INSECTS. 371 



the table, they were fully awake and livel) T . The half torpid 

 animals consume about three times, and the perfectly wakeful 

 animals nine times the amount of oxygen required by those which 

 were in a perfectly rigid state. The same relation exists in respect 

 to the absolute quantity of the excreted nitrogen,, although we 

 very frequently meet with the opposite condition in these animals, 

 namely, with absorption of nitrogen, as was several times noticed 

 in frogs by Regnault and Reiset. 



We now proceed to investigate the products of respiration of 

 those animals which do not respire through lungs, namely, insects 

 and fishes. Although the former of these absorb atmospheric air 

 directly, the mechanism of inspiration and expiration is not the 

 same as in lung-breathing animals. Their pneumatic apparatus 

 consists of extremely elastic ramifying tubes, intersected by 

 vessels of communication designed for the uniform distribution of 

 the air. The expiration in insects is effected by muscular action 

 only, while the act of inspiration is accomplished solely through 

 the elasticity of the tracheal walls, which not only consist of chitin, 

 but are surrounded by a spiral thread of that substance for the pur- 

 pose of increasing their elasticity. The air in the tracheae is 

 brought into free and direct contact with the external atmosphere 

 by means of the so-called stigmata, in which there is not often any. 

 appearance of muscularity. By every motion of the insect the 

 universally distributed tracheae are compressed, and a portion of 

 the air which they contain is thus expelled ; when the muscular 

 contraction ceases, the tracheae, in consequence of their extreme 

 elasticity, resume their former volume, and fresh air again enters 

 through the open stigmata into the spaces containing rarefied air. ; 

 Insects are also provided with a special muscular apparatus for 

 expiration, but this is limited to the abdominal rings, and exhibits 

 even in beetles only 15 or 25 contractions in a minute, correspond- 

 ing pretty nearly with those of the dorsal vessel. The voluntary 

 and irregular motions undoubtedly exert the most important 

 influence on the expiration of the air in insects, and hence we find 

 that the various degrees of animation in the motions of insects pro- 

 duce the most extraordinary differences in respect to the quantity 

 of carbonic acid which they excrete, and the degree of animal heat 

 which they exhibit. On this account pupae expire only 1-1 90th or 

 1- 160th part of the carbonic acid which is exhaled by a caterpillar 

 of equal weight ; but yet, according to Regnault and Reiset, the 

 consumption of oxygen by the larva of the silkworm is only 

 about l-2()th less than that by the caterpillar. 



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