Evolution and Adaptation

by Thomas Hunt Morgan

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.

Night-flying moths were placed in a box and exposed in a room to ordinary light. As twilight approached the moths became active and began to fly always toward the window side of the box. They were positively heliotropic to light of this intensity. If let out of the case, they flew toward the window, where they remained even during the whole of the next day, fully exposed to light. If the moth is disturbed in the daytime, so that it flies, it goes always toward the light, and never away from it. These facts show that the moth is always positively heliotropic, and also that the flight toward the lamp is a natural response, misapplied in this case. That the moths do not fly by day is due to another factor, namely, the alternation in the degree of their sensitiveness at different times. But this condition alone does not seem to account fully for all the facts.

If the moths are given the alternative of flying toward the evening light, or toward the lamp, they always go toward the brighter light. Thus if, when they swarm at dusk, they are set free in the middle of the room, at the back of which a lamp is burning, the moths fly toward the window. If, however, they are set free within a metre of the lamp, they fly toward it.

The explanation that Loeb offers of the habit of these moths to fly only in the evening is, that, although they are at all times positively heliotropic, they respond to light only in the evening. In other words, it is assumed that there is a periodic change in their sensitiveness to light, which corresponds with the change from day to night. Loeb says that, just as certain flowers open only at night, and others only during the day, so do moths become more responsive in the evening, and butterflies during the day. Both moths and butterflies are positively heliotropic, and the sensitiveness of moths to light may be even greater in the evening than is that of butterflies, for the light of the evening to which the moth reacts is less than the minimal to which the butterfly responds.

Moths appear to pass into a sort of sleep during the day, while butterflies are quiescent only at night. The periodicity of the sleeping time continues, at least for several days, when the insects are kept in the dark. For instance, moths kept in the dark become restless as the evening approaches, as Réaumur observed long ago. It has been found in plants that this sort of periodicity may continue for several days, but gradually disappears if the plants are kept in the dark. By using artificial light, and exposing the plants to it during the night, and putting them in the dark during the day, a new periodicity, alternating with the former one, may be induced; and this will continue for some days if the plants are then kept continually in the dark.

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.