206 Trans. Acad. Sci. of St. Louis 
the hands of the clock, but by optimum conditions of light. This 
has been adequately proven in the experiments. 
In order carefully to study the light conditions of this period 
I set the alarm at 3:25*, and was on the roof at 3:30, waiting 
for the first moth to aprive: At that time there was in my 
estimation, absolute darkness, broken only by the stars and arti- 
ficial lights of the city. My brief notes were made only by 
means of a flashlight, which I could turn on for only an instant. 
Not until 4:08 did it become sufficiently light that I could 
elumsily guide my pencil on the paper without the aid of the 
flashlight. By the time their flight ceased, about 4:30, the gray 
light of day awaited the sunrise. Hence, to my senses, dawn was 
at about 4 a. m. Of course it is easy to understand that if the 
conditions at 3:30 aroused the moths to flight, those which were 
near would come in at once, while those which came from great 
distances would arrive later, unless the dazzling light of sunrise 
overtook them en route and stupified or lulled them into quiet 
for the day. This sounds logical, yet in my attempts at imi- 
tating in the laboratory this condition of intense darkness, I 
eould get no reaction, but when I produced a condition of light 
similar to that at 4 o’clock, there was some response. Of course 
the artificial lights of the city modified the darkness, but they 
had been there all night, as had also the females in the cages, 
which were emanating their odors from my windows all through 
the night when no males came in. Hence it seems that in na- 
ture there is an optimum condition of diffused light to which 
alone the cecropias react. Some supersensitive males can per- 
ceive the coming of the dawn before it is evident to others; 
some are so dull that the light of day is not perceived until it 
is well upon them, and come in at the last end of the proces- 
sion, in very human fashion. Then there is the great class 
of respectable citizens who do the right thing at the right time, 
and come in midway in the period. Then there is the excep- 
tional individual who reacts to the light he sees or thinks he 
sees long before his fellows. Lastly there are those of dull sen- 
sibilities who never arrive at the goal; how great this horde is 
I shall never know. So while the moths as a class respond to 4 
certain intensity of light, their response is modified by individ- 
ual temperament. 
*On many nights, as the data show, I was up on the roof all —_ 
and —- that (with the exceptions recorded) none came in before 
