HELIOTROPISM OF ANIMALS 33 
in a test-tube open on the room side; but when I turned the 
vessel through an angle of 180° in the daytime, hardly two 
minutes elapsed before all the animals had moved to the 
open end of the vessel which was now turned toward the 
window. Under these conditions they of course escaped 
from the test-tube. A position which the animals have 
assumed under the influence of light is usually not changed 
when the light is removed, unless some other stimulus comes 
into play. 
5. On negative geotropism and contact-irritability in the 
caterpillars of Porthesia chrysorrheea.—The reader may 
perhaps have noticed that in all of these experiments on 
caterpillars the test-tubes were always placed with their 
longitudinal axes horizontal. This was due to the fact that 
the animals behave like plant structures, not only in regard 
to their heliotropic, but also in regard to their geotropic, 
irritability. Just as is frequently the case in positively 
heliotropic plants, we find that the caterpillars are also nega- 
tively geotropic; that is, they are compelled by gravity to 
creep vertically upward until they come to rest in the highest 
part of the test-tube. These experiments were made in a 
dark room, with the long axis of the test-tube in a verti- 
cal direction. If the test-tube is inverted, the animals again 
creep to the top; if left undisturbed, the animals remain in 
the uppermost regions of the test-tube. It is necessary in 
these experiments, as in those on heliotropism, to have the 
temperature of the room at least 15°, preferably as high as 
20-22°. It is simplest to put the test-tube in one’s pocket 
with its longitudinal axis vertical. In a few minutes the 
animals are found at the highest point in the tube. An 
increase in temperature increases the geotropic irritability 
of the animals. 
it must now seem questionable whether in our former 
discussion of the heliotropism of these animals we were 
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