268 Annals Entomological Society of America  [Vol. XIV, 
gregate in groups in shady, slow-moving parts of streams, at the tree 
roots projecting from banks into the water, in the shadow of bridges, 
and in general in almost any place where they have some shelter from 
the burning rays of the summer sun. 
Again referring to Gerris remigis, he (1917, p. 201) makes 
the following statement in connection with some of their habitat 
responses: 
These beasties are common and familiar sights to the lover of the 
quiet flowing waters running to the distant sea. In these haunts, in 
some still bay or moveless backwater, under a bridge, or in the shadow 
of a tree, or in the cool recesses of an overhanging bank, you may 
see remigis gathered in numbers, rowing silently about. . . . Here they 
rear large families and spend at ease the sultry dog-days. 
2. RESPONSES TO SUNLIGHT AND LABORATORY 
OBSERVATIONS. 
Although the habitat responses of the gerrids, referred to in 
the previous section of this paper, eventually may be proved 
to be responses to temperature rather than to light—sunlight— 
nevertheless, there is some positive evidence in connection 
with their behavior with respect to sunlight. During the sum- 
mer, numbers of Gerris remigis frequently have been kept in 
the laboratory in large aquaria, of the kind already described. 
On several different occasions, it was observed that when a 
strong beam of sunlight entered an aquarium, containing gerrids 
of this species, that they were found congregated at the end of 
the receptacle farther away from the entering beam of light. 
Sometimes they did not crowd to the end of the aquarium, but 
simply moved out of the sunlight into the more shaded parts of 
the vessel. One afternoon, two aquaria accidently were left 
on a laboratory table near a window, in such a position that 
they were practically parallel to each other. The aquarium 
next to the window was almost full of water, while the one 
farther away from the window contained only a few inches of! 
water. There were gerrids in both aquaria and in each case 
they were congregated at the end farther from the beam of 
sunlight that passed through both aquaria. Such behavior 
suggested that the water-striders were responding to the 
sunlight, but there remained the possibility that the response 
might be due to temperature, at least on the part of the gerrids 
