Experiment confirms what we have learnt in the actual nests. Over and over again, I bring Wasp-grubs and Volucella-grubs together in small test-tubes, which are easy to observe. The first are well and strong; I have just taken them from their cells. The others are in various stages, from that of the snow-flake born the same day to that of the sturdy Porcupine. There is nothing tragic about the encounter. The Flies’ grubs roam about the test-tube without touching the live tit-bit. The most that they do is to put their mouths for a moment to the morsel; then they take it away again, not caring for the dish. They want something different: a wounded, a dying creature; a corpse dissolving into sanies. Indeed, if I prick the Wasp-grub with a needle, the scornful ones immediately come and sup at the bleeding wound. If I give them a dead larva, brown with putrefaction, the grubs rip it open and feast on its humours. Better still: I can feed them quite satisfactorily with Wasps that have turned putrid under their horny rings; I see them greedily suck the juices of decomposing Cetonia-larvæ; I can keep them thriving with chopped-up butcher’s meat, which they know how to liquefy by the method of the common maggot. And these unprejudiced ones, who accept whatever comes their way, provided that it be dead, refuse it when it is alive. Like the true Flies and frank body-snatchers that they are, they wait, before touching a morsel, for death to do its work. Inside the Wasps’-nest, robust larvæ are the rule and weaklings the rare exception, because of the assiduous supervision which eliminates anything that is like to die. Here, nevertheless, Volucella-grubs are found, on the combs, among the busy Wasps. They are not, it is true, so numerous as in the charnel-house below, but still they are pretty frequent. Now what do they do in this abode where there are no corpses? Do they attack the healthy? Their continual visits from cell to cell would at first make one think so; but we shall soon be undeceived if we observe their movements closely; and this is possible with my caged colonies. I see them fussily crawling on the surface of the combs, swaying their necks from side to side and taking stock of the cells. This one does not suit, nor that one either; the bristly creature passes on, still in quest of something, thrusting its pointed fore-part now here, now there. This time, the cell appears to fulfil the requisite conditions. A larva, glowing with health, opens wide its mouth, believing its nurse to be approaching. It fills the hexagonal chamber with its bulging sides. The gluttonous visitor bends and slips its slender fore-part, a blade of exquisite suppleness, between the wall and the inhabitant, whose slack rotundity yields to the pressure of this animated wedge. It plunges into the cell, leaving no part of itself outside but its wide hind-quarters, with the red dots of the two breathing-tubes.