HELIOTROPISM OF ANIMALS 41 
first two or three days if the animals are kept in the dark. 
Under these circumstances the moths become restless at the 
usual time. Réaumur showed that moths begin to fly in the 
evening when kept in a box. I must, leave it undecided for 
the present whether this periodicity finally disappears if the 
animals are kept still longer in the dark. I have tried 
repeatedly to cause Sphinx euphorbie to fly in the daytime 
by a sudden diminution in the intensity of the light. When 
I protected the animals from all jarring I never succeeded 
between 6 and 12 o'clock in the morning. Yet I was easily 
successful in the afternoon, long before the beginning of 
twilight. I will cite here several of my experiments. One 
morning I placed a Sphinx euphorbiz, which had begun to 
fly at 9 o’clock on the previous evening, on the window cur- 
tain, where it remained quietly. At 2:45 I returned it to its 
glass box, which stood in a dark corner and into which light 
fell only through a narrow slit. An hour went by, but the 
animal did not leave its place. It then moved to the light 
side of the box, without flying. I carried the animal back to 
the window, where it remained quietly. After twenty min- 
utes I returned it again to the dark box. Half an hour later, 
at half-past 4, it finally began to fly. 
The next day I allowed it to remain at rest near the win- 
dow, and it did not begin to fly until 9 Pp. mM. at well-advanced 
twilight. On the following day I kept it in the dark box, 
and at half-past 3 in the afternoon it had already begun to 
fly. At noon on the succeeding day a heavy storm came up 
and it grew quite dark. The moth, which until then had 
remained quietly at the window, began to fly. I have had 
the same experience with other examples of this species. 
These facts seem to indicate that it is possible to influence 
the time of waking of Sphinx euphorbie by diminishing the 
intensity of the light, but only when they would soon wake 
up without artificial interference. 
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