606 PHYSIOLOGY: W. E. CARREY 
animal may push itself up and back until poised vertically on the tips 
of the wings and abdomen. The tendency to fly is very pronounced 
in this condition and upon the slightest disturbance the fly soars up- 
ward and backward, striking the top of a confining glass dish or com- 
pleting a circle by "looping the loop" backward. If it falls upon its 
back it rights itself by turning a backward somersault. Unequal black- 
ening of the lower parts of the two eyes results in a combination of the 
effects just described, with those described for blackening one eye, for 
the animal also performs circus motions. 
4. With the upper halves of the eyes blackened the attitude is the re- 
verse of that described in the preceding section. The anterior and 
middle pairs of legs are flexed. The anterior and posterior ends of the 
body bend ventral ly with the body in emprostho tonus. The head is 
bent far down until its anterior aspect is parallel to the surface of the 
table. The animal may actually stand on its head, but the abdomen 
retains its ventral curvature, leaving a considerable angle open between 
its dorsum and the wings which normally rest on it. 
In both walking and flying it continually keeps close to the table, 
and upon encountering an obstacle it frequently does a forward somer- 
sault. If it gets on its back it rights itself with greatest difficulty as 
its efforts simply result in bending the tail and head ventrally until 
they may form a complete ring. In galvanotropism the same general 
picture is presented by Palcemonetes and Amhly stoma when the anode 
is at the head end, the tonus changes involved being identical in the 
two conditions. 
5. By blackening the upper half of one eye and the lower half of the 
other, circus motions are performed toward the side with the lower half 
blackened. If for example the lower half of the right eye and the upper 
half of the left eye of the robber fly are blackened flexion of the anterior 
leg on the right side results. There is extension of the anterior leg on 
the left side. The body is thus somewhat twisted on its long axis 
for there is also flexion of the posterior leg on the left side and exten- 
sion of its mate on the right side. Such a bizarre effect is hardly ex- 
pHcable by any recourse to the assumption that the animal is ''avoiding 
the darkened field." In reahty the tilting of the body and the twist 
of the head are toward the blackened part of the eye viz., down on the 
side on which the lower half of the eye is blackened, and up on the 
side on which the upper half has been blackened. The result is readily 
explained on the basis of a crossed innervation from the eyes to the 
extensor muscles of the opposite side, a view for which much evidence is 
at hand. 
