1912] Ecology of Dragon-fly Nymphs 291 
ficial. Such responses not only aid in concealment from 
enemies, but also assist in obtaining food. Beneficial reactions 
are frequently “‘pleasureable’’; at least they are not usually 
‘painful’. On the other hand injurious responses are often 
““painful’’; certainly they are not “pleasant’”’. The nymphs 
are guided fairly well if they follow their “‘likes’’ and “‘dislikes”’, 
if such terms may be used. Spencer (1885) has maintained, 
and Holmes (1910) and (191la) has discussed the probability 
that the connection between “pleasure” and “pain’’ has 
arisen through natural selection. If this should prove to be 
true with respect to other animal groups, there seems no reason 
why it should not apply to Agrionid nymphs. 
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