AQUATIC BEETLES 



37 



in which the gills are bathed contains much dissolved 

 oxygen, and this oxygen is supposed to diffuse through 

 the delicate wall, while the carbonic acid formed within 

 the body passes out. Little 

 attention has been paid to the 

 circumstance that in a number 

 of aquatic Insects the tracheal 

 tubes become thereby filled with 

 a gas, probably rich in oxygen. 

 How this is accomplished I for 

 one cannot understand. I be- 

 lieve that no physicist would 

 undertake to set up an apparatus 

 which would without rise of 

 temperature or diminution of 

 pressure remove dissolved oxy- 

 gen from water, and store it up 

 in a gaseous form within a closed 

 chamber. To imitate the appa- 

 ratus of the Insect, there should 

 be no gas whatever within the 

 closed chamber to begin with. 

 In the fresh-hatched larva of 

 certain aquatic Insects, or in the 

 young pupa of others, tiny bub- 

 bles can be seen to form inside 

 the air-tubes, and gradually ex- 

 pel the watery fluid, with which 

 they were previously filled. 



The separation of a gas from 

 water or blood is not peculiar to Insects. Fishes 

 nil their swim-bladders, the Pearly Nautilus the 



Fig. 2. — Larva of Gyrinus 

 marinus. From Schiodte. 



