274 Annals Entomological Society of America [Vol. XIV, 
the light had gone. Then, as it swung back and passed them 
again, some oriented themselves with their heads toward the 
light and moved for short distances in its direction along lines 
parallel with the end of the aquarium. In many instances— 
as the light swung past the gerrids—orientation toward the 
source of illumination was attempted, but this was not com- 
pleted before the light was well on its way back again toward 
them. On such occasions, the water-striders did not orient, but 
continued along the paths that they already had taken. It is 
evident, therefore, that there was a tendency for those gerrids, 
which had reached the glass end of the aquarium, to follow after 
the oscillating light and so to move, frequently for distances of 
one or two inches, back and forth in lines roughly parallel 
with the end of the aquarium. 
6. RESPONSES TO OSCILLATING ARTIFICIAL LIGHT OF 
WEAKER INTENSITY. 
Experiments were conducted with the view of discovering 
the character of the behavior of these water-striders when 
subjected to the stimuli of an oscillating artificial light of lesser 
intensity than 44 ca. m. in the experimental field. For this 
purpose, an incandescent electric light of 22 ca. m. in the field 
of experimentation was suspended in front of the glass end of an 
aquarium. Then, the light was made to oscillate as a pendulum 
in a plane parallel with the end of the receptacle. 
There were twenty gerrids on the surface of the water. 
When the light first was flashed onto the insects, it appeared 
as if all of them responded by so orienting their bodies that the 
long axes were parallel with the sides of the aquarium and their 
heads were pointed toward the source of illumination, but this 
soon was observed not to be the case. Five of them did not 
orient in this manner and fifteen of them did assume such 
positions as have been mentioned. Fifteen individuals responded 
positively to the light by striding along the surface-film toward 
the source of illumination. Five of the gerrids remained behind 
and did not move toward the light. Two out of the five 
responded by turning their heads toward the positive end of 
the aquarium, but they did not move in the direction of the 
light. The other three appeared to be rather indifferent with 
reference to the directive influence of the photic stimuli. The 
