56 ROBERT STANLEY McEWEN 
states that the females are nearly twice as responsive as the males. 
Concerning the latter point Moenkhaus has reached exactly the 
opposite conclusion, while the writer’s early work failed to con- 
firm the former. It therefore became necessary to settle these 
questions before reliable results could be obtained in which 
other factors were involved. 
After a number of preliminary trials the following experiment 
was devised to test thoroughly the effect of age and sex upon the 
insect’s light reaction. Incidentally the influence of these factors 
upon fatigue is also brought out. Method II was used for measur- 
ing the response of the flies by groups, while mechanical agita- 
tion by means of the pendulum was employed throughout the ex- 
periment. The animals were introduced into the tube from the 
padded vials as described above, with as little disturbance as pos- 
sible. They were returned to the starting point between each test 
by means of light, with only such slight shaking as was absolutely 
necessary to get them in place inside of a minute and a half 
after the preceding test. Usually very little was required. The 
groups of flies to be tested were removed from the culture bot- 
tles late in the afternoon, and placed in vials with a small quan- 
tity of food. The females were a trifle older than the males in 
most cases and have been designated as 17 to 19 hours, while the 
males were recorded as 16 to 18. The test was always made 
the following morning, beginning at nine o’clock. The males 
were given 32 successive tests while the females received 50. As 
‘indicated above, there was an interval of a minute and a half 
between each test during which the flies were transferred to the 
starting point by taking the tubes to a window and allowing 
them to crawl toward the light. After this series of experiments 
was completed, a second series was run with flies from 4 to 6 
days old. In the case of groups A, B, C, D, and E of this series, the 
insects had not been tested before, but groups F, and G were flies 
which had been tested in the first series under groups D and EH 
respectively. Thus in these two instances it is possible by refer- 
ence to the tables to get a direct comparison between the reac- 
tions of the same flies at different ages. Suffice it to say that 
the results are in entire accord with those obtained by averag- 
