In the sea-anemone affixing itself to the shell of the hermit-crab, who becomes its friendly and interested landlord, we have seen one of the more pleasing instances of association between two or more different species of animals. There are many others, such as that between the shark and the pilot-fish, the honey-guide and the ratel, the rhinoceros and its little bird, etc., etc., which we can dwell upon with equal pleasure. Some, however—and, unfortunately, they are much more numerous—are of a darker character, repelling us almost as much by the picture which they present of nature’s unbending cruelty as they arouse our admiration by their wonderful ingenuity and adaptation of means to ends. The most salient examples of this kind of living together—partnership we can hardly call it—are to be found, perhaps, in the insect world. Parasitism is the proper word for it, and the most salient, or at least the most repulsive, examples are furnished perhaps by the hymenoptera—that genus of insects in which the bees, wasps, and ants are included. Thus almost all caterpillars—perhaps all—are victimised by some species of ichneumon-fly—a wasp-like creature that seeks it out, pierces its soft body with a long ovipositor, with which it is provided for the purpose, and lays a number of eggs inside it. Having done this, it goes away, and the caterpillar goes on feeding. It is, however, doomed, and destined never to enter into the moth or butterfly state of existence. In due time the eggs are hatched by the warmth of its own body, and on this body to which they are so highly indebted, the young ichneumons, now in their own caterpillar state, begin with unconscious ingratitude to prey. They feast upon it day and night, but the creature, ordained by the iron laws of nature to suffer in this way, is long-lived, and though sickening from day to day, has often sufficient strength to become full-fed, and make its cocoon, and pass into the chrysalis, or pupal, state. How long it lives after that it is difficult to say. Probably some vitality remains as long, or almost as long, as any part of itself does. All that we know is that after a longer or shorter interval a score or so of ugly, evil-looking ichneumon-flies issue from the dry shell of the chrysalis, instead of the innocent and radiant creature that would otherwise have done so.
It is curious—gratifying, too, if one allows oneself to give way to a natural, though unreasonable feeling—to learn that some of these very ichneumons themselves become, in a similar manner, the victims of others of their own species. Thus from two corpses, on the slowly dying bodies of both of which it has directly, or indirectly, fed, the third life in death emerges, like some ghoul from a double tomb. Could the caterpillar know that the being which so remorselessly preyed upon its tissues and juices had a similar parasite within its own body, doing it to death in the same horrid way, how relieved and almost happy it might feel! But Nature, though she often brings in her revenges, seldom grants to her suffering children the proverbial sweetness of revenge. Caterpillars, however, do not always submit to the machinations of the ichneumon-fly without a struggle, and in some cases they may be successful—how often it is not easy to say—in guarding themselves against their attacks. The puss-moth, for instance, which is especially liable to them, is furnished, no doubt for that very reason, with a special weapon for its defence. The end of its body is forked, and each fork is prolonged into a sort of tail, from which a red filament can be extruded and waved about at the will of the creature. In this way, and by its violent contortions, it may sometimes succeed in whipping off, as it were, the ichneumon that is attacking it, but it has another and more efficacious means of defending itself. An aperture in the skin behind the head communicates with a gland containing a clear fluid, forty per cent. of which is formic acid, and the rest water. This the caterpillar can eject with great force, and so pungent is it that a few drops falling on an unwary ichneumon-fly are sufficient to incapacitate, if not actually to kill it. Lizards, indeed, and monkeys, as has been experimentally ascertained, are affected by this powerful irritant, nor, as far as we know, is there any other animal secretion which contains so large a proportion of strong acid. It is probable that the caterpillar of the puss-moth is not the only one which has this power of spurting a noxious fluid over its enemies, whilst many are protected in other ways: some by their hairiness, others by being coloured like the leaves they eat, or resembling, when at rest, a twig of the plant on which they sit immovable. By these latter means they certainly avoid being eaten by birds, and there seems no reason why they should not sometimes deceive the ichneumon-fly also. But in spite of all defences, whether consciously or unconsciously brought into play, a large proportion of most caterpillars yield to destiny, and are slowly eaten alive by the special parasite which Nature has provided for them.
Still worse, perhaps, though very similar, is the fate which various insects—caterpillars, grasshoppers, etc.—as well as spiders, experience at the hands of several species of wasps. These wasps are not social, like the ones we are familiar with, and make no nest other than a cell in which to place their eggs, together with the nourishment which the young, when hatched, will require. This nourishment is the creatures aforesaid. In the best-known instance the female wasp first makes a long tunnel in the earth, with three or four separate chambers or cells at the end of it, in which she deposits her eggs. She then seizes an insect, which, if it is a grasshopper, or something equally active, struggles violently to escape, and being often as large as, or larger, than the wasp herself, the contest may be a long one. Invariably, however, the wasp—or sphex, to give her her generic name—is victorious. She could, indeed, sting at once, if she were so minded, but this she does not do. She reserves her fire, so to speak, till, after more or less violent exertion, she has succeeded in throwing her victim on its back. Then she stings it in two particular spots, the throat, namely, and between the thorax and abdomen. Instantly the struggles of the wretched creature cease: a ganglion, or nerve centre, has, with each sting, been pierced—it is paralysed, but not by any means dead. To kill it, indeed, is far from the intention of the sphex herself, and, kind and thoughtful mother, she does not allow the meat which she provides for her offspring to go bad. It will last, as she has managed it, for the whole time that it is wanted. All now is quite satisfactory. She can be grateful for mercies vouchsafed. She rests for a little, then seizing the helpless, living food by a leg or a wing, she drags it—for it is usually too heavy for her to fly with—to the mouth of her nursery larder. Here she leaves it for a little, while she enters to see that all is well. Re-emerging, she seizes it again, drags it down the tunnel, deposits it in one of the chambers, plasters it up, and leaves it for a while. In due time she returns with, and inters, another paralytic, and, having thus successively filled all the four chambers, she closes the mouth of the tunnel and flies merrily away—doubtless
“With the gratifying feeling that her duty has been done.”
Of course, after a certain number of days, the eggs are hatched, each young sphex caterpillar immediately falls to, and the grasshoppers that have been previously buried alive are now eaten alive—two kinds of deaths which are equally unpleasant, and each of which lasts a long time. However, they are paralysed—insensible, we may hope, therefore, to such pain as insects feel. Whether the paralysis is mental as well as corporeal it is impossible to say, but it may, in any case, be doubted whether grasshoppers can suffer through the mind. Assuming that they cannot, the inability, though it would involve another of an opposite nature, must yet here be considered advantageous, since that sort of pleasure which arises out of a just sense of the beautiful contrivances and adaptations of nature, must always, we may suppose, be beyond the capacity of an insect; and even were it not, it is the sphex in this case whose mind would, in all probability, be most open to such reflections.
The habits of the sphex have been studied in Europe by “that inimitable observer,” as Darwin calls him, M. Fabre, from whose writings the foregoing account has been compiled. In India its place is taken by a very large wasp of a uniform steely-blue colour and a most venomous aspect. This kind makes a clay nest, of about the size and shape of a very large Brazil-nut, on the outside of some perpendicular surface, and often chooses for this purpose the walls of houses or bungaloes. I have watched it time after time flying in first with little glistening round balls of moist clay, and afterwards with curled up balls of caterpillars of about the same colour, but larger. With these she filled up one large cell, the entrance to which she then closed with more mud. It was an interesting sight, but, to enjoy it thoroughly, one ought to be an optimist.
In the above-mentioned instances the cell which serves as cradle and tomb combined is made by the provident mother. Some wasps, however, have learnt to save time and trouble by walling the victim up in a cell of its own manufacture, or, at least, of its own choosing. This happens to a certain spider in South America, which sits in a little hole in the ground waiting for insects either to pass or come in. The wasp, which is blue like the other, but smaller than our common one, goes about from hole to hole, and when it finds one occupied by a spider, goes a little way into it, and then rushes out, hotly pursued by the owner. When on the point of being overtaken it suddenly turns, grapples with the spider, stings it, drags it back, paralysed, into its own hole, lays an egg by it, and departs, having previously blocked up the entrance with earth. The entrance of the wasp into the spider’s hole, with its retreat, some time afterwards, in feigned alarm, so as to draw the spider out, is certainly an act of great intelligence. The intelligence, however, is surpassed, or exhibited in a more entertaining manner, in the case of another wasp which has been seen to creep noiselessly round to the entrance of a spider’s nest, and then wriggle one of its antennæ in front of the opening. Upon this, the owner of the nest, a very large spider, came out, and was at once stung to death by the wasp. The latter then wriggled an antennæ again, and upon no notice being taken, entered the nest and killed all the young spiders, which he then carried off at his leisure.
Other wasps lay their eggs in the nests of humble bees, and the young growing up there prey upon the honey and comb. Amongst ants, again, some of the smaller species are parasitic upon the larger ones, an enforced association which may be very much to the disadvantage of the latter. Especially is this the case where an ant whose Latin name is Solenopsis fugax—if it has an English one I do not know it—is the unbidden guest. Lord Avebury tells us that it “makes its chambers and galleries in the walls of the nests of larger species, and is the bitter enemy of its hosts. The latter cannot get at them because they are too large to enter the galleries. The little solenopses, therefore, are quite safe, and, as it appears, make incursions into the nurseries of the larger ant, and carry off the larvæ as food. It is as if we had small dwarfs, about eighteen inches to two feet long, harbouring in the walls of our houses, and every now and then carrying off some of our children into their horrid dens.” The insect world is particularly rich in these parasitic relations, but space will not allow me to enlarge upon them further.
Turning to birds, we meet with instances not less interesting, whilst very much less painful, since here the victimised species is only robbed by the other, and not so frequently as to prevent its making a living. The osprey, for instance, which preys almost exclusively on fish, which it hooks with its claws out of the water, is forced, though itself a large bird, to give up much of its booty to the still more powerful white-headed eagle. The latter sits on some rocky crag or peak “that beetles o’er its base into the sea,” and watches with a greedy eye the “inferior fiend,” as, far below, it hovers on broad wings above its destined prey. At once the wings are closed, and the spray dashes over them as the bird precipitates itself upon a gleaming light amidst the waves. For a moment it is almost hidden in the foam and swirl, the next it emerges out of it, and mounts with powerful beats into the air, its head stretched shorewards, and its bent claws struck deep into the body of a large fish, beneath the weight of which it labours. Slowly at first, but gaining strength and speed as it ascends, it heads towards the cliff’s face. Already it can see the crag on which its eyrie hangs, when, like a thunderbolt, and with the shriek or laugh of a demon, the lonely watcher, who has marked it all, hurls itself downwards on spoiler and spoil. With a quick turn the startled bird avoids the furious rush, but almost at the same moment another maniac laugh, answering the first, drowns its own note of anger and despair, as the mate of the eagle that has commenced the attack swoops towards it from a neighbouring pinnacle. All striving now on the osprey’s part is in vain. Like storm-clouds the two strong robbers gather above him and descend like the jagged lightning out of them. Their screams sound almost in his ears, their claws have cut his feathers, when his own reluctantly relax their grip, and the glittering booty falls. Something falls with it—over it. There is a rushing wind of wings, an overshadowing darkness in the air, the trail of light is checked in its descent, and out of that whirlwind of excessive speed an eagle soars serenely to the sky bearing a fish in its claws. In their eyrie, or on a ledge of the precipice, the pair of imperial brigands share their meal, or distribute it to their eaglets. The osprey tries again, and may, perhaps, catch another fish before they have finished.
