350 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 cesar and of many other 
flies are negatively phototropic as a rule (Fig. 289, 4), 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. 
Phototaxis and Photopathy.—A phototropic organism, if 
bilaterally symmetrical, orients itself with the head directly 
toward or else directly away from the source of hght 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 mtensity of light and those of its direction. Thus by in- 
geniously contrived experiments, it has been found, apparently, 
