70 STUDIES IN GENERAL PHYSIOLOGY 



X. THE NEGATIVE HELIOTROPISM OF THE LARVAE OF TENEBRIO 



MOLITOR 



The larvae of a beetle Tenebrio molitor, which can easily 

 be collected in large quantities, are also suitable animals 

 upon which to demonstrate negative heliotropism. When 

 such animals are placed on a plate, they creep to the room 

 side of it; if the intensity of the light is sufficiently great, 

 they remain there. If the plate be covered with dark-blue 

 glass, the result of the experiment is the same. If the plate 

 be covered with red glass, the animals move in the concave 

 edge of the plate both toward the window and away from it ; 

 a definite orientation does not occur. Under red glass they 

 behave just as in the dark ; under blue glass, just as in the 



light. 



I covered one-half of a plate with blue glass and one-half 

 with red glass, so that the boundary between them lay in the 

 direction of the rays. The animals were distributed equally 

 over the plate at the beginning of the experiment. All the 

 animals in the blue half moved to the room side of the plate, 

 but none in the opposite direction; in the red half they 

 moved in all directions. The animals moved from the blue 

 into the red, but never from the red into the blue. When I 

 covered one-half of the plate with opaque cardboard, the 

 other half with red glass, so that the boundary between them 

 again coincided with the direction of the rays of light, the 

 animals scattered in all directions in the two halves of the 

 plate. After a long time, however, the greater number col- 

 lected under the cardboard. 



The experiments which have been described were made in 

 direct sunlight. If on a dark day the plate is some distance 

 from the window and the light is not very intense, the ani- 

 mals, which at the beginning of the experiment were in the 

 middle of the plate, will gradually creep toward the room 

 side; when, however, they reach the shallow groove in the 



