RESPIRATION OP INSECTS. 103 



organ for its aeration. If the Respiration of Insects were effected 

 in the usual manner, by means of lungs, or at the external sur- 

 face of the body, it would have been consequently extremely 

 incomplete ; but the disadvantage which would appear to be the 

 necessary result of this great imperfection in a function so import- 

 ant as the Circulation, does not really exist. Nature has made 

 up the deficiency in the transmission of the blood, by conducting 

 the air itself into all the parts of the body, by the aid of a multi- 

 tude of canals, which communicate with the exterior, and which 

 ramify minutely in the substance of the organs. These air- 

 conveying tubes, known, as we have already said, under the 

 name of trachea, present a very complicated structure : we 

 can usually distinguish in them three coverings, 

 of which the middle one is composed of a carti- 

 laginous filament, rolled in a spiral, like an 

 elastic spring. Sometimes they are simple ; 

 but at other times they have a certain number 

 of large dilatations, in the form of soft vesicles, 

 which act as reservoirs of air. The openings by 

 which the air penetrates into the tracheae are 

 TUBE OK INSECT, called stigmata or spiracles ; they are generally 

 simple slits, like button-holes ; but sometimes 

 they have two valves, which open and shut like the leaves of a 

 folding-door ; and they are frequently furnished with a kind of 

 sieve or grating, to prevent particles of dust, &c., being drawn 

 inwards by the air. We usually see one pair upon the lateral 

 and upper part of each segment ; but they are often wanting on 

 the two last segments of the thorax. The means by which the 

 air is renewed in the interior of this respiratory apparatus, 

 appears to consist generally only of the movements of contrac- 

 tion and dilatation of the abdomen. As we have already said, 

 Respiration is very active amongst these animals j they consume 

 a considerable quantity of air in comparison with their size, and 

 they quickly die when they are deprived of oxygen ; but when 

 they are in this state of apparent death, they may remain in it a 

 very long time, without losing the power of being restored to life. 

 620. The greater number of Insects produce but very little 



