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) pyramid oides, 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. Fishe? swimming or heading directly 

 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, 

 either positive or negative. 



Anemotfopism. Various flies orient the body with refer- 



