THE SENSATIONS OF AXTS. 515 



question from the standpoint of mechanical stimulation rather than 

 attempt to set up questionable distinctions based upon human sensations." 



In Chapter IV attention was called to the fact that the eyes of ants, 

 especially those of the workers, exhibit remarkable differences of 

 development in different species. That this implies an equally wide 

 range in the reactions of these insects to light is proved by observation 

 and experiment. There can be little doubt that these reactions are both 

 phototropic and visual ; in other words, that they involve not only an 

 adaptive orientation of the ants' movements as a whole with reference 

 to the direction of the light rays, but also some discrimination of colors 

 and objects. We are not surprised to find that the workers of hypo- 

 gaeic ants, like the smaller species of Solcnopsis, all species of Acan- 

 thomyops andDorylus and certain species of Eciton, which are virtually 

 cave-dwellers and either have no eyes or mere vestiges of these organs, 

 are strongly photophobic, or negatively phototropic ; whereas certain 

 tropical tree ants (Gigantiops, Dimorphomyrmex, Mynnotcras, Gcso- 

 myrmex and Pscudomyrma}, which have large convex eyes, are posi- 

 tively phototropic. The w r orkers of the common ants of temperate 

 regions (Camponotus, Formica, Lasius, Mynnica) stand about mid- 

 way between these two extremes in the development of the eyes, but 

 are, on the whole, rather strongly photophobic. This is shown by their 

 actions when suddenly exposed to the light, especially when they happen 

 to be in possession of their brood. However slowly or reluctantly they 

 may react to sudden illumination when alone, they make the greatest 

 haste to remove their brood to a dark place. 



The phototropic response, both in the workers and the sexual forms, 

 may be reversed rather suddenly, apparently as the result of changes 

 in physiological condition. This is very clearly seen in the males and 

 females, which in their callow stages are negatively phototropic, but as 

 the time for the marriage flight approaches, exhibit a strong positive 

 phototrophism, as Loeb (1890) has shown. After their marriage 

 flight and the loss of their wings, the females again become negatively 

 phototropic and this reaction seems to become the more pronounced the 

 longer the insect lives. It is evident that these reactions are all highly 

 adaptive and depend on important physiological changes in the wing- 

 musculature and reproductive organs. The dependence of the photo- 

 tropism of the workers on physiological conditions is not so clear. It 

 is possible, however, that hunger may have something to do with chang- 

 ing the usual negative to a positive phototropism and impelling the 

 worker to leave the nest and forage, while repletion or exposure to the 

 light for several minutes may reverse the reaction and induce the ant 

 to return to the nest. But this is, perhaps, too simple an explanation, 



