1921] Riley: Responses of Water-Strider 257 
openings and came to rest there, often staying in a quiescent 
condition for several hours and on a few occasions for several 
days. In these various observations, it appeared as if the 
angles of the aquaria and the close proximity of the stones 
exerted contact stimuli to which the water-striders responded 
by coming to rest in such positions. In situations of this 
character large areas of their bodies were in close application 
to the substrata. 
Certain members of the class Crustacea, belonging to the 
order Amphipoda, for example some of the species of the 
family Orchestiide, evince thigmotactic responses which are 
very similar to those of Gerris remigis. Because of this simi- 
larity, attention is directed to the careful observations made 
by Holmes (1903, pp. 194, 195) on three species of the Amphi- 
poda, Orchestia agilis, Allorchestes littoralis and Orchestia 
palustris. Brief quotations from the paper by Holmes are 
given here: 
O. agilis generally continues hopping until it alights in a place 
where it can readily get under some object or wedge itself between 
bodies, so that it secures contact on a considerable surface of its body. 
Writing of Allorchestes littoralis, he mentions how it glides away 
when it is disturbed and continues his remarks as follows: 
The efforts are continued until they bring the creature into some 
niche or crevice where the contact sought for is obtained; then it 
curls up and lies quiet. 
With reference to Orchestia palustris, Holmes states that: 
The tendency to get under or between objects is as strongly devel- 
oped in this species as in agilis, and contact has apparently a stronger 
quieting effect upon it. When lying quiet O. palustris may be poked 
about more or less without being aroused from its thigmotactic lethargy. 
While it is true that the observations on Gerris remigis were 
made on individuals confined in aquaria and the records of 
Holmes were obtained in the habitats of the Amphipods, yet 
the responses in the two cases were so much unlike—and 
similar behavior on the part of the gerrids has been observed 
in their own environment—that a brief comparison seemed to be 
desirable. 
