If identity of shape and costume do not save the Polistes, how will the Volucella fare, with her clumsy imitation? The Wasp’s eye, which is able to discern the dissimilar in the like, will refuse to be caught. The moment she is recognized, the stranger is killed on the spot. As to that there is not the shadow of a doubt. In the absence of Volucellæ at the moment of experimenting, I employ another Fly, Milesia fulminans, who, thanks to her slim figure and her handsome yellow bands, presents a much more striking likeness to the Wasp than does the fat V. zonaria. Despite this resemblance, if she rashly venture on the combs, she is stabbed and slain. Her yellow sashes, her slender abdomen deceive nobody. The stranger is recognized behind the features of a double. My experiments under wire-gauze, which vary according to the captures which I happen to make, all lead me to this conclusion: so long as there is mere propinquity, even around the honey, the other prisoners are tolerated fairly well; but, if they touch the cells, they are assaulted and often killed, without distinction of shape or costume. The grubs’ dormitory is the sanctum sanctorum which no outsider must enter under pain of death. With these caged captives I experiment by daylight, whereas the free Wasps work in the absolute darkness of their crypt. Where light is absent, colour goes for nothing. Once, therefore, that she has entered the cavern, the Volucella derives no benefit from her yellow bands, which are supposed to be her safeguard. Whether garbed as she is or otherwise, it is easy for her to effect her purpose in the dark, on condition that she avoid the tumultuous interior of the Wasps’-nest. So long as she has the prudence not to hustle the passers-by, she can dab her eggs, without danger, on the paper wall. No one will know of her presence. The dangerous thing is to cross the threshold of the burrow in broad daylight, before the eyes of those who go in and out. At that moment alone, protective mimicry would be convenient. Now does the entrance of the Volucella into the presence of a few Wasps entail such very great risks? The Wasps’-nest in my enclosure, the one which was afterwards to perish under a bell-glass in the sun, gave me the opportunity for prolonged observations, but without any result upon the subject of my immediate concern. The Volucella did not appear. The period for her visits had doubtless passed; for I found plenty of her grubs when the nest was dug up.