THE LEAF-CUTTING BEE
If you know how to use your eyes in your garden you may observe, some
day or other, a number of curious holes in the leaves of the lilac-
and rose-trees, some of them round, some of them oval, as if idle but
skillful hands had been at work with the pinking-iron. In some places
there is scarcely anything but the veins of the leaves left. The author
of the mischief is a gray-clad Bee. For scissors, she has her jaws; for
compasses, she has her eye and the pivot of her body. The pieces cut
out are made into thimble-shaped bags, meant to contain the honey and
the egg: the larger, oval pieces make the floor and sides; the smaller,
round pieces are kept for the lid. The Leaf-cutter’s nest consists of a
row of a dozen, more or less, of these thimbles, placed one on top of
the other.
One species of the Leaf-cutting Bee whom we will notice is called the
White-girdled Leaf-cutter. She usually takes for her dwelling the
tunnel of some Earthworm opening off a claybank. The tunnel is too
deep for her purpose. At the bottom of it the climate is too damp, and
besides, when the Bee-grub is hatched, it would be dangerous for it to
have to climb so far through all sorts of rubbish to reach the surface.
The Leaf-cutter, therefore, uses only the front part of the Worm’s
gallery, seven or eight inches at the most. What is to be done with the
rest of the tunnel? It would never do to leave it open, because some
underground burglar, a worm or other insect, might come that way and
attack the cells at the rear.
The little Bee foresees this danger. She sets to work to block the
passage with a strong barricade of fragments of leaves, some dozens of
pieces rolled into screws and fitting into each other. You can see that
the insect has cut out these pieces carelessly and hurriedly, and on a
different pattern from that of the pieces which are to make the nest.
Next after the barricade of leaves comes the row of cells, usually
about five or six in number. These are made of round and oval pieces,
as we have seen; oval for the sides, round for the lid. There are two
sizes of ovals, the larger ones for the outside and bottom of the bag;
the smaller ones for the inside, to make the walls thicker and fill up
the gaps.
The Leaf-cutter therefore is able to use her scissors according to the
task before her; she makes large or small pieces as they are needed.
She is especially careful about the bottom of the bag. As the natural
curve of the larger pieces is not enough to make a cup without cracks
in it, the Bee improves the work with two or three small ovals applied
to the holes.
The cover of the pot consists solely of round pieces, and these are cut
so exactly by the careful Bee that the edges of the cover rest upon
the brim of the honey-bag. No one could do better with the help of
compasses.
[Illustration]
When the row of cells is finished, the entrance to the gallery must be
blocked up with a safety stopper. The Bee then returns to the free and
easy use of her scissor-jaws which we noticed at the beginning when she
was fencing off the back part of the Earthworm’s too-deep burrow; she
cuts out of the foliage irregular pieces of different shapes and sizes;
and with all these pieces, very few of which fit at all closely the
opening to be blocked, she succeeds in making a door which cannot be
forced open, thanks to the huge number of layers.
Let us leave the Leaf-cutter to finish laying her eggs, and consider
for a moment her skill as a cutter. What model does she use, when
cutting her neat ovals out of the delicate Robinia-leaves, which she
uses for her cells? What pattern that she carries in her mind guides
her scissors? What system of measurement tells her the correct size?
One would like to picture the insect as a living pair of compasses,
able to trace curves by swaying her body, even as our arm traces a
circle by swinging from the shoulder. This explanation might do if she
made only one size of oval; but she makes two, large and small. A pair
of compasses which changes its radius of its own accord and alters
the curve according to the plan before it appears to me an instrument
somewhat difficult to believe in. Besides, the Bee cuts out round
pieces also. These rounds, for the most part, fit the mouth of her
jar almost exactly. When the cell is finished, the Bee flies hundreds
of yards away to make the lid. She arrives at the leaf from which the
round pieces are to be cut. What picture, what recollection has she of
the pot to be covered? Why, none at all; she has never seen it; she
does her work underground, in utter darkness! At the utmost, she can
only remember how it felt.
And yet the circular piece to be cut out must be of a certain size: if
it were too large, it would not go in; if too small, it would close
badly, it would slip down on the honey and suffocate the egg. The Bee
does not hesitate a moment. She cuts out her circle as quickly as she
would cut out any shapeless piece; and that circle, without further
measurement, is of the right size to fit the pot. Who can explain this
geometry?
One winter evening, as we were sitting round the fire, whose cheerful
blaze unloosed our tongues, I put the problem of the Leaf-cutter to my
family:
“Among your kitchen utensils,” I said, “you have a pot in daily use;
but it has lost its lid, which was knocked over and broken by the cat
playing on the shelves. To-morrow is market-day and one of you will
be going to Orange to buy the week’s provisions. Would she undertake,
without a measure of any kind, with the sole aid of memory, which we
would allow her to refresh by a careful examination of the object
before starting, to bring back exactly what the pot wants, a lid
neither too large nor too small, in short, the same size as the top?”
It was admitted with one accord that nobody would accept such a
commission without taking a measure with her, or at least a bit of
string giving the width. Our memory for sizes is not accurate enough.
She would come back from the town with something that “might do”; and
it would be the merest chance if this turned out to be the right size.
[Illustration: “What pattern that she carries in her mind guides her
scissors?”]
Well, the Leaf-cutting Bee is even less well off than ourselves. She
has no mental picture of her pot, because she has never seen it; she is
not able to pick and choose in the crockery dealer’s heap, which acts
as something of a guide to our memory by comparison; she must, without
hesitation, far away from her home, cut out a disk that fits the top
of her jar. What is impossible to us is child’s play to her. Where we
could not do without a measure of some kind, a bit of string, a pattern
or a scrap of paper with figures upon it, the little Bee needs nothing
at all. In housekeeping matters she is cleverer than we are.
The insect excels us in practical geometry. I look upon the
Leaf-cutter’s pot and lid as an addition to the many other marvels of
instinct that cannot be explained by mechanics; I submit it to the
consideration of science; and I pass on.
[Illustration]