RESPIRATION 



131 



connexion with this it should be recollected that the outer 

 integument consists of chitin, and is cast and renewed several 

 times during the life of the individual. Now as chitin consists 

 largely of carbon and nitrogen, it is evident that the moulting 

 must itself serve as a carbonaceous and nitrogenous excretion. 

 If, as is suggested by Bataillon's researches,^ the condition 

 accompanying metamorphosis be that of asphyxia, it is probable 

 that the secretion of the new coat of chitin may figure as an 

 act of excretion of considerable importance. If there be any 

 truth in this suggestion it may prove the means of enabling us 

 to comprehend some points in the development of Insects that 

 have hitherto proved very perplexing. 



Peyrou has shown ^ that the atmosphere extracted from the 

 bodies of Insects {Melolontlia) is much less rich in oxygen than 

 the surrounding atmosphere is, and at ordinary temperatm-es 

 always contains a much larger proportion of carbonic acid : he 

 finds, too, that as in the leaves with which he makes a comparison, 

 the proportion of oxygen augments as the protoplasmic activity 

 diminishes. Were such an observation carried out so as to dis- 

 tinguish between the air in the tracheal system and the gas in 

 other parts of the body the result would be still more interesting. 



We know very little as to the animal heat produced by 

 insects, but it is clear from various observations * that the 

 amount evolved in repose is very small. In different conditions 

 of activity the temperature of the insect may rise to be several 

 degrees above that of the surrounding medium, but there seems 

 to be at present no information as to the physiological mode of 

 its production, and as to the channel by which the products — 

 whether carbonic acid or other matters — may be disposed of 



In the order Aptera (Thysanura and CoUembola) the tracheal 

 system is highly peculiar. In some CoUembola it apparently 

 does not exist, and in this case we may presume with greater 

 certainty that transpiration of gases occurs through the integu- 

 ment : in other members of this Order tracheae are present in a 

 more or less imperfect state of development, but the tracheae 

 of different segments do not communicate with one another, 



1 C. M. Ac. Sci., cxv. 1892, p. 61, and Bull. Sci. Frame Belgiquc, xxv. 1893, 

 p. 18. 



^ Cmnpt. rend. Ac. Paris, cii. 1886, p. 1339. 



^ See Newport, Phil. Trans. 1837, and Lubbock Linn. Trans, xxiii. 1860, 

 p. 29, etc. 



