THE HABITS OF NOTONECTIDAE 387 



At temperature of sixteen to eighteen degrees the insects are 

 slower in their motions. At a temperature of twenty to twenty- 

 two degrees they become more active, and with a further increase 

 of temperature they are still more active, moving toward the 

 light hurriedly and remaining crowded there. In a temperature 

 of thirty-two degrees and above, the water bugs are slower in 

 their movements, dying in a temperature of forty to forty- 

 two degrees. 



With increasing need for oxygen as is the case when Noto- 

 nectas are placed in a high jar which is kept in diffused light 

 and the temperature gradually increased, the bugs always come 

 to the surface. But if the jar, containing the insects, is put 

 in a dark room and a light is placed below the jar and then the 

 temperature gradually increased as before, the bugs, attracted 

 by the light, remain at the bottom, becoming more and more 

 positively phototactic with the increase of the temperature, 

 beating their heads against the bottom of the aquarium until 

 they die in an effort to get nearer the light. Rarely one or 

 more escape from the center of attraction and rise to the surface. 

 If, however, two lights are placed, one above, the other below 

 the aquarium, and the temperature gradually increased, the 

 backswimmers rise to the surface and remain there even after 

 the light above has been turned off, regardless of the fact that 

 the light below is still shining. The last experiment indicates 

 that although the creatures are strongly positively phototactic, 

 after they have departed once from the source of light, the need 

 of oxygen overcomes their phototaxis. 



The question arises, Why do the insects rise to the surface 

 with the increase of temperature ? As I have explained above, 

 the animals carry air under the hair surrounding the body and 

 in the spacious air tubes. Thus they are rendered lighter than 

 the water and must keep their appendages in motion in order 

 to keep beneath the surface. The increase in temperature 

 brings with it an increase in the activity of the animal with a 



