282 Annals Entomological Society of America [Vol. XIV, 
These gerrids respond to contact in a different manner on 
different occasions to similar stimuli, although the general 
conditions are not changed. When two of them come in 
contact with each other, they may become stationary; they 
may move apart slowly; or they may dart rapidly away from 
each other. These variations in responses probably are due 
to the differences in the internal conditions of the animals at 
different times. 
Through the influence of contact, groups of various sizes are 
formed. The animals respond by remaining quietly on the 
surface-film, with their legs overlapping and their bodies 
closely applied to each other. These aggregations may origi- 
nate near some solid object, extending above the surface of the 
water, or they may arise through various individuals impinging 
against each other. Surface breezes assist in the formation 
of these assemblages. The physical conditions of the environ- 
ment, the physiological states of the gerrids, and the season of 
the year, all seem to have a bearing on this matter. Such 
groupings appear to be more common in the fall than is the 
case at any other season. They are larger and more compact 
at this time. The groups may consist of a few gerrids, as many 
as seventy-five to a hundred, or even more. Such aggregations 
are formed on the surfaces of pools, through which pass gentle 
currents, but usually they are not formed on the swiftest water, 
and they appear as dark patches on the surface-film. They may 
continue to exist from a few minutes to an hour and a half or 
for longer periods. 
The disintegration of the groups of gerrids appear to occur 
mainly through the modification or inhibition of thigmotaxis by 
means of other stimuli. Mechanical stimulation appears to 
have the greatest influence in this process. This is due, fre- 
quently, to the activities of certain individuals within the 
groups. Their activities disturb other gerrids in the immediate 
vicinity and eventually result in the disassociation of the groups. 
Those gerrids near the peripheries of the groups generally leave 
first. It is usually more difficult for the individuals within the 
groups, near the centers, to break loose from the clusters. 
Very strong wind currents; agitation of the surface by means of 
water-currents; objects drifting with the stream, such as dead 
leaves and driftwood; all these seem to have much the same 
