1921] Riley: Responses of Water-Strider 241 
In the early and late fall, there appears to be a much greater 
tendency for these groups of water-striders, Gerris remigis, to be 
formed than is the case during any other season of the year. 
Long and continued observations, extending over a number of 
years, seem to indicate that the gerrids are much more respon- 
sive to thigmotactic stimuli just previous to and while migrating 
onto the land to ‘‘seek’’ hibernation quarters, than they are 
under most other conditions.* Early in the fall, even on the 
open surface of the brook-pools, they appear to congregate 
more thickly and more compactly, although engaged in their 
usual normal responses. As the fall advances, these somewhat 
loose formations—more dense in appearance, however, than 
those of the early summer—are found nearer to the banks 
of the streams (Figs. 5, 6). In the late fall, large numbers of 
gerrids, frequently, are observed close to the land, to rocks 
rising above the water surface, to tree-roots extending into the 
water, to dams of driftwood, in fact in close proximity to such 
objects and places as afford them opportunity for contact, and 
onto which they can crawl from the surface-film. Sometimes, 
for days together, they are found in such situations, forming 
large aggregations, with their legs and not infrequently their 
bodies touching and the former even overlapping (Figs. 2, 
Se GOVE 
Curiously enough, strong air-currents and surface-breezes 
blowing along the surface-film—which in the next section of 
this paper are designated as agents that are active in the 
disintegration of groups—assist in the formation of clusters of 
gerrids. Such currents frequently move the water-striders 
nearer to each other, thus making the loose groups more com- 
pact. In addition to the mere drifting together of the insects, 
the winds act as stimuli that produce active responses on the 
part of the gerrids, for under such circumstances, they usually 
quickly stride close up against each other or against some solid 
object in the vicinity (Figs. 2, 3, 5, 6). Such stimuli not only 
invoke the usual contact responses, but they also induce clinging 
responses on the part of the insects as soon as they come in 
touch with their neighbors (Figs. 2, 3). Such behavior is 
especially noticeable when the water-striders are gathered 
along the leeward side of a brook or creek (Fig. 5). Onetine 
* Data are being accumulated and experiments are being conducted with the 
expectation of obtaining precise evidence on this subject. 
