CHAPTER XI

TROPISMS AND INSTINCTS AS ADAPTATIONS

Of the different kinds of adaptation none are more remarkable than those connected with the immediate responses of organisms to external agents. These responses are usually thought of as associated with the nervous system; and while in the higher forms the nervous system plays an important rôle in the reaction, yet in many cases it is little more than the shortest path between the point stimulated and the muscles that contract; and in the lower animals, where we find just as definite responses, there may be no distinct nervous system, as in the protozoa, for instance.

Many of the so-called instincts of animals have been shown in recent years to be little more than direct responses to external agents. Many of these instincts are for the good of the individual, and must be looked upon as adaptations. For example: if a frog is placed in a jar of water, and the temperature of the water lowered, the frog will remain at the top until the water reaches 8 degrees C., when it will dive down to the bottom of the jar; and, if the temperature is further lowered, it will remain there until the water becomes warmer again, when it will come to the surface again. It is clear that, under the ordinary conditions of life of the frog, this reaction is useful to it, since it leads the animal to go to the bottom of the pond on the approach of cold weather, and thus to avoid being frozen at the surface.

Another illustration of an instinct that is a simple response to light is shown by the earthworm. During the day the worm remains in its burrow, but on dark nights it comes out

of its hole, and lies stretched out on the surface of the ground. It procures its food at this time, and the union of the individuals takes place. In the early morning the worm retires into its burrow.

This habit of the earthworm is the direct result of its reaction to light. It crawls away from ordinary light as bright as that of diffuse daylight, and, indeed, from light very much fainter than that of daylight. If, however, the light be decreased to a certain point, the worm will then turn and crawl toward the source of light. This lower limit has been found by Adams to be about that of .001 candle-metre. This corresponds to the amount of light of a dark night, and gives an explanation of why the worm leaves its burrow only at night, and also why it crawls back on the approach of dawn. It is also obvious that this response is useful to the animal, for if it left the burrow during the day, it would quickly fall a prey to birds.

The blow-fly lays its eggs on decaying meat, on which the larvæ feed. The fly is drawn to the meat by its sense of smell, a simple and direct response to a chemical compound given off by the meat. The maggot that lives in the decaying meat is also attracted by the same odor, as Loeb has shown, and will not leave the meat, or even a spot on a piece of glass that has been smeared with the juice of the meat, so long as the odor remains. Here again the life of the race depends on the proper response to an external agent, and the case is all the more interesting, since the response of the fly to the meat is of no immediate use to the fly itself, but to the maggot that hatches from the egg of the fly.

The movement toward or from a stimulating agent is, in some cases, brought about in the following way. Suppose an earthworm is lying in complete darkness, and light be thrown upon it from one side. The worm turns its head, as it thrusts it forward, to the side away from the light; and

as it again moves forward, it continues to bend its head away from the light, until it is crawling directly away from the source. When the light first strikes the worm, the two sides will be differently illuminated. This causes a bending

of the head, as it stretches forward, toward the side of less illumination, and the bending is due to a stronger contraction of some of the muscles on the less illuminated side; at least the reaction appears to be due to a simple response of this kind. When the body has been so far turned that the two sides are equally illuminated, the muscles of the two sides will contract equally, and the movement will be straight forward and away from the light. If the reaction is as simple as this (which is in principle the explanation advanced by Loeb), the result is a simple reflex act, and need not involve any consciousness or intentional action on the part of the worm to crawl away from the light. In fact, the same reaction takes place when the brain is removed, not so quickly or definitely, it is true, but this may be due to the removal of the anterior segments of the worm, in which part the skin appears to be more sensitive to light than elsewhere.

Another factor that plays an important rôle in the habits of the earthworm is the response to contact, - the so-called stereotropism. If, in crawling over a flat surface, the worm comes in contact with a crevice, it will crawl along it, and refuse to leave until the end is reached. The contact holds the worm as strongly as though it were actually pulled into the crevice. It can be forced to leave a crevice only by strong sunlight, and then it does not do so at once. If the worm crawls into a small glass tube, it is also held there by its response to contact, and the smaller the tube, the more difficult is it to make the worm leave by throwing strong sunlight upon it.

Loeb has found that when winged aphids, the sexual forms, are collected in a tube, and the tube is kept in a room, the aphids crawl toward the light.

This happens in

ordinary diffuse light, as well as in lamplight. It is stated that the animals orientate themselves towards the light more quickly when it is strong than when it is weak. They turn their bodies toward the light, and then move forward in the direction from which the rays come. It can be shown by a simple experiment that the aphids are turned by the direction of the light, and not by its intensity. If they are placed in a tube, and the tube laid obliquely before a window in such a way that the direct sunlight falls only on the inner end of the tube, the aphids will, if started at the inner end of the tube, first crawl toward the outer surface of the tube, and then wander along this wall, passing out of the region of sunlight into the end of the tube nearest the window, where they come to rest at the end. They have moved constantly towards the direction from which the rays come, passing, as it were, from ray to ray, but each time toward a ray nearer the source of the light.

If the tube be turned toward the window, and the window end be covered with blue glass, the aphids crawl into this end of the tube, as they would have done had the tube been uncovered. If, on the other hand, the end of the tube be covered with red glass, they do not crawl into the part of the tube that is covered, unless they are very sensitive to light. Even in the latter case they may remain scattered in the red part, and do not all accumulate at the end, as they do when blue glass is used. In other words, while they respond to blue as they do to ordinary light, they behave toward red as they do towards a very faint light.

In diffuse daylight the aphids, as has been said, crawl toward the light, but if they come suddenly into the sunlight they begin to fly. Thus they remain on the food plant until the sun strikes it, and then they fly away.

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.