The Hunting Wasps

by Jean-Henri Fabre

THE FLY-HUNT

After our list, in the last chapter, of the fare on which the Bembex feed in the larval form, it behoves us to seek the motive that induces these Wasps to adopt a method of victualling so exceptional among the digger-insects. Why, instead of previously storing a sufficient quantity of provisions on which the egg could be laid—which would enable the mother to close the cell immediately afterwards and never to return to it—why, I ask, does she tie herself down for a fortnight to this incessant, toilsome coming and going from the burrow to the fields and from the fields to the burrow, forcing her way each time through the unstable sand, either to go hunting or to bring the larva her latest capture? It is, first and foremost, a question of having fresh victuals for her larva: an all-important question, for the grub absolutely refuses any high or tainted game. Like the grubs of the other Diggers, it wants fresh meat and nothing but fresh meat. [[272]]

We have seen in the case of the Cerceres, the Sphex and the Ammophilæ how the mother solves the problem of preserved food-stuffs, the problem of stocking a cell with the requisite quantity of game for its future occupant and keeping the meat fresh for whole weeks at a time; indeed, it is something more than fresh, for the victims are kept in an almost living state, except that they are incapable of movement, an essential condition if the grub is to feed on them in safety. The miracle is performed by the most cunning methods known to physiology. The poisoned lancet is driven into the nerve-centres once or oftener, according to the structure of the nervous system. Thus operated upon, the victim retains all the attributes of life, short of the power of moving.

Let us see if the Bembex make use of this profound science of slaughter. The Flies taken from between the legs of the kidnapper as she enters her burrow present, in most cases, every appearance of death. They are motionless; occasionally we can detect in a few of them some faint convulsions of the tarsi, the last vestiges of a life that is passing away. The same appearance of complete death is usually found in the insects which are not actually killed but paralysed by the adroit dagger-thrust of a Cerceris or a Sphex. The question whether they are [[273]]alive or dead can therefore be decided only according to the manner in which the victims keep fresh.

Placed in little screws of paper or in glass tubes, the Crickets and Grasshoppers of the Sphex, the caterpillars of the Ammophilæ, and the Beetles and Weevils of the Cerceres preserve their flexibility of limb, their freshness of colouring and the normal condition of their intestines for weeks and months. They are not corpses but bodies sunk in a lethargy from which there is no awaking. The Flies of the Bembex behave quite differently. The Eristales, the Syrphi—in short, all those whose livery is at all brightly coloured—soon lose the brilliancy of their attire. The eyes of certain Gad-flies, magnificently gilded, with three purple bands, very quickly grow pale and dim, like the eyes of a dying man. All these Flies, large and small, when placed in little paper bags through which the air circulates freely, dry up in two or three days and become brittle; all, when preserved against evaporation in glass tubes in which the air is stationary, go mouldy and decay. They are dead, therefore, really and truly dead, when the Wasp brings them to her larva. Should some of them still retain a remnant of life, a few days or even hours put an end to their agony. Consequently, for lack of talent in the use of her dagger or for [[274]]some other reason, the murderess kills her victims outright.

In view of this fact, that the prey is quite dead at the moment when it is carried off, who would not admire the logic of the Bembex’ procedure? How methodical and consistent everything is in the actions of the cunning Wasp! As the provisions cannot keep beyond two or three days without going bad, they must not be stored entire in the first stages of an infancy which will last at least a fortnight; and the hunting and distribution must necessarily be done day by day, bit by bit, as the larva grows up. The first ration, the one that receives the egg, will last longer than the others; the budding grub will take several days to eat its flesh. It must therefore be small, otherwise the joint would begin to putrefy before it was all finished. This joint therefore will not be a bulky Gad-fly or a corpulent Bombylius, but rather a tiny Sphærophoria, or something similar, making a dainty meal for the larva which is still so delicate. Later, getting bigger and bigger in time, will come the larger joints of venison.

The burrow must be kept shut during the mother’s absence, to save the larva from regrettable intrusions; nevertheless the entrance must be one that can be opened very frequently and hurriedly, without much difficulty, when [[275]]the Wasp returns laden with her prey and watched by the sharp eyes of daring parasites. These conditions could not be obtained with a compact soil such as that in which the Digger-wasps usually make their abodes: the door, left to itself, would stay open; and so, each time, there would be the long and toilsome job of either blocking up the entrance with earth and gravel or unblocking it, as the case might be. The house therefore must be dug in ground with a very loose surface, in fine dry sand, which will at once yield to the slightest effort on the mother’s part and, as it slides down, will close the door of its own accord, like a curtain which, when you thrust it aside with your hand, lets you pass through and then falls back again. There you have the series of actions as deduced by man’s reason and as practised by the Wasp’s sagacity.

Why does the spoiler kill the captured prey instead of simply paralysing it? Is it for want of skill in the use of her sting? Is it because of some difficulty due to the structure of the Flies or to the methods employed in the chase? I must begin by confessing that I have failed in my attempts to place Flies, without killing them, in that state of complete immobility to which it is so easy to reduce a Buprestis, a Weevil or a Scarab by injecting a tiny drop of [[276]]ammonia with a needle into the thoracic ganglia. In making the experiment, it is difficult to render the insect motionless; and, by the time that it has ceased to move, death has actually occurred, as is proved by its speedy corruption or desiccation. But I have too much confidence in the resources of instinct and have witnessed the ingenious solution of too many problems to believe that a difficulty which baffles the experimenter can bring the insect to a standstill. Therefore, without throwing doubt upon the Bembex’ talents as a slaughterer, I should be inclined to look for other reasons.

Perhaps the Fly, so thinly covered, so devoid of any plumpness, in a word, so lean, could not, if paralysed by the sting, resist evaporation long enough and would shrivel up during the two or three weeks of waiting. Consider the puny Sphærophoria, the larva’s first mouthful. How much liquid has that body to satisfy the needs of evaporation? An infinitesimal drop, a mere nothing. The abdomen is a thin strip; its two sides touch. Can such game as this form the basis of preserved food, seeing that evaporation would dry up its juices in a few hours when these are not renewed by nutrition? It is doubtful, to say the least.

Let us examine the method of hunting, so as to throw some final light on the subject. In [[277]]the quarry removed from between the legs of the Bembex, it is not rare to observe signs of a hurried capture, made anyhow, according to the chances of a rough-and-tumble fight. The Fly sometimes has her head turned the wrong way round, as though the spoiler had wrung her neck; her wings are crushed; her fur, when she possesses any, is ruffled. I have seen some that had their bellies ripped open by their assailant’s mandibles and had lost their legs in the battle. As a rule, however, the victim is intact.