REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 59 
As has been noted, the older females gain rather less rela- 
tively in speed than the older males. This, it appears, might 
very likely be due to the fact that the female is gradually weighed 
down by the growth of her eggs. Indeed, it is known that these 
do not reach their full size for 4 or 5 days, the time varying some- 
what with the abundance of food. This suggested the experi- 
ment of running some insects of both sexes through a series of 
tests to be made daily for a period of two weeks, during which 
time they would be given fresh food each day. ‘Three groups of 
flies with ten males and ten females in each group were selected 
for this purpose. Five trials were given to every set of ten flies 
at each of the testing periods, and the results of these five trials 
averaged for the period in question. This average has been 
taken as the index for the sef of ten flies for this test. It is from 
an average of these averages that table 2 is made up. The tests 
for the first day were given at the ages of 4, 12, 15, 18, 21, and 
24 hours. Afterward there was one test a day consisting of the 
usual five trials. The results indicated in table 2 and the cor- 
responding graph show that the females, though starting ahead 
of the males, fell away much more rapidly than usual, thus tend- 
ing to confirm the conclusion that a good share of their falling 
off with age is due to the increased weight of their ovaries. In- 
cidentally, it is again evident that the strongest reaction does 
not come at 18 hours. 
The next step was to run a similar series of daily tests for flies 
whose food was not changed daily. In this case a small amount 
of banana food was put in the vial in which a group of flies was 
kept and allowed to remain there. It gradually dried up so that 
the flies could derive less nourishment from it each day. That 
they must have derived some is certain, for Drosophila can not 
live 24 hours without food. Besides the drying, however, there 
is also a chemical change in food in which larvae are not work- 
ing. This, as well as the drying, tends after a few days to make 
the food unfit for the insects. This fact accounts for the death 
of those flies to which a little fresh food was not given on the 
sixth day. From this, as well as other experiments, it appears 
that 6 days is about the average time that flies will live under 
