PARASITES
In August or September, let us go into some gorge with bare and
sun-scorched sides. When we find a slope well-baked by the summer heat,
a quiet corner with the temperature of an oven, we shall call a halt;
there is a fine harvest to be gathered here. This tropical land is the
native soil of a host of Wasps and Bees, some of them busily piling
the household provisions in underground warehouses—here a stack of
Weevils, Locusts or Spiders, there a whole assortment of Flies, Bees,
or Caterpillars,—while others are storing up honey in wallets or clay
pots, cottony bags or urns made with pieces of leaves.
With the Bees and Wasps who go quietly about their business, mingle
others whom we call parasites, prowlers hurrying from one home to the
next, lying in wait at the doors, watching for a chance to settle their
family at the expense of others.
It is something like the struggle that goes on in our world. No sooner
has a worker by means of hard labor gotten together a fortune for his
children than those who have not worked come hurrying up to fight
for its possession. To one who saves there are sometimes five, six
or more bent upon his ruin; and often it ends not merely in robbery
but in black murder! The worker’s family, the object of so much care,
for whom that home was built and those provisions stored, is devoured
by the intruders. Grubs or insect-babies are shut up in cells closed
on every side, protected by silken coverings, in order that they may
sleep quietly while the changes needed to make them into full-grown
insects take place. In vain are all these precautions taken. An enemy
will succeed in getting into the impregnable fortress. Each foe has
his special tactics to accomplish this—tactics contrived with the most
surprising skill. See, some strange insect inserts her egg by means
of a probe beside the torpid grub, the rightful owner; or else a tiny
worm, an atom, comes creeping and crawling, slips in and reaches the
sleeper, who will never wake again, because the ferocious visitor will
eat him up. The interloper makes the victim’s cell and cocoon his own
cell and cocoon; and next year, instead of the mistress of the house,
there will come from below ground the bandit who stole the dwelling and
ate the occupant.
[Illustration]
Look at this one, striped black, white, and red, with the figure of
a clumsy, hairy Ant. She explores the slope on foot, looks at every
nook and corner, sounds the soil with her antennæ. She is a kind of
Wasp without wings, named Mutilla, the terrible enemy of the other
Wasp-grubs sleeping in their cradles. Though the female Mutilla has no
wings, she carries a sharp dagger, or sting. If you saw her, you might
think she was a sort of sturdy Ant, gayer in dress than other Ants. If
you watched her for some time, you would see her, after trotting about
for a bit, stop somewhere and begin to scratch and dig, finally laying
bare a burrow underground, of which there was no trace outside; but she
can see what we cannot. She goes into the burrow, stays there for a
while, and at last reappears to replace the rubbish and close the door
as it was at the start. The abominable deed is done: the Mutilla’s egg
has been laid in another’s cocoon, beside the slumbering grub or larva
on which it will feed.
Here are other insects, all aglitter with gleams of gold, emerald,
blue, and purple. They are the humming-birds of the insect-world, and
are called the Golden Wasps. You would never think of them as thieves
or murderers; but they, too, feed on the children of other Wasps. One
of them, half emerald and half pale-pink, boldly enters the burrow
of a Fly-hunting Wasp at the very moment when the mother is at home,
bringing a fresh piece of game to her babies, whom she feeds from
day to day. The elegant criminal, the Golden Wasp who does not know
how to dig, takes this moment when the door is open to enter. If the
mother were away, the house would be shut up, and the Golden Wasp, that
sneak-thief in royal robes, could not get in. She enters, therefore,
dwarf as she is, the house of the giantess whose ruin she is planning;
she makes her way right to the back, never bothering about the Wasp,
with her sting and her powerful jaws. The Wasp-mother either does not
know the danger or is paralyzed with terror. She lets the strange Wasp
have her way.
Next year, if we open the cells of the poor Fly-hunting Wasp, we shall
find some which contain a russet-silk cocoon, the shape of a thimble,
with its opening closed with a flat lid. In this silky covering, which
is protected by the hard outer shell, is a grub of the Golden Wasp.
As for the grub of the Fly-hunter, that grub which wove the silk and
encrusted the outer casing with sand, it has disappeared entirely, all
but a few tattered shreds of skin. Disappeared how? The Golden Wasp’s
grub has eaten it.
[Illustration]
One of these splendid-appearing, criminal Golden Wasps is dressed in
lapis-lazuli on the front part of the body and in bronze and gold
on the abdomen, with a scarf of blue at the end. When one of the
Mason-wasps has built on the rock her heap of dome-shaped cells, with
a covering of little pebbles set in the plaster, when the grubs have
eaten up their store of Caterpillars and hung their rooms with silk,
we see the Golden Wasp settle on the outside of the nest. Probably
some tiny crack, some defect in the cement, allows her to insert her
probe and lay her egg. At any rate, about the end of the following May,
the Mason-wasp’s chamber holds a cocoon which again is shaped like a
thimble. From this cocoon comes a Golden Wasp. There is nothing left of
the Mason-wasp’s grub; the Golden Wasp has gorged herself upon it.
Flies, as we have seen, often act the part of robbers. They are not the
least to be dreaded, though they are weak, sometimes so feeble that
one cannot take them in his fingers without crushing them. One species
called Bombylii are clad in velvet so delicate that the least touch
rubs it off. They are fluffs of down almost as frail as a snowflake,
but they can fly with wonderful quickness. See this one, hovering
motionless two feet above the ground. Her wings vibrate so rapidly one
cannot see the motion at all, and they seem to be in repose. The insect
looks as though it were hung at one point in space by some invisible
thread. You make a movement, and your Fly has disappeared. You look
about for her. There is nothing here, nothing there. Then where is she?
Close by you. She is back where she started, before you could see where
she went to. What is she doing, there in the air? She is up to some
mischief; she is watching for a chance to leave her egg where it will
feed on some other insect’s provisions. I do not know yet what sort of
insect she preys upon, nor what she wishes for her children, whether
honey, game, or the grubs themselves.
I know more about the actions of certain tiny, pale-gray Flies, called
Tachinæ, who, cowering on the sand in the sun, near a burrow, patiently
wait for the hour at which to strike the fell blow. When the different
Wasps return from hunting, one kind with her Gad-fly, another with a
Bee, another with a Beetle, another with a Locust, at once the Gray
Flies are there, coming and going, turning and twisting with the Wasp,
always behind her and never losing her. At the moment when the Wasp
huntress goes indoors, with her captured game between her legs, they
fling themselves on her prey, which is on the point of disappearing
underground, and quickly lay their eggs upon it. The thing is done in
the twinkling of an eye; before the Wasp has crossed the threshold
of her home, the food for her babies holds the germs of a new set of
guests, who will feed on it and starve the children of the house to
death.
Perhaps, after all, we should not blame too much these insects which
feed on others, or on the food of others. An idle human being who
feeds at other people’s tables is contemptible; we call him a parasite
because he lives at his neighbor’s expense. The insect never does
this; that is to say, it does not live on the food of another of the
same species. You remember the Mason-bees: not one of the Bees touches
another’s honey, unless the owner is dead or has stayed away a long
time. The other Bees and Wasps behave in the same way.
What we call parasitism in insects is really a kind of hunting. The
Mutilla, for instance, is a huntress, and her prey is the grub of
another kind of Wasp, just as the game of this other kind of Wasp
may be a Caterpillar or a Beetle. When it comes to this, we are all
hunters, or thieves, whichever way you look at it, and Man the greatest
of all. He steals the milk from the Calf, he steals the honey from the
children of the Bee, just as the Gray Fly takes the food of the Wasps’
babies. She does it to feed her children; and Man helps himself to
everything he can find to feed his.
[Illustration]