24 Trans. Acad. Sci. of St. Louis. 



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 1 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 

 undiscovered, 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 6 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 



e Excluding the Fall River cecropias, which were insufficient in num- 

 bers to be reliable, and the ice-box cecropies, which were under artificial 

 conditions. 



