Vol.. 8, 1922 
ZOOLOGY: S. O. MAST 
241 
lated when these insects with but one functional eye are oriented as is 
demanded by the Ray-Verworn theory. And the same may be said in 
reference to the fiddler-crab and a number of other forms that move side- 
wise toward the light. 
In the robber-fly, Erax rufibarbis, with one eye covered the body assumes 
a tilted posture. The legs on the blind side are much more extended than 
those on the normal side and the feet consequently tend to move faster on 
the former than on the latter side, resulting in turning toward the normal 
side. Similar reactions are obtained if the lower half of one eye, the 
right e.g., and the upper half of the other eye are covered, i.e. they lean 
toward the right side and turn to the right; but if such specimens are 
laterally illuminated from the left, they will, under certain circumstances, 
turn to the left. Thus they turn toward the side on which the extension 
of the legs is greater, and in the direction precisely opposite to that which 
would obtain if it were in accord with the tonus hypothesis. 
If Eristalis on the wing is illuminated from above or below it turns 
directly upward or downward. This turning can not be due to unequal 
illumination of the two eyes in accord with the Ray-Verworn theory 
for the two eyes may be continuously equally illuminated. Similar re- 
sponses have been observed in Caprella (Mast, 1911, p. 224). 
In light from two sources both Eristalis and Erax, go toward a point 
between the sources. The location of this point depends upon the rela- 
tion of the luminous intensity of the light received from the two sources. 
The greater the difference the nearer the more intense illumination the 
point is located. When the insects are oriented in such a field of light 
the two eyes are not equally illuminated except when the light received 
from the two sources is equal. Consequently, when insects are oriented 
under natural conditions where they are nearly always subjected to light 
from various sources of unequal intensity, the eyes are rarely if ever equally 
illuminated. Under such conditions orientation is therefore not in accord 
with the Ray-Verworn theory as applied to insects by Bohn, Loeb, and 
others. 
In specimens of Eristalis or Erax with one of the front legs removed 
orientation is practically normal. If such specimens are laterally illumi- 
nated they turn toward the light either to the right or to the left. If 
the front and the middle legs on the same side are removed, Eristalis 
deflects strongly toward the normal side, but after a few days it orients 
fairly accurately. Erax usually goes fairly directly toward the light at 
once, but only for a short distance after which it deflects rather sharply 
toward the normal side ; then it ordinarily attempts to turn in the opposite 
direction, but it usually fails, and topples over. The movements of the 
legs are not well coordinated and there is consequently much difficulty 
in locomotion. Orientation in these specimens obviously can not be due 
