Experiments in Rhythmetic Periodicity 173 
correlated with sleep or rest, while dim light suggests to them 
activity and mating. 
In other cages nearby while this work was in progress were 
moths of both sexes bred from cocoons in the laboratory, and 
other native males that had been attracted by our females, but 
none of these showed any signs of activity at this time of day. 
They were exposed all the time to the normal changes of night 
and day, and their program was not disrupted by artificial imi- 
tations of darkness, daylight and dawn. Hence it appears that 
the activity of these creatures is regulated by the changing in- 
tensities of light and not, as has been thought by many investi- 
gators, by the clock. Of course it was not the normal time for 
activity of those in the glass box, any more than it was for the 
others in the room, but they were easily fooled into activity by 
fake dawns, at any time of day we wished to stage them, so long 
as we did not work them to the point of fatigue. The fact that 
their periods of rhythmic response is altered by light conditions 
shows that this habit is not so deeply ingrained in their psychol- 
ogy (or is it their physiology, or is it both?), else it could not 
be so readily changed. The fact that cynthia does not so readily 
change its periods under similar conditions makes one suspect 
that cynthia is phylogenetically older, since her habits are more 
deeply ingrained. 
Exp. 17. June 2,1:30 p.m. Temperature, 66° F. If any- 
one should offer the criticism that my experiments were made 
early in the day, too soon after the moths had come in from 
their flight at dawn and their reactions were merely a result of 
the momentum acquired in that flight, or that some of them 
were made too late in the evening, at a time too near to the ap- 
proach of their next flight at dawn, Exp. 17, which was made 
at 1:30 p. m., a time half-way between the two, should set them 
at ease. : 
Thirty-five males that had flown in at dawn were kept in the 
darkened glass box until 1:30, when they were all gently placed 
at the north end of the box and the curtain was lifted from the 
south end. They were so utterly lethargic that they seemed 
fast asleep. Since, as we know, insects do indulge in sleep, 
why should not this time of day be most suitable for them? 
Within six minutes, however, six males were fluttering at 
the light end of the box, and during the next few minutes this 
