Chapter vi
THE LARVA AND THE NYMPH
The egg of the Yellow-winged Sphex is white, elongated, cylindrical, slightly bow-shaped and measures three to four millimetres[1] in length. So far from being laid anywhere on the victim, at random, it is deposited on a specially favoured spot, which is always the same; in short, it is placed across the Cricket’s breast, a little to one side, between the first and second pair of legs. The egg of the White-edged Sphex and that of the Languedocian Sphex occupy a similar position: the first on the breast of a Locust, the second on the breast of an Ephippiger.[2] The point selected must present some peculiarity of great importance to the young larva’s safety, for I have never known it to vary.
The egg hatches after three or four days. A very delicate wrapper tears asunder; and there lies before our eyes a feeble grub, transparent as crystal, a little attenuated and as it [[87]]were compressed in front, slightly swollen at the back and adorned on either side with a narrow white thread formed of the principal trachean ducts. The frail creature occupies the same position as the egg. Its head is, so to speak, planted at the very spot where the upper end of the egg was fixed; and all the remainder simply rests upon the victim, without being fastened to it. The grub’s transparency enables us readily to distinguish rapid undulations inside it, ripples which follow one upon the other with mathematical regularity and which, beginning in the middle of the body, spread some forward and some backward. These fluctuating movements are due to the digestive canal, which takes long draughts of the juices drawn from the victim’s body.
Let us dwell for a moment upon a sight which cannot fail to attract our attention. The Wasp’s prey lies on its back, motionless. In the cell of the Yellow-winged Sphex it is a Cricket, or rather three or four Crickets stacked one atop the other; in the cell of the Languedocian Sphex it is a single head of game, but large in proportion, a fat-bellied Ephippiger. The grub is lost should it happen to be torn from the spot whence it derives life; a fall would be the end of it, for, weak as it is and deprived of all means of motion, how could it [[88]]make its way back to the spot at which it slakes its appetite? The slightest movement would enable the victim to rid itself of the atom gnawing at its entrails; and yet the gigantic prey submits meekly, without the least quiver of protest. I well know that it is paralysed, that it has lost the use of its legs through the sting of its murderess; but still, recent victim that it is, it retains more or less power of movement and sensation in the regions not affected by the dart. The abdomen throbs, the mandibles open and close, the abdominal filaments wave to and fro, as do the antennæ. What would happen if the worm were to bite into one of the still impressionable parts, near the mandibles, or even on the belly, which, being more tender and more succulent, seems as though it ought, after all, to supply the first mouthfuls of the feeble grub? Bitten to the quick, the Cricket, Locust or Ephippiger would at least shiver; and this faint tremor of the skin would be enough to shake off the tiny larva and bring it to the ground, where it would no doubt perish, for it might at any moment find itself in the grips of those dreadful mandibles.
But there is one part of the body where no such danger is to be feared, the part which the Wasp has wounded with her sting—in short, [[89]]the thorax. Here and here alone, on a victim of recent date, the experimenter can rummage with a needle, driving it through and through, without producing a sign of suffering in the patient. Well, it is here that the egg is invariably laid; it is here that the young larva always takes its first bite at its prey. Gnawed at a point no longer susceptible to pain, the Cricket remains motionless. Later, when the wound has reached a sensitive point, he will doubtless toss about to such extent as he can; but then it will be too late: his torpor will be too deep; and besides the enemy will have gained strength. This explains why the egg is laid on a spot which never varies, near the wounds caused by the sting—in short, on the thorax: not in the middle, where the skin would perhaps be too thick for the new-born grub, but on one side, towards the juncture of the legs, where it is much thinner. What a judicious choice, how logical on the part of the mother when, underground, in complete darkness, she discerns the one suitable spot on the victim and selects it for her egg!
I have reared Sphex-grubs by giving them, one after the other, the Crickets taken from the cells; and I was then able to follow day by day the rapid progress of my nurselings. The first Cricket, the one on whom the egg was [[90]]laid, is attacked, as I have said, near the point where the huntress administered her second sting, that is to say, between the first and second pair of legs. In a few days the young larva has dug in the victim’s breast a hollow large enough to admit half its body. It is not uncommon to see the Cricket, bitten to the quick, uselessly waving his antennæ and his abdominal threads, opening and closing his mandibles on space and even moving a leg. But the enemy is safe and is ransacking his entrails with impunity. What an awful nightmare for the paralysed Cricket!
The first ration is finished in six or seven days’ time; none of it remains but the framework of skin, with all its parts more or less in position. The larva, whose length is now twelve millimetres,[3] leaves the Cricket’s body through the hole in the thorax which it made to start with. During this operation it moults; and its cast skin often remains caught in the opening through which it made its exit. It rests after the moulting and then attacks a second ration. Being stronger now, the larva has nothing to fear from the feeble movements of the Cricket, whose daily-increasing torpor has had time to extinguish the last glimmers of resisting-power during the week and more [[91]]that has elapsed since the dagger-thrusts were given. It is therefore assailed with no precautions, usually at the belly, which is the tenderest part and the richest in juices. Soon the turn comes of the third Cricket and lastly of the fourth, who is devoured in ten hours or so. Of these last three victims all that remains is the tough integuments, whose various parts are severed one by one and carefully emptied. If a fifth ration be presented, the larva scorns it, or hardly touches it, not from abstemiousness, but from imperious necessity. For observe that hitherto the larva has ejected no excrement and that its intestines, into which four Crickets have been crammed, are distended to bursting-point. A new ration cannot therefore tempt its gluttony; and henceforth it thinks only of making itself a silken tabernacle.
In all, its repast has lasted from ten to twelve days without cessation. At this period the larva’s length measures from twenty-five to thirty millimetres[4] and its greatest breadth from five to six.[5] Its general outline, spreading a little at the back and gradually tapering in front, conforms with the usual type of Hymenopteron-grubs. Its segments are fourteen in number, including the head, which is very [[92]]small and armed with weak mandibles that would appear unequal to the part which they have just played. Of these fourteen segments the middle ones are supplied with stigmata, or breathing-holes. Its livery consists of a yellowish-white ground, studded with innumerable dots of a chalky white.