In the normal environment in which they live, organisms are almost always, if not always, subject to different external stimulations to which they may be more or less sensitive. They are therefore subject to many tropic reactions, and if one supposes that no other reaction manifests itself with them, it may be isaid that their behavior results from an equilibrium between these reactions, or rather from the predominance of one over the others.
That is what is perfectly shown by the pretty experiments of Mayer and Soule (1906) on the caterpillars of an American butterfly, Danais archippus. Under natural conditions these caterpillars feed extensively on the leaves of Asclepias, choosing the upper ones. They differ from the larvae of Argynnis and of Catocala in that they seem incapable of searching far for their food, and, once put on the ground, they perish if a clump of Asclepias is not found in the immediate neighborhood. So each individual passes his whole life on the same plant. The behavior of these caterpillars is explicable, not by their tastes as one would be led to believe, but by their tropisms. (1) Placed in darkness on a vertical cord, the head below, they turn around to climb toward the top, and again make the turn when the cord is reversed. A potted Asclepias was suspended upside down and larvae placed in the terminal leaves crawled upward and over the pot; when the plant was reversed, they walked up the pot and to the summit of the plant. In both of these experiments they are guided solely by their negative geotropism, for their behavior is the same whether they are young or well grown. (2) In a horizontal tube closed at one end and open at the other they went to the open and thus showed positive phototropism. This tropism lasts through their life, but it is especially provoked by the ultra-violet rays, for it does not appear at all when the light is from a kerosene lamp or daylight passing through a bisulphide of carbon screen which excludes these rays. (3) The geotropism of these caterpillars is stronger than their phototropism, for if the tube is held vertically, the closed end upward, they go to this perfectly dark end. So much for the behavior of this species. They feed indifferently on the natural leaves of Asclepias and on those artificially colored, so that coloration does not guide them. They do not use associative memory, for they quickly renew attempts to eat unfit leaves which are offered them when they are on their proper food plant. They are poor creatures, badly endowed, which stay and feed in the place where the predominant power of their negative geotropism has placed them.
We have seen in the preceding chapter that a little burrowing crustacean, Cuma ratkkei, although it passes all its life in its burrow, has a very intense positive phototropism, as intense as that of the Euglenas. To explain its behavior in a state of nature, Loeb tried the following experiment:
I placed a little glass vessel filled with mud in the same jar which contained the crustaceans. The animals did not seem to notice the vessel, as none of them moved toward the mud which it contained. When I disturbed them (touching them with a pencil), they swarmed upward at first, then, if not too greatly excited, slowly fell back to the bottom. If one fell into the little vessel it became active as soon as it touched it. It dug in actively, after which it could not be induced to react to light. Thus we see that the effect of contact with a little receptacle predominates that of light. The stimulus of contact (stereotropism) is more intense than heliotropism (phototropism). It is thus that an animal, although a poor swimmer, lives far from the light in spite of its positive heliotropism.