INSECT BEHAVIOR 287 



in general, the rays of shorter wave length, as violet or blue, are more 

 effective than those of longer wave length, as yellow or red; the latter in 

 fact acting like darkness. Ants avoid violet rays as they would avoid 

 direct sunlight, but carry on their operations under yellowish red light 

 as they would in darkness. Miss Fielde has made use of this fact in 

 studying the habits of ants, by using as a cover for her artificial formi- 

 caries an orange-red sheet of glass such as the photographer uses for his 

 dark room. Though ants avoid violet rays, they prefer them to ultra- 

 violet rays, as Lubbock found. 



These responses to light are inevitable on the part of the organism, 

 whether they are beneficial or harmful, and it is now becoming recognized 

 that the reactions of both plants and animals to light are fundamentally 

 the same. 



Phototaxis and Photopathy. A phototropic organism, if bilater- 

 ally symmetrical, orients itself with the head directly toward or else 

 directly away from the source of light and moves toward or away from 

 the light, as the case may be. In either event the long axis of the or- 

 ganism becomes parallel with the rays of light. Now a ray of light is 

 ever diminishing in intensity from its source, and it would seem that 

 differences of intensity along the paths of light rays determine the orien- 

 tation and consequent direction of locomotion of the organism. Some 

 investigators, however, distinguish between the effects of intensity of 

 light and those of its direction. Thus by ingeniously contrived experi- 

 ments, it has been found, apparently, that Protista (Strasburger), Daph- 

 nia (Davenport and Cannon) and the caterpillars of Porthesia (Loeb) 

 move toward a source of light even while, in so doing, they are passing 

 into regions of less intensity of illumination. For this migration as de- 

 termined by the direction of the light rays, the term phototaxis is by some 

 authors (as Davenport) reserved. Usually, however, the direction of 

 locomotion does depend on differences of intensity, without regard to the 

 direction whence the light comes. This " migration towards a region of 

 greater or less intensity of light" has been termed photopathy, and or- 

 ganisms are said to be photophil or photophob, according as they move, 

 respectively, toward or away from a more intensely illuminated area. 



Verworn and others maintain that differences of intensity are suf- 

 ficient to account for all phototropic phenomena. 



Optimum Intensity. It has been found that there is a certain 

 optimum degree of light, differing according to the organism, toward 

 which the organism will move, from either a region of greater illumina- 

 tion or one of less. The organism appears to be attuned to a " certain 



