STUDIES ON THE DURATION OF LIFE 189 



only with flies in the adult or imago stage, after the com- 

 pletion of the larval and pupal periods. The curve is 

 based upon 3,216 female and 2,620 male flies, large enough 

 numbers to give reliable and smooth results. We note at 

 once that in general the curve has the same form as the 

 corresponding l x curve from human mortality tables. The 

 most striking difference is in the absence from the fly 

 curves of the heavy infant mortality which characterizes 

 the human curve. There is no specially sharp drop in the 

 curve at the beginning of the life cycle, such as has been 

 seen in the l x curve for man in an earlier chapter in this 

 book. This might at first be thought to be accounted 

 for by the fact that the curve begins after the infantile life 

 of the fly, but it must be remembered that the human I x 

 line begins at birth, and no account is taken of the mortal- 

 ity in utero. Really the larval and pupal stages of the 

 fly correspond rather to the foetal life of a human being 

 than to the infant life, so that one may perhaps fairly take 

 the curves as covering comparable portions of the life span 

 in the two cases and reach the conclusion that there is not 

 in the fly an especially heavy incidence of mortality in 

 the infant period of life, as there is in man. The explana- 

 tion of this fact is, without doubt, that the fly when it 

 emerges from the pupal stage is completely able to take 

 care of itself. The baby is, on the contrary, in an almost 

 totally helpless condition at the same relative age. 



It is further evident that at practically all ages in 

 Drosophila the number of survivors at any given age 

 is higher among the female than among the males. This, 

 it will be recalled, is exactly the state of the case in human 

 mortality. The speed of the descent of the Drosophila 

 curve slows off in old age, just as happens in the human 

 life curve. The rate of descent of the curve in early 

 middle life is somewhat more rapid with the flies than 



