388 CHRISTINE ESSENBERG 



resultant greater need for oxygen for the metabolic changes 

 which are taking place, hence the bugs rise to the surface in 

 order to obtain the oxygen from the atmosphere. That the 

 bugs remain at the bottom of the aquarium in spite of the high 

 temperature, indicates that there are two opposite forces in 

 action, viz., the need of oxygen and the positive phototaxis. 

 The latter is increased with the increase of the temperature 

 and is evidently so strong that the animals, after they have 

 once been attracted by the light, can not depart, but struggle 

 to get to the light at the bottom until they die, while, after 

 they have once departed from the source of light the tendency 

 of the bugs, with the increase in temperature, is to gather at 

 the upper light. And even after the upper light is extinguished 

 and the lower light is shining, the bugs still persist in remaining 

 at the surface, because the demand for oxygen is greater than 

 the positive phototaxis. The bugs are probably led by instinct 

 to seek the surface when they become aware of the need of 

 oxygen, and again they are only led by their strong positive 

 phototaxis to go to the bottom of the aqtiarium. In normal 

 conditions the Notonectas rise to the surface at more or less 

 regular intervals. The same is true in diffused light when the 

 temperature is increased, the bugs always rise to the surface 

 and remain there. The buoyant force plays an important role 

 here, but it may be of secondary importance only. 



If the insects are sealed in ordinary water, they first swim 

 about, but later they remain at the surface all the time. They 

 may go down for a moment but return immediately to the 

 surface, usually remaining there until they die. 



If Notonectas are sealed in water from which the oxygen has 

 been expelled by boiling, the air carried by the bugs is absorbed 

 by the water, hence the insects drop to the bottom and do not 

 rise, although they try hard to reach the surface. In this con- 

 dition they die in from five to ten minutes. In this case buoyant 

 force is of least importance and it is only this specific response 

 which leads the animals to the surface in their quest for oxygen. 



If Notonectas are placed in boiled water in an open dish they 

 immediately come to the surface, remaining there until the 

 amount of oxygen in the water has increased. When a light was 

 placed below the aquarium containing boiled water, the Noto- 

 nectas collected around it, the majority of them dying in a few 



