3o THE TR AC HEAL 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 tracheae, 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 tracheae 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 tracheae 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 tracheae- 

 just as a 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 trachese 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 Dewit/ [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 accessor)- respiratory 

 organs, serving to renew tin- oxygen in the air, contained in the 



