THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 381 
tracheze, 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 trachez 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 trachee 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 trachez 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. 
