Chapter xvi
A PARASITE OF THE BEMBEX. THE COCOON
I have shown the Bembex hovering with her cumbrous prize above the nest and then dropping vertically and very slowly: a hesitating descent accompanied by a sort of plaintive hum. This cautious arrival might suggest that the insect is examining the ground from above in order to find its door and trying to recall the locality before alighting. But another motive is at work, as I propose to demonstrate. Under ordinary conditions, when no sign of danger is apparent, the Wasp comes suddenly, at full tilt, without any hovering, hesitating or whimpering, and settles at once on her threshold or very near it. Her memory is so faithful that she has no need to search about. Let us then look into the cause of that hesitating approach which I described in the last chapter.
The Wasp hovers, descends slowly, ascends again, flies away and returns, because the nest is threatened by a very grave danger. Her plaintive hum denotes anxiety: she never [[285]]emits it when there is no peril. But who is the enemy? Can it be I, sitting here and watching? Why, no: I am nothing to her, nothing but a shapeless mass unworthy of her attention. The formidable enemy, the fearsome foe that must be avoided at all costs, is there, sitting motionless on the sand, near the house. It is a miserable little Fly, feeble and inoffensive in appearance. This insignificant Gnat is the terror of the Bembex. The scourge of the Fly-tribe, the fierce slayer who so swiftly wrings the necks of colossal Gad-flies sated with blood from an Ox’s back, does not enter her own residence because she sees herself watched by another Fly, a regular pigmy, who would make scarcely a mouthful for her larvæ.
Why does she not pounce upon her and get rid of the little wretch? The Wasp is quick enough on the wing to catch her; and, small though the capture be, the larvæ will not scorn it, since any sort of Fly suits them. But no: the Bembex flees from a foe whom she could cut to bits with a single stroke of her mandibles; it is to me as though I saw my Cat fleeing in terror from a Mouse. The ardent huntress of Flies is hunted by a Fly, and a small one at that. I bow before the facts without hoping ever to understand this inversion of the parts played by each insect. To be able to rid yourself [[286]]easily of a mortal enemy who is contemplating the ruin of your family and would furnish a nice little meal for it, to be able to do that and not do it when the enemy is there, within reach of you, watching you, defying you: this is the height of animal aberration. But aberration is not the right word; let us rather speak of the harmony of created things, for, since this wretched little Fly has her tiny part to play in the general order, the Bembex must needs respect her and like a craven flee before her, else there would long since have been none of her left in the world.
Let us now tell the history of this parasite. Among the nests of the Bembex, we find very frequently some that are occupied at the same time by the larva of the Wasp and by other larvæ, strangers to the family and gluttonous companions of the first. These strangers are smaller than the Bembex’ nurseling, tear-shaped and of a purplish colour, due to the tint of the baby-food that shows through the transparent body. They vary in number: there are sometimes half-a-dozen of them, sometimes ten or more. They belong to a species of Fly, as is evident from their shape and also confirmed by the pupæ which we find in their place. Home-breeding completes the proof. When reared in boxes, on a layer of [[287]]sand, with Flies renewed from day to day, they turn into pupæ from which, a year later, there issues a small Fly, a Tachina of the genus known as Miltogramma.
It is the same Fly that caused the Bembex such lively fears by lying in ambush near the burrow. The Wasp’s terror is but too well founded. This is what happens inside the dwelling: around the heap of food which the mother exhausts herself in keeping up to the requisite quantity, seated in company with the lawful offspring, are from six to ten hungry guests, who dip their sharp-pointed mouths into the common dish with no more restraint than if they were at home. Harmony seems to prevail at the table. I have never seen the lawful larva grow indignant at the indiscretion of the alien grubs, nor have I seen these appear to wish to interfere with the other’s repast. All help themselves indiscriminately and eat away peaceably without seeking a quarrel with their neighbours.
So far all would be well, if a serious difficulty did not now arise. However active the mother-nurse may be, she is obviously not equal to such an output. She had to be constantly hunting to feed one larva, her own; how could she possibly manage to provide for a dozen greedy mouths? The result of this [[288]]enormous increase of family can only be want, or even starvation, not for the Fly’s maggots, which, developing more quickly than the Bembex’ larva, get ahead of it and profit by the days when there is still plenty for everybody, as their host is too young to need much, but certainly for that unfortunate host, who arrives at the transformation period without being able to make up for lost time. Besides, even if the first visitors, in becoming pupæ, leave him the free run of the table, others appear upon the scene, so long as the mother continues to come to the nest, and complete his starvation.
In burrows invaded by numerous parasites, the Bembex’ larva is in point of fact much smaller than one would suppose from the heap of food consumed, the remains of which encumber the cell. Limp, emaciated, reduced to a half or a third of its normal size, it vainly tries to weave a cocoon for which it does not possess the silk; and it perishes in a corner of the house among the pupæ of its more fortunate companions. Its end may be more cruel still. Should the provisions fail, should the mother-nurse delay too long in returning with food, the Flies devour the larva of the Bembex. I verified this black deed by rearing the brood myself. All went well so long as there was plenty to eat; but, if the daily portion was [[289]]omitted by accident or design, next day or the day after I was sure to find the Fly’s grubs greedily slicing up the larva of the Bembex. So, when the nest is invaded by the parasites, the lawful larva is doomed to perish, either by hunger or by a violent death; and this is what makes the Bembex hate the sight of the Miltogrammæ prowling around her home.
The Bembex are not the only victims of these parasites: all the Digger-wasps without distinction have their burrows plundered by Tachinæ and especially Miltogrammæ. Different observers, notably Lepeletier de Saint-Fargeau, have spoken of the wiles of these bold-faced Flies; but none of them, so far as I know, has remarked this very curious instance of parasitism at the expense of the Bembex. I say very curious, because the conditions are quite different. The nests of the other Digger-wasps are stocked beforehand and the Miltogramma drops her eggs on the pieces of game as they are taken in. When the Wasp has finished her catering and laid her egg, she closes the cell, where henceforth the lawful larva and the alien larvæ hatch and live together without ever being visited in their solitude. The mother therefore is not aware of the parasites’ brigandage, which remains unpunished because it is unknown. [[290]]