies the alimentary tract and the 

 reproductive organs; (3) a lower, or ventral, branch, which 

 pertains to the ventral ganglia and muscles. 



In many swiftly-flying insects (dragon flies, beetles, moths, 

 flies and bees) there occur tracheal pockets, or air-sacs, which 



