Part 13

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.

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.

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 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 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.

(2) B. oculata, Jur. The Dipteron upon which the egg is laid is generally a Sphærophoria, especially S. scripta; sometimes it is a Geron gibbosus. Further provender consisted in Stomoxys calcitrans, Pollenia ruficollis, P. rudis, Pipiza nigripes, Syrphus corollæ, Onesia viarum, Calliphora vomitoria, Echinomyia intermedia, Sarcophaga agricola, Musca domestica. The usual food was Stomoxys calcitrans, of which I have found fifty or sixty in a single burrow.

(3) Bembex tarsata, Lat. It, too, lays its egg on Sphærophoria scripta; but it also hunts Anthrax flava, Bombylius nitidulus, Eristalis æneus, E. sepulchralis, Merodon spinipes, Syrphus corollæ, Helophilus trivittatus, Zodion notatum. Its favourite prey consists in Bombylius and Anthrax.

(4) Bembex Julii (a new species). The egg hatches either on a Sphærophoria or a Pollenia floralis, and the provender is a mixture of Syrphus corollæ, Echinomyia rubescens, Gonia atra, Pollenia floralis, P. ruficollis, Clytia pellucens, Lucilia Cæsar, Dexia rustica, Bombylius.

(5) Bembex rostrata. This is above all a captor of gadflies. It lays its egg on a Syrphus corollæ, or a Lucilia Cæsar, but then only brings to the larva large game belonging to the various kinds of Tabanus.

(6) Bembex bidentata. Another ardent hunter of gadflies. I have never seen it with other game, and do not know on what the egg is laid.

This variety of provisions shows that the Bembecids have no exclusive tastes, and attack one and all of the species of Diptera which are offered by the chances of the chase. They seem, however, to have some favourites—one species especially choosing Bombylius, another Stomoxys, and a third and fourth, Gadflies.

XVII

HUNTING DIPTERA

After this bill of fare for Bembecids in the larva state, we must seek the motive which causes these Hymenoptera to adopt a mode of storage exceptional among Fossors. Why, instead of laying up sufficient food and dropping an egg on it, which would allow the cell to be closed at once without need of returning, does the Hymenopteron oblige itself to come and go perpetually for a fortnight from the fields to the burrow and back again, toiling every time through the sand to issue forth and hunt, or bring back prey? The explanation is that the food must be fresh—an all-important matter, for the grub absolutely refuses game which is at all high, with a hint of decay; like the larvæ of all Fossors, it must have fresh provisions—always fresh provisions.

We have seen in the case of the Cerceris, Sphex, and Ammophila how the mother resolves the feeding problem, by placing beforehand in the cell a sufficient quantity of game, and also that of keeping it for weeks perfectly fresh—nay, almost alive, though motionless—in order to secure the safety of the grub which feeds on the prey. This marvel is brought about by the most skilful means known to physiology. The poisoned sting is sent into the nerve centres once or oftener, according to the construction of the nervous system, and the victim retains all which we call life, except power of motion.

Let us see if the Bembex practises this deep science of murder. Diptera taken from between the feet of their captor as the latter enters the burrow mostly seem quite dead. They are motionless; only in rare cases are there some slight convulsions of the tarsi—the last vestiges of life soon to be extinct. The same appearance of complete death is found, as a rule, in insects not really killed but paralysed by the skilful stab of a Cerceris or a Sphex. The question as to life or death can, therefore, only be decided by the manner in which the victims keep fresh.

Placed in little paper twists or glass tubes, the Orthoptera of the Sphex, the caterpillars of the Ammophila, the Coleoptera of the Cerceris, preserve flexibility of limb and freshness of colour, and the normal state of their intestines, for weeks and months. They are not corpses, but bodies plunged in a lethargy from which there will be no awakening. The Diptera of the Bembex behave quite otherwise. Eristalis, Syrphus,—in short, all which are brightly coloured,—soon lose their brilliance; the eyes of certain gadflies, magnificently gilded, and with three purple bands, soon grow pale and dim, like the gaze of a dying man. All these Diptera, great and small, placed in paper twists where air circulates, dry up and grow brittle in two or three days, while all kept from evaporation in glass tubes, where the air is stagnant, grow mouldy and decay. So they are dead—really dead—when carried to the larva. If some few preserve a little life, a few days, a few hours ends all. Not being clever enough to use its sting, or for some other reason, the assassin kills its victims outright.

Knowing this complete death of the prey at the moment when it is seized, who would not admire the logic of the Bembecid’s manœuvres? How methodical all is, and how one thing brings about another in all which the wary Hymenopteron does! As the food could not be stored without its decaying at the end of two or three days, it cannot be laid in wholesale at the beginning of a phase of life destined to last at least a fortnight, and there must be a hunt and distribution of provisions day by day, in proportion to the larva’s growth. The first ration—that on which the egg is laid—will last longer than the others, and must be small, for the little grub will take several days to eat it, and if too big it would go bad before it was finished. Therefore it will not be a huge gadfly or a corpulent Bombylius, but a small Sphærophoria, or something of that kind, as a tender meal for a still frail larva. Later, and gradually larger, will come the bigger joints.

In the mother’s absence the burrow must be closed to prevent awkward intrusions, but the entrance must be one opened quickly, without serious difficulty, when the Hymenopteron returns loaded with prey, and laid in wait for by audacious parasites. These conditions would be wanting in a tenacious soil, such as that in which the mining Hymenoptera habitually establish themselves. The wide-open entrance would each time require long and painful labour, whether to close it with earth or gravel, or to clear it. The domicile, therefore, must be hollowed in earth with a very light surface, in dry, fine sand, yielding at once to the least effort of the mother, and which slips and closes the entrance like floating tapestry, which, pushed back by the hand, allows entrance and then drops back. Such is the sequence of acts, deduced by human reason, and put into practice by the wisdom of the Bembex.

Why does the spoiler kill the prey instead of paralysing it? Is it want of skill with the sting? Is it a difficulty arising from the organisation of the Diptera or from the manœuvres of the chase? I must own, at once, that I have failed to put a Dipteron, without killing it, into that state of complete immobility into which it is so easy to plunge a Buprestis, a Weevil, or a Scarabæus, by injecting a little drop of ammonia, on the point of a needle, into the thoracic ganglia. It is difficult to render your subject motionless; when it no longer moves, actual death has occurred, as is proved by its speedy decay or desiccation. But I have too much confidence in the resources of instinct,—I have seen the ingenious solution of too many problems,—to believe that a difficulty, though insurmountable for the experimenter, can baffle an insect; therefore, without casting doubt on the Bembex’s capacity for murder, I should be inclined to seek other motives.

Perhaps the Dipteron, so thinly cuirassed, of so little substance,—so lean, in short,—could not, when paralysed by a sting, resist evaporation, and would dry up in two or three weeks. Consider the slender Sphærophoria—the larva’s first mouthful. What is there in this body to evaporate? An atom—a mere nothing. The body is a thin strip—its two walls touch. Could such prey form a basis for preserved food when a few hours would evaporate its juices, unrenewed by nutrition? To say the least, it is doubtful.

Let us proceed to consider the manner of hunting, by way of throwing a final light on the subject. In prey withdrawn from the clasp of a Bembex, one may not infrequently observe indications of a capture made in haste, as best might be, in the chances of a wild struggle. Sometimes the Dipteron has its head turned backward, as if its neck had been twisted, its wings are crumpled, and its hairs, if it have any, are ruffled. I have seen one with the body ripped open by a bite from the mandibles, and legs lost in the battle. Usually, however, the prey is intact.

No matter. Considering that the game has wings prompt in flight, the capture must be made with a suddenness which it seems to me hardly allows of obtaining paralysis without death. A Cerceris with its heavy weevil, a Sphex engaged with a corpulent grasshopper or a paunched ephippiger, an Ammophila holding its caterpillar by the nape of its neck, have all three the advantage over a prey too slow to avoid attack. They may take their time, choose at leisure the exact spot where the sting shall penetrate, and, in short, can act with the precision of a physiologist who uses his scalpel on a patient laid upon the operating table; but for the Bembex it is another matter. At the least alarm the prey is off, and its power of wing defies that of the pursuer. The Hymenopteron must pounce on its prey, without measuring its attack or calculating its blow, like a hawk hunting over the fallows. Mandibles, claws, sting—all weapons—must be used at the same moment in the hot battle, to end as fast as possible a struggle in which the least indecision would give the prey time to escape. If these conjectures agree with facts, the Bembex can only secure a dead body, or, at all events, a prey wounded to death.

Well, my calculations are right. The Bembex attacks with an energy which would do honour to a bird of prey. To surprise one on the chase is no easy matter, and it would be useless to lay in a stock of patience and watch near the burrow, for the insect flies to a distance, and it is impossible to follow its rapid evolutions, and doubtless its manœuvres would be still unknown to me but for the help of an article from which I should assuredly never have expected a like service—namely, the umbrella which served me as a tent amid the sands of Issarts.

I was not the only one to profit by its shade; my companions were usually numerous. Gadflies of different kinds would take refuge under the silken canopy, and roost peacefully here and there on the outspread silk, rarely failing to appear when the heat was overpowering. To pass the hours when I was unemployed, I used to observe with pleasure their great gilded eyes shining like carbuncles under my canopy, or their grave movements when some spot of their ceiling became too much heated, and they were forced to move a little way.

One day—ping! ping! the tense silk was resounding like the parchment of a drum. Perhaps an acorn has fallen on my umbrella. Soon after, close together, came ping! ping! Has some idle jester come to disturb my solitude, and fling acorns or little pebbles on my umbrella? I came out of my tent and inspected the neighbourhood. Nothing! The blow was repeated. I looked upward, and the mystery was explained. The Bembecids of the neighbourhood, which prey on gadflies, had found out the rich store of food which was keeping me company, and were darting audaciously under my shelter to seize the gadflies on the ceiling. Nothing could have been better. I had only to keep quiet and observe.