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 euphorbise. 

 It was therefore not to be expected that in lamplight any 

 other than the more refrangible rays would bring about 

 movements. I 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 



