INSECT BEHAVIOR 303 



center of diffusion in much the same way that a moth follows a ray of 

 light to its source. In both cases a stimulus affects muscular tissue; 

 the animal orients its body until the muscular tension is symmetrically 

 distributed, and then locomotion brings the animal to the source of the 

 stimulus, whether it be food or light. 



The remarkable "instinctive" action of the fly in laying her eggs on 

 meat is due, according to Loeb, simply to the fact that both the fly and 

 the maggot have the same kind of positive chemotropism. Similarly 

 also in the case of such butterflies or other insects as lay their eggs on a 

 special kind of plant. It is certain that " neither experience nor volition 

 plays any part in these processes." 



W. M. Barrows determined experimentally that the well-known 

 pomace fly, Drosophila ampelophila, is positively chemotropic to amyl 

 alcohol, ethyl alcohol, acetic acid, lactic acid and other chemical sub- 

 stances, all of which occur in fermenting fruits. The fly finds its food, 

 not by sight, but by smell, and when this sense is lost it reaches its food 

 only by accident. The olfactory sense organs that are concerned with 

 finding food are located in the third or terminal segment of the antenna. 

 When one antenna is lost and the other antenna is stimulated by food 

 odor, circus movements are carried out in such a way as to prove that 

 the fly orients normally by an unequal stimulation of the antennae. 

 Drosophila, when stimulated by a weak food odor, first shows random 

 movements, i.e., it attempts to find the food by the method of trial and 

 error, but as the fly passes into an area of greater stimulation, these 

 movements give way to a direct orientation. 



Hydrotropism. Wheeler observed that beetles of the genera Hali- 

 plus and Hydroporus were positively hydrotropic; that when released on 

 the shore from a bunch of water plants, they scrambled toward the lake, 

 twenty feet away. Collectors take advantage of the negative hydro- 

 tropism of Bembidion, Elaphrus, Omophron and other shore-dwelling 

 beetles by splashing the water upon the dry bank, when the beetles leave 

 their places of concealment and are easily caught. 



It is well known that after a rain ants carry their young out into the 

 sunshine, though when the upper parts of the nest become too dry, the 

 ants transfer their eggs, larvae and pupae to lower and moister galleries. 

 In these instances, however, we have to deal with thermotropism as well 

 as hydrotropism. 



Thigmotropism. Negative thigmotropism (stereotropism) as dis- 

 played in the withdrawal from contact, is a common phenomenon 

 among animals, from Protozoa to Vertebrata, and is often conducive 



