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 



