178 Trans. Acad. Sct. of St. Louis 
twenty-four hour period, is as deeply rooted physiologically as 
the rhythmic periodicity of the mammalian heart-beat. 
The room was darkened to the best of my ability; even the 
troublesome crack under the door was plugged. Twenty-eight 
polyphemus of both sexes were used; some of these were 2 to 5 
days old, and had been used in previous experiments and had 
reacted to light stimuli, and some were young moths, 1%4 day or 
less, which had never had such an experience. All of the moths 
were placed on a blanket on the south wall and left undisturbed 
for the night. 
At 7:30 next morning all of the moths were in their places; 
a few of the energetic ones had crept up the wall a few inches, 
but among the older ones not one had moved at all. A second 
examination at 7:45 p. m. showed that there had been no flight 
during the day. Some had crept up the wall a few inches 
more, and a few had fallen to the floor, but lay just beneath the 
blanket. Thus it was evident that they had not at any time in 
the twenty-four hour period been overtaken by the wild 
paroxysms of excitement and flight that they display out under 
the stars every night, or they would have been scattered all over 
the room. Even though the 14 older ones had during the last 
three or four days flown repeatedly to the window in other 
experiments, not one of them now so much as started toward 
the same window, either by day or night, when the light was 
excluded. At 9 p. m. there had been no movement, so the ex- 
periment was changed. 
At the opposite (north) end of the room was a closet with a 
drop electric light. Here a 60-watt lamp was turned on and 
the door almost closed, so that only a pencil of light one-fourth 
inch wide and as high as the door shone out. Between 9 p. m. 
and midnight, 7 moths came to this light (8 males and 4 fe- 
males), and the next morning 6 more were there. These 18 in- 
eluded both young and old of both sexes. When I examined 
those which did not respond, I found 10 dead in their tracks 
and 5 aged females; hence there is evident reason why more 
did not react. 
Thus this simple test shows that these polyphemus do not 
act merely with the recurrence of a certain interval, but they 
must have light rays (of a low intensity), of course, to stim- 
ulate or enable them to go through the usual activity. This 
