1921] Riley: Responses of Water-Strider 239 
and the various physical conditions of the immediate environ- 
ment remain unchanged. The natural assumption that occurs 
to the observer is that these different responses, to similar 
stimuli at different times, is due, probably, to the physiological 
conditions of the bodies of these insects varying at different 
times and, therefore, because of these differences in the physio- 
logical conditions, the gerrids do not respond in the same manner 
at all times to similar stimuli. Therefore, it may be assumed that 
the behavior of these animals is affected by the changes which 
occur within their bodies, and yet these changes can not take 
place unless there are either internal or external stimuli that 
bring about such changes. In this connection it is perhaps 
worth while to recall a brief statement by Mast (1911, p. 287): 
It is evident that such changes must be regulated by internal 
factors, that they must be due to alterations within the organism itself. 
As a matter of fact, all reactions are directly controlled by internal 
factors which are in turn influenced by external factors. 
The general subject of ‘‘nhysiological states’? in lower 
organisms has been elaborated by Jennings (1904, pp. 109-127), 
(1906, pp. 283-292). 
3. ASSEMBLING OF GROUPS. 
Not infrequently, after coming in contact with objects 
protruding above the surface-film (Figs. 5, 6) the legs of the 
gerrids, and sometimes their bodies, become closely applied; 
there may be several individuals taking part in such responses 
(Figs. 2, 3): In this manner a number of water-striders may 
stop their locomotor movements and attach themselves to 
the group, until, eventually, a large aggregation is formed. 
As many as seventy-five to a hundred individuals have been 
enumerated in groups of this character. Usually such responses 
occur, not on those parts of the stream where the current is the 
swiftest, but rather on the surfaces of small pools, with gentle 
currents passing through them (Figs. 2, 3, 4, Ose We 
gerrids may remain in such formations from a few minutes to 
an hour and a half, or for much longer periods. The general 
physical conditions of the environment, the season of the year, 
and the physiological conditions of the animals themselves, 
all these, seem to have a bearing on this matter. As viewed 
