1912] Ecology of Dragon-fly Nymphs 285 
their phototactic proclivities. Later on the nymphs may 
again form a group which in turn will be dispersed. Observa- 
tions very similar to these have been recorded by Holmes 
(1901, p. 212) upon Gammarus locusta. When these Crus- 
taceans are exposed to light, they swim away from the Cerato- 
phyllum, in the dish of water, to which they were clinging. 
They move rapidly to the negative end of the vessel, where they 
dart actively about, as if attempting to get farther away from 
the light. If these movements bring the animals in contact 
with the Ceratophyllum again, there they remain. If there are 
no objects in the water, the contact response is not invoked, and 
the organisms are found at the negative end of the dish. If 
thait aro objects in the water, the Amphiopods react to contact 
stimuli * 2never their chance movements bring them against 
a solid suiiace. The creatures remain in such a position until 
the phototactic impulse again causes them to swim away. 
INDEFINITENESS AND CHANGE OF PHOTIC RESPONSE. 
Sometimes, immediately following the breaking up of the 
aggregation of dragon-fly nymphs, the movements of the 
animals appear to lack definiteness. They swim away from the 
groups at various angles to the rays of light. While most of 
them sooner or later move toward the negative end of the glass 
trough, there are some whose reactions are indifferent, and a 
few evidence a tendency to positiveness in their responses to the 
light. This again the writer attributes to a change in the 
internal condition of the creatures, induced perhaps either by 
thigmotaxis or by the effect of the light from the projection 
lantern. Holmes (1905, pp. 318-325) has performed some very 
interesting experiments with Ranatra along similar lines. The 
insect is usually strongly positive in its responses to light; but 
this worker has been able to cause the animal to become nega- 
tively phototactic through the agency of contact stimuli. This 
change in the photic response, he was able to bring about many 
times. On one occasion the Ranatras were exposed to the light 
from a window for more than an hour and half, and it was 
found at the end of that time that all the specimens had become 
negatively phototactic. Previously they had all reacted posi- 
tively to the light, yet they had become negative in their 
reactions although the intensity of the light had increased. He 
has also demonstrated that the negative phototaxis of these 
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