The passers-by, intent upon their business, may not perceive the intruder; but these three certainly do. During their work of demolition, they touch the grub with their legs, their antennæ, their palpi; and yet none of them minds it. The fat grub, so easily recognized by its queer figure, is left alone; and this in broad daylight, where everybody can see it. What must it be when the profound darkness of the burrows protects the visitor with its mysteries! I have been experimenting all along with big Volucella-grubs, coloured with the dirty red that comes with age. What effect will pure white produce? I sprinkle on the surface of the combs some larvæ that have lately left the egg. The tiny, snow-white grubs make for the nearest cells, go down into them, come out again and hunt about elsewhere. The Wasps peaceably let them go their way, as heedless of the little white invaders as of the big red ones. Sometimes, when it enters an occupied cell, the little creature is seized by the owner, the Wasp-grub, which nabs it and turns it over and over in its mandibles. Is this a defensive bite? No, the Wasp-grub has merely blundered, taking its visitor for a proffered mouthful. There is no great harm done. Thanks to its litheness, the little grub escapes intact from the grip and continues its investigations. It might occur to us to attribute this tolerance to some lack of penetration in the Wasps’ vision. What follows will undeceive us: I place separately, in empty cells, a larva of the Scalary Saperda and a Volucella-larva, both of them white and selected so as not to fill the cell entirely. Their presence is revealed only by the paleness of the hind-part, which serves as a plug to the opening. A superficial examination would leave the nature of the recluse undecided. The Wasps make no mistake: they extirpate the Saperda-grub, kill it, throw it into the rubbish-pit; they leave the Volucella-grub in peace. The two strangers are quite well recognized in the secrecy of the cells: one is the intruder that must be turned out; the other is the regular visitor that must be respected. Sight helps, for things take place in the daylight, under my cage; but the Wasps have other means of information in the dimness of the burrow. When I produce darkness by covering the apparatus with a screen, the murder of the trespassers is accomplished just the same. For so say the police-regulations of the Wasps’-nest: any stranger discovered must be slain and thrown on the midden.