Experiments in Rhythmetic Periodicity 177 
light intensities of the period of early dawn, when their normal 
flight takes place. In nature, bright light means enemies and 
danger if they are in flight; therefore daylight means motion- 
less rest and sleep. The reactions which the moths displayed in 
our tests would be very fitting in the wilds of nature. 
Activity in a Dark Room. 
If the rhythmic periodicity is something innate and is not 
influenced by surroundings, then we shall find in these moths 
the activity returning with the recurring hour each night, re- 
gardlesss of external conditions. If we shut out the stimulus 
of light, by keeping the room entirely dark, we should expect 
to find that the flight has occurred at the regular time, and in 
the morning the moths will be scattered about the room. To 
this end the following experiments were done: 
Exp. Di6. June 4, 9 p. m. Fifty-two moths, comprising 
thirteen each of male eynthias, female eynthias, male cecropias 
and female cecropias, were placed on the south wall and the 
room darkened. The next morning I found 36 moths just where 
I had left them, and 16 were crowded around the door; this 
door was closed, but did not fit tightly at the bottom, and where 
a small pencil of light penetrated, the only streak of light in 
the room, they had congregated. This simple test shows that 
the moths do not act merely because it is their appointed hour to 
act, but without the stimulus of a precise condition of diffused 
light they pass the hour of dawn unmoved. It also shows that 
some supersensitive individuals can sense a meager trace of 
light and respond to it. 
Exp. D17. May 5, 11 p.m. We know that polyphemus are 
abroad in quest of mates during the entire night, and that their 
activity is influenced by moonlight. In this experiment we want 
to see if they can be induced to inhibit their activity during 
the night if they are placed in a totally — If 
they fail to move about when the luminous rays are eliminated, 
then we know that the rays influence or perhaps induce the 
Movement. If they react in spite of the absence of any varia- 
tion in the light, then we shall feel that their reactions are in- 
nately bound up with the physiology of their beings and reap- 
pear at recurring periods, regardless of any outside stimuli; in 
other words, their rhythmic periodicity recurring once In each 
