Sensory Discrimination : Vision 137 



On the color sense of insects there are first the old ex- 

 periments of Graber by the Preference Method, whose most 

 definite result was to show that positively phototropic insects 

 prefer colors containing the ultra-violet rays, while the nega- 

 tively phototropic ones prefer red, from which these rays are 

 absent. No proof that the discriminations were made on the 

 basis of color proper rather than brightness was forthcoming 

 (151). Similar observations were made by Lubbock on ants, 

 which in their underground life are negatively phototropic, 

 the eggs and larvas apparently needing darkness in order to 

 develop, but on their foraging expeditions are comparatively 

 indifferent to light. They showed a preference for red when 

 tested, and a tendency to avoid the ultra-violet rays, so marked 

 that they preferred bright daylight from which these rays 

 had been extracted by chemical screens, to darkness that con- 

 tained the ultra-violet (248, pp. 207 fL). Graber suggested 

 that the ultra-violet rays produce a skin sensation in the ants ; 

 but Forel agrees with Lubbock that the effect is visual, be- 

 cause he found that varnishing the eyes made the ants in- 

 different to ultra-violet (130). Ants of the family Lasius seem 

 to be normally insensitive to these rays (134). It is just pos- 

 sible, then, that a visual sensation of quality wholly foreign 

 to our experience may accompany the action of ultra-violet 

 rays on insects. Loeb has noted that the relative effect of 

 violet and ultra-violet vibrations, as compared with that of the 

 rest of the spectrum, is greater, the less developed the visual 

 organ (233). 



Lubbock's experiments on the color sense of bees are more 

 to the point than those on ants, for they were made not by the 

 Preference Method, but by associating a color with food. No 

 precaution, however, was taken against the brightness error. 

 He found that bees which had eaten honey from blue paper 

 would pick out the blue pieces from a number of differently 



