66 ROBERT STANLEY McEWEN 
In conclusion, it may be said that females are never twice as 
active as males. They are, however, somewhat more active, 
particularly when only 1 or 2 days old. As age advances the 
difference between the sexes decreases until in some cases at 8 or 
10 days the males actually surpass the females. Moreover, in- 
stead of the maximum period of phototropic response occurring 
at 18 hours, it would seem rather that both males and females, 
if not too heavily fed, increase their response with age, reaching 
a maximum in the neighborhood of 4 or 5 days. After this 
point both sexes tend to become less active, the females more 
rapidly than the males. It may also be added that the young 
flies fatigue much more rapidly than do older insects. 
EFFECTS OF OPERATIONS ON THE REACTIONS TO LIGHT 
a. Removal of wings 
The operation of removal of the wings was suggested by Dr. 
T. H. Morgan as a laboratory experiment for one of his classes. 
Mr. 8. Safir was the first to try the experiment, and obtained 
the rather surprising result that flies so treated no longer showed 
any response to light. This effect was so unexpected that it 
was determined by the writer to investigate the matter as thor- 
oughly as possible. 
The first experiments performed in-this connection were under- 
taken with a view to determining whether the insects would 
recover their normal response if kept a sufficient length of time 
after the operation. As these tests were made at the beginning 
of the work no apparatus was employed except the tube and 
light from the north window. One fly was tested at a time and 
its record calculated according to method I, for instance, the fly 
was placed alternately in the end of the tube toward the light 
and in the end away from the light, and the algebraic sum of 
the average of the two sets of records in inches crawled was 
taken as the index of the fly in question. Five groups of animals 
were tested, in which the number of insects varied from one to 
five. When there was more than one fly in a group, the index 
of the group as a whole was computed by adding algebraically 
