REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 55 
thest from the light counted zero, the second 25, the third 50, the 
fourth 75 and the last which came nearest the light, 100. <A 
given number of flies, usually ten, was introduced into the zero 
end and the test lasting one minute was begun. During the min- 
ute a record was taken of the furthest sections reached by any 
of the flies. The number of flies in each section was then multi- 
plied by the value assigned to the section in question. The total 
number of values thus obtained were then added together and 
divided by the number of flies used. Thus, let us suppose that 
ten flies were undergoing a test. Eight flies reached the section 
nearest the light valued at 100. Two flies ofly reached the sec- 
ond section valued at 25. The process for obtaining the photo- 
tropic index was as follows. 100 x 8 = 800, 25 x 2 = 50, 
800 + 50 = 850. Divide by 10 the number of flies and we have 
85. This, then, will be the phototropic index for these flies in 
this test. It will be noted that there is not in this method any 
means of detecting actually negative flies. When this system 
was started, however, enough work had already been done to 
show that none of the insects worked with were actually nega- 
tive under any circumstances. The problem had therefore re- 
solved itself into one of determining the relative positivity of the 
various types experimented with, and for that purpose the sys- 
tem just described was superior to the first. 
Finally, it is evident that virtually the same general method 
could be used in studying the relative reactions of different groups 
of flies to gravity. The detailed changes introduced in connec- 
tion with the gravity experiments as well as those employed in 
some of the special light tests will be dealt with in the descrip- 
tion of the particular experiment in question. 
EFFECTS OF AGE AND SEX 
In a paper entitled “Biological Notes Concerning Drosophila 
ampelophila,’”’ published in the Journal of the New York Ento- 
mological Society for June, 1914, vol. 22, F. E. Lutz of the Amer- 
ican Museum of Natural History, states that Drosophila shows 
the maximum phototropic response at the age of 18 hours, after 
which the reaction gradually diminishes in intensity. He also 
