1912] Ecology of Dragon-fly Nymphs 289 
predaceous insects. They feed upon small Crustaceans and 
the nymphs of May-flies. When kept in the aquarium, they 
prey upon each other. They also feed readily upon freshly 
killed Physa and small pieces of fresh beef. The nymphs 
usually wait for their prey to approach them before seizing it, 
but they may move a distance of 30—40 mm. toward it. The 
food is seized by the lobes of the labium as this organ is suddenly 
darted toward the prey. 
Agrionid nymphs respond negatively to the light from a 
Thompson projection lantern. They swim away from the 
source of illumination. This occurs both in the case of single 
individuals and also when a number of specimens are in a 
glass trough at the same time. If the position of the trough is 
reversed, the nymphs again swim away from the light. This 
sort of response continues even when the experiment is repeated 
a number of times. The movement away from the light is a 
photic response, and not a reaction to temperature.  Fre- 
quently, when a number of specimens are placed in the glass 
trough, the response to light is modified by contact with the 
sides of the dish and with other individuals. This contact 
causes the nymphs to become more or less motionless, and to 
remain in close contact with each other, with their bodies more 
or less parallel. Such responses are examples of their thigmo- 
tactic proclivities. They also exhibit a ‘‘clasping response,” 
seizing each other around the thorax and abdomen by means 
of their thoracic appendages. The thigmotactic and ‘‘clasping 
responses’ result in the grouping of the nymphs in clusters. 
These groups tend to form at the end of the trough farthest 
from the source of light, although they also occur at other 
points. Sometimes there are as many as fifty individuals in a 
group. The origin and permanence of the groups are due to 
the contact and ‘‘clasping responses.’”’ The response of 
Agrionid nymphs to photic stimuli may be overcome by the 
response to contact stimuli; there is an inhibition of the one by 
the other. The explanation of this phenomenon seems to rest 
in part at least, with certain changes—bodily conditions— 
which take place within the animal concerned. In many 
instances such changes probably occur rapidly. The clusters 
of nymphs break up from time to time. The disintegration 
seems to be due to several causes. The “‘spontaneous’’ move- 
ments of the insects in the groups are a factor in this. Such 
