280 Annals Entomological Society of America [Vol. V, 
light now passed through a wall of water approximately 15 cm. 
in thickness. My records show that, in a period of 30 minutes, 
the increase in temperature is very small. The dragon-fly 
nymphs react negatively to the light, as they did before either of 
the two cells were placed in position. This shows their movement 
to be a response to light, and not a reaction to temperature. 
RESPONSE TO CONTACT. 
When experiments are performed with a number of individ- 
uals in the glass trough, it is found that their movements are 
often very much modified. 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 to 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 
7” 
response’’ results. 
FORMATION OF GROUPS. 
The reactions above described often result in the formation 
of groups of nymphs. In this manner one large cluster may be 
formed, or a number of smaller ones. These collections may be 
formed at various points in the glass trough, but seldom at the 
end next to the source of light. They frequently form first at 
the end of the vessel farthest from the source of illumination. 
They may, however, form at other points nearer the light. As 
many as forty and fifty individuals, clinging together, are not 
infrequently counted in one cluster. The origin and perma- 
nence of the groups are due to the contact and clasping reactions. 
As various individuals swim away from the light, they may come 
in contact with other individuals, thus eliciting the thigmotactic 
response. Locomotion ceases and, as a greater bodily surface 
is applied in the case of the various nymphs, the “‘clasping 
response”’ is invoked, causing the creatures to hold each other 
tightly by means of their thoracic appendages. 
