1912] Ecology of Dragon-fly Nymphs 283 
must conclude then that contact with solids so alters the physio- 
logical condition of the organisms that they no longer react to the 
other stimuli.’”’ This change in the internal condition of dragon- 
fly nymphs must take place very rapidly, for as soon as an 
organism—that is swimming along under the influence of photic 
stimuli from the projection lantern—comes in contact with other 
nymphs, locomotion stops, and the animal responds to contact 
stimuli. If the electric arc should be turned off—thus doing 
away with photic stimuli—it would not be surprising to see the 
nymphs react in a different manner. Such a change is undoubt- 
edly physiological, and must be largely explained by the new 
external stimuli acting upon the organism. Moreover, it may 
be possible that the stimuli from the electric arc have so modi- 
fied the animal’s internal condition that it responds to contact 
stimuli more readily than if it had not previously been sub- 
jected to photic influence. As Jennings (1906, p. 96) has well 
brought out in his work upon Paramecia, ‘‘The essential 
factor in the interference is a physiological one. When reacting 
to the contact stimulus, the animal is less easily affected by 
other stimuli, and when reacting to the other stimuli, it is less 
easily affected by the contact stimulus. Since the two stimuli 
in question require behavior of opposite character, it is indeed 
inevitable that one should give way to the other, or at least 
modify the behavior toward it; both cannot receive the usual 
reaction.”’ Mast (1911, p. 287) in his discussion of ‘‘ the effect 
of internal changes’’ makes the following interesting statement: 
““As a matter of fact, all reactions are directly controlled by 
internal forces which are in turn influenced by external factors.”’ 
Jennings (1904, pp. 109-127) and (1906, pp. 283-313) has 
written full and elaborate discussions concerning ‘‘ physiological 
states."’ Both of which are extremely interesting and sug- 
gestive. 
DISINTEGRATION OF GROUPS. 
Holmes (1905, p. 308) has found that Ranatras form a 
cluster in an aquarium. ‘‘In this way they may lie for hours 
in an almost motionless state.’’ While somewhat similar 
conditions may be observed in the case of dragon-fly nymphs, 
the periods of quiet are very much shorter and the disintegra- 
tion of the groups occurs from time to time. This is brought 
about by mechanical—contact—stimuli and response to the 
