HELIOTROPISM OF ANIMALS 63 
those cases occasionally observed in plants where in light of 
great intensity the heliotropism of an organ is the opposite 
of that in light of less intensity. A closer examination, 
however, showed this not to be the case. When the test- 
tube lay perpendicular to the plane of the window, positively 
heliotropic animals contained in it moved, as we have seen, 
not only to the upper, but also to the window side of the test- 
tube. This was not the case with the newly hatched fly 
larve. They all turned their ventral surfaces toward the 
source of light, but otherwise moved about irregularly. I 
placed the animals in a test-tube which was covered with 
black paper, except for a small slit, and let direct sunlight 
enter the tube only through the slit. The animals which were 
on the lower side of the tube left it as soon as the light 
struck their backs, and crept upward; but no animal which 
was sheltered from the light was attracted to the upper, 
lighted side of the tube, as was the case under similar con- 
ditions in the positively heliotropic caterpillars of Chrysor- 
thea. When I held the glass vertically, more animals 
collected on the window side, but they did not all creep up- 
ward, as did the positively heliotropic animals. When I 
placed the animals on the outside of a test-tube, they did 
not move upward, but collected for the most part on the 
under side of the tube. This experiment was not very 
decisive, however, as the animals easily fell off the tube. 
These facts can be interpreted in no other way than that 
the intense light compels the fly larva to turn their ventral 
surfaces toward the source of light, in which condition 
they are indifferent to the orientation of their median planes 
toward the rays of light. The ventral position is assumed 
only when the animals are exposed to light. With this, 
however, the striking features of the movements of orien- 
tation in fly larve are by no means exhausted. While the 
movements of orientation in all the other animals go on 
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