304 ENTOMOLOGY 



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 auto- 

 matically as the collapse of a sensitive plant at a touch. 



Positive thigmotropism is less common, though nevertheless wide- 

 spread 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/or the purpose of self-con- 

 cealment. That this is not a case of negative phototropism, 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 corner 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." 



Among the water-striders, Gerridae, thigmotropism is strongly in 

 evidence at the inception of and during the hibernation period. The 

 gerrids hibernate in large groups or clusters under dead leaves, in holes 

 in banks of streams, under logs, etc., with their bodies in close contact 

 with the substratum. The acts of crawling into and remaining in such 

 places are evidently due to the contact stimuli that impinge on them at 

 such times (C. F. C. Riley). 



Rheotropism. Fishes swimming or heading directly against a cur- 

 rent of water illustrate positive rheotropism. When facing the current, 

 the resistance of the water is symmetrically distributed on the body of 

 the animal and is met by symmetrical muscular action, in the most eco- 

 nomical manner. Many aquatic insects offer such examples of rheo- 

 tropism, either positive or negative. 



E. P. Lyon gives, however, a different explanation. He found that 

 fishes orient themselves just as well when they are put into a closed 

 glass bottle, which is dragged through the water, although in this case 

 they are not under the influence of any friction from the current. When 

 the bottle is not moved the fishes swim in any direction inside the 



