I 



INSECT BEHAVIOR 307 



either positive or negative according as the organism moves, respec- 

 tively, toward or away from the source of light. Maggots of Lucilia 

 ccesar and of many other flies are negatively phototropic as a rule (Fig. 

 296, A), but in the absence of light (other directive stimuli being 

 excluded, of course) wander about indifferently (Fig. 296, B). 



Do the different rays of the spectrum differ in phototropic power? 

 This question has occurred to many investigators, who have found that, 

 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 recog- 

 nized that the reactions of both plants and animals to light are funda- 

 mentally 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 

 organism 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), 

 Daphnia (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 determined 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 organisms are said to be photophil or photophob, accord- 



