INSECT BEHAVIOR 347 
the withdrawal from contact, is a common phenomenon among 
animals, from Protozoa to Vertebrata, and is often conducive 
to the safety of an organism; though the negative response 
occurs none the less, whether it is to prove useful or not, and 
occurs as automatically as the collapse of a sensitive plant at 
a touch. 
Positive thigmotropism is less common, though nevertheless 
widespread among animals. Protozoa and Infusoria cling to 
solid bodies and become aggregated about them. Cockroaches 
squeeze themselves into crevices until their bodies come into 
close contact with surrounding surfaces. A moth, Pyrophila 
(Amphipyra) pyramidoides, is accustomed to squeeze into 
crevices under loose bark or elsewhere, though this habit, 
though doubtless protective, is not performed for the purpose 
of self-concealment. That this is not a case of negative photo- 
tropism, it was proved by Loeb, who wrote: ‘‘ I placed some 
of these animals in a box, one-half of which was covered with 
a non-transparent body, the other half with glass. I covered 
the bottom of the box with small glass plates which rested on 
small blocks, and were raised just enough from the bottom to 
allow an Amphipyra to get under them. Then the Amphipyra 
collected under the little glass plates, where their bodies were 
in contact with solid bodies on every side, not in the dark cor- 
ner where they would have been concealed from their enemies. 
They even did this when in so doing they were exposed to 
direct sunlight. This reaction also occurred when the whole 
box was dark. It was then impossible for anything but the 
stereotropic [thigmotropic| stimuli to produce the reaction.” 
Rheotropism. 
against a current of water illustrate positive rheotropism. 
When facing the current, the resistance of the water is sym- 
metrically distributed on the body of the animal and is met 
by symmetrical muscular action, in the most economical man- 
ner. Many aquatic insects offer such examples of rheotropism, 
Fishes swimming or heading directly 
either positive or negative. 
Anemotropism.— Various flies orient the body with retfer- 
