We wish that we might witness the struggle between the two adversaries and see at first hand what happens, but we cannot hope to succeed. It all takes place in the mysterious darkness of the soil; in broad daylight, the attack would not be delivered, for the victim must remain where it is and then and there receive the egg, which is unable to thrive and develop except under the warm cover of vegetable mould. If direct observation is impracticable, we can at least foresee the main outlines of the drama by allowing ourselves to be guided by the warlike manoeuvres of other burrowers.
I picture things thus: digging and rummaging through the heap of mould, guided perhaps by that singular sensibility of the antennae which enables the Hairy Ammophila to discover the Grey Worm (The caterpillar of the Turnip Moth. Cf. "The Hunting Wasps": chapters 18 to 20.--Translator's Note.) underground, the Scolia ends by finding a Cetonia-larva, a good plump one, in the pink of condition, having reached its full growth, just what the grub which is to feed on it requires. Forthwith, the assaulted victim, contracting desperately, rolls itself into a ball. The other seizes it by the skin of the neck. To unroll it is impossible to the insect, for Imyself have some trouble in doing so. One single point is accessible to the sting: the under part of the head, or rather of the first segments, which are placed outside the coil, so that the grub's hard cranium makes a rampart for the hinder extremity, which is less well defended. Here the Wasp's sting enters and here only can it enter, within a narrowly circumscribed area. One stab only of the lancet is given at this point, one only because there is no room for more; and this is enough: the larva is absolutely paralysed.
The nervous functions are abolished instantly; the muscular contractions cease; and the animal uncoils like a broken spring. Henceforth motionless, it lies on its back, its ventral surface fully exposed from end to end. On the median line of this surface, towards the rear, near the brown patch due to the alimentary broth contained in the intestine, the Scolia lays her egg and without more ado, leaves everything lying on the actual spot where the murder was committed, in order to go in search of another victim.
This is how the deed must be done: the results prove it emphatically. But then the Cetonia-grub must possess a very exceptional structure in its nervous organization. The larva's violent contraction leaves but a single point of attack open to the sting, the under part of the neck, which is doubtless uncovered when the victim tries to defend itself with its mandibles; and yet a stab in this one point produces the most thorough paralysis that I have ever seen. It is the general rule that larvae possess a centre of innervation for each segment. This is so in particular with the Grey Worm, the sacrificial victim of the Hairy Ammophila. The Wasp is acquainted with this anatomical secret: she stabs the caterpillar again and again, from end to end, segment by segment, ganglion by ganglion. With such an organization the Cetonia-grub, unconquerably coiled upon itself would defy the paralyser's surgical skill.
If the first ganglion were wounded, the others would remain uninjured; and the powerful body, actuated by these last, would lose none of its powers of contraction. Woe then to the egg, to the young grub held fast in its embrace! And how insurmountable would be the difficulties if the Scolia, working in the profound darkness amid the crumbling soil and confronted by a terrible pair of mandibles, had to stab each segment in turn with her sting, with the certainty of method displayed by the Ammophila! The delicate operation is possible in the open air, where nothing stands in the way, in broad daylight, where the sight guides the scalpel, and with a patient which can always be released if it becomes dangerous. But in the dark, underground, amidst the ruins of a ceiling which crumbles in consequence of the conflict and at close quarters with an opponent greatly her superior in strength, how is the Scolia to guide her sting with the accuracy that is essential if the stabs are to be repeated?
So profound a paralysis; the difficulty of vivisection underground; the desperate coiling of the victim: all these things tell me that the Cetonia-grub, as regards its nervous system, must possess a structure peculiar to itself. The whole of the ganglia must be concentrated in a limited area in the first segments, almost under the neck. I see this as clearly as though it had been revealed to me by a post-mortem dissection.
Never was anatomical forecast more fully confirmed by direct examination.
After forty-eight hours in benzine, which dissolves the fat and renders the nervous system more plainly visible, the Cetonia-grub is subjected to dissection. Those of my readers who are familiar with these investigations will understand my delight. What a clever school is the Scolia's! It is just as I thought! Admirable! The thoracic and abdominal ganglia are gathered into a single nervous mass, situated within the quadrilateral bounded by the four hinder legs, which legs are very near the head. It is a tiny, dull-white cylinder, about three millimetres long by half a millimetre wide. (.117 x .019 inch.--Translator's Note.) This is the organ which the Scolia's sting must attack in order to secure the paralysis of the whole body, excepting the head, which is provided with special ganglia.
