HELIOTKOPISM OF ANIMALS 27 



same diameter, is covered with dull black paper, except for 

 a strip about 2mm. wide. The two test-tubes are placed 

 together on a table so that their longitudinal axes lie per- 

 pendicular to the plane F of the window, and the transparent 

 side cd of the glass 6 is turned up; the animals move along 

 the illuminated side cd from a to 6, with- 

 out stopping at the boundary between 

 them, until they reach the window side c 

 of the cylinder. The total amount of 

 light which strikes a caterpillar in the 

 glass b, however, is less than in the glass a, 

 since all lateral rays are cut off in the 

 former and the animal is struck by rays 

 of light only on its ventral side; in test- 

 tube a light falls upon the animals from 

 all sides, though the rays from above and 

 in front are of course the most intense. The animals there- 

 fore move toward the source of light in the direction of the 

 rays of light, even if by so doing to judge from human 

 sensations they are led from a "bright" to a "dark" 

 place. 



In such an experiment no animals are found, as a rule, 

 scattered over the rest of the surface of the glass b. If 

 both glasses are turned around so that a is nearest the 

 window side, the animals of course again move from b to a. 



The experiments described here were carried on in diffuse 

 daylight. In sunlight, however, the results are the same as 

 in diffuse daylight. When the glass is placed with the 

 longitudinal axis in the direction of the rays, the animals 

 move in the direction of the rays toward the sun and collect 

 at the end of the glass which is turned toward the sun, even 

 though in their hungry state they cannot bear the high tem- 

 perature. When the test-tube is placed with the longitudinal 

 axis perpendicular to the rays, the animals scatter over the 



