Seeing that identity of shape and costume does 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 bumblebee flies 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 Volucella 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 glass, which varied according to the captures which I happened to make, all lead me to this conclusion: as long as there is more propinquity, even around the honey, the other occupants 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 underground retreat. Where light is absent, color goes for nothing. Once, therefore, that she has entered the cavern, the bumblebee fly 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 avoids 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 in the sun under a bell glass, gave me the opportunity for prolonged observations, but without any result upon the subject of my immediate concern. The bumblebee fly did not appear. The period for her visits had doubtless passed; for I found plenty of her grubs when the nest was dug up.
Other flies rewarded me for my assiduity. I saw some—at a respectful distance, I need hardly say—entering the burrow. They were insignificant in size and of a dark gray color, not unlike that of the housefly. They had not a patch of yellow about them and certainly had no claim to protective mimicry. Nevertheless, they went in and out as they pleased, calmly, as though they were at home. As long as there was not too great a number at the door, the wasps left them alone. When there was anything of a crowd, the gray visitors waited near the threshold for a less busy moment. No harm came to them.
Inside the establishment, the same peaceful relations prevail. In this respect I have the evidence of my excavations. In the underground charnel house, so rich in Fly grubs, I find no corpses of adult flies. If the strangers had been slaughtered in passing through the entrance hall, or lower down, they would fall to the bottom of the burrow anyhow, with the other rubbish. Now in this charnel house, as I said, there are never any dead bumblebee flies, never a fly of any sort. The incomers are respected. Having done their business, they go out unscathed.
This tolerance on the part of the wasps is surprising. And a suspicion comes to one's mind: can it be that the Volucella and the rest are not what the accepted theories of natural history call them, namely, enemies, grub killers sacking the wasps' nest? We will look into this by examining them when they are hatched. Nothing is easier, in September and October, than to collect the Volucella's eggs in such numbers as we please. They abound on the outer surface of the wasps' nest. Moreover, as with the larvae of the wasp, it is some time before they are suffocated by the petroleum fumes; and so most of them are sure to hatch. I take my scissors, cut the most densely populated bits from the paper wall of the nest and fill a jar with them. This is the warehouse from which I shall daily, for the best part of the next two months, draw my supply of nascent grubs.
The Volucella's egg remains where it is, with its white color always strongly marked against the brown of the background. The shell wrinkles and collapses; and the fore end tears open. From it there issues a pretty little white grub, thin in front, swelling slightly in the rear and bristling all over with fleshy protuberances. The creature's papillae are set on its sides like the teeth of a comb; at the rear, they lengthen and spread into a fan; on the back, they are shorter and arranged in four longitudinal rows. The last section but one carries two short, bright red breathing tubes, standing aslant and joined to each other. The fore part, near the pointed mouth, is of a darker, brownish color. This is the biting and motor apparatus, seen through the skin and consisting of two fangs. Taken all round, the grub is a pretty little thing, with its bristling whiteness, which gives it the appearance of a tiny snowflake. But this elegance does not last long: grown big and strong, the bumblebee fly's grub becomes soiled with sanies, turns a russety brown and crawls about in the guise of a hulking porcupine.
What becomes of it when it leaves the egg? This my warehousing jar tells me, partly. Unable to keep its balance on sloping surfaces, it drops to the bottom of the receptacle, where I find it, daily, as hatched, wandering restlessly. Things must happen likewise at the wasps'. Incapable of standing on the slant of the paper wall, the newborn grubs slide to the bottom of the underground cavity, which contains, especially at the end of the summer, a heaped up provender of deceased wasps and dead larvae removed from the cells and flung outside the house, all nice and gamy, as proper maggot's food should be. The Volucella's offspring, themselves maggots, notwithstanding their snowy apparel, find in this charnel house victuals to their liking, incessantly renewed. Their fall from the high walls might well be not accidental, but rather a means of reaching, quickly and without searching, the good things down at the bottom of the cavern. Perhaps, also, some of the white grubs, thanks to the holes that make the wrapper resemble a spongy cover, manage to slip inside the Wasps' nest. Still, most of the Volucella's grubs, at whatever stage of their development, are in the basement of the burrow, among the carrion remains. The others, those settled in the wasps' home itself, are comparatively few.
These returns are enough to show us that the grubs of the bumblebee fly do not deserve the bad reputation that has been given them. Satisfied with the spoils of the dead, they do not touch the living; they do not ravage the wasps' nest: they disinfect it.