Loeb tried the experiment of exposing the quiescent moths suddenly to a lower intensity of light, in order to see if they would respond equally well at any time of day. It was found that if the change was made in the forenoon, between six o’clock and noon, it was not possible to awaken the moths by a sudden decrease in the intensity of the light. But it was possible to do so in the afternoon, long before the appearance of dusk. It appears, therefore, that in this species, _Sphinx euphorbiæ_, it is possible to influence the period of awakening by decreasing the intensity of light, but this can be done only near the natural period of awakening. It seems to me that this awaking of a positively heliotropic animal by decreasing the light needs to be further investigated. The day butterflies are also positively heliotropic. Butterflies of the species _Papilio machaon_, that have been raised from the pupa, remain quietly on the window in the diffuse daylight of a bright day. They can be carried around on the finger without leaving it, but the moment they come into the direct rays of the sun they fly away. Butterflies that have just emerged from their pupa case exhibit a marked negative geotropic reaction, and this appears to be connected with the necessity of unfolding their wings at this time. Loeb says that the same cause that determines the direction of the falling stone and the paths of the planets, namely, gravity, also directs the actions of the butterfly that has just left its pupa case. The geotropic response is especially strong at first. The animal wanders around until it reaches a vertical wall, which it immediately ascends, straight upward, and remains hanging at the top until its wings have unfolded. A similar response occurs in the final stage of the larva of the May-fly, which leaves the water and crawls up a blade of grass, or other vertical support, and there, bursting the pupa skin, it dries its wings and flies away. That this is a reaction to gravity and not to light is shown by Loeb’s observation, that their empty skins are sometimes observed under a bridge where the light does not come from above. “This observation on the larva of the May-fly contradicts the assumption that the ‘purpose’ of the geotropic response of the butterfly is that it may the better unfold its new wings, for in the ephemerid larva the negative geotropism appears at a time when no wings are present.” On the other hand, it should not be overlooked that the reaction is important for the May-fly larva in other ways, because it leads the larva to leave the water at the right period, and come out into the air, where the flying insect can more safely emerge.