1921] Riley: Responses of Water-Strider 271 
extended dry weather. Attention is directed to a quotation 
from the paper just referred to: 
On perhaps three occasions, when droughts of this character extended 
into the fall, I have witnessed the flight of a few isolated specimens [of 
Gerris marginatus]. I wish to state that it was several years, after I 
first became interested in the family Gerride, before I saw a water- 
strider fly without some artificial stimulation. Flight occurs at dusk 
and during moonlight nights. This fact I observed for the first time in 
Illinois. Flight may take place at other times also, but it has not been 
seen by me. 
Essenberg (1915a, p. 400) has observed the flight of another 
closely related species, Gerris orba, in response to stimuli from 
artificial light. She states that: 
-Gerris remigis [orba] is positively phototactic. If it takes to its . 
wings once in a while it always flies toward the light, producing a 
buzzing sound as it flies. 
4. RESPONSES TO ARTIFICIAL LIGHT OF WEAKER 
INTENSITY. 
These water-striders respond not only to the stimuli of an 
electric light of 44 ca. m. in the field of experimentation, but 
they also respond to photic stimuli from a light of much less 
intensity. An electric incandescent light bulb, which gave 
22 ca. m. illumination in the experimental field, was hung at 
one end of the aquarium containing the gerrids. The general 
responses of the animals, with respect to the photic stimuli, 
were noted. When the light was flashed onto them, the 
majority so oriented themselves that their heads pointed toward 
the source of illumination and the long axes of their bodies were 
parallel with the sides of the aquarium and with many of the 
incoming rays. There were thirty water-striders in the 
receptacle. Twenty-two responded by striding over the surface- 
film to the positive end of the aquarium. Most of them 
congregated there, but they did not remain stationary. Some 
crawled up the perpendicular glass end; others remained with 
their heads directed toward the light; while still others moved 
back and forth at the lighted end of the aquarium. A number 
of similar experiments were performed and the results, with 
certain slight modifications, were much the same as those that 
have been described. 
