244 Annals Entomological Society of America  [Vol. XIV, 
light. In some of my own experiments (1912, pp. 279, 280), 
it has been observed that Agrionid nymphs, Hexapoda belonging 
to a totally different order, the Odonata, exhibit responses 
during the process of forming groups that are decidedly similar 
to those that have been described for Gerris remigis, and for 
that reason and also for a basis of comparison, they are men- 
tioned here. 
4. DISASSOCIATION OF GROUPS. 
It has been pointed out that water-striders gather in groups 
on the surfaces of streams, the legs and bodies of different 
individuals being in contact with each other (Figs. 2, 3). It 
also has been stated that these aggregations, probably, are due 
to the thigmotactic propensities of the animals. This thig- 
motaxis may be overcome, or at least it may be modified, by 
other stimuli, with the result that the groups of gerrids are 
broken up. Frequently, this occurs because of mechanical 
stimuli acting on them. Usually, this is brought about by the 
activity of certain individuals in the clusters disturbing others 
around them; or it may be accomplished by wind-currents, of 
considerable strength, blowing against the groups and thus 
causing some of the water-striders to disentangle themselves; 
while in other instances, members of the aggregations become 
active from the stimuli received from the agitation of the surface 
of the stream by water-currents. Driftwood, or a bunch of dead 
leaves, drifting with the current, occasionally strikes against one 
of these groups of gerrids and acts as the initial stimulus which 
results in its disintegration. _Other Heteroptera exhibit similar 
responses, for Holmes (1905, pp. 318-325) has pointed out, in 
his experimental investigations on the water-scorpion, Ranatra 
fusca, that members of this species, responding to stimuli of 
one character, not only may have these responses modified, but 
also may have them inhibited through the influence of stimuli 
of a different character. It is interesting to notice that 
behavior of this general character is not limited to the Hexopoda, 
but that it is evinced by members of a less specialized class, 
the Crustacea, as Holmes (1901, p. 212) has demonstrated in 
his experiments on the freshwater shrimp, Gammarus locusta, 
belonging to the order Amphipoda and to the subclass Malaco- 
straca. 
