The aphid also shows another response; it is negatively geotropic, _i.e._ it tends to crawl upward against gravity. If placed on an inclined, or on a vertical, surface, it will crawl upward. Such an experiment is best made in the dark, since in the light the aphid also responds to the light. If put on a window it crawls upward never downward. Aphids are also sensitive to heat. If they are placed in a darkened tube and put near a stove, they crawl away from the warmer end; but if they are acted upon by the light at the same time, they will be more strongly attracted by the light than repulsed by the heat. We thus see that there are at least three external agents that determine the movements of this animal, and its ordinary behavior is determined by a combination of these, or by that one that acts so strongly as to overpower the others. The swarming of the male and female ants is also largely directed by the influence of light. Loeb observed that when the direct sunlight fell full upon a nest in a wall the sexual forms emerged, and then flew away. Other nests in the ground were affected earlier in the day, because the sun reached them first. These ants, when tested, were found to respond to light in the same way as do the aphids. The wingless forms, or worker ants, do not show this response, and the winged forms soon lose their strong response to light after they have left the nest. Thus we see that the heliotropism is here connected with a certain stage in the development of the individual; and this is useful to the species, as it leads the winged queens and males to leave the nest, and form new colonies. Even the loss of response that takes place later may be looked upon as beneficial to the species, since the queens do not leave the nest after they have once established it. It is familiar to every one that many of the night-flying insects are attracted to a lamplight, and since those that fly most rapidly may be actually carried into the flame before they can turn aside, it may seem that such a response is worse than useless to them. The result must be considered, however, in connection with other conditions of their life. The following experiments carried out by Loeb on moths show some of the responses of these insects to light.