The same shut. 



BREATHING MECHANISM. 161 



these branch tubes which open to the air, the 



large tubes send off a great number of smaller 



ones, which extend 



to all parts of the 



body, penetrating 



even to the legs and 



wings. In some in- 



sects, the spiracles, 



or breathing holes, 



^ 



are defended by a 

 pair of flaps, which swing to and fro, so as to 

 let in the air, and close upon it after each 

 inspiration. By this simple means the breathing 

 of insects is carried on ; the air enters the tubes, 

 passes along them, and is then conveyed to every 

 part of the body ; after which, having fulfilled its 

 duties, it escapes again. In man, and the higher ani- 

 mals, there is a separate place for the air in the 

 body, which we call the lungs, and here the blood 

 is conveyed to the air, in order to be purified. 

 But, as we have just seen, in insects it is very 

 different, for in them the air is conveyed by in- 

 numerable minute pipes to every portion of the 

 frame. Such is the breathing mechanism common 

 in Iarva3 which live in the air. 

 M 



