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 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 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 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.