The ophthalmologist Hess[238] has utilized the heliotropic reactions of animals in an attempt to prove that all animals from the lowest invertebrates up to the fishes inclusive suffer from total colour-blindness. This statement was based on the observation that for most positively heliotropic animals the region in the yellowish-green near λ = 520 µµ seems the most efficient. Since this region of the spectrum appears also as the brightest to a totally colour-blind man, he concluded that all these animals are totally colour-blind. There is no reason why heliotropic reactions should be used as an indicator for colour sensations; if totally colour-blind human beings were possessed of an irresistible impulse to run into a flame Hess’s assumption might be considered, but no such phenomenon exists in colour-blind man. Moreover, v. Frisch[239] has shown by experiments on the influence of the background on the colouration of fish as well as by experiments on bees and on _Daphnia_ that the reactions of these animals to light of different wave-lengths indicate different effects besides those of mere intensity. Thus v. Frisch could train bees to go to a blue piece of cardboard distributed among many cardboards of different shades of grey. Bees thus trained would alight on any blue object even if it contained no food. It would be impossible to do this with totally colour-blind organisms. [238] Hess, C., “Gesichtssinn,” _Winterstein’s Handb. d. vergl. Physiol._, 1913, iv. [239] v. Frisch, K., “Der Farbensinn und Formensinn der Biene,” _Zoöl. Jahrb. Abt. f. allg. Zoöl. u. Physiol._, 1914, xxxv. See also Ewald, W. F., _Ztschr. f. Sinnesphysiol._, 1914, xlviii., 285. 9. Heliotropic reactions play a great rôle in the preservation of individuals as well as of species. In order to understand this rôle it must be stated that the photosensitive substances appear often only under certain conditions and that their effect is inhibited under other conditions. Thus among ants the winged males and females alone show positive heliotropism,[240] while the wingless workers are free from this reaction. This positive heliotropism becomes violent at the time of the nuptial flight and this phenomenon itself seems to be a heliotropic phenomenon since it takes place in the direction of the light. When the queen founds her nest she loses her wings and becomes negatively heliotropic again. Kellogg[241] has shown that the nuptial flight of the bees is also a purely heliotropic phenomenon. When a part of the hive remote from the entrance is illuminated the bees rush to the light and can thus be prevented from swarming. These phenomena suggest that the presence of some substance secreted by the sex glands may cause the intensification of the heliotropism which leads to the nuptial flight. [240] Loeb, J., _Der Heliotropismus der Tiere_, 1889. [241] Kellogg, V. L., _Science_, 1903, xviii., 693.