REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 83 
striking point which distinguishes these data, however, is the 
failure of wing removal to affect light reaction in Group B. 
Indeed, the test after the operation shows an actual increase in 
the reaction to light. In view of the rather low light indices in 
all the flies of this variety, I am inclined to explain this as fol- 
lows: Even normal insects whose wings have been removed, 
vary a good deal from time to time in their degree of response 
to light. Also the general variability is such that a fly with 
three-fourths of its wings gone will not always show a lower re- 
sponse than one with only one-half gone. I therefore suggest 
that since these flies already have a low index on account of their 
imperfect wings, the removal of the remainder of the wing might 
not have sufficient additional effect to counterbalance some un- 
known change in the physiological condition of the animal. 
This statement is partially borne out by the fact that the males 
in Group A which were not operated on, also showed a markedly 
higher index for both light and gravity in the second test. 
Further experiments now under way will serve to show whether 
this is the true explanation or not. If it is not, we should have 
to accept the rather astonishing hypothesis that a fly with short 
wings and a low index to begin with, actually has its photo- 
tropism increased instead of diminished by the removal of such 
Wings as it has. 
Finally, we have to consider the reaction of the flies desig- 
nated as curled. As usual, three groups of insects, constituted 
as in the previous tests of this series, were tested when 5 days 
old. As this particular group was really the first of the series 
the use of the single lamp in the gravity test was still customary. 
The change to the new system was, indeed, made during the 
work on this group, which accounts for the fact that only the 
males in Group A were subjected to the improved treatment. 
That this feature was really of no great significance, at least in 
the case of these flies can be told by comparison of Group <A’s 
record with those of the others. The males of both Groups A 
and B had their wings removed after the first test and were re- 
tested eight hours later. Group C males were not operated on, 
but were retested after the same interval of time as a control. 
