We have seen that this wasp stung the caterpillar between the segments of its body, and, as we will assume—for the result seems to warrant the inference—in the central part of it, so that the sting, entering the great nervous cord or ganglion, which is situated in this region, with little swellings at each of the segmental rings, produced the described paralysis. It was Fabre’s view that this must always be the case, and he thought likewise, in accordance with his own observation, that the caterpillar received a sting at the junction of all or nearly all the segments of its body. Otherwise it would be imperfectly stung, and in consequence not sufficiently paralysed to prevent its struggling, and so detaching the young larva, or perhaps the egg, which, as it would seem, is laid on, and not inside, the body of its living provisions. On the other hand, were the caterpillar stung too severely, so as to be killed outright, the grub when hatched would only have putrid meat to feed upon, and this again, it was assumed, would be fatal to its existence. On these grounds Fabre concluded that we had here an instinct which must have been perfect from the beginning, since as anything short of such perfection would be followed by the death of the larva, those gradual steps by which, on the theory of natural selection, all excellence either of structure or instinct has been attained, could not in a case like this have had any existence.
But all this has been exploded by subsequent observation. What Fabre saw he knew, but in all that he inferred without seeing he was entirely mistaken. As observed by the Peckhams, a caterpillar may be either stung so slightly as to be quite lively, and yet not succeed in shaking off the wasp larva hatched on its body, or so severely as to die almost immediately, yet without detriment to the larva who feeds on its discoloured and more or less putrified body, with the same gusto, and apparent benefit, as though it were warm with life.[[87]] Thus the question seems not so much to be, how can such perfection of instinct as was observed by Fabre have been attained through the process of natural selection, as why it should have been attained; or perhaps we may even go further and ask if this supposed perfection exists at all, and whether Fabre did not deceive himself. A wasp having secured a caterpillar is, of course, at liberty to sting it as often as it pleases. Why, then, should one wasp behave quite like another one in this respect? Here, as elsewhere, there would be some amount—perhaps a considerable amount—of variation in individual disposition, and wasps of milder or less savage mood would sting less frequently than their fiercer fellows. There might, therefore, as it appears to me, be a large amount of fluctuation both in the number and degree of severity of the stings—if indeed there is any regulation in this respect—and also in the consequent injury to the caterpillar or other insect, without any particular scope being offered for natural selection to play a part. What room, indeed, for such a force can there be if it makes no difference to the wasp-grub whether the caterpillar which is to be its food, is stung badly or slightly, or whether it lives or dies?
That this is really the case seems to be implied in the following paragraph which I quote from the same interesting work that I have before referred to[[87]] “The conclusions that we draw from the study of this genus differ in the most striking manner from those of Fabre. The one pre-eminent, unmistakable, and ever-present fact” (the invariable fact, as one might say) “is variability. Variability in every particular—in the shape of the nest and the manner of digging it, in the condition of the nest (whether closed or open) when left temporarily, in the method of stinging the prey, in the degree of malaxation, in the manner of carrying the victim, in the way of closing the nest, and last and most important of all, in the condition produced in the victims of the stinging, some of them dying and becoming ‘veritable cadavers,’ to use an expressive term of Fabre’s, long before the larva is ready to begin on them, while others live long past the time at which they would have been attacked and destroyed if we had not interfered with the natural course of events. And all this variability we get from the study of nine wasps and fifteen caterpillars”![[87]] Fabre’s ideas therefore seem totally disproved, but though natural selection—the counter-theory to his own—has no doubt produced the Sphex and Ammophila, with their habit of stinging and storing caterpillars, to serve as food for their young, it does not follow that it has done anything more than this; for though variation be the stuff in which natural selection works, it need not always work in it—any more than a tailor need always make clothes because there is an abundance of cloth.
SOLITARY WASPS
In the upper part of the picture a solitary wasp is seen attacking a caterpillar on a leaf. Beneath is another of the same species busy pounding the entrance to its burrow with a pebble.
In the digging and closing of her burrow—her nesting-habits, as we may call them—our Ammophila is almost as interesting to watch as in her mode of proceeding with caterpillars, though here a certain well-known stimulus to human enjoyment which I need not enlarge upon is wanting. Having found a convenient spot for her nursery, she digs, with her mandibles and front pair of legs, a little tunnel in the ground, to about the length of her own body, and at the end of it hollows out a round chamber or cavern just large enough to make comfortable quarters for a pair of invalid caterpillars—a hospital for incurables, we may call it to begin with, but soon to become their tomb. Having dug to about her middle, the wasp backs out, with a little pellet of collected earth held firmly in her mandibles. With this she flies to a little distance and then, letting it drop, alights on the ground and takes a little rest before returning to continue her work. She may either fly or run back, for her legs are as highly developed as her wings—she is in fact a very perfect athlete. The process of excavation is now continued, there is more burrowing, more flying away with the earth dug out, and before long the nursery-vault is completed. The next thing is to find caterpillars for it, but before flying away to look for one, Ammophila carefully conceals the entrance to her tunnel with pellets of earth, which she often brings from a distance, and will not be satisfied with unless they seem well adapted for their purpose. At last, when the aperture is both blocked and hidden, she starts off upon the still more important undertaking which has been already described, and after a longer or shorter interval—if her quest is successful—returns with a nicely stung caterpillar. As two are required there must be another journey and another stopping up of the burrow, before the final one, which is of a more solid nature, occupying, sometimes, as much as twenty minutes. In thus bringing her labours to a conclusion, Ammophila often shows a wonderful degree of intelligent foresight—foresight we must term it if we admit the intelligence; for sometimes she will drag a leaf over the entrance to the tunnel, though now filled in, or taking a stone as large as her head in her mandibles, will pound down the earth with it to make it firm and compact.[[87]]
It used to be said—and may be still by that large class of people who are for ever making false parallels and artificial distinctions—that man was the only animal that made intelligent use of an instrument, but Darwin instanced a monkey cracking a nut with a stone, and an elephant breaking off a bough to fan itself with. Here, in an insect, we have a case which is perhaps even more to the point—more extraordinary, that is to say; for certainly the idea of flattening and pressing down earth over a general surface, and of taking something to do it with, seems a little less obvious than that of cracking a nut, in a similar manner, and therefore to require more thought in the planner of such a process. No wonder that the delighted witnesses of this interesting fact flung themselves on the ground on each side of the unconscious inspirer of their wonder, in order to have a better sight of it; but that a previous observer of the same thing should have waited a year before publishing what he had seen, because he feared such a statement would not be believed,[[87]] is to my mind a display of prudence almost as wonderful, though not nearly so edifying, as that of Ammophila herself. If we are not to make known what we see, because people who believe in their own and nobody else’s eyesight are not likely to credit it, how is evidence to accumulate for the benefit of the more intelligent part of the community? It is only of this small minority that we should think, or, rather, we should not think of anything but the truth, where truth is concerned.
Returning to bees of the solitary kind, “the operations of the wood-piercers,” says Bingley, “merit our careful attention.” They shall have it for a moment, but space is against them. However, the female of the species, Xylocopa violacea, which for some reason is disliked by householders, bores in the springtime, by the aid of her strong mandibles alone, neat little circular tunnels in such objects as garden-seats, gates, front doors, arbours, window-shutters, rustic tables, and the like. At first, it is stated, she “bores perpendicularly, but when she has advanced about half an inch she changes her direction, and then proceeds nearly parallel with its sides for twelve or fifteen inches. If the wood of the seat, door, table, etc., be sufficiently thick, she sometimes forms three or four of these long holes in its interior, a labour which for a single insect seems prodigious, and in the execution of it some weeks are sometimes employed. On the ground, for about a foot from the place in which one of these bees is working, little heaps of timber-dust are to be seen. These heaps daily increase in size, and the particles that compose them are almost as large as those produced by a hand-saw.”[[88]] When the tunnels are finished, the mother bee divides them into some dozen little rooms about an inch deep, making the divisions of wood-dust, which she cements together by aid of a glutinous secretion with which she is furnished. Before each cell is closed it is filled with a paste composed of the farina of flowers, mixed with honey (it makes one envy the grub), and an egg is deposited in it. As each cell takes some time to make and provision, it is obvious that the egg in the lower one will hatch a little sooner than the one above it, and so on right up to the top. If, therefore, any one of the larvæ “were to force its way upwards, which it could easily do, it would not only disturb, but would infallibly destroy all those lodged in the superior cells.”[[88]] Here, however, natural selection (called Providence in Bingley’s time) steps in, and “has wisely prevented this devastation, for the head of the nymph (chrysalis), and consequently of the emerging bee, is always placed in a downward direction.”[[88]] Of course, therefore, the insect moves forward in the direction towards which it looks at birth—its new birth, that is to say; and, moreover—this is the really astonishing thing—“the mother digs a hole at the bottom of the long tube, which makes a communication between the undermost cell and the open air. By this contrivance, as all the bees instinctively endeavour to cut their way downward, they find an easy and convenient passage, for they have only to pierce the floor of their cells in order to make their escape, and this they do with their teeth very readily.” As regards this communicating passage, however, it is presumably a mere continuation of the tunnel, and was probably once occupied with cells, like the upper portion. Why the lower end of the tunnel should have come in time to be left empty—what advantage, that is to say, its being so represents—it is not easy to see, but possibly it stood in danger of being reached by a bird’s beak, through what has now become the exit of escape merely.