THE FLY-HUNTING WASP
You have read about the Wasps who store up paralyzed Caterpillars and
Crickets for their babies’ food, then close up the cells and fly away;
now you shall hear about a Wasp who feeds her children with fresh food
from day to day. This is the Bembex, or the Fly-hunting Wasp, as I
shall call her.
This Wasp digs her burrows in very soft, light sand, under a blazing
sun and a blue sky. I go out and watch her sometimes on an unshaded
plain where it is so hot that the only way to avoid sunstroke is to
lie down at full length behind some sandy knoll, put one’s head down a
rabbit-burrow, or provide one’s self with a large umbrella. The latter
is what I did. If the reader will sit with me under the umbrella at the
end of July, he will see the following sight.
A Fly-hunting Wasp arrives suddenly and alights, without any
hesitation, at a spot which to my eyes looks exactly like the rest of
the sandy surface. With her front feet, which are armed with rows of
stiff hairs and remind one at the same time of a broom, a brush, and a
rake, she works at clearing her underground dwelling. The insect stands
on her four hind-legs, while the front ones first scratch and then
sweep the shifting sand. She shoots the sand backwards so fast that it
gushes in a curve like a stream of water, falling to the ground seven
or eight inches away. This spray of dust is kept up evenly for five or
ten minutes at a time by the swift, graceful Wasp.
Mingled with this dust are tiny bits of wood, decayed leaf stalks,
particles of grit and other rubbish. The Wasp picks them up in her
mouth and carries them away. This is really the purpose of her digging.
She is sifting out the sand at the entrance to her home, which is all
ready underground, having been dug some time before. The Wasp wishes
to make the sand at the entrance to her burrow fine, light, and free
from any obstacle, so that when she alights suddenly with a Fly for her
children, she can dig an entrance to her home quickly. She does this
work in her spare time, when her larva has enough food to last it for
a while, so that she does not need to go hunting. She seems happy as
she works so fast and eagerly, and who knows that she is not expressing
in this way her mother’s satisfaction in watching over the roof of her
house where her baby lives?
If we should take a knife and dig down into the sand where the
Wasp-mother is scratching, we should find, first, an entrance corridor,
as wide as one’s finger, and perhaps eight to twelve inches long, and
then a room, hollowed out down below where the sand is damper and
firmer. It is large enough to contain two or three walnuts; but all it
does hold at present is a Fly, a golden-green Greenbottle, with a tiny
white egg laid on the side. This is the Wasp’s egg. It will hatch out
in about twenty-four hours, into a little worm, which will feed on the
dead Fly. For the Fly is dead, and not paralyzed, as the food of other
Wasp-babies often is.
[Illustration]
At the end of two or three days the Wasp-grub will have eaten up the
little Fly. Meanwhile the mother Wasp remains in the neighborhood and
you see her sometimes feeding herself by sipping the honey of the field
flowers, sometimes settling happily on the burning sand, no doubt
watching the outside of the house. Every now and then she sifts the
sand at the entrance; then she flies away for a while. But, however
long she may stay away, she never forgets the young larva who has food
enough to last only a short time; her mother’s instinct tells her the
hour when the grub has finished its food and wants more. She therefore
returns to the nest, which, you must remember, does not show in the
least from the surface of the ground, as the shifting sand has filled
in the entrance; she knows, however, exactly where to look for it; she
goes down into the earth, this time carrying a larger piece of game.
After leaving this in the underground room she again leaves the house
and waits outside until the time comes to serve a third course. This
is not long, for the little worm is getting a larger appetite all the
time. Again the mother appears with another Fly.
For nearly two weeks, while the larva is growing up, the meals thus
follow in succession, one by one, as needed, and coming closer together
as the infant grows larger. Towards the end of the two weeks, the
mother is kept as busy as she can be satisfying her hungry child, now a
large, fat grub. You see her at every moment coming back with a fresh
capture, at every moment setting out again upon the chase. She does not
cease her efforts until the grub is stuffed full and refuses its food.
I have counted and found that sometimes the grub will eat as many as
eighty-two Flies.
I have wondered sometimes why this Wasp does not lay up a store of
food, as the other Wasps do, close the door of her burrow and fly
away, instead of waiting about, as she does so patiently. I realize
that she does not do so because her Flies would not keep; they would
spoil and be unfit for eating. But why does she kill the Fly instead of
paralyzing it? Possibly because the Fly would not make a satisfactory
preserved food; it is so slight and frail, it would shrivel up and
there would be nothing of it; it must be eaten fresh to be worth
anything. Another reason almost certainly is that the Fly has to be
captured very quickly, on the wing. There is not time for the Wasp to
aim her sting, as the Wasps do who are killing clumsy Worms or fat
Crickets on the ground. She must attack with claws, mouth or sting
wherever she can, and this method of attack kills at once.
It is not easy to surprise a Wasp hunting, as she flies far away from
where her burrow lies; but one day I had a quite unexpected experience
as I was sitting in the hot sun under my umbrella. I was not the only
one to enjoy the shade of the umbrella. Gad-flies of various kinds
would take refuge under the silken dome and sit peacefully on every
part of the tightly stretched cover. To while away the hours when I
had nothing to do, it amused me to watch their great gold eyes, which
shone like carbuncles under my umbrella; I loved to follow their solemn
progress when some part of the ceiling became too hot and obliged them
to move a little way on.
One day, bang! The tight cover resounded like the skin of a drum.
Perhaps an oak had dropped an acorn on the umbrella. Presently, one
after the other, bang, bang, bang! Can some practical joker be flinging
acorns or little pebbles at my umbrella? I leave my tent and look
around: nothing! I hear the same sharp sounds again. I look up at the
ceiling and the mystery is explained. The Fly-hunting Wasps of the
neighborhood, who all eat Gad-flies, had discovered the rich game that
was keeping me company and were impudently coming into my shelter to
seize the Flies on the ceiling. Things were going to perfection: I had
only to sit still and look.
Every moment a Wasp would enter, swift as lightning, and dart up to
the silken ceiling, which resounded with a sharp thud. Some rumpus was
going on aloft, where so lively was the fray that one could not tell
which was attacker, which attacked. The struggle did not last long: the
Wasp would soon retire with a victim between her legs. The dull herd of
Gad-flies would not leave the dangerous shelter. It was so hot outside!
Why get excited?
[Illustration: “One day, bang!”]
Let us watch the Wasp as she returns to the burrow with her capture
held under her body between her legs. As she draws near her home, she
makes a shrill humming, which has something plaintive about it and
which lasts until the insect sets foot to earth. The Wasp hovers above
the sand and then dips down, very slowly and cautiously, all the time
humming. If her keen eyes see anything unusual, she slows up in her
descent, hovers for a second or two, goes up again, comes down again
and flies away, swift as an arrow. We shall see in a few moments what
it is that makes her hesitate. Soon she is back again, looks at things
once more from a height, then comes down slowly and alights at a spot
which looks exactly like the rest of the sandy surface.
I think she has landed more or less on chance, and will now look about
for the entrance to her home. But no; she is exactly over her burrow.
Without once letting go her prey, she scratches a little in front of
her, gives a push with her head, and at once enters, carrying the Fly.
The sand falls in, the door closes, and the Wasp is at home. It makes
no difference that I have seen this Wasp return to her nest hundreds of
times; I am always astonished to behold the keen-sighted insect find
without hesitation a door which does not show at all.
The Wasp does not always hesitate in the air before alighting at
her house, and when she does, it is because she sees her nest is
threatened by a very grave danger. Her plaintive hum shows anxiety; she
never gives it when there is no peril. But who is the enemy? It is a
miserable little Fly, feeble and harmless in appearance, whom we have
mentioned in another chapter. The Wasp, the scourge of the Fly-tribe,
the fierce slayer of large Gad-flies, does not enter her home because
she sees herself watched by another Fly, a tiny dwarf, who would make
scarcely a mouthful for her larvæ.
I feel just as I should if I saw my Cat fleeing in terror from a Mouse.
Why does the Wasp not pounce upon the little wretch of a Fly and get
rid of her? I do not know. It must be because this wretched little Fly
has her tiny part to play in the universe, as well as the Wasp. These
things are ordered somehow, in a way we do not understand.
[Illustration]
As I shall mention elsewhere, this is the Fly that lays her eggs on
the game the Wasp puts in the nest for her own baby; and the Fly’s
offspring eat the food of the Wasp-grub, and sometimes eat the grub
itself, if provisions are scarce. The way the Fly manages her business
is interesting. She never enters the Wasp’s burrow, but she waits with
the greatest patience for the moment when the Wasp dives into her home,
with her game clasped between her legs. Just as she has half her body
well within the entrance and is about to disappear underground, the
Fly dashes up and settles on the piece of game that projects a little
way beyond the hinder end of the Wasp; and while the latter is delayed
by the difficulty of entering, the former, with wonderful swiftness,
lays an egg on the prey, or even two or three in quick succession. The
hesitation of the Wasp, hampered by her load, lasts but the twinkling
of an eye. No matter: the Gnat has accomplished what she wished to,
and now she goes and squats in the sun, close to the burrow, and plans
fresh deeds of darkness.
A number of these Flies, usually three or four, are apt to station
themselves on the sand at one time near a burrow, of which they well
know the entrance, carefully hidden though it be. Their dull-brown
color, their great blood-red eyes, their astonishing patience, have
often reminded me of a picture of brigands, clad in dark clothes,
with red handkerchiefs around their heads, waiting in ambush for an
opportunity to hold up some travelers.
[Illustration]
It is when the poor Wasp sees these brigands that she hesitates. At
last she comes nearer, however. The Midges then take flight and follow
behind the Wasp. If she turns, they turn also, so as to keep exactly
behind her; if she advances, they advance; if she retreats, they
retreat. She cannot keep them off. At last she grows weary and alights;
they also alight, still behind her. The Wasp darts off again, with an
indignant whimpering; the Midges dart after her. The Wasp tries one
more way to get rid of them. She flies far away at full speed, hoping
that they will follow and lose their way. But they know too much for
that. They settle down on the sand again near the burrow and wait for
her to come back. Come she does; the pursuit begins all over again; the
mother’s patience is worn out, and at last they have a chance to lay
their eggs as she goes into the burrow.
We shall end our chapter with the story of the Wasp-grub to whom no
accidents happen, into whose burrow no nasty Fly-eggs enter. For two
weeks it eats and grows; then it begins to weave its cocoon. It has
not very much silk in its body to use for this, so it uses grains of
sand to strengthen it. First it pushes away the remains of its food and
forces them into a corner of the cell. Then, having swept its floor, it
fixes to the different walls of its room threads of a beautiful white
silk, forming a web which makes a kind of scaffold for the next work.
It then weaves a hammock of silk in the center of the threads. This
hammock is like a sack open wide at one end and closed at the other in
a point. The grub, leaning half out of its hammock, picks up the sand
almost grain by grain with its mouth. If any grain found is too large,
it is thrown away. When the sand is sorted in this way, the grub brings
some into the hammock in its mouth, and begins to spread it in an even
layer on the lower side of the hammock-sack; it adds grains also to the
upper side, fixing them in the silk as one would place stones in putty.
The cocoon is still open at one end. It is time to close it. The grub
weaves a cap of silk which fits the mouth of the sack exactly, and
lays grains of sand one by one upon this foundation. The cocoon is all
finished now, except that the grub gives some finishing touches to the
inside by glazing the walls with varnish to protect its delicate skin
from the rough sand. It then goes peacefully to sleep, to wait for its
transformation into a Wasp like its mother.
[Illustration]