INSECT BEHAVIOR 285 



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 [thigmo tropic] stimuli to produce the reaction." 



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. 



Anemotropism. Various flies orient the body with reference to the 

 direction of the wind. Wheeler observed swarms of the male of Bibio 

 albipennis poising in the air, with all the flies headed directly toward the 

 gentle wind that was blowing. If the wind shifted, the insects at once 

 changed their position so as again to face to windward; a strong wind, 

 however, blew them to the ground. The males of an an thorny iid (Ophyra 

 leucostoma) , according to the same naturalist, hover in swarms in the 

 shade for hours at a time; if the breeze subsides they lose their definite 

 orientation, but if it is renewed they face the wind with military precision. 

 In Syrphidae, he finds, either males or females are positively anemotropic. 

 The midges of the genus Chironomus, which on summer days dance in 

 swarms for hours over the same spot, orient themselves to every passing 

 breeze. So also in the case of Empididae, which Wheeler has observed 

 swarming in one spot every day for no less than two weeks, possibly on 

 account of "some odor emanating from the soil and attracting and arrest- 

 ing the flies as they emerged from their pupae." 



The Rocky Mountain locusts "move with the wind and when the air- 

 current is feeble are headed away from its source"; .when the wind is 

 strong, however, they turn their heads toward it. 



Anemotropism and rheotropism are closely allied phenomena. As 

 Wheeler says, "The poising fly orients itself to the wind in the same way 

 as the swimming fish heads upstream," adjusting itself to a gaseous 

 instead of a liquid current. "In both cases the organism naturally as- 

 sumes the position in which the pressure exerted on its surface is sym- 

 metrically distributed and can be overcome by a perfectly symmetrical 

 action of the musculature of the right and left halves of the body." 



