HELIOTROPISM OF ANIMALS 51 
collected at the highest point in the test-tube. This experi- 
ment must, of course, be made in a dark room. When the 
animals are first brought into the dark, the experiment can 
be repeated many times with exactly the same result; every 
change in the position of the test-tube with reference to the 
vertical compels the animals to creep upward and to collect 
at the highest point in the tube. When, however, the ani- 
mals were kept permanently in the dark, the reaction ceased 
soon, and the animals remained motionless, no matter how 
often the position of the test-tube was reversed. The ani- 
mals were in a sort of rigor. When they were placed on an 
inclined or vertical plane, they moved upward. Geotropic 
orientation occurred as soon as the plane made an angle of 
30° with the horizontal; the geotropic movements were the 
more certain and precise the nearer the plane approached the 
vertical. When light fell on the animals at the same time, 
their orientation was determined by the resultant of the 
direction of the rays of light and gravitation, in which, how- 
ever, the light was the stronger force even at a great distance 
from the window. 
The winged animals behave toward a source of heat in the 
same manner as the caterpillars of Porthesia chrysorrhcea. 
When I brought the animals in an opaque vessel into a room 
having a temperature of 18° and placed them near a stove, 
they left the side of the vessel which was turned toward the 
stove, as soon as its temperature increased a few degrees. At 
a temperature of 9° the animals were so sluggish that a 
definite reaction to light or gravity did not take place. A 
temperature of 20-24° is the most suitable for the experi- 
ments. When I surrounded one-half of the vessel with a 
water-bag having a temperature of 20°, the other half with 
one having a temperature of 10°5 the animals moved, under 
the influence of light, from the warmer into the cooler area. 
But they did not move far into the latter, as their movements 
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