HELIOTROPISM 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 / of the window, and the transparent 
side cd of the glass b is turned up; the animals move along 
the illuminated side cd from a to b, with- 
out stopping at the boundary between f 
them, until they reach the window side ¢ 
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 «. 
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 
FIG. 2 
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