30 RESPIRATORY ORGANS. 



The want of a vascular system also renders the 

 mode in which respiration is effected very different 

 from that in which this necessary function is per- 

 formed by most other air-breathing animals, in which 

 the blood, deteriorated by use, is carried to and from 

 the heart by a special set of vessels, which pass 

 through the breathing organs (or lungs) where the vital 

 fluid is exposed to the action of the air. Of course 

 nothing of this kind can take place in insects, and we 

 find that in them the function of respiration is effected 

 by the agency of numerous minute tubes, called 

 tracheae, which spring from openings on the sides of 

 the body, and ramify to all parts of the interior, thus 

 enabling the air to penetrate freely to the most hidden 

 organs. Thus the blood, during the whole course of 

 its passage through the minute spaces left between the 

 internal organs and the fatty mass which fills up the 

 cavity of the body, is constantly exposed to the influ- 

 ence of the air, and the specific gravity of the insect 

 is at the same time greatly reduced, a matter of no 

 small consequence to creatures destined to pass a con- 

 siderable portion of their existence in the air. Insects 

 have often been regarded as the representatives of the 

 birds amongst articulated animals, and this fact of the 

 penetration of air to all parts of their bodies speaks 

 strongly in favour of such a view, for it is well known 

 that in birds, the lungs communicate with numerous 

 air-sacs imbedded amongst their internal organs, and 

 even penetrating to the cavities of their bones. 



The apertures by which the tracheae or air-tubes 

 communicate with the external air are called stigmata, 

 or spiracles. They are usually placed one on each 

 side of every segment, with the exception of the 

 head, and are furnished with a muscular apparatus 



