46 STUDIES IN GENERAL PHYSIOLOGY 
the older wingless animals I could notice an inclination to 
move toward the source of light. When their wings had 
sprouted, however, the orientation of the plant lice was 
extraordinarily definite. In this state they are perhaps 
the most suitable animals we have for demonstrating the 
phenomena of heliotropism. Not all species are equally 
irritable ; Cineraria afforded me the best specimens. I have 
never found a species of plant louse which was not definitely 
positively heliotropic. I kept the plants near a closed 
window. The animals were attracted by the sun to the 
window, where they crept upward. When the animals are 
lightly touched with the point of a pen, they fall down a 
second or two later. If a glass vessel is held under them, a 
large number of these animals can be collected in an unin- 
jured condition in a short time. I found it much better to 
work with such animals as have already flown from the 
plant, than to collect the winged animals from the plant 
itself. To obtain the winged plant lice in great numbers it 
is necessary only to allow a plant which is covered with them 
to dry out gradually. Under such conditions the wings 
grow out very rapidly. 
All the experiments which were made with Porthesia 
chrysorrhoea can be repeated with exactly similar results 
on winged plant lice contained in a test-tube. 
As in the heliotropism of caterpillars, the heliotropism of 
plant lice is determined chiefly by the more refrangible rays, 
which compel the animals to move in the direction of the 
rays toward the source of light. If we place the test-tube 
containing the animals on a horizontal table, they always 
move toward the source of light, whether this be lamplight, 
diffuse daylight, or direct sunlight. The orientation occurs 
the more rapidly the more intense the light. If the intensity 
of the light is constant, the plant lice, like the caterpillars of 
Porthesia chrysorrhcea, are compelled to remain perma- 
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