1912] Ecology of Dragon-fly Nymphs 279 
from a different viewpoint—“‘ visual perception of movement’’— 
than that presented in the present paper. Further, Plateau’s 
work was done with the imagoes and not with the nymphs. 
Some research has been carried on by Sondheim (1901) in con- 
nection with damsel-fly nymphs, regarding the power of asso- 
ciating certain appearances with food. Radl (1903) is the only 
author, of which the writer is aware, who has published any 
observations of this nature upon dragon-fly nymphs. Atten-_ 
~tion should also be called to the very interesting work of von 
Uexktll. One of his investigations in a series of Studies on 
Tonus was on the dragon-fly (1908). 
Agrionid nymphs react strongly to the light from the 
electric arc of a Thomson projection lantern, swimming away 
from the source of illumination. Experiments were performed 
with separate individuals and‘also with a number of nymphs in 
the glass trough at the same time. After placing the vessel, 
containing the specimens, in the beam of light entering the dark 
room, it is seen that they swim away rapidly from the light to 
the far end of the dish—to the end farthest from the source of 
illumination. If the position of the trough is now reversed— 
the far end being placed in such a position that it is in the beam 
of light and facing its source—the creatures again swim away 
from the light. This experiment was repeated many times. 
Fresh nymphs were used from time to time.* At each experi- 
ment the animals swim away from the source of light and tend 
to congregate at the end of the glass trough most distant from 
the lantern. 
MOVEMENT AWAY FROM THE LIGHT NOT A RESPONSE TO 
TEMPERATURE. 
An observer of my experiments, who was at the time engaged 
upon some temperature studies of hydra, believed that the 
reactions described were responses to heat. That such was not 
the case was readily demonstated. A cell containing distilled 
water was placed in front of the projection lantern. The 
animals respond to the light in the manner previously stated. 
and swim to the far end of the glass trough. A second cell was 
placed immediately in front of the first, so that the beam of 
: *The specimens were taken from aquaria standing in a moderate light at some 
distance from a south window. Such nymphs had not been subjected to the kind 
of stimuli used in the experiments. Of course, this does not mean that no stimuli 
had been acting upon them, for stimuli constantly impinge upon all animals. 
