1921] Riley: Responses of Water-Strider 245 
Some of the gerrids in these gatherings are more active than 
others and thus, probably, less responsive to contact stimult. 
These from time to time move their legs and bodies, in this way 
disturbing other .water-striders next to them. Such dis- 
turbances act as mechanical stimuli on still other individuals 
in the groups. In this manner, whole clusters may be affected 
and may become disorganized, so that there is a tendency for 
them to disintegrate. The insects that have become the most 
active are most likely to leave the aggregations first. Then 
others in their immediate vicinity follow, so that eventually 
all the members of any individual aggregation free themselves 
from it and are found striding back and forth on the surface-film 
as separate units. Often the gerrids on the outside of the 
groups are the first to break away, but this is not always the 
case, for individuals crowded into the centers of the masses of 
water-striders have been observed to pull loose from the others 
in contact with them and then to stride over, or to push between, 
those forming the peripheral parts of the groups, until they 
reached the free surface-film. Generally, it appears to be 
more difficult for the gerrids in the centers of the clusters to 
free themselves, than is the case for those nearer the outer 
margins to do so. 
With respect to the manner in which the disintegration of 
the aggregation of gerrids occurs, it is interesting to notice 
that members of a related family, Nepide, have been observed 
to evince behavior of a somewhat similar character. Holmes 
(1905, p. 308), writing of the responses of Ranatra fusca, found 
that individuals of this species formed groups in aquaria, and 
he states that: 
In this way they may lie for hours in an almost motionless state. 
He (1905, p. 323) also has observed that an aggregation of 
this character is more likely to disintegrate as the animals 
exhibit more activity. Again, this writer (1905, p. 308) remarks 
that in these clusters, which are formed by Ranatra fusca, the 
insects 
are often so closely aggregated and so tangled together that those 
which are near the center of the group experience much difficulty in 
disengaging themselves. 
Other Hexapoda display responses not dissimilar to those of 
Ranatra fusca and Gerris remigis in the disintegration of clusters. 
