REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 85 
these organs carefully in order to seeif they might possibly contain 
any sort of light receiving structures. A number of minute or- 
gans were found (fig. 3). Except for the seven larger ones which 
occur well out on the veins, the majority are arranged in groups 
near the base. They have in fact very much the same arrange- 
ment and appearance as have the so called olfactory organs de- 
scribed by McIndoo for the honey bee (14). 
In order to discover whether these organs have anything to 
do with the reaction to light three groups of twenty flies each 
were selected and kept for the usual 5 days. In one group the 
Fig. 3. A B, Line of cut made to isolate the larger sense organs. EF, CD, 
Line of cuts made as a control, on veins which contained no sense organs. 
vein upon which occured the largest number of organs was cut 
as shown by the line AB. In the other group the veins were 
cut along the lines CD and EF, while in the third group no 
operation was performed, though the flies were etherized as in 
the first two cases. Table 19 gives the result. 
These operations were performed on the assumption that if 
the structures on the veins were light receiving organs, the nerv- 
ous connection for such organs must pass along these veins. If 
such were the case, then the insects which had those veins cut 
on which some of the organs occurred, should have been most 
affected by the operation. As a matter of fact, however, flies in 
which the veins were cut which contained no sense organs were 
as much affected as the others. Furthermore, it should be noted 
