Comparative Psychology

Earthworms have the habit of dragging into their tubes leaves, pine-needles, bits of straw and other small objects, in order that the decaying plant-remains may serve them as nourishment. As early as 1881 Charles Darwin, who devoted a comprehensive work to the activities of worms, drew attention to the fact that the dragging down into the earth of these objects is done in a re markably purposeful way. The worms tend to catch broad-based leaves, such as those of limes or cherry trees, by the top, so that the leaf rolls itself up on being drawn into the narrow worm tube. It would have been useless to catch hold of the leaf by the stalk, for the broad base would have offered an almost insur mountable obstacle. The worms behave in an opposite manner with pine-needles, which are attached in pairs to a short basal piece. The worms take hold of them at the point where the two needles are joined together. Had one needle been dragged by its tip, the other would have been prevented from entering the hole. Mangold (1924) lately showed that there is a difference in the chemical make-up of leaf-blade and stalk, of needle-tip and basal piece. The worms scorn the taste of the leaf-stalks or of the needle-tips. In addition, further help is afforded by the fact that the leaf usually turns round on being drawn along on the ground, so that the point goes foremost while the stalk acts as a rudder.

The strange marine crab

Dromia possesses the instinct of hold ing a sponge or an anemone, as if for protection or as a conceal ment from view, with its last pair of legs which are bent up towards the back. On strong stimulation however, and when fleeing from enemies, the crabs drop these foreign bodies and scamper away hastily. In the aquarium they just as readily take up living or dead companions, heads of fishes, or even transparent pieces of glass which naturally could not conceal them from the gaze of their enemies. The spider-crab, Maia, conceals itself from its enemies by tearing off with its claws pieces of seaweed grow ing in the neighbourhood and sticking these on to the spines and hairs growing on its back. These spines are provided with back turned hooks. A well-known habit of hermit-crabs is to plant the surface of their shelly dwelling with actinians, and indeed with particular species of anemones. Many of these crabs transfer their companion to their new dwelling when they move. Violent fights often take place between the inhabitants of an aquarium for possession of the anemones. Lloyd Morgan (1908) describes a very specialized adaptation to the circumstances of life in the yucca moth (Pronuba yuccasella) of tropical America. The in stinct involves the most minute details of the creature's life history. The caterpillars of this species pupate a fortnight before the commencement of the flowering season of the yucca plants (q.v.). Each of the large, yellowish-white, bell-shaped flowers, opens for a single night only. This occurs at the same time as the straw-coloured yucca moths with their silvery sheen, hatch out. A female of this moth fetches the sticky pollen from the flowers, kneads it together and holds the big ball so formed with her strong, hairy labial palps. She then visits another flower and with

her ovipositor lays an egg in the rudiment of the seed in between the egg cells of the plant. Then she betakes herself to the stigma on the pistil and there sticks the pollen she has brought with her. It is asserted that the same individual several times repeats the alternate egg-laying and pollen-seeking in the same flower. Four or five days afterwards, the larvae creep out and begin to devour the seed-rudiments, of which about 200 are present in the flower. Three or four larvae are found in one flower and each needs about 20 eggs for its food. Thus, about too eggs are left for the yucca for its own reproduction. As soon as the larvae have grown to full size, they creep through a hole they have gnawed in the wall of the fruit and let themselves down on a thread to the ground. Here they spin an egg-shaped cocoon in the earth. They remain there until the following summer, when they pupate. The yucca plants can be pollinated by no other kind of moth. Conversely, the cater pillars of the yucca moth require the seed-rudiments of their own particular species of yucca as nourishment. Each species of yucca has a special species of moth adapted to itself alone. The yucca moth brings her business to completion without seeing or hearing what happens, without knowing the result, the fate of her eggs or the necessity for plant-pollination.

Even the most complicated behaviour of the social insects (q.v.) turns out in most instances to consist of inborn instinctive actions. Thus bees and wasps have been isolated as soon as they hatch, so that they could learn nothing of the life and activities in the hives or nests whence they came. Nevertheless, these animals occupied themselves in exactly the same way as all workers of the particular species, as soon as the necessary material was placed at their dis posal. They built, collected food, and fed larvae which were given them. Ants and termites, too, afford an excellent example of the fact that instincts may be purposeless. Quite a number of definite species of lower animals, particularly beetles of the family Staphy linidae, habitually steal into the nests of social insects. These ant guests and termite guests are not only tolerated but often even cared for and fed. Ants, indeed, care for the brood of many of these guests. They are induced to do this by certain secretions furnished to them by special glands of their guests in compensa tion for the trouble they have taken. This tolerance of and caring for definite guests has become with them a definite instinct, the so-called symphily instinct. And this is essentially harmful to the success of the ants' or termites' own colony. For the ants are often so intensely occupied by their guests and offspring of these, that they go so far as to neglect the proper care of their own brood and thus jeopardise the continued existence of their colony.