1921] Riley: Responses of Water-Strider 235 
found on both plants in the angles formed by the whorls of leaves 
arranged around the stem. 
When experiments are performed with a number of individuals in 
the glass trough, it is found that their movements are often very much 
impeded. As they swim away from the source of illumination, they 
frequently come in contact with the sides of the vessel and with other 
individuals. This contact, in many instances, impedes the movement 
away from the light, and causes the nymphs to become practically 
motionless. This is the result of the contact stimulus. They usually 
assume a position with the long axes of their bodies parallel with each 
other and in close contact, although this relation may be modified 
considerably. Another response, which the writer designates as the 
“clasping response,”’ quickly follows. The nymphs clasp each other 
closely around the thorax and abdomen. The preliminary contact of 
their bodies causes locomotion to cease, being an example of true 
thigmotaxis. Then as the full surface of the body of one is applied to 
the body of another the ‘“‘clasping response”’ results. 
Before passing from these brief references to the writings of 
other observers on thigmotaxis in aquatic Heteroptera and 
Odonata, attention well may be directed to certain observations 
on the Crustacea, a less specialized class than the Hexapoda. 
Holmes (1903), in a very valuable and suggestive paper, has 
pointed out that this form of response is exhibited in a high 
degree by certain genera and species of the order Amphipoda, 
belonging to the family Orchestiide, of the subclass Malacos- 
traca. Certain statements of his, with respect. to two species, 
Orchestia agilis and Allorchestes littoralis, will be given here. 
He (1903, pp. 194-195) remarks that: 
This instinct of O. agilis to get into close contact with solid objects 
is an expression of the strong thigmotactic tendency found among 
amphipods in general. It is a tendency especially marked in the aquatic 
representative of the Orchestide, Allorchestes littoralis. . . . The thig- 
motactic reactions of amphipods keep these animals among the sea- 
weeds and rocks where they secure protection and obtain food. The 
behavior of the terrestrial O. agilis in relation to solid objects is little 
modified beyond that of the aquatic species. The thigmotaxis of this 
form is certainly protective in function, not only by enabling the animal 
to escape detection by lying quiet, but by leading it into situations such 
as under stones or into crevices which are inaccessible to its enemies. 
