Dec, 1920 Bulletin of the Brooklyn Entomological Society 133 



air. The other four have sternal hair which is hydrofuge at the 

 base, but not hydrofuge at the tip. The tips of these sternal hairs 

 actually extend beyond the air plastron and form a coat for the 

 plastron. This outer film of hair tips operates precisely like a 

 membrane: oxygen is absorbed through it from the water 

 (Brocher). A bubble coming in contact with such a " hair mem- 

 brane " is absorbed, it decreases in size until it disappears. Hy- 

 drophilus and all other plastron insects differ in that they fuse 

 an air-bubble with the plastron. 



Now, it is decidedly interesting that Hydrophilus tries to 

 achieve a similar result, i.e., oxygen absorption from the water, 

 from a different angle, but by the same means, namely by partly 

 aquifuge hair, on the antennae. The hairs have aquifuge bases 

 and miscible tips, — hence Hydrophilus is able to absorb oxygen 

 from the water by means of the antennae. In this fashion the 

 antennae constitute a tunnel leading the air to the base of the an- 

 tennae, and then to the plastron of the head. Hcenionia, it is in- 

 teresting to note, has both this antennal " hair membrane " be- 

 sides the sternal "hair membrane." The relation of these five 

 genera in their types of respiration may be indicated in the fol- 

 lowing table: 



With miscible plastron hair membrane, 

 Elmts i_e., sternal hair aquifuge at base, miscible 



Stenelmis [with water at tips, forming " hair mem- 

 Limnius brane " for absorption of oxygen in solu- 



tion in the water. 



With miscible C Hcemonia , 



"hair membrane ""{ Hydrophilus — plastron of ordinary type, i.e., hairs 

 on antennae L aquifuge to tips. 



6. The Entry of Air into the Closed System of the Embryo. 



One of the puzzling problems of respiration of aquatic insects 

 has been to ascertain how the air gets into the tracheae in the first 

 place. In the insect embryo the tracheae are filled with a serous 

 fluid. When the young larva leaves the egg, the fluid is still 

 within the tracheae. Suddenly these tracheae can be seen to fill 

 with air. This filling takes place rapidly, it is a matter of a few 

 seconds. The problem had been investigated by Weisman, Pla- 

 teau, Sadones, Calvert, Dewitz, and others, without a satisfactory 



