HELIOTROPISM OF ANIMALS 45 
Several days later the animals assume every possible position 
with reference to the vertical. They prefer to remain on 
vertical walls, yet they will creep just as readily into hori- 
zontal folds and crevices. 
VII. THE POSITIVE HELIOTROPISM OF PLANT LICE 
Anyone closely studying a rose covered with wingless 
plant lice will notice that they are arranged in a definite 
way on the plant. On a vertical stem they rest with the 
head downward; on the leaves they are usually found on 
the underside, mostly on the principal veins. Here one also 
notices a certain regularity in their orientation, in so far as 
the animals on the principal vein turn their oral poles toward 
the stem, and their aboral poles toward the point of the 
leaf. The orientation of the animals seems therefore to be 
controlled by the structure of the plant, and not directly by 
external forces. 
But the plant lice do not behave on all plants as on the 
rose. On a palm, for example, I found no such definite 
orientation of the animals toward the plant, even though in 
this case also they show a preference for the lower surfaces 
of the leaves. 
Yet it might seem reasonable to suppose that light or 
gravity compels the plant lice to seek the lower surfaces of 
the leaves. I twisted several leaves of Cineraria, the dorsal 
sides of which were covered with plant lice, so that the 
dorsal sides were directed upward and toward the window, 
and fixed the leaves in this position. I watched the animals 
for two days and found by actual count that the animals 
remained at rest. I repeated the same experiment on the 
plant lice of palm leaves, but also with negative results. 
My experiments on the orientation of new-born wingless 
plant lice were practically negative when I removed them 
from the plant and placed them in a glass vessel. Yet in 
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