Insect life: Souvenirs of a naturalist

by Jean-Henri Fabre

This peculiarity of beginning to lay in food by a single small piece of game is not peculiar to Bembex rostrata; all the other species do the same. Open any cell after the egg is laid, and you always find it glued to the side of a Dipteron—all the food there is; moreover, this first ration is invariably small, as if the mother had sought some specially tender mouthful for her frail nursling. Another motive, the freshness of the food, may also have guided her choice. Later we will look further into the matter. This first ration—always a moderate one—varies much, according to the frequency of such or such a kind of game in the neighbourhood. It is sometimes a Lucilia Cæsar, sometimes a Stomoxys, or some small Eristalis, or a delicate Bombylius clad in black velvet, but the commonest is a Sphærophoria with a slender abdomen. This fact (and it has no exception) of storing the nest with but a single Dipteron,—a ration far too meagre for a larva with a voracious appetite,—at once puts us on the track of the most remarkable habit of the Bembecidæ. Hymenoptera whose larvæ live on prey heap into each cell the whole number of victims needed by the grub, which is hatched and lives alone,—an egg having been laid on one fly and the dwelling closed up. The larva has before it its whole store of food. But the Bembex is an exception to this rule. First a head of game is brought to the cell and an egg dropped on it. Then the mother leaves the burrow, which closes of its own accord; besides which she takes care to rake the surface smooth, and hide the entrance from every eye but her own.

Two or three days pass: the egg hatches and [[226]]the small larva eats up its choice ration. Meanwhile, the mother remains near: one may see her licking the sugary exudations on the flower-heads of Eryngium campestre for nourishment, then settling with enjoyment on the burning sand, whence she doubtless surveys the exterior of her dwelling, or she sifts the sand at its entrance, then flies off and vanishes—perhaps to excavate other cells to be stored in a like manner. But however prolonged her absence, she does not forget the young larva so scantily provided for; maternal instinct teaches her the hour when the grub has finished its food and needs new sustenance. Then she comes back to the nest whose invisible entrance she knows right well how to find, and penetrates the hollow—this time laden with a larger prey. This deposited, she goes out again, and awaits outside the time for a second expedition. It soon comes, for the larva shows a devouring appetite. Again the mother arrives with fresh provender.

During almost a fortnight, while the larva is growing, the meals follow each other thus, one by one, as it needs them, and so much the nearer together as the nursling grows stronger. Toward the end of the fortnight the mother requires all her activity to supply the glutton’s appetite as it crawls heavily amid the remains of its repasts—wings, feet, and horny rings of abdomens. Each moment she returns with a new capture or comes forth for the chase. In short, the Bembex brings up her family from hand to mouth without storing provisions, like the bird which brings a beakful of food to the little ones still in the nest. Among the numerous proofs [[227]]of this method of upbringing—one very singular in a Hymenopteron which feeds its family on prey—I have already mentioned the presence of the egg in a cell where but one little fly is found as provender—always one—never more. Another proof is the following one, which does not require any special moment for its ascertainment.

Let us examine the burrow of a Hymenopteron, which provides beforehand for its larvæ. If we choose the moment when the insect enters with a captive, we shall find in the cell a certain number of victims already stored, but never a larva—not even an egg, for this is only laid when the provisions are complete. The egg deposited, the cell is closed, and the mother returns no more. It is, therefore, only in burrows where the mother’s visits are no longer needed that one can find larvæ amid the larger or smaller heap of food. Visit, on the other hand, the dwelling of a Bembex as she enters with the produce of her chase, and you are sure to find a larva, larger or smaller, amid the remains of food already devoured. The ration now brought is to continue a repast which has been going on for several days, and is to be prolonged upon the produce of future expeditions. If we can make this examination towards the end of the larva’s upbringing,—an advantage which I have enjoyed at pleasure,—we shall find upon a great heap of fragments a portly larva, to which the mother is still bringing food. The Bembex only ceases to do so and to leave the cell definitely when the larva, distended by a wine-coloured pap, refuses to eat, and reclines, thoroughly stuffed, on the remains of wings and feet of the game which it has devoured. [[228]]

Each time that she penetrates into the burrow on returning from the chase, the mother brings but a single fly. Were it possible by means of the remains contained in a cell where the larva is full grown to count the victims served up, one would at least know how often the Hymenopteron visited its burrow after the egg is laid. Unfortunately, these broken meats—munched and munched again in moments of scarcity—are for the most part unrecognisable. But on opening a cell with a less advanced nursling, one can examine the provisions, some of the prey being yet whole or nearly so, and others, more numerous, being trunks in sufficiently good preservation to be distinguishable. Incomplete as it is, the enumeration thus obtained strikes one with surprise, as showing what activity the Hymenopteron must display to satisfy the demands of such a table. Here is one of the bills of fare observed.

At the end of July around the larva of Bembex Julia, which had almost reached the third of its full size, I found the prey of which the following is the list:—Six Echinomyia rubescens—two whole and four in pieces; four Syrphus corollæ—two whole, two in fragments; three Gonia atra—all intact, and one just brought by the mother, which had enabled me to discover the burrow; two Pollenia ruficollis—one whole, one attacked; a Bombylius reduced to pulp; two Echinomyia intermedia in bits; and finally two Pollenia floralis, also in bits—total, twenty. Certainly we have here a bill of fare as abundant as varied, but as the larva had only attained to a third of its complete size, the entire bill of fare might well amount to sixty articles. [[229]]

The verification of this magnificent sum-total is easily obtained. I myself will undertake the maternal cares of the Bembex, and feed the larva until it is thoroughly satisfied. I place the cell in a little cardboard box furnished with a layer of sand. On this bed is placed the larva with due regard to its delicate epidermis. Around it, without omitting a single fragment, I arrange the provender with which it was supplied, and return home with the box still in my hand, to avoid any shake which might turn it topsy-turvy and endanger my charge during a journey of several miles. Any one who had seen me on the dusty road to Nîmes, exhausted with fatigue and bearing religiously in my hand, as the only result of my painful journey, a wretched grub, distending itself with a heap of flies, would assuredly have smiled at my simplicity. The journey was achieved without hindrance; when I got home the larva was peacefully consuming its flies as if nothing had happened. On the third day the provisions taken from the burrow were finished, and the grub with its pointed mouth was searching in the heap of remains without finding anything to its taste. The dry, horny, juiceless pieces which it got hold of were rejected with disgust. The moment had come for me to continue the food supply. The first Diptera within reach must content my prisoner; I slew them by squeezing them between my fingers, but did not crush them. Three Eristalis tenax composed the first ration, together with a Sarcophaga. In twenty-four hours all were devoured. The next day I provided two Eristalis and four house-flies. This sufficed for that day, but nothing was left over. I [[230]]went on thus for a week, giving the grub each morning a larger ration. On the ninth day it refused to eat and began to spin its cocoon. The bill of fare for the week’s high feeding amounted to sixty-two items, chiefly Eristalis and house-flies, which, added to the twenty items found entire or in fragments in the cell, formed a total of eighty-two.

Possibly I may not have brought up my larva with the wholesome frugality which the mother would have shown; there may have been some waste in the daily rations, provided all at once and left entirely to the discretion of the grub. I fancied that in some particulars things did not go on exactly as in the cell, for my notes have such details as: “In the alluvial sands of the Durance I discovered a burrow into which Bembex oculata had taken a Sarcophaga agricola. At the bottom of the gallery was a larva, numerous fragments, and some Diptera entire—namely, four Sphærophoria scripta, one Onesia viarum, and two Sarcophaga agricola, counting that which the Bembex had brought under my very eyes.” Now it must be remarked that one half of this game, the Sphærophoria, was quite at the bottom of the cell—under the very jaws of the larva, while the other half was still in the gallery—on the threshold of the cell—consequently out of the grub’s reach, as it could not leave its place. It would seem that when game abounds, the mother disposes provisionally of her captures on the threshold of the cell, and forms a reserve on which she draws as need arises, especially on rainy days, when all labour is at a standstill. This economy in distributing food would prevent the waste unavoidable with my larva perhaps too sumptuously [[231]]treated. I subtract then from the sum obtained, and reduce it to sixty pieces of medium size, between that of the house-fly and Eristalis tenax. This would be about the number of Diptera given by the mother to the larva when the prey is middle-sized, as is the case with all the Bembecids of my district except B. rostrata and B. bidentata, which especially favour the gadfly. For these the number of slain would be from one to two dozen, according to the size of the Dipteron, which varies greatly in the gadfly species.

In order not to return to the kind of provisions, I give a list of the Diptera observed in the burrows of the six kinds of Bembex, which are the subject of this essay.

(1) B. olivacea, Rossi. Once only have I seen this species, at Cavaillon, preying on Lucilia Cæsar. The five next are common round Avignon.