In the same way the instinct to seek for food wakes in the kitten at the sight of a mouse. I once set before a kitten which had never seen a mouse a living one in a trap. The kitten became greatly excited, and when I opened the trap and let the mouse run away she overtook and caught it in a few bounds. The instinct in this case does not express itself as in the chicken by the rapid lowering of the head and seizing the food, but in quite a different combination of movements, in running after and grasping the fleeing victim. But that is not all that is included in the instinctive action in the case of the cat, for there is also the whole wild and gruesome play, the familiar letting go and catching again, the passionate growling of satisfaction which, in its wildness, reminds us much more of a blood-thirsty tiger than of a tame domestic animal.
As the egg-laying instinct of the female butterfly is excited only by the sight and odour of a particular plant, so also is the food instinct of the caterpillar. If we put a silkworm caterpillar (Bombyx mori) just out of the egg upon a mulberry leaf it will soon begin to gnaw at it; but put it on a beech leaf or on that of any other indigenous tree, shrub, or herb and it will not touch it, but simply die of hunger. And yet it could quite well eat many of these leaves, and thrive on them too, but the smell and perhaps also the sight of them is not the appropriate stimulus to liberate the instinct of eating. There are many species of caterpillar which are 'monophagous,' that is to say, restricted to a single species of plant in a country. One may ask how such a restriction of the liberating stimulus to a single species could have been brought about by natural selection, since it could not possibly be advantageous to be so much restricted in food. The answer to this will be found in the following facts. On the Belladonna plant there lives a little beetle whose feeding instinct is aroused by this plant alone. But as Atropa belladonna is avoided entirely by other animals on account of its poisonousness, this beetle is, so to speak, sole proprietor of the Belladonna; no other species disputes its food, and in this there must assuredly be a great advantage, as soon as the other instincts, above all that of egg-laying, are so regulated as to secure that the larva shall have access to its food-plant; and this is the case. The monophagy of many caterpillars is to be understood in the same way; it is an adaptation to a plant otherwise little sought after, and it is combined with a more or less complete loss of sensitiveness to the stimulus of other plants. The establishment of such a specialized food-instinct depends on its utility, and has resulted from the preference given, through natural selection, to those individuals in which the food-instinct responded to the stimulus of the smallest possible number of plants, and at the same time to those which showed themselves best adapted to a plant especially favourable to their kind, whose food-instinct was not only most strongly excited by this one plant, but also whose stomach and general metabolism made the best use of it. So we understand why so many caterpillars live on poisonous plants, not only some of our indigenous Sphingidæ, like Deilephila euphorbiæ, but whole groups of tropical Papilionidæ, Danaides, Acræides, and Heliconiidæ. With this again is connected the poisonousness or nauseousness of these butterflies.
How diversely the instinct to procure food may be developed in one and the same group of animals is shown by the fact that not infrequently plant-eating, saphrophytic, and flesh-eating animals occur in a single group of organisms, as, for instance, in the order of water-fleas or Daphnidae, or in the class of Infusorians. Many species find their food by making an eddy in the water, which brings a stream towards the mouth, and with it all sorts of vegetable or dead particles; others live by preying upon other animals like themselves.
But even when the food-instinct in all the species of a group is directed towards living prey, the procuring of it may be achieved by means of quite different instincts. Such finer graduations of the food-instinct are found not infrequently within quite small groups of animals, as in the Ephemeridæ or Day-flies. All their larvæ live by preying on other animals, but those of one family, represented by the genus Chloëon, seek to secure their victims by agility and speed, while the larvæ of the second family, with the typical genus Baëtis, have the instinct to press their smooth broad bodies, with large-eyed head, close to the brook pebbles on which they sit. They are exactly like these in colour, and thus they lurk almost invisible, until a victim comes within their reach, when they throw themselves upon it with a bound. The third group, with the typical genus Ephemera, follows its instinct to dig deep tubes in the mud at the bottom of the water, and to lurk in these for their prey. We have thus within this small group of Day-flies three distinct modifications of the food-instinct, which differ essentially from one another, are made up of quite different combinations of actions, and, consequently, must have their foundation in essentially different directive brain-mechanisms. All these cases have only one feature in common; the animals all throw themselves upon their prey as soon as they are near enough.
Fig. 31. Sea-cucumber (Cucumaria), with
expanded tentacles (a), and protruded
tube-feet (b); after Ludwig.
But even this common feature is not everywhere part of the food-instinct. The sea-cucumber (Cucumaria) (Fig. 31), according to the observations made on it by Eisig in the Aquarium of the Zoological Station at Naples, gets its food in the following manner. The animal sits half or entirely erect on a projecting piece of rock and unfolds its ten tree-like tentacles which surround the mouth. These are branched, and have quite the effect of little tufts of seaweed. They are probably taken for such by many minute animals; for larvæ of all kinds, Infusorians, Rotifers and worms settle down on them. But the sea-cucumber bends inwards first one tentacle and then another, so slowly as barely to be noticeable, brings the point to its mouth, lets it glide gradually deeper into the gullet, until the whole tentacle is within, and after a time draws it out again equally slowly and unfolds it anew. Obviously it wipes the tentacle inside the gullet, and retains everything living that was upon it. This performance is repeated continually, day and night, and it is usually the only externally visible sign of life which the animal displays.
This remarkable instinct is associated with a structural peculiarity, for without the arborescent tentacles the fishing would not be nearly so successful. Other sea-cucumbers or Holothurians have different tentacles, and use them in quite a different manner, filling the mouth with mud by means of them.
Very frequently, indeed, there are visible structural changes associated with the modified food-instinct. Most predatory fishes chase their prey, like the pike, the perch, and the shark, but there are also lurkers, and these show in addition to the lurking instinct certain definite bodily adaptations, without which this instinct could not have such full play.