578 General Notes. [June 
the surface. The ventral aspect of the body at such times is 
covered with a film of air; and the space between the head and 
prothorax, and that between the prothorax and mesothorax, are 
also filled with air, as well as a space beneath the wings. t 
the quantity of air thus carried about by the insect is insufficient 
to account for the long period during which it remains beneath 
the surface of the water. By observing a Corisa when anchored 
near the bottom of a pond or aquarium, it will be seen that it 
clings by means of its anterior legs, and that the posterior legs 
are extended laterally and are frequently moved as when swim- 
‘ming. The result of this movement is to cause a current of 
little, and is not, therefore, purified to any considerable extent by 
the free gases in the water. Correlated with this are habits very 
different from those of Corisa. The Notonecta, when at rest, 
almost invariably floats with the tip of its abdomen projecting 
from the water, in order that it may easily change the air under 
its wings. 
that the Notonecta can, in an emergency, avail itself of a method 
_ of purifying its supply of air, which, with the Corisa, is normal. 
_ I doubt, however, whether the Votonecta could keep itself alive 
in this way for a long period —¥ H. Comstock. 
