34 STUDIES IN GENERAL PHYSIOLOGY 
justified in taking as the effect of light the movements of 
the animals to the top of the test-tube ; it might, indeed, be 
a geotropic phenomenon. To decide this the animals were 
placed in a test-tube which was lined with thick black paper 
except for a strip 2 mm. wide. The uncovered strip was 
turned downward, so that light could enter the vessel only 
from below. Diffuse daylight was reflected through the slit 
from below by means of a mirror. The animals collected in 
the lower, lighted portion of the glass vessel. Their helio- 
tropism is therefore more powerful than their geotropism, 
even when only weak diffuse daylight is used. 
The geotropic experiments succeed only when the animals 
have been in the light for some time and have not yet come 
to rest. When the animals are kept in the dark for a long 
time and the test-tube is not disturbed, they do not creep 
upward. The orienting effect of the light always exceeds 
that of gravity. The effects of gravity, like the effects of 
light, usually appear only during certain periods in the life 
of the animals; at any rate, they cannot always be demon- 
strated with certainty. 
The contact-irritability of the caterpillars of Porthesia 
chrysorrhcea shows itself by the way in which the animals 
remain in the corners and convex sides of solid bodies. I 
covered the boxes in which I cultivated my caterpillars with 
large, square glass plates. These did not close the box 
tightly, so that the animals could creep out and creep upon 
the glass plates. Only rarely, however, were they found on 
the free surface of the plates. The animals moved along the 
rough edges of the plate until they reached the window side 
of the dish. I confirmed this observation almost daily for 
months. When I placed the animals upon the outside of a 
cubical block, they collected by hundreds in one of the 
upper corners. Of course, only a few have room in the 
corner itself, but, as is generally the case with these ani- 
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