42 STUDIES IN GENERAL PHYSIOLOGY 
The day butterflies are positively heliotropic like the night 
butterflies. The only striking feature is that in certain day 
butterflies the intensity of the light must be very great to 
bring about heliotropic movements. Specimens of Papilio 
machaon (which I had raised) remained at rest during the 
day at a window where they were exposed to the diffuse day- 
light and could be carried around on the finger; as soon, 
however, as they were brought into direct sunlight, they flew 
toward the window in the direction of the rays of light, 
and this with such force that they dropped down as if stunned. 
In direct sunlight they pressed themselves closely against 
the window pane. In diffuse daylight the animals, if they 
moved at all, crept toward the source of light; but in direct 
sunlight they flew toward it. My attempts to attract Papilio 
machaon by the weak light of a kerosene lamp were unsuc- 
cessful. 
I will add at this point my general observations on the 
caterpillars of butterflies. I have not found these periodic 
variations in heliotropic irritability in most caterpillars, not 
even those of Sphinx euphorbiae. The caterpillars which 
I studied reacted to light at all times of the day and night. 
The caterpillars agree, however, with the day and night 
butterflies in so far as they are all, without exception, 
positively heliotropic. 
This positive heliotropism is most marked in the cater- 
pillar of the willow-borer, which lives in the stems of the 
willow where it is not at all exposed to light. Such cases 
are also known in plants. Roots, for instance, are helio- 
tropically irritable, and yet, as Sachs points out, under nor- 
mal conditions their heliotropism is of no use to them. 
They can certainly not have acquired it through natural 
selection. According to the Darwinian theory, we would 
expect that the caterpillars of willow-borers should be nega- 
tively heliotropic, or at least indifferent to light. But the 
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