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, fling it on
the dust heap; 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 glass; 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.

To thwart this vigilance, the real enemies need to be masters of the art
of deceptive immobility and cunning disguise. But there is no deception
about the Volucella grub. It comes and goes, openly, wheresoever it
will; it looks round amongst the wasps for cells to suit it. What has it
to make itself thus respected? Strength? Certainly not. It is a harmless
creature, which the wasp could rip open with a blow of her shears, while
a touch of the sting would mean lightning death. It is a familiar guest,
to whom no denizen of a wasps' nest bears any ill will. Why? Because
it renders good service: so far from working mischief, it does the
scavenging for its hosts. Were it an enemy or merely an intruder, it
would be exterminated; as a deserving assistant, it is respected.

Then what need is there for the Volucella to disguise herself as a
wasp? Any fly, whether clad in drab or motley, is admitted to the burrow
directly she makes herself useful to the community. The mimicry of the
bumblebee fly, which was said to be one of the most conclusive cases,
is, after all, a mere childish notion. Patient observation, continually
face to face with facts, will have none of it and leaves it to the
armchair naturalists, who are too prone to look at the animal world
through the illusive mists of theory.

CHAPTER XII. MATHEMATICAL MEMORIES: NEWTON'S BINOMIAL THEOREM

The spider's web is a glorious mathematical problem. I should enjoy
working it out in all its details, were I not afraid of wearying the
reader's attention. Perhaps I have even gone too far in the little that
I have said, in wh