24 Trans. Acad. Sci. of St. Lows. 
the tables), we find considerable variation, but all within 
the limits of 7.71 and 10.72 days. 
What can be the cause of this variation in the length 
of life? Why, for instance, should the New York Cecro- 
pias show a mean life for the whole population in 1911 
of 8.06 days, and in 1913 of 10.24 days? The difference 
may seem trivial in itself, but it is significant enough to 
assure us that there must be some potent factors, as yet 
undiseovered, working singly or together determining 
these conditions. How can we account for it unless it 
be that one crop of insects acquire more or another less 
food as larvae, or perhaps the succulence or the chem- 
ical composition of the food plants may vary from year 
to year, or a thousand fine adjustments in nature of 
which we are yet ignorant. 
Comparing next the mean length of life of all males 
and all females, we find in two lots the duration of life 
of both sexes almost equal, and in seven lots* we find the 
mean for the males to be the greater. This shows that 
the male lives as long or even longer than the female. 
This is in direct contrast with what we have already ob- 
served in Polyphemus, where the male is much shorter 
lived. 
Then again when we compare the means for all mated 
insects of both sexes with all unmated, we find that for 
every one of the nine lots the life was longer in the un- 
mated groups, but which sex contributes most to this 
higher mean can be ascertained by inspecting the first 
four columns of the two tables. In considering the length 
of life of the mated and unmated males we find that in 8 
of the 9 lots of Cecropias the mean duration of life was 
practically equal for the mated and unmated males. In 
only one case, the 1913 St. Louis Cecropias, do we find the 
life longer for the mated males, by a difference of 1.14 
days. Thus we see that in this species copulation hardly 
6 Excluding the Fall River cecropias, which were insufficient in num- 
bers to be reliable, and the ice-box cecropies, which were under artificial 
conditions. 
