270 Annals Entomological Society of America  [Vol. XIV, 
surface-film in the direction of the source of illumination, 
remaining at the end of the aquarium next to the light. None 
of the water-striders evinced a total indifference to the light, 
for even those that did not stride to the positive end of the 
aquarium, exhibited some sort of response. In certain instances 
there were movements of the antenne, of the legs, or of the 
bodies. In other instances two or three of these sets of move- 
ments were observed. Two out of the four gerrids oriented 
themselves with their heads directed toward the source of light 
and they moved a few inches in the direction of the positive end 
of the aquarium. On the part of the two remaining insects, 
there was no orientation with respect to the light, but there 
were such movements of the legs, antennz and bodies as already 
have been mentioned. 
When the light was placed at the opposite end of the 
aquarium, the resulting responses of the gerrids were similar 
to those that have been described. The animals oriented with 
their heads toward the light and moved to the end of the 
vessel next to the source of illumination. Many individuals 
strode over the surface-film to the light very rapidly, the 
photic stimuli, apparently, exerting its influence on them 
immediately. By moving the light first to one end of the 
aquarium and then to the other, the gerrids were made to 
respond by moving to that end of the receptacle where the light 
temporarily was placed. Such experiments were performed 
many times with similar results. It is evident that Gerris 
vemigis responds positively to photic stimuli from an incan- 
descent light, with a field of illumination of 44 ca. m. 
It has been pointed out previously that the majority of the 
members of this species are apterous (Fig. 1), but that occasion- 
ally a few alate forms are found (Fig. 11). In the various 
experiments performed up to the present time, it has not been 
possible to make the gerrids leave the surface of the water and 
fly toward the light. However, it is probable that, under 
certain favorable conditions, they would do so. In a former 
paper by me (1920a, pp. 4-5), attention was directed to 
the fact that a related species, Gerris marginatus, has been 
observed to migrate by flight during moonlight nights, a 
probable response to photic stimuli. Drought may have some 
bearing on such flights, as they were noticed in an interval of 
