50 STUDIES IN GENERAL PHYSIOLOGY 



This time it took ten minutes for the animals to orient them- 

 selves a longer time, therefore, than in white light. 



At 10 : 29 I covered the test-tube with red glass, and since 

 I knew that in diffuse light the heliotropic movements take 

 place only very slowly under red glass, I brought the animals 

 at once into direct sunlight. It required seventeen minutes 

 before the majority of the animals had passed to the window 

 side of the mark. In diffuse light it required an hour for 

 orientation to take place under red glass ; in a new experiment 

 it required twelve minutes under blue glass. 



I noticed no periodic change in irritability in plant lice 

 such as that observed in Lepidoptera, but I did notice a 

 decrease in heliotropic irritability, a kind of rigor when the 

 animals have been left undisturbed in the dark for some 

 time. 



If the test-tube remained undisturbed, the animals 

 remained permanently on the side nearest the window. 

 When I very carefully turned the test-tube through an angle 

 of 180 in the daytime, the animals again moved toward the 

 window, even when they had been left undisturbed for 

 hours. When, however, I kept the animals quietly in the 

 dark, and after some hours carefully placed the tube near a 

 lamp, the animals did not move from the position which they 

 had maintained through the day. They seemed to be asleep. 

 But when I shook the tube so that the animals began to 

 move, they promptly oriented themselves toward the light as 

 often as I turned the tube around. 



I found that winged plant lice are negatively geotropic 

 as well as positively heliotropic, as is the case in the larvse 

 of Chrysorrhcea. If the animals in the test-tube were very 

 vigorous, a change in the position of the tube with reference 

 to the vertical brought about a change in the orientation of 

 the animals toward the center of the earth; they traveled 

 upward at as small an angle as possible with the vertical, and 



