1921] Riley: Responses of Water-Strider 283 
sort of influence in disintegrating the groups of assembled 
gerrids. 
Responses to contact stimuli are strongly in evidence at 
the inception of and during the hibernation period. These 
gerrids hibernate in many different situations, for example, 
under masses of dead leaves, in holes in the banks of streams, 
under tree roots, under the bark of fallen dead trees, under logs 
and driftwood, and under piles of brush. The acts of crawling 
into and remaining in such places are evidently due to the 
contact stimuli that impinge on them at such times. They 
usually hibernate in large groups or clusters, sometimes as 
many as two hundred and fifty gerrids being found in such 
assemblages, although they are taken in smaller numbers. 
On such occasions, they are in close contact with each other, 
their legs and bodies being’ closely applied. They remain 
very quiet, with their bodies in close contact with the sub- 
stratum. At such times, the bodies of the gerrids appear to 
be very sensitive to contact stimuli. They hibernate in close 
proximity to their own habitat. 
During severe droughts, the water in the habitat of these 
gerrids may disappear entirely. At such times they migrate 
from their dry habitat, often along the beds of streams. During 
such migrations, they frequently come in contact with various 
objects, such as stones, driftwood, clumps of dead leaves, and 
lumps of dry mud. Their thigmotactic propensities are in 
evidence at such times, for contact stimuli influence the gerrids 
to come to rest against and underneath these objects. They 
also jump into cracks in the dry beds of streams. Stimulation 
of this sort inhibits locomotor activities, and the responses of 
the animals to such stimuli influence them to remain quietly in 
these places, with their bodies closely applied to the objects 
with which they are in contact. Such thigmotactic responses 
keep the gerrids in places of this character from a few minutes 
to an hour or more and even for longer periods. 
In laboratory aquaria the gerrids frequently leave the 
surface-film and crawl onto the sides and upper surfaces of 
stones, extending above the surface of the water, often covering 
the stones several layers in thickness. They remain motionless 
in clusters for long periods of time. They crawl up the glass 
walls of the aquaria into the angles formed by the sides of the 
