1921] Riley: Responses of Water-Strider 243 
surface of a large pool (Fig. 5), near the headwaters of a small 
brook, in the vicinity of Syracuse, New York, and on the 
water-film of pools of intermittent brooks, of permanent 
brooks (Fig. 6), and of creeks, in the region roundabout Urbana, 
Illinois, such gatherings frequently have been observed. Often 
they were so large and the gerrids were so numerous that two 
to five hundred insects could be taken in less than half an hour. 
It should be pointed out that the best and most satisfactory 
evidence, for the general facts stated above, concerning the 
formation of groups, has been obtained in the fall. The ‘“‘habit’”’ 
of this species of forming groups was pointed out by me (1912, 
p. 281), several years ago in the following statement: 
This tendency to cluster together frequently has been observed by 
the writer in the case of Gerris remigis Say. 
Other aquatic Heteroptera evince responses much like those 
that have been described, as Severin and Severin (1911la, 
pp. 100-101) have demonstrated on the part of several different 
species and genera. In connection with certain observations 
on the thigmotaxis of Belostoma flumineum Say, they state 
éhaits 
Again, it was not unusual to find two or more Belostomas or some- 
what larger clusters clinging together at the surface or bottom of the 
water, a characteristic which is also noticed with Lethocerus (Belostoma 
aucct.) americanum, Benacus griseus, Nepa apiculata, Ranatra americana, 
and Ranaira kirkaldyi. This habit is probably a manifestation of their 
thigmotactic responses. 
Holmes (1905, pp. 324-325) in his phototactic experiments on 
Ranatra fusca, Pal. B., of the family Nepide, states that: 
Efforts to go toward the light are frequently inhibited by contact 
stimuli. When several individuals are put into a dish of water near 
a window they commonly cease, after a time, to swim towards the light 
and form a cluster in which they lie at all possible angles to the direction 
_ of the rays. 
While the experiments referred to here are related to the 
consideration of the subject of the inhibition of one form of 
response by stimuli that result in responses of another character, 
yet they indicate that Ranatra fusca also forms aggregations 
through the action of contact stimuli. Holmes (1905, pp. 320, 
323) states that Ranatras group themselves into compact 
groups, in cool water, at the ends of dishes farthest from the 
