35 ENTOMOLOGY 



totropic response is either positive or negative according as 

 the organism moves, respectively, toward or away from the 

 source of light. Maggots of Lucilia ccesar and of many other 

 flies are negatively phototropic as a rule (Fig. 289, A), but in 

 the absence of light (other directive stimuli being excluded, of 

 course) wander about indifferently (Fig. 289, 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 formicaries 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 ; though the latter rays produce no sensible effect on the 

 human organism. 



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 ani- 

 mals to light are fundamentally the same. 



Photo taxis and Photopathy. A phototropic organism, if 

 bilaterally 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 in- 

 geniously contrived experiments, it has been found, apparently, 



