48 



AUSTRALASIAN 



RESPIRATORY ORGANS. 



It has been already mentioned, as one of the main charac- 

 teristics of the class Insecta, that they breathe air through a 

 peculiar system of tubes. These tubes are called trachea, and 

 are of a very beautiful formation. They are thus illustrated 

 and described by Mr. Cheshire : — 



Fig. 9.— TRACHEA, MAGNIFIED. 



6. Elastic Spiral of Trachea. 



" The tubes consist of two membranes, between which lies 

 the elastic spiral thread, which prevents the closing of the tube 

 through movement. In the same intervening space the fluids 

 of the insect become aerated, so that the purpose of the lungs 

 of the higher animals is answered." The outer openings of 

 these tubes, in the sides of the insect, are called spiracles. The 

 bee has fourteen of these spiracles, two on each side of the 

 thorax, and five on each side of the abdomen. The trachea 

 expand into large lung-like sacs in the abdomen of the bee, as 

 shown in the following illustration, where a represents one of 

 the air sacs, bbb the spiracles, and c c some of the trachea. 



