242 
ZOOLOGY: S. 0. MAST 
Proc. N. a. 
to equal action in locomotor appendages on opposite sides as the theory 
in question demands. Specimens of Bristalis with the front and middle 
legs on one side removed and either eye covered may under certain condi- 
tions proceed fairly directly toward the light. If the direction of the 
rays is changed while they are thus proceeding toward the light, they 
reorient by turning either to the right or to the left. This shows that the 
movements of the legs on either side may be controlled by impulses origi- 
nating in either eye. These responses are obviously not in accord with 
any theory of orientation that demands balanced or equal action either in 
the receptors or in the locomotor appendages on opposite sides. 
In certain fire-flies the male, in response to a flash of light produced 
by the female, turns thru the required angle, no matter in which direction 
he may be going in reference to the location of the flash, and proceeds 
directly toward her. Thus after momentary illumination, he may turn 
in the total absence of light in any direction, thru angles varying from 
0 to 180 degrees; and this turning is, under many conditions, a response 
to the illumination of but one eye. It is consequently evident that it 
cannot be accounted for by any theory which demands balanced or equal 
illumination of receptors on opposite sides as the basis of orientation. 
The facts presented above show that there are a number of phenomena 
of fundamental importance in the process of orientation in insects that 
are not in accord with the Ray-Verworn theory. How can these pheno- 
mena be explained ? 
Insects have image-forming eyes, so that when an eye is exposed to a 
source of light an image is produced on the retina, and if the source is 
concentrated the image is very small, resulting in definite localization 
of the stimulus. Thus while the whole surface of the eye is illuminated 
the stimulus may be confined to a minute portion of the retina, the location 
of which depends upon the location of the source of light in relation to the 
longitudinal axis of the insect. If the light is in front of the insect the image 
is near the anterior edge of the retina; if it is to the side the image is on 
the lateral surface of the retina, etc. In general it is always directly below 
the ommatidium whose longitudinal axis is parallel with the rays of light. 
The flash produced by a female fire-fly forms on the retina of the male 
a minute luminous image so that the stimulation is practically confined 
to a point. This point varies in location from the posterior to the anterior 
edge of the retina depending upon the axial position of the male in refer- 
ence to the female, and it also varies in a dorso-ventral direction. There 
is then for every axial position of the male a definite location of the stimulus 
in the eye. The fact that in response to a flash of light produced by the 
female, the male turns after the flash has disappeared until he faces the 
female regardless of the extent of turning required, indicates that momen- 
tary stimulation of a point on the retina sets up a series of turning reflexes,. 
