284 Annals Entomological Society of America [Vor ae 
strong electric ight. At irregular intervals the nymphs in the 
cluster make “‘spontaneous’’ movements—or at least movements 
that are often difficult to interpret from any external cause, though 
some of them may be due to the continued action of the electric 
arc—and such movements act as mechanical—contact—stimuli 
on other members of the group. This naturally tends to cause 
the animals to change their relative positions to some extent; and 
no matter how slight a change in position this may be, there is, 
for a short space of time, a slackening of the grip of the append- 
ages. At such moments an opportunity is presented for photic 
stimuli to act upon the nymphs, and they frequently break 
away from the aggregation, responding with the negative reac- 
fon to tisht.© This is more readily understood if ~ © realize 
that the organisms are probably in a different pry Gological 
state than they were when the group was first formed—a state 
which now results in a response to the photic stimuli, and not 
to thigmotactic stimuli. The “‘spontaneous”’ or other slight 
movements of the animals in the closely packed masses are of 
great importance in initiating the breaking up of the clusters. 
Holmes (1095, p. 323) has observed in his experiments with 
Ranatra that the groups are more apt to be broken up as the 
insects become more active. The dragon-fly nymphs in the 
peripheral portion of the clusters are more likely to swim away 
first, while those in the central part are the last to leave the 
collections, as it is more difficult for the electric light to affect 
them in such a position. They are largely shut off from the 
light because of the nymphs surrounding them, besides being 
more largely influenced by contact stimuli. Holmes (1905, p. 
308) states that in these clusters formed by Ranaitra, the insects. 
‘are often so closely aggregated and so tangled together that 
those which are near the center of the group experience much 
difficulty in disengaging themselves.”’ 
We have noticed that dragon-fly nymphs respond nega- 
tively to photic stimuli. It has also been shown that they are 
positively thigmotactic. Their negative response to light often 
brings them against the sides of the experimentation dish and 
against each other. This contact invokes thigmotaxis, and the 
animals become grouped together. Mechanical—contact— 
stimuli plus the influence of the strong electric light inhibits the 
contact reactions for the time being, and the aggregation is broken 
up, the organisms swimming away from each other in response to 
