284 ENTOMOLOGY 



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 or something else. 



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." 



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 Bembidium, 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, as displayed in the with- 

 drawal from contact, is a common phenomenon among animals, from 

 Protozoa to Vertebrata, and is often conducive to the safety of an or- 

 ganism; 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 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 sur- 

 faces. 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 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 



