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 [234]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, [235]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 [236]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 [237]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 [238]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 [239]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.

Every moment a Bembex entered like a sudden flash, and darted up to the silken ceiling, which resounded with a dull thud. A tumult went on aloft, in which one could not distinguish attacker from attacked, so lively was the mêlée. The struggle was very brief; almost at once the Hymenopteron retired with a captive between its feet. The dull band of gadflies drew a little back all round on this sudden irruption, which decimated them, but without leaving the treacherous shelter. It was so hot outside; wherefore move? Plainly, such swift attack and prompt departure with the prey does not allow the Bembex to use a poignard according to rules. The sting no doubt fulfils its office, but is directed with no precision towards such spots as are exposed by the chances of the combat. To slay outright the [240]half-murdered gadfly, still struggling between the feet of its assassin, I have seen the Bembex chew the head and thorax of her victim. This habit, peculiar to the Bembecids, shows that the Bembex desires death, not paralysis, since she ends the life of the Diptera with so little ceremony. Everything considered, I think that on the one side the nature of the prey, so quickly dried up, and on the other, the difficulties of so vehement an attack, are the reasons why the Bembecids serve up dead prey to their larvæ, and consequently provide it daily.

BEMBEX ROSTRATA TAKING GADFLY TO ITS NEST; BEMBEX ROSTRATA MINING

Let us follow the Hymenopteron when it returns with its captive closely clasped to the burrow. Here is one—B. tarsata—coming loaded with a Bombylius. The nest is placed at the sandy foot of a vertical slope, and the approach of the Bembex is announced by a sharp humming, somewhat plaintive, and only ceasing when the insect has alighted. One sees her hover above the bank, then descend, following the vertical line slowly and cautiously, still emitting the sharp hum. If her keen gaze should discover anything unusual, she delays her descent, hovers a moment, ascends again, redescends, then flies away, swift as an arrow. In a few moments she returns. Hovering at a certain height she appears to inspect the locality, as if from the top of an observatory. The vertical descent is resumed with most circumspect deliberation; finally, she alights without hesitation at a spot which to my eye has nothing to distinguish it from the rest of the sandy surface. The plaintive note ceases at once. She must have alighted somewhat by chance, since the most practised eye could not distinguish one spot from another [241]on the sandy tract. She will have dropped down somewhere near her hole, whose entrance she will now seek, marked since her last exit not only by the natural falling in of materials, but by her scrupulous sweeping. No! she does not hesitate in the least—does not feel about—does not seek. All have agreed that the organs fitted to direct insects in their researches reside in the antennæ. At the moment of returning to the nest I see nothing special in their play. Without once losing hold of the prey the Bembex scratches a little in front of her just where she alighted, pushes with her head, and straightway enters clasping the Dipteron to her body. The sand falls in, the door closes, and the Hymenopteron is at home.

I have watched the Bembex return home a hundred times, yet it is always with fresh astonishment that I see the keen-sighted insect at once detect an entrance which nothing indicates, and which indeed is jealously hidden—not indeed when she has entered (for the sand, more or less fallen in, does not become level, and now leaves a slight depression, now a porch incompletely obstructed), but always after she comes out, for when going on an expedition she never neglects to efface the traces of the sliding sand. Let us await her departure, and we shall see that she sweeps before her door and levels everything scrupulously. When she is gone, I defy the keenest eye to rediscover the entrance. To find it when the sandy tract was of some extent I was forced to have recourse to a kind of triangulation, and how often did my triangle and efforts of memory prove vain after a few hours’ absence! I [242]was obliged to have recourse to a stake—in other words, a grass stalk planted before the entrance—a means not always effectual, for it often disappeared during the frequent settings to rights of the outside of the Bembex’s nest.