These returns are enough to show us that the grubs of the Volucella 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.
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. [[302]]
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 [[303]]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.
It remains in this posture for some time, occupied with its work at the bottom of the cell. Meanwhile, the Wasps present remain [[304]]impassive, do not interfere, a clear proof that the grub visited is in no peril. The stranger, in fact, retires with a soft, gliding motion. The chubby babe, a sort of india-rubber bag, resumes its original volume without having suffered any harm, as its appetite soon shows. A nurse offers it a mouthful, which it accepts with every sign of unimpaired vigour. As for the Volucella-grub, it licks its lips for a few moments after its own fashion, pushing its two fangs in and out; then, without further loss of time, it goes and repeats its probing elsewhere.
What it wants down there, at the bottom of the cells, behind the grubs, cannot be decided by direct observation; it must be guessed at. Since the visited larva remains intact, it is not prey that the Volucella’s grub is after. Besides, if murder formed part of its plans, why dive to the bottom of the cell, instead of attacking the defenceless recluse straightway? It would be much easier to suck the patient’s juices through the actual orifice of the cell. Instead of that, we see a dip, always a dip and never any other tactics.
Then what is there behind the Wasp-grub? Let us try to word all this as decently as we can. In spite of its exceeding cleanliness, [[305]]the grub is not exempt from the physiological ills inseparable from the work of the stomach. Like all that eats, it has intestinal waste matter in regard to which its confinement compels it to behave with extreme discretion. Like so many other close-cabined larvæ of Wasps and Bees, it waits until the moment of the transformation to rid itself of its digestive refuse. Then, once and for all, it casts out the unclean accumulation whereof the pupa, that delicate, reborn organism, must not retain the least trace. This is found later, in any empty cell, in the form of a dark-purple plug. But, without waiting for this final purge, this lump, there are, from time to time, slight excretions of fluid, clear as water. We have only to keep a Wasp-grub in a little glass tube to recognize these occasional discharges. Well, I see nothing else to explain the action of the Volucella’s grubs when they dip into the cells without wounding the larvæ. They are looking for this liquid, they provoke its emission. It represents to them a dainty which they enjoy over and above the more substantial fare provided by the corpses.