The Hunting Wasps

by Jean-Henri Fabre

‘Yes, that’s it, of course! The creature must be paralysed; it must be deprived of movement, without being deprived of life.’

There is only one way of achieving this result: to injure, cut or destroy the insect’s nervous system in one or more skilfully-selected places. But, even at that stage, if left in hands unfamiliar with the anatomical secrets of a delicate organism, the question would not have advanced much further. What in fact is the disposition of this nervous system which has to be smitten if we would paralyse the insect without at the same time killing it? And, first of all, where is it? In the head, no doubt, and down the back, like the brain and the spinal marrow of the higher animals.

‘You make a grave mistake,’ our congress would say. ‘The insect is like an inverted animal, walking on its back; that is to say, instead of having the spinal marrow on the top, it has it below, along the breast and the belly. The operation on the insect to be paralysed must therefore be performed on the lower surface and on that surface alone.’ [[45]]

This difficulty once removed, another arises, equally serious in a different way. Armed with his scalpel, the anatomist can direct the point of his instrument wherever he thinks fit, in spite of obstacles, for these he can eliminate. The Wasp, on the contrary, has no choice. Her victim is a Beetle in his stout coat of mail; her lancet is her sting, an extremely delicate weapon which would inevitably be stopped by the horny armour. Only a few points are accessible to the fragile implement, namely, the joints, which are protected merely by an unresisting membrane. Moreover, the joints of the limbs, though vulnerable, do not in the least fulfil the desired conditions, for the utmost that could be obtained by means of them would be a partial paralysis and not a general paralysis affecting the whole of the motor organism. Without a prolonged struggle, which might be fatal to the patient, without repeated operations, which, if too numerous, might jeopardize the Beetle’s life, the Wasp has, if possible, to suppress all power of movement at one blow. It is essential, therefore, that she should aim her sting at the nervous centres, the seat of the motor faculties, whence radiate the nerves scattered over the several organs of movement. Now these sources of locomotion, these nervous centres, consist of a certain [[46]]number of nuclei or ganglia, more numerous in the larva, less numerous in the perfect insect and arranged along the median line of the lower surface in a string of beads more or less distant one from the other and connected by a double ribbon of the nerve-substance. In all the insects in the perfect state, the so-called thoracic ganglia, that is to say, those which supply nerves to the wings and legs and govern their movements, are three in number. These are the points to be struck. If their action can be destroyed, no matter how, the power of movement will be destroyed likewise.

There are two methods of reaching these motor centres with the Wasp’s feeble instrument, the sting: through the joint between the neck and the corselet; and through the joint between the corselet and the rest of the thorax, in short, between the first and second pair of legs. The way through the joint of the neck is hardly suitable: it is too far from the ganglia, which are near the base of the legs which they endow with movement. It is at the other point and there alone that the blow must be struck. That would be the opinion of the academy in which the Claude Bernards were treating the question in the light of their profound knowledge. And it is here, just here, between the first and second pair of legs, on [[47]]the median line of the lower surface, that the Wasp inserts her dirk. By what expert instinct is she inspired?

To select, as the spot wherein to drive her sting, the one vulnerable point, the point which none save a physiologist versed in insect anatomy could determine beforehand: even that is far from being enough. The Wasp has a much greater difficulty to surmount; and she surmounts it with an ease that stupefies us. The nerve-centres governing the locomotory organs of the insect are, we were saying, three in number. They are more or less distant from one another; sometimes, but rarely, they are close together. Altogether they possess a certain independence of action, so that an injury done to any one of them induces, at any rate for the moment, the paralysis only of the limbs that correspond with it, without affecting the other ganglia and the limbs which they control. To strike in succession these three motor centres, each farther back than the one before it, and to do so between the first and second pair of legs, seems an impracticable operation for such a weapon as the Wasp’s sting, which is too short and is besides very difficult to guide under such conditions. It is true that certain Beetles have the three ganglia of the thorax very near together, almost touching, [[48]]while others have the last two completely united, soldered, welded together. It is also a recognized fact that, in proportion as the different nervous nuclei tend towards a closer combination and greater centralization, the characteristic functions of animal nature become more perfect and consequently, alas, more vulnerable. Here we have the prey which the Cerceris really needs. Those Beetles with motor centres brought close together or even gathered into a common mass, making them mutually dependent on one another, will be at the same instant paralysed with a single stroke of the dagger; or, if several strokes be needed, the ganglia to be stung will at any rate all be there, collected under the point of the dart.

Which Beetles are they, then, that constitute a prey so eminently convenient for paralysing? That is the question. The lofty science of a Claude Bernard, concerning itself only with the fundamental generalities of organism and life, would not suffice here; it could never tell us how to make this entomological selection. I appeal to any physiologist under whose eyes these lines may come. Without referring to his library, could he name the Beetles in whom that centralization of the nervous system occurs; and, even with the aid of his books, would he at once know where to find the desired [[49]]information? The fact is that, with these minute details, we are now entering the domain of the specialist; we are leaving the public road for the path known to the few.

I find the necessary information in M. Émile Blanchard’s fine work on the nervous system of the Coleoptera.[4] I see there that this centralization of the nervous system is the prerogative, in the first place, of the Scarabæidæ, or Chafers; but most of these are too large: the Cerceris could perhaps neither attack them nor carry them away; besides, many of them live in the midst of ordure where the Wasp, herself so cleanly, would refuse to go in search of them. Motor centres very close together are found also in the Histers, who live on carrion and dung, in an atmosphere of loathsome smells, and who must therefore be eliminated; in the Scolyti, who are too small; and lastly in the Buprestes and the Weevils.

What an unexpected light amid the original darkness of the problem! Among the immense number of Beetles whereon the Cerceres might seem able to prey, only two groups, the Weevils and the Buprestes, fulfil the indispensable conditions. They live far removed from stench and filth, two qualities perhaps [[50]]invincibly repugnant to the dainty huntress; their numerous representatives vary considerably in size, in much the same way as their kidnappers, who can thus pick and choose the victims that suit them; they are far more vulnerable than any of the others at the one point where the Wasp’s dart can penetrate, for at this point the motor centres of the feet and wings are crowded together, all easily accessible to the sting. At this point, in the Weevils, the three thoracic ganglia are very close together, the last two even touching; at the same point, in the Buprestes, the second and third are mingled in one large mass, very near the first. And it is just Buprestes and Weevils that we see hunted, to the absolute exclusion of all other game, by the eight species of Cerceres whose provisions have been found to consist of Beetles! A certain inward resemblance, that is to say, the centralization of the nervous system, must therefore be the reason why the lairs of the different Cerceres are crammed with victims bearing no outward resemblance whatever.

The most exalted knowledge could make no more judicious choice than this, by which so great a collection of difficulties is magnificently solved that we wonder if we be not the dupes of some involuntary illusion, whether preconceived [[51]]theoretic notions have not obscured the actual facts, whether, in short, the pen have not described imaginary marvels. No scientific conclusion is firmly established until it has received confirmation by means of practical tests, carried out in every variety of way. We will therefore subject to experimental proof the physiological operation of which the Great Cerceris has just apprised us. If it be possible to obtain artificially what the Wasp obtains with her sting, namely, the abolition of movement and the continued preservation of the patient in a perfectly fresh condition; if it be possible to work this wonder with the Beetles hunted by the Cerceris, or with those presenting a similar nervous centralization, while we are unsuccessful with Beetles whose ganglia are far apart, then we shall be bound to admit, however hard to please we may be in the matter of tests, that in the unconscious inspiration of her instinct the Wasp has all the resources of consummate art. Let us see what experiment has to tell us.