Experiments in Rhythmetic Periodicity 179 
leads to the conclusion that their movements are caused by ex- 
ternal stimuli and not by something within. 
Relation of the Movement of Male Cecropia and Polyphemus 
to Moonlight 
While waiting through the night hours for the moths to come 
in, it often seemed to me that on moonlit nights an unusual 
number of moths were abroad. To test this point, I remained on 
watch all night, or all but an hour, for seven nights, and kept 
records of the moths appearing on the roof, together with the 
weather and moon-light conditions at the various times. The 
resulting data are too bulky to be presented here, but the fol- 
lowing table gives a summary of the seven nights’ observations: 
No Bright Half Dense 
moonlight moonlight moonlight fog Total 
Bred cecropias.......... 5 $14 35. 1 eS 
Wild cecropias.............. 0.2% 4 4-2 8 7 
Bred polyphemus: 3 "4, 3, 74 <1, © 0 
Wild polyphemus..2 4 0,295 0,4 0 46 
0 
Total number flights 8 86 19 113 
Immediately the figures show that my impressions were not 
in error; the average number in flight on the bright nights 
was more than twice the number on dark nights. The times 
when the moonlight was subdued or when light and darkness 
fitfully alternated brought forth an intermediate number of 
moths. It is easy to imagine that they awoke and started on 
their journey during an interval of brightness, and the tem- 
porary darkness did not arrest them. On one night the moon 
was completely obscured by a dense fog, and all night long I 
waited in vain, for not a single moth of any description came in. 
I cannot believe that the severity of the weather, the difficulty 
of flying in the moist atmosphere, could have prevented their 
flight, since I have seen them come in in large numbers through 
a drenching rain. Rather, this instance may be explained by 
the lack of wind. Our previous evidence indicates that wind is 
the chief factor in inducing the flight of moths in the open, 
and fog indicates a total absence of wind; hence the moths could 
