Well, I avow, in all sincerity, this is asking a little too much of chance. When the Ammophila first found herself in the presence of her caterpillar, there was nothing, you would have it, to guide the sting. The choice was made at random. The pricks were directed at the upper surface of the captured prey, at the lower surface, at the sides, the front and the back indiscriminately, according to the fortunes of a close struggle. The Hive-bee and the Social Wasp sting those points which they are [[369]]able to reach, without showing a preference for one part over the other. That is how the Ammophila must have acted, when still ignorant of her art.
Now how many points are there in a Grey Worm, above and below? Mathematical accuracy would answer, an infinity; a few hundreds will serve our purpose. Of this number, nine or perhaps more have to be selected; the needle must be inserted there and not elsewhere: a little higher, a little lower, a little to one side, it would not produce the desired effect. If the favourable event is a purely accidental result, how many combinations would be needed to bring it about, how much time to exhaust all the possible cases? When the difficulty becomes too pressing, you take refuge behind the mist of the ages; you retreat into the shadows of the past as far as fancy can carry you; you call upon time, that factor of which we have so little at our disposal and which, for this very reason, is so well suited to hide our illusions. Here you can let yourselves go and lavish the centuries. Suppose we shake up hundreds of figures, all of different values, in an urn and draw nine at random. When shall we, in this way, obtain a sequence fixed beforehand, a sequence that stands alone? The chance is so slight, answers [[370]]mathematics, that we may as well put it down as nil and say that the desired arrangement will never come about. For the Ammophila of the prehistoric age, the attempt was renewed only at long intervals, from one year to the next. Then how did this sequence of nine stings at nine selected points emerge from the urn of chance? When I am driven to appeal to infinity in time, I am very much afraid of running up against absurdity.
‘But,’ say you, ‘the insect did not attain its present surgical dexterity at the outset: it went through experiments, apprenticeships, varying degrees of skill. There was a weeding-out by natural selection, eliminating the less expert, retaining the more gifted; and instinct, as we know it, developed gradually, thanks to the accumulation of individual capacities, added to those handed down by heredity.’
The argument is erroneous: instinct developed by degrees is flagrantly impossible in this case. The art of preparing the larva’s provisions allows of none but masters and suffers no apprentices; the Wasp must excel in it from the outset or leave the thing alone. Two conditions, in fact, are absolutely essential: that the insect should be able to drag home and store a quarry which greatly surpasses it in size and strength; and that the newly-hatched [[371]]grub should be able to gnaw peacefully, in its narrow cell, a live and comparatively enormous prey. The suppression of all movements in the victim is the only means of realizing these conditions; and this suppression, to be complete, requires sundry dagger-thrusts, one in each motor centre. If the paralysis and the torpor be not sufficient, the Grey Worm will defy the efforts of the huntress, will struggle desperately on the road and will not reach the journey’s end; if the immobility be not complete, the egg, fixed at a given spot on the worm, will perish under the contortions of the giant. There is no via media, no half-success. Either the caterpillar is treated according to rule and the Wasp’s family is perpetuated; or else the victim is only partially paralysed and the Wasp’s offspring dies in the egg.
Yielding to the inexorable logic of things, we will therefore admit that the first Hairy Ammophila, after capturing a Grey Worm to feed her larva, operated on the patient by the exact method in use to-day. She seized the creature by the skin of the neck, stabbed it underneath, opposite each of the nerve-centres and, if the monster threatened further resistance, munched its brain. It must have happened like this; for, once more, an unskilled murderess, doing her work in a perfunctory and haphazard [[372]]fashion, would leave no successor, since the rearing of the egg would become impossible. Save for the perfection of her surgical powers, the slayer of fat caterpillars would die out in the first generation.
Again I hear you say:
‘The Hairy Ammophila, before hunting the Grey Worm, may have picked out feebler caterpillars and heaped up several in one cell, until they represented the same bulk of provender as the big prey of to-day. With puny game, a few thrusts of the needle, perhaps one, would be enough. Gradually, large-sized prey came to be preferred, as reducing the number of hunting expeditions. Then, as successive generations went after bigger game, the dagger-strokes were multiplied, in proportion to the victim’s power of resistance; and, by degrees, the elementary instinct of the outset became the highly-developed instinct of our time.’
To these arguments we may begin by replying that the larva’s change of diet and the substitution of one morsel for a number are diametrically opposed to what happens before our eyes. The Hunting Wasp, as we know her, is extremely loyal to old customs; she has sumptuary laws which she never transgresses. She who fed on Weevils in her youth puts Weevils and naught else in her larva’s cell; she who was [[373]]supplied with Buprestis-beetles persists in the fare which she has adopted and serves her larva with Buprestis-beetles. One Sphex must have Crickets; a second, Grasshoppers; a third, Locusts. Nothing is accepted but these particular dishes. The Bembex who hunts Gad-flies revels in them and refuses to do without them, whereas Stizus ruficornis, who fills the larder with Praying Mantes, scorns any other game. And so with the rest. They have each their own taste.
It is true that many allow themselves a more varied bill of fare, but only within the limits of one entomological group: thus the Weevil and Buprestis hunters prey upon any species proportioned to their strength. Were the Hairy Ammophila to make a change in her diet, that would be her case too. Whether small and sundry to each cell or large and single, the prey would always consist of caterpillars. So far, so good. But there remains the question of the many replaced by the unit; and I do not yet know one instance of such an alteration in the Wasp’s habits. She who stocks the burrow with a single joint never thinks of heaping up several of smaller size; she who goes on repeated expeditions to stack a quantity of game in the same cell does not know how to limit herself to one head by choosing larger meat. The [[374]]result of my observations never varies in this regard. The prehistoric Ammophila, who abandoned her multiplicity of small game for one colossal head, has nothing to warrant her existence.
If the point were conceded, would the question be advanced? Not in the least. Let us accept as the initial prey a feeble caterpillar, paralysed with a single sting. Even then that sting must not be given at random, else the act would be more harmful than profitable. Irritated, but not subdued by the wound, the animal would but become more dangerous. The dart must strike a nerve-centre, probably in the middle region of the string of ganglia. This, at any rate, is how the present-day Ammophilæ seem to go to work when they are addicted to the rape of frail and slender grubs. What chance would the operator have of striking that one particular point, if her lancet were wielded without method? The probability is ludicrously remote: it is as one to the countless number of points whereof the caterpillar’s body is made up. And yet, according to the theorists, it is on this probability that the Wasp’s future depends. What an edifice to balance on the point of a needle!