BY R. J. TILLYARD. 435 



for some hours after death or dissection. This seems to be clear 

 proof that their contained gas is at a pressure equal to, or slightly 

 lower than, that of the atmosphere. In any case, what more is 

 needed as a driving force than the steady consumption of oxygen 

 by the tissues ? If this consumption is rapid, surely the play of 

 the rectal pulsations will increase correspondingly; more water 

 will pass over the gills, and more oxygen will pass into the 

 tracheae from it. If the consumption is slow, the rectal pulsa- 

 tions will slow down also. Here indeed we note a beautiful 

 interplay of action. For the larva uses the expulsion of water 

 from the rectum as a means of propulsion through the water, and 

 thus economises its store of energy by utilising the very means 

 which supply it with oxygen ! 



The elimination of Nitrogen. — TYiough. this paper is not pri- 

 marily concerned with this problem, it seems advisable to point 

 out that the question offers little difficulty, if we accept the 

 Diffusion-Theory. For this gas will only diffuse into the capil- 

 laries in sufficient quantity to keep its partial pressure equal on 

 both sides of the gill- wall. As the free nitrogen in the body of 

 the larva is used little, if at all, in the process of metabolism, 

 very little of the gas, if any, will diffuse inwards. There is, 

 therefore, no question of the elimination of immense quantities 

 of this gas, such as puzzle our minds in the case of the Open 

 Tracheal System. 



The eliTYiination of Carbonic Acid Gas. — There seems to be 

 little doubt that this is chiefly dissolved in the blood, whence it 

 passes to the exterior through the chitin of the integument. A 

 small amount probably gets into the tracheae by diffusion from 

 the hsemoccele, but the quantity would never be sufficient to 

 affect the steady diffusion of oxygen from the gills in a supply 

 sufficient for all requisite purposes. 



In conclusion, we may say that what is now wanted, in order 

 to make this interesting problem clearer, is a careful and accu- 

 rate analysis of the gas contained in the large tracheal trunks of 

 an Anisopterid larva (care being taken to exclude the carbonic 

 acid gas present in the blood). Analyses made of this gas, in 

 the case of certain insects with an Open Tracheal System, show 



