The one observed fact which can throw any light on the problem is this. Amid a colony of Sphegidæ in full activity, whence all other Hymenoptera are habitually excluded, I one day surprised a sportsman of a different kind, Tachytes nigra, carrying one by one, without any haste and with the greatest composure, amid the crowd where he was but an intruder, grains of sand, little bits of dry stalk, and other small materials, to stop up a burrow of the same shape and size as the neighbouring ones of the Sphegidæ. This labour was pursued too conscientiously to admit of any doubt as to the presence of the worker’s egg in the underground dwelling. A Sphex with anxious movements, apparently the legitimate owner of the burrow, never failed each time that the Tachytes entered the gallery to dart in pursuit, but emerged swiftly, as if frightened, followed by the other, who continued her task unmoved. I visited this burrow, the evident cause of strife between them, and found a cell provisioned with four crickets. Suspicion almost gave place to certainty, for this allowance far exceeded the needs of a Tachytes’ larva, which is at least one-half smaller than the Sphex. The calm insect whose care to stop up the burrow at first suggested that it was the owner was really a usurper. How comes it that the Sphex, larger and more robust than her adversary, allows herself to be robbed with impunity, limiting herself to a fruitless pursuit, and flying like a coward when the intruder, who seems not even to perceive her, turns round to come out of the burrow? Is it with insects as with men, the first quality needed for success is audacity—audacity—audacity? Certainly the usurper had no lack of it. I can still see that Tachytes, imperturbably calm, going and coming before the meek Sphex, which stamped with impatience, but did not venture to fall upon the thief.