THE HABITS OF NOTONECTIDAE 389 



minutes, while a few repeatedly returned to the surface for 

 oxygen, sometimes picking up their dead companions on the 

 way and carrying them to the surface. These experiments 

 were tried with adult Notonectas and with nymphs with the 

 same results. 



Cold does not destroy the phototaxis of Notonectas, but when 

 the insects become chilled, they move more slowly toward the 

 light. If kept in ice-water for some time, the insects become 

 so chilled that they drop to the bottom. If the water is shallow' 

 the insects come to the surface when the temperature is increased 

 but if the water is from fifteen to thirty cm. in depth, while they 

 show signs of life as soon as the temperature of the water is 

 increased, they fail in their attempts to reach the surface. Some- 

 times they succeed in rising a few inches, swimming obliquely, 

 falling back after each successive effort. It is an interesting 

 fact that, while under ordinary circumstances the bugs must be 

 in a constant motion in order to remain beneath the surface, 

 here the reverse is true, the bugs working in the greatest effort 

 to reach the surface and falling back each time. Evidently they 

 have lost their air in some way or another. Thus the animals 

 may lie until an increase in temperature arouses them to greater 

 activity. Metabolism goes on, the remaining oxygen is used 

 up, and when the animals attempt to rise there is no surround- 

 ing air to buoy them up, and death from lack of oxygen is the 

 result. While in shallower water they have more access to free 

 oxygen and can more easily reach the surface of the water. 

 Hoppe believes that the cold water dissolves the carbon dioxide 

 more readily and that, therefore, the animals, losing the sur- 

 rounding gas, are rendered heavier and sink down. The fact 

 of the bugs' trying to reach the surface, leads one to believe 

 that the effort is an instinctive one when the animals are in 

 need of oxygen. 



If Notonectas are exposed to an arc light they first show a 

 tendency to negative phototaxis for a moment, then they fly 

 to the light and are burned. Young nymphs are positively 

 phototactic from the very first day. I have experimented with 

 light reactions on Notonecta insulata which were only three 

 or four hours old. When exposed to light in a dark room, they 

 crowded to the lighted side of the aquarium. 



Notonectas are negatively geotropic and are usually found at 



