Experiments in Rhythmetic Periodicity 205 
May. 3:30t03:50 3:50t04:10 4:10to4:30 Total 
21 25 31 37 
93 
22 58 30 11 99 
23 5 2 1 8 
24 33 28 12 73 
25 13 22 6 41 
27 16 4] 12 69 
28 aes 21 12 33 
29 16 6 6 28 
ees 166 181 97 444 
Thus, of these 444 males, 38 per cent came in during the 20 
minutes when the light was scarcely distinguishable from night, 
and 40 per cent during the medium light, leaving only 22 per 
cent of the moths abroad as the full light of day approached. 
We are safe in assuming that the majority of those which ar- 
rived after daybreak had probably aroused themselves and 
started on their journey during the darker portions of the hour. 
Some further records were kept to see if this distribution over 
the period covered the various classes, the wild old males, the 
wild young males and the bred males, but no evidence was 
found to indicate that one period influenced one class more 
than another; all three periods contained members of all three 
groups. 
I have frequently referred to the cecropias coming in at dawn. 
This would imply that the moths become active with the first 
streaks of light, which was not true, for at 3:30 a, m. the blackest 
night prevails, that proverbial blackness which precedes the 
dawn. This brings up the question, ‘‘When does dawn begin 
and when does it end?’’ At first this seems a foolish question, 
but since these creatures are very sensitive to light, and it is a 
factor of so great importance in their existence, this is an im- 
portant question. It is difficult indeed to believe that light 
alone at 3:30 a. m., can be the stimulus to their action, when it 
seems to the human eye in no wise different from the light of 
other night hours. : 
A ecritie of course might say, that this is the hour of their 
activity, and come what may, when that period recurs they must 
and will fly, and they ean not evade it. If he made that state- 
ment about cynthias, he would be more nearly right, but it is 
not true for cecropias, for their movements are actuated not by 
