86 ROBERT STANLEY McEWEN 
TABLE 19 
VEIN CUT 
NO OPERATION 
A B. CD: £E 
Males Females Males Females Males Females 
Before operation.....| 100.0 99'.1 93.3 95.8 93.3 80.8 
After operation...... 43.1 41.6 40.8 46.6 92.5 83.3 
that in neither case was that part of the wing injured where the 
chief groups of organs occur. These points make it pretty clear 
that the effect produced on light reaction due to injuring the 
fly’s wing is not the result of injury to these particular organs. 
The fact that the stimulus for the light reaction is received 
in large part at least through the eyes and not through the wings 
or other organs is attested by the following experiment. In one 
of the mutant stocks known as eyeless, the compound eyes are 
very poorly developed, and in the case of many of the females 
are entirely lacking. Two groups of twenty flies each were, 
therefore, selected, one group containing only the individuals 
with the best developed eyes, and the other only those males 
with poorly developed eyes, and those females with no eyes at 
all. At the age of 5 days a light and gravity test was given 
these insects with the following results (table 20). 
TABLE 20 
EYES PRESENT EYES POORLY DEVELOPED OR LACKING 
Light Gravity Light Gravity 
Male Female Male Female Male Female Male Female 
9i<5 | 65.8 65.8 46.6 74.1 25.0 70.8 39.1 
The data seems to indicate that at least a large share of the 
stimulus causing the light reaction is received through the com- 
pound eyes, since when these are undeveloped the response is 
greatly reduced, whereas that for gravity remains approximately 
the same. 
We may now summarize the work which has been done on 
the relation of the fly’s wing to its phototropic response as follows. 
