The Hunting Wasps

by Jean-Henri Fabre

Three general methods are employed by the Digger-wasps in constructing the sanctum in which the metamorphosis is to take place. Some dig their burrows at great depths, under shelter: their cocoon then consists of a single envelope, so thin as to be transparent. This is the case with the Philanthi and the Cerceres. Others are content with a shallow burrow in open ground; but in that case they sometimes have enough silk to increase the number of wrappers for the cocoon, as we see with the Sphex, the Ammophilæ and the Scoliæ, or sometimes the quantity of silk is insufficient, when they have recourse to gummed sand, this being the method practised by the Bembex, the Stizi and the Palari. A Bembex-cocoon is so compact and strong that it might be taken for the kernel of some seed. The form is cylindrical, with one end rounded and the other pointed. The length is about three-quarters of an inch. On the outside it is slightly wrinkled and rather coarse to look at; but the inner walls are glazed with a fine varnish. [[299]]

My experiments in indoor breeding have enabled me to observe every detail of the construction of this architectural curiosity, a regular strong-box inside which the inclemencies of the weather can be braved in safety. The larva first pushes away the remains of its food and forces them into a corner of the cell or compartment which I have arranged for it in a box with paper partitions. Having swept the floor, it fixes at the different walls of its dwelling threads of a beautiful white silk, forming a spidery web which keeps off the cumbrous heap of broken victuals and serves as a scaffolding for the next work.

This work consists of a hammock slung far from any dirt, in the centre of the threads stretched from wall to wall. Nothing but silk, magnificently fine, white silk, enters into its composition. Its shape is that of a sack open at one end with a wide circular mouth, closed at the other and ending in a point. An eel-trap would give a very fair picture of it. The edges of the mouth are kept apart and permanently stretched by numerous threads starting from there and fastened to the adjoining walls. Lastly, the texture of this sack is extremely fine and allows us to see all the grub’s proceedings.

Things had been in this condition since the day before, when I heard the larva scratching [[300]]in the box. I opened it and found my prisoner engaged in scraping the cardboard wall with its mandibles, while its body was half outside the sack. The cardboard had already suffered considerably and a heap of tiny fragments were piled in front of the opening of the hammock, to be used later. For lack of other materials, the grub would doubtless have employed these scrapings for its building. I thought it better to provide something in accordance with its tastes and to give it sand. Never had Bembex-larva built with such sumptuous materials. I poured before the captive sand from my ink-stand: blotting-sand, blue sand sprinkled with little gilt mica spangles.

This supply is placed in front of the mouth of the bag. The bag itself is in a horizontal position, which is convenient for the coming task. The larva, leaning half out of the hammock, picks up its sand almost grain by grain, rummaging in the heap with its mandibles. If any grain is found to be too bulky, the grub takes it and throws it away. When the sand is thus sorted, the larva introduces a certain quantity into the silken edifice by sweeping it with its mouth. This done, it retires into the eel-trap and begins to spread the materials in a uniform layer on the lower surface of the sack; then it gums the different grains and inlays them in the [[301]]fabric, using silk as cement. The upper surface is built more slowly: the grains are carried up one by one and fixed on with the silken putty.

This first layer of sand as yet embraces only the front half of the cocoon, the half that ends at the mouth of the bag. Before turning round to work at the back half, the grub renews its supply of materials and takes certain precautions so as not to be hindered in its mason’s work. The sand outside, heaped up in front of the entrance, might slip inside and embarrass the builder in so narrow a space. The grub foresees this possibility: it glues a few grains together and makes a rough curtain of sand, which stops up the orifice very imperfectly, but sufficiently to prevent an accident. Having taken these precautions, the larva works at the back half of the cocoon. From time to time it turns round to fetch fresh supplies from outside, tearing a corner of the curtain that protects it against the outer sand and grabbing through this window the materials which it requires.

The cocoon is still incomplete, wide open at the big end; it wants the spherical cap that is to close it. For this final labour the grub takes a plentiful supply of sand, the last supply of all, and then pushes away the heap outside the entrance. At the opening it now weaves a silken cap, which fits the mouth of the primitive [[302]]eel-trap precisely. Lastly, grains of sand, kept in reserve inside, are laid one by one upon this silken foundation and glued together with silky slime. Having finished this lid, the larva has nothing else to do but give the last finish to the inside of the abode and glaze the walls with varnish to protect its delicate skin against the rough sand.

The hammock of pure silk and the hemisphere that closes it later are, as we see, but a scaffolding intended to support the masonry of sand and give it a regular curve; they might be compared with the wooden moulds which builders set up when constructing an arch, a vault. Once the work is done, the timber frame is taken away and the vault is sustained by virtue of its perfect balance. Even so, when the cocoon is finished, the silken support disappears, partly lost in the masonry, partly destroyed by contact with the coarse earth; and not a trace remains of the ingenious method followed in welding together materials with so little consistency as sand into a building of such perfect regularity.

The round cap closing the mouth of the original eel-trap is a work apart, adjusted to the main body of the cocoon. However well the two parts are fitted and soldered, the solidity is not the same as the larva would obtain if it [[303]]built its whole dwelling continuously. The circumference of the lid therefore has a circular line of least resistance. But this is not a fault of construction; on the contrary, it is a fresh improvement. The insect would find grave difficulty in issuing later from its strong-box, so stout are the walls. The line of junction, weaker than the others, would seem to save it a good deal of effort, for it is mostly along this line that the cover is removed when the Bembex emerges from the ground in the perfect state.

I have called this cocoon a strong-box. It is indeed a very solid piece of work, both from its shape and from the nature of its materials. Landslips or subsidences cannot alter its outline, for the strongest pressure of one’s fingers does not always succeed in crushing it. Therefore it matters little to the larva if the ceiling of its burrow, dug in loose soil, should fall in sooner or later; it does not care much if a passing foot should press upon it under its thin covering of sand; it has nothing to fear once it is enclosed in its stout bulwark. Nor does damp endanger it. I have kept Bembex-cocoons immersed in water for a fortnight at a time without afterwards discovering the least trace of dampness inside them. Why have we no such waterproofing for our dwellings!