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 larve 
of Chrysorrhoea. 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 
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