380 THE TRACHEAL SYSTEM OF THE IMAGO. 
tracheal walls and the skin of insects is a complex phenomenon, 
and depends partly upon the solution of the gas, and partly 
upon transpiration. The external chitinous integument prob- 
ably acts like a porous plate of gypsum, whilst its deeper 
layers, and the intima of the trachez, are moist films. The 
condition and tension of the carbon dioxide, of the oxygen and 
of the nitrogen of the absorbed air are different, and need 
separate consideration. 
Absorption of Oxygen.—If, as I hold, the air is pumped into 
the trachez by the contractile vestibules, the condensed gas is 
placed under circumstances which are especially favourable for 
its solution in the blood. The oxygen, once in solution, is 
probably immediately appropriated by the tissues, in which the 
oxygen tension, if we may judge by the phenomena observed in 
vertebrates, is reduced to zero; the blood is therefore always 
ready to absorb more oxygen, and the percentage of oxygen in 
the trachez would therefore, probably, always be very low, as it 
is in the alveoli of the vertebrate lung. It is probable that all 
the oxygen is capable of being absorbed from the trachee— 
just asa mouse in a confined space absorbs all the oxygen 
before it dies. 
The Action of Tracheal Gills has always offered a difficulty, 
which, so far as I know, has as yet remained unexplained ; but this 
difficulty vanishes as soon as the low percentage of oxygen in 
the trachez is admitted ; as the oxygen, dissolved in the water, 
has the same tension as the oxygen in the air it would diffuse 
into the tracheal vessels, where they are exposed to the water 
in the gills. 
Such diffusion would not occur into and distend empty tubes 
or tubes containing fluids, but Dewitz [159] has, I think, 
established the fact that the so-called closed tracheal tubes of 
these insects are not closed; at least the anterior thoracic 
spiracles are open; and it appears probable that air is taken in 
from time to time, so that the tubes always contain a gas, into 
which oxygen diffusion from the water is possible. It is almost 
certain that the tracheal gills are only accessory respiratory 
organs, serving to renew the oxygen in the air, contained in the 
