1921] Riley: Responses of Water-Strider 233 
II. RESPONSES TO CONTACT. 
1. GENERAL RESPONSES. 
It is not uncommon for Gerris remigis, when striding along 
the surface-film to come in contact with some more or less solid 
body, such as a leaf, a piece of driftwood, a rock projecting 
above the surface of the water, the bank of the stream, or 
another gerrid (Figs. 2, 3, 6). Sometimes such contact does 
not appear to impede the movements of the insect. At other 
times the gerrid remains quiet for a few seconds only, while, 
on the other hand, there are instances when the animal keeps 
in contact with such surfaces for varying periods, extending 
from a few minutes to an hour or more. On some occasions 
only one leg may be in touch with the solid surface, the gerrid 
remaining there as if it were anchored, swaying gently with the 
breeze, or falling and rising with the movements of the surface 
of the water. Then again, one side of the body may be closely 
applied to some solid object, the animal staying quietly in one 
position, as if suddenly paralyzed. Such responses are very 
common in the daily life of this species and they appear to be 
due to its thigmotactic propensities (Figs. 2, 3). 
Members of other groups of aquatic Heteroptera exhibit 
habitat responses to contact of a somewhat similar character 
to those described for Gerris remigis. Holmes (1907, p. 163) 
found that Ranatra quadridentata Stal. of the family Nepide 
is responsive to contact stimulation. He directs attention to 
the fact that: 
As a tule Ranatra inhabits more or less shaded retreats among 
submerged grass or weeds near the water’s edge. It is kept in such 
situations, partly through the direct effect of its positive thigmotaxis, 
and partly because contact stimuli (as shown in a previous paper) 
cause it to become negatively phototactic. 
According to Essenberg (1915, pp. 381-382, 383, 390), sev- 
eral species and genera—Notonecta insulata Kirby, Notonecta 
undulata, var. charon, Notonecta indica, and Notonecta sp.—of 
the family Notonectide are thigmotactic. She states that: 
The two pairs of forelegs are sparsely covered with hairs and are 
provided with claws. The latter serve for the capture of food and for 
attachment to the surface-film, from which they hang with their heads 
downward, the posterior part of the ventral surface being exposed to 
