At last, here we are at last! See, the Odynerus arrives, with a flight as silent as the Eumenes’. She disappears into the curved cylinder of the vestibule, bringing home a grub beneath her abdomen. I place a small glass test-tube at the entrance to the nest. When the insect emerges, it will be caught. Done! The Wasp is caught and at once decanted into the asphyxiating-flask, with its strips of paper steeped in bisulphide of carbon. And now, my Dog, still lolling your tongue and frisking your tail, we can be off; the day has not been wasted. We will come back to-morrow.
Upon investigation, my Odynerus does not correspond with what I expected to see. This is not the species of which Réaumur speaks (O. spinipes); nor is it the species studied by Dufour (O. Reaumurii); it is another. (O. reniformis, Latr.), a different one, though addicted to the same arts. Already the naturalist of the Landes had [[36]]allowed himself to be deceived by that similarity in architecture, provisions and habits; he thought that he was observing Réaumur’s Solitary Wasp, whereas in reality his tube-builder presented specific differences.
We know the worker; it remains for us to become acquainted with her work. The entrance to the nest opens in the perpendicular wall of the bank. It is a round hole, on the edge of which is built a curved tube, with the orifice turned downwards. Made with the materials cleared from the burrow under construction, this tubular vestibule is composed of grains of earth, not arranged in continuous courses, but leaving small vacant intervals. It is a species of open-work, a lacework of clay. Its length is about an inch and its internal diameter a fifth of an inch. This portico is continued by the gallery, of the same diameter, which slants into the soil to a depth of nearly six inches. Here this main gallery branches into short corridors, each giving access to a cell which is independent of its neighbours. Each larva has its chamber, which can be reached by a special passage. I have counted as many as ten of them; and there may be more. These chambers have nothing [[37]]remarkable about them, either in construction or in capacity; they are just culs-de-sac ending the corridors that give access to them. Some are horizontal, some more or less sloping; there is no fixed rule. When a cell contains what it is meant to contain, the egg and the provisions, the Odynerus closes the entrance with a little earthen lid; she then digs another near it, on one side of the principal gallery. Lastly, the common road to the cells is blocked with earth; the tube at the entrance is demolished, to furnish material for the work done inside the nest; and every vestige of the habitation disappears.
The surface of the bank is of clay baked in the sun; it is almost brick. I break into it with difficulty, making use of a small pocket-trowel. Underneath, it is much less hard.
How does the frail miner manage to sink a gallery in this brick? She employs, I cannot doubt, the method described by Réaumur. I will therefore reproduce a passage from the master’s writings, to give my younger readers a glimpse into the habits of the Odyneri, habits which my very small colony did not enable me to observe in all their details: [[38]]
“It is at the end of May that these Wasps set to work; and one can see them busily labouring during the whole of June. Though their actual object is only to dig in the sand a hole a few inches deep and not much wider than their bodies, one might suppose that they had another end in view; for, to make this hole, they build on the outside a hollow tube, which has as its base the circumference of the entrance to the hole and which, after following a direction perpendicular to the surface containing that aperture, turns downwards. This tube becomes longer in proportion as the hole becomes deeper; it is built of the sand drawn from the hole; it is fashioned in coarse filigree, or a sort of guilloche. It is made of big, granular, winding fillets, which do not touch at all points. The gaps left in between make it look as if it were artistically constructed, whereas it is only a sort of scaffolding by means of which the mother’s tactics are rendered swifter and surer.
“Though I knew these insects’ two teeth to be capital instruments, capable of breaking into very hard substances, the task which they had to perform appeared to me rather severe for them. The sand on which they had to act was scarcely less hard than ordinary [[39]]stone; at least, one’s finger-nails made but a poor impression upon its outer layer, which the sun’s rays had dried more thoroughly than the rest. But, when I succeeded in observing these workers at the moment when they were beginning to bore a hole, they taught me that they did not need to subject their teeth to so harsh an ordeal.
“I saw that the Wasp begins by softening the sand which she proposes to remove. Her mouth discharges upon it a drop or two of water, which is promptly swallowed by the sand, turning it instantly into a soft paste which her teeth scrape and remove without difficulty. Two of her legs, the foremost pair, immediately proceed to gather it into a little pellet, about the size of a currant-seed. It is with this pellet, the first one removed, that the Wasp lays the foundations of the tube which we have described. She carries her pellet of mortar to the edge of the hole which she has just made by removing it; her teeth and feet turn it about, flatten it and make it stand up higher than it did before. This done, the Wasp again sets about removing sand and loads herself with another pellet of mortar. Soon she contrives to have extracted enough [[40]]sand to make the entrance of the hole perceptible and to have laid the foundation of the tube.
“But the work can proceed quickly only so long as the Wasp is able to moisten the sand. She is obliged to take trouble to renew her store of water. I do not know whether she simply went to take in water at some stream, or whether she drew, from some plant or fruit, a more sticky fluid; what I do know is that she returned without delay and set to work with renewed zeal. I observed one Wasp who managed, in about an hour, to sink a hole the length of her body and who raised a chimney as tall as the hole was deep. At the end of a few hours the tube stood two inches high and she was still deepening the hole that lay underneath.
“It did not appear to me that she had any rule respecting the depth which she gives it. I have found some whose hole ran more than four inches from the orifice; others whose hole measured only two or three inches. Again, over one hole you will find a tube twice or three times as long as that over another. Not all the mortar removed from the hole is invariably employed to prolong it. In cases where the [[41]]Wasp has given the tube a length which she considers sufficient, you see her simply arrive at the opening to the tube, put her head beyond its edge and forthwith drop her pellet, which falls to the ground. In this way I have often observed a quantity of rubbish at the foot of certain holes.
“The object for which the hole is pierced in a solid mass of mortar or sand cannot appear in doubt: it is plainly intended to receive an egg, together with a store of foodstuffs. But we do not so easily see to what end the mother has built the mortar shaft. By continuing to follow her labours, we shall discover that it means to her what a stack of well-laid stones means to the masons building a wall. Not the whole of the tunnel which she has excavated is intended as a lodging for the larva which will be born inside; a portion will be quite enough. Yet it was necessary that the hole should be dug to a certain depth, in order that the larva may not find itself exposed to too great a heat when the sun’s rays fall on the outer layer of sand. It will occupy only the end of the tunnel. The mother knows what space she must leave vacant and this space she retains; but she fills up all the remainder and replaces in the upper [[42]]portion of the hole as much of the sand removed from it as is necessary to stop it up. It is to have this mortar within reach that she has built that shaft. Once the egg is laid and the store of victuals placed within its reach, we see the mother come and gnaw the end of the shaft, after first moistening it, carry the pellet inside and next return for more, in the same manner, until the hole is blocked right up to the opening.”
Réaumur goes on to speak of the victuals heaped up in the cells, the “green grubs[9],” as he calls them, heedless of the ugly alliteration. Not having seen the same things, because my Odynerus is of a different species, I will continue my story. I counted the head of game in three cells only: the colony was a small one; I had to deal tenderly with it if I would follow its history to the end. In one of the cells, before the provisions were broached, I counted twenty-four pieces; in each of the two others, which were likewise intact, I counted twenty-two. Réaumur found only eight to twelve pieces in the larder of his Odynerus; and Dufour, [[43]]in the store-room of his, discovered a batch of ten to twelve. Mine requires twice as many, a couple of dozen, which may be due to the smaller size of the game. No predatory Wasp of my acquaintance, apart from the Bembeces,[10] who obtain their supplies from day to day, approaches this prodigality in numbers. Two dozen grub-worms to make a meal for only one! How far removed are we from the single caterpillar of the Hairy Ammophila! And what delicate precautions must be taken for the safety of the egg in the midst of such a crowd! A scrupulous vigilance is necessary here, if we would obtain a true conception of the dangers to which the Odynerus’ egg is exposed and of the means that save it from danger.
And, in the first place, what variety of game is this? It consists of worms as thick as a knitting-needle and varying slightly in length. The biggest measure a centimetre.[11] The head is small, of an intense, glossy black. The segments, unlike those of the caterpillars, have no legs, either true or false, but all, without exception, are furnished with ambulatory organs in the [[44]]shape of a pair of small fleshy nipples. These worms, though of the same species, to judge by their general characteristics, differ in colouring. They are a pale, yellowish green, with two wide longitudinal stripes of pale pink in some and of a more or less deep green in others. Between these two stripes, on the back, runs a streak of pale yellow. The whole body is sprinkled with little black tubercles, each bearing a hair on its crest. The absence of legs proves that they are not caterpillars, not the larvæ of Butterflies or Moths. According to Audouin’s experiments, Réaumur’s “green grubs” are the larvæ of a Weevil, Phytonomus variabilis, an inhabitant of the lucerne-fields. Can my worms, pink or green, also belong to some little Weevil? It is quite possible.
Réaumur described the grubs composing the victuals of his Odynerus as alive; he tried to rear some, hoping to see a Fly or a Beetle appear from them. Léon Dufour, on his side, called them live caterpillars. The mobility of the game provided escaped neither of the two observers; they had before their eyes grubs that moved about and gave full signs of life.
What they saw I also see. My little [[45]]larvæ frisk and fidget; curled at first in the shape of a ring, they uncurl themselves and curl again, if I do no more than slowly turn the small glass tube in which I have imprisoned them. When touched with the point of a needle, they struggle abruptly. Some succeed in shifting their position. While engaged in rearing the Odynerus’ egg, I opened the cell lengthwise, so as to reduce it to a semicylinder; in the little trench thus made, which was kept horizontal, I placed a few head of game. Next day usually I found that one of them had fallen out, a proof of movement, of a change of position, even when nothing was disturbing its repose.