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 Porth<-xi<t 

 chrysorrhoea can be repeated with exactly similar result* 

 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 chrysorrhoea, are compelled to remain perma- 



