34^ ENTOMOLOGY 



their food or their mates; and negative chemotropism enables 

 them to avoid injurious substances. This negative reaction 

 on the part of other organisms is made use of also by such 

 insects as emit repellent odors. 



A maggot orients its body with reference to a source of 

 food and then moves toward the food just as mechanically as 

 a moth flies to a flame. The maggot, as Loeb maintains, is 

 influenced chemically by the radiating diffusion from a piece 

 of meat, and follows a line of diffusion to the center of diffu- 

 sion 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 sym- 

 metrically 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 posi- 

 tive chemotropism. Similarly also in the case of such butter- 

 flies 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 gen- 

 era Haliplus 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 hydrotropism 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, larvse 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 



