THE PHYSIOLOGY OF RESPIRATION AY INSECTS. 381 



tracheas, as fast as it is removed from them by the blood. If, 

 as appears probable, the oxygen tension in the fluids of the 

 insect is reduced to zero, the tendency of the general integu- 

 ment will be to admit oxygen through its porous substance. 

 With a pressure externally of 153 mm. of mercury, the normal 

 oxygen pressure of the air, and a pressure of zero internally, 

 unless the integument is impervious to air, oxygen must be 

 absorbed through it. The transpiration of oxygen through the 

 integument can hardly fail to occur, and is probably an im- 

 portant factor in the respiration of insects. 



Excretion of Carbon Dioxide. The received view, that expira- 

 tion occurs through the spiracles, is rendered untenable by the 

 physical difficulty involved in the entrance of the carbon dioxide, 

 formed by tissue metabolism, into the tracheal tubes. There 

 can be no doubt that the blood contains carbon dioxide in 

 solution at a high tension, and that diffusion must occur into 

 the adjacent air. That some may diffuse into the tracheae is 

 possible, but owing to the small quantity of air they contain, a 

 very small quantity would raise the carbon dioxide tension in 

 them to that of the blood, whilst the diffusion into the external 

 air, through the body wall, is constant, the carbon dioxide 

 tension of the surrounding air being practically zero. This 

 difficulty is so great, that the idea that the tracheae are con- 

 cerned in the excretion of carbon dioxide must be entirely 

 abandoned as soon as it is seriously considered. 



That the carbon dioxide tension in the blood is considerable 

 follows from the large percentage of carbon dioxide in the 

 gases which can be extracted from the insect ; and as its 

 transpiration through the integument, from this high tension 

 into the atmosphere in which the tension of carbon dioxide 

 is practically zero, must take place as a physical necessity ; 

 there is no difficulty in the view that carbon dioxide is excreted 

 from the general surface of the insect. 



As I have already shown, the quantity of carbon dioxide which 

 can pass into the tracheae is limited, whilst its escape from the 

 body surface is practically unlimited, and depends entirely on 

 the rate of transfusion under a pressure from within outwards. 



