The Mason-Wasps

by Jean-Henri Fabre

As I should have expected, the laying-peculiarities of the Nest-building Odynerus are precisely identical with those of O. reniformis, the object of my earlier observations. I again witness, with the satisfaction that results from verifying an interesting fact, the curious arrangements already described. The egg is laid first, right at the back of the cell. Next comes the stacking of the provisions in the order of capture. In this way the eating proceeds from the oldest to the most recent.

I was above all anxious to ascertain whether the egg was pendulous, that is to say, whether it hung by a thread at one point of the cell, in accordance with what I had learnt from the Eumenes and from O. reniformis. A kinswoman of the latter must, I felt certain beforehand, conform to the method of the suspension-cord; but there was reason to fear that the journey from Orange and the jolting of the cart might have broken the delicate pendulum. I recalled my anxieties, my minute precautions, during the removal of the cells with [[202]]the egg of O. reniformis swinging from the ceiling. The cart, ignorant of its precious burden, might have undone everything.

But no, to my great surprise. In most of the cells which were sufficiently recent I find the egg in place, slung sometimes from the arched roof of the reed, sometimes from the upper edge of the partition, by a thread which is just visible and about one twenty-fifth of an inch long. The egg is itself cylindrical and measures about an eighth of an inch. The reeds, opened wide and placed in glass tubes, enable me to witness the hatching, which takes place three days after the closing of the cell and probably four days after the laying.

I see the new-born grub enclosed almost wholly, head downwards, in the sheath provided by the pellicle of the egg. Very slowly it slides forward in this scabbard and the suspension-cord stretches to the same extent. It is extremely fine in the part consisting of the original thread, but very much thicker in the portion resulting from the slough of the egg. The grub’s head reaches the nearest piece of game at one point or another; and the fragile creature takes its first mouthful. If anything startles it, if I tap the reed, it lets go and [[203]]withdraws a little way into the sheath of the egg; then, reassured, it once more glides forward and resumes the point attacked. At other times a jerk leaves it indifferent. This suspension-stage of the new-born larva continues for about twenty-four hours, after which the grub, now somewhat fortified, lets itself drop and eats in the ordinary manner. The victuals last it for twelve days. Immediately afterwards comes the working of the cocoon, in which the insect remains, a yellow larva, until next May. It would be tedious to follow the Odynerus in its career of eating and weaving. The consumption of dishes highly spiced with nitrobenzene and the spinning of the cocoon, of a fine amber-coloured fabric, involve nothing so remarkable as to deserve special mention.

Before leaving this subject, I will state a problem which the pendulous egg sets to the embryogenist. Every insect’s egg, if cylindrical in form, has two poles, the front and back, the cephalic and the anal pole. By which of the two does the insect see the light?

By the hinder pole, the Eumenes and the Odyneri tell us. The end of the egg fastened to the wall of the cell was evidently the first to issue from the oviduct, in view [[204]]of the mother’s absolute need first to glue the suspension-thread somewhere, before abandoning her egg to space. In the ovarian tubes and in the oviduct, which are too narrow to allow of an inversion, the anal pole therefore passes first. Pointing in the same direction as the egg, the new-born grub will thus hang head downwards, with its hinder end uppermost, at the end of its thread.

By the front pole, the Scoliæ, the Spheges and the Ammophilæ in their turn reply, as do all the Hunting Wasps that fix the egg to some portion of the victim. It is, indeed, always by the cephalic end that the egg adheres to the prey, at a definite point selected by the mother’s prudence; for the safety of the nurseling and the preservation of the victuals demand that the first bites shall be taken here and here only. For the same reasons as above, the extremity fastened to the game has emerged into the light of day before the other.

Both these opposite testimonies are equally truthful. According as its destiny is to be glued to the wall of the cell or to be kept away from it on another support, the egg takes its plunge into life by the front pole or the rear pole, which requires [[205]]an inverse direction in the ovaries and the oviduct. In this manner the new-born grub always has its food under its mandibles; and its utter lack of experience does not expose it to the danger of death from inanition in front of a heap of provisions which its mouth would not yet be able to seek and find. There is the problem. I beg and entreat the embryogenists to solve it, without reference to preordination, with the sole aid of protoplastic energy.

To know the Odynerus in the privacy of her home was not enough: the thing was to see her also at work as a huntress. How does she capture her game? How does she operate on it, in order to keep it fresh while deprived of life and movement? What is her surgical method? As, for the moment, I knew of no smallest colony of the Chrysomela’s persecutor in my neighborhood, I put the matter to Claire. She was on the spot, in daily contact with the Hen-house where the memorable events that form the subject of this essay occurred; and—a most important circumstance—I knew her to be both quick-witted and willing. She accepted the burdensome task with enthusiasm. I, on my side, was, if possible, to attempt certain observations with the captive [[206]]insect. So as not to influence each other in our appreciation of facts which, by their rapidity, might leave room for doubt, we each agreed to keep our results secret until we were both certain of our data.

Fully instructed as to what to do, Claire begins. She soon discovers on the banks of the Aygues some poplars covered with Chrysomela-larvæ. From time to time an Odynerus arrives, alights upon a leaf and goes off again with her capture in her legs. But things are happening too high up; detailed inspection of the struggle between the huntress and the victim is impracticable. Moreover, the appearances of the Odynerus on the tree which was being watched among so many others, all equally propitious to the chase, occur at long intervals, which try the patience beyond all bounds. Tenacious in her desire to see, to learn and to be useful to me, my zealous collaborator bethinks herself of an ingenious expedient. A young poplar, with a wealth of Chrysomelæ, is pulled up, together with the lump of earth clinging to it. Lavish precautions are taken to avoid the shocks which, during the uprooting and the removal, might cause the herd of larvæ to drop off. The business is so successfully done that the tree [[207]]arrives without a hitch at its destination, in front of the Hen-house. It is put back in the earth immediately facing the reeds wherein the Odynerus makes her dwelling. No matter whether it takes root again or not, provided that the little tree keep fresh for a few days with abundant watering: that is all that is wanted.