244 
ZOOLOGY: S. O. MAST 
Proc. N. a. S. 
this point. If the two sources of Hght differ in intensity it will turn until 
it faces a point nearer the more intense source and the greater the differ- 
ence the nearer this source the point will be. Under the first conditions 
the two eyes are equally illuminated when the insect is oriented; under 
the second they are not; the region stimulated in one eye is more intensely 
illuminated and it is farther forward than that in the other eye, and the 
greater the difference in the intensity of the two sources, the greater the 
difference in the location and in the intensity of the illumination of the 
regions stimulated in the two eyes. Consequently the turning effect of 
stimulation of a given region of the retina in one eye is obliterated by 
simultaneous stimulation of the same region in the opposite eye, provided 
the stimuli are of the same magnitude, and by simultaneous stimulation 
of any other region in the retina of the opposite eye provided the stimuli 
in the two eyes bear the proper relation in magnitude. If the stimulus in 
one eye is located relatively farther forward than that in the other eye,, 
the former in order to produce complete inhibition, must be stronger than 
the latter, if farther backward it must be weaker. 
The elimination of the effect of stimulation in one eye by simultaneous 
stimulation in the other eye is not due to antagonistic action of the legs 
on opposite sides as demanded by the Ray theory of orientation as appHed 
to animals by Verworn, Loeb, Bohn, and others. The elimination is due- 
to the total absence of any appreciable effect of the stimulating agent on-, 
the muscles of the legs. When an insect is oriented in light, the light: 
has no immediate observable effect on the muscles. 
Photic orientation in these organisms is the result of series of coordi- 
nated reflexes in the legs of both sides specifically related to the localiza- 
tion of the stimulus in either eye and inhibition or modification of the- 
effect of the illumination in one eye by simultaneous illumination in the: 
other. 
This view of the process of orientation is in full accord with all of the- 
phenomena presented above. It accounts for orientation in insects with 
one eye covered or with legs on one side removed as well as for orientation 
in normal specimens. It accounts for orientation on the wing in a vertical 
plane. It accounts for orientation in Caprella and in the fiddler crab. 
It accounts, moreover, for the facts observed by Dolley (1916) in Vannessa. 
with one eye covered that the degree of deflection toward the functional 
eye in a horizontal beam of light is independent of the intensity, and de- 
creases as the edge of the beam is approached where it becomes zero and. 
that the degree of deflection may, in illumination from a concentrated, 
source directly above, be greater in weak than in strong light, if reflection 
from the back ground is eliminated. 
Photic orientation in insects is as a whole adaptive. It is rarely in^ 
jurious except under unnatural conditions, as, e.g., in light produced hyr 
