第71章 THE METHOD OF THE SCOLIAE(1)

After the Ammophilae, the paralysers who multiply their lancet-thrusts to destroy the influence of the various nerve-centres, excepting those of the head, it seemed advisable to interrogate other insects which also are accustomed to a naked prey, vulnerable at all points save the head, but which deliver only a single thrust of the sting. Of these two conditions the Scoliae fulfilled one, with their regular quarry, the tender Cetonia-, Oryctes-or Anoxia-larva, according to the Scolia's species. Did they fulfil the second? I was convinced beforehand that they did. From the anatomy of the victims, with their concentrated nervous system, I foresaw, when compiling my history of the Scoliae, that the sting would be unsheathed once only; I even mentioned the exact spot into which the weapon would be plunged.

These were assertions dictated by the anatomist's scalpel, without the slightest direct proof derived from observed facts. Manoeuvres executed underground escaped the eye, as it seemed to me that they must always do.

How indeed could I hope that a creature whose art is practised in the darkness of a heap of mould would decide to work in broad daylight? I did not reckon upon it all. Nevertheless, to salve my conscience, I tried bringing the Scolia into contact with her prey under the bell-glass. I was well-advised to do so, for my success was in inverse ratio to my hopes.

Next to the Philanthus, none of the Hunting Wasps displayed such ardour in attacking under artificial conditions. All the insects experimented upon, some sooner, some later, rewarded me for my patience. Let us watch the Two-banded Scolia (S. bifasciata, VAN DER LIND) operating on her Cetonia grub.

The incarcerated larva strives to escape its terrible neighbour. Lying on its back, it fiercely wends its way round and round the glass circus.

Presently the Scolia's attention awakens and is betrayed by a continued tapping with the tips of the antennae upon the table, which now represents the accustomed soil. The Wasp attacks the game, delivering her assault upon the monster's hinder end. She climbs upon the Cetonia-grub, obtaining a purchase with the tip of her abdomen. The quarry merely travels the more quickly on its back, without coiling itself into a defensive posture. The Scolia reaches the fore-part, with tumbles and other accidents which vary greatly with the amount of tolerance displayed by the larva, her improvised steed. With her mandibles she nips a point of the thorax, on the upper surface; she places herself athwart the beast, arches herself and makes every effort to reach with the end of her abdomen the region into which the sting is to be driven. The arch is a little too narrow to embrace almost the whole circumference of her corpulent prey; and she renews her attempts and efforts for a long time. The tip of the belly tries every conceivable expedient, touching here, there and everywhere, but as yet stopping nowhere. This persistent search in itself demonstrates the importance which the paralyser attaches to the point at which her lancet is to penetrate the flesh.

Meanwhile, the larva continues to move along on its back. Suddenly it curls up; with a stroke of its head it hurls the enemy to a distance.

Undiscouraged by all her set-backs, the Wasp picks herself up, brushes her wings and resumes her attack upon the colossus, almost always by mounting the larva's hinder end. At last after all these fruitless attempts, the Scolia succeeds in achieving the correct position. She is seated athwart the Cetonia-grub; the mandibles grip a point on the dorsal surface of the thorax; the body, bent into a bow, passes under the larva and with the tip of the belly reaches the region of the neck. The Cetonia-grub, placed in serious peril, writhes, coils and uncoils itself, spinning round upon its axis. The Scolia does not interfere. Holding the victim tightly gripped, she turns with it, allows herself to be dragged upwards, downwards, sidewards, following its contortions. Her obstinacy is such that I can now remove the bell-glass and follow the details of the drama in the open.

Briefly, in spite of the turmoil, the tip of the abdomen feels that the right spot has been found. Then and only then the sting is unsheathed. It plunges in. The thing is done. The larva, at first plump and active, suddenly becomes flaccid and inert. It is paralysed. Henceforth there are no movements save of the antennae and the mouthparts, which will for a long time yet bear witness to a remnant of life. The point wounded has never varied in the series of combats under glass: it occupies the middle of the line of demarcation between the prothorax and the mesothorax, on the ventral surface. Note that the Cerceres, operating on Weevils, whose nervous system is as compact as the Cetonia-grub's, drive in the needle at the same spot. Similarity of nervous organization occasions similarity of method. Note also that the Scolia's sting remains in the wound for some time and roots about with marked persistence. Judging by the movements of the tip of the abdomen, one would certainly say that the weapon is exploring and selecting. Free to shift in one direction or the other, within narrow limits, its point is most probably seeking for the little mass of nerve-tissue which must be pricked, or at least sprinkled with poison, to obtain overwhelming paralysis.

I will not close this report of the duel without relating a few further facts, of minor importance. The Two-banded Scolia is a fierce persecutor of the Cetonia. In one sitting the same mother stabs three larvae, one after the other, in front of my eyes. She refuses the fourth, perhaps owing to fatigue or to exhaustion of the poison-bag. Her refusal is only temporary.

Next day, she begins again and paralyses two grubs; the day after that, she does the same, but with a zeal that decreases from day to day.