40 STUDIES IN GENERAL PHYSIOLOGY 
red, they flew under it to the window side of the box. At 
fifteen minutes past 9 o’clock they came to rest and no longer 
reacted to light. When exposed to daylight on the follow- 
ing day, they did not stir, and made no attempt to creep away 
from the light, although sufficient opportunity was offered. 
I repeatedly established the fact that the movements of 
night butterflies are determined by the more refrangible rays 
of the spectrum on other specimens of Sphinx euphorbie. 
It was therefore not to be expected that in lamplight any 
other than the more refrangible rays would bring about 
movements. JI have convinced myself that the moths of 
Geometra piniaria are readily attracted by the light of a 
lamp when behind blue glass, but not when behind red glass. 
The night butterflies, therefore, shun neither diffuse nor 
intense light, nor do they prefer artificial light to diffuse 
daylight; the correct expression of the facts is rather this, 
that most species react to light only at night, when they are 
positively heliotropic like the day Lepidoptera. We find in 
butterflies periodic variation in irritability (as in many 
plants), and these variations correspond to the changes of 
day and night. As certain flowers open their calices only by 
night, while others open theirs by day, so certain butterflies 
fly only by day, while others fly only by night. Both classes 
of butterflies, however, are positively heliotropic; and it 
seems as if the irritability of the night butterflies toward 
light is not less, but even greater, than that of the day but- 
terflies ; for the intensity of the light which causes heliotropic 
phenomena in moths is apparently much less than the mini- 
mal intensity which stimulates day butterflies to heliotropic 
movements. 
The phenomena of sleep in butterflies are perhaps more 
complex than the corresponding phenomena in plants. One 
thing is, however, certain—that the periodicity of the noc- 
turnal movements of butterflies does not change during the 
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