1921] Riley: Responses of Water-Strider 267 
III. RESPONSES TO LIGHT. 
1. HABITAT RESPONSES TO SUNLIGHT. 
The responses of Gerris remigis to photic stimuli have not 
yet been worked out fully. Groups of these gerrids, in their 
habitats on the surfaces of brooks and creeks, frequently have 
been observed to assemble in the shade of the banks, of over- 
hanging rocks, and of trees. These aggregations are evident 
especially during the heated days of summer. Such gatherings 
are very common on the surfaces of streams in the prairie 
regions of Illinois, where, during the late summer, there is often 
a very high temperature and glaring sunlight untempered by 
any wind. On cloudless days, with the temperature at 100° F., 
the unshaded portions of streams frequently were entirely free 
from water-striders, but they were found on those parts of 
such waters that were shaded by trees, shrubs and other vegeta- 
tion. Occasionally, there were days of this character when no 
gerrids could be seen on the open surface of a brook, but often 
on reaching some situation where the water practically was 
concealed by overhanging willows, alders and herbaceous 
vegetation, it was found, when such vegetation was disturbed, 
that the water-striders darted out from under it in many 
different directions. Hungerford (1919, p. 116), writing of 
Gerris remigis in Kansas, states that: 
They are gregarious fellows, seeking a resting place in the shade of 
overhanging bank or bush, but taking wildly to the open when alarmed. 
However, this “‘seeking’’ of shaded situations may be the 
result of a combination of responses to the sunlight and to 
the heat, or it may be entirely a response to the latter form of 
stimulation. Although this needs to be tested in detail experi- 
mentally,* yet certain statements by de la Torre-Bueno (1911, 
p. 246) seem to suggest that temperature is a factor in influencing 
these gerrids to congregate in shaded situations. This observer 
remarks that: 
It [Gerris remigis] is to be found most frequently on running waters, 
although it also frequents still, but to a less extent. . . . They con- 
* Some experimental evidence already has been obtained and further experi- 
ments are under way. It is hoped that the results obtained will be published in a 
later paper. 
