24 STUDIES IN GENERAL PHYSIOLOGY 
Vv. THE POSITIVE HELIOTROPISM OF THE CATERPILLARS OF 
PORTHESIA CHRYSORRH@A 
I will enumerate the observations which show the identity 
of animal and plant heliotropism in the caterpillars of Por- 
thesia chrysorrheea. I shall mention only such experiments 
as in my experience were always successful under the given 
conditions, and which may be taken as the prototype of the 
experiments made upon all the animals treated of in this 
discussion. 
1. The direction of the progressive movement in animals 
is determined by the direction of the rays of light.—I placed 
a large number—about a hundred specimens of the small 
gregarious caterpillars of Porthesia chrysorrhcea which had 
just crept out of the web in which they had passed the win- 
ter—into a test-tube. They had not fed as yet, and in this 
hungry condition they were exposed to the light. The tem- 
perature of the room was necessarily more than 12°-15° C., 
as otherwise they would have crowded together and fallen 
asleep again—a state in which they react neither to light 
nor to gravity. 
Experiment 1.—If the test-tube is laid on a dark table, 
so that the longitudinal axis of the tube is perpendicular to 
the plane of the window, the animals, which are at first scat- 
tered about irregularly, all assume the same orientation. 
They creep to the upper portion of the test-tube, turn their 
heads toward the window, and with their ventral surfaces and 
their heads turned toward the light creep in a straight line 
toward the window side of the test-tube. The process 
requires from one to five minutes, according to the tempera- 
ture and the condition of the hibernated animals. All with- 
out exception, provided they are not sickly, move in the 
direction of the rays of light to the window side of the test- 
tube. If the tube is turned about an angle of 180°, the 
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