BENEFICENT PARASITES
Continuing the subject of the day before, Uncle Paul laid emphasis on
the distinction between those hymenopterous insects that sting and
those that do not.
“At the end of the abdomen,” said he, “the one class have a poisoned
weapon for self-defense, as in bees and wasps, while the other class
are furnished simply with the implement called a terebra, sometimes
concealed in a fold of the skin, sometimes standing out in full view,
and used, not for stinging, but for introducing the creature’s eggs
into such places as will provide the nourishment required by the future
brood. Insects of this latter class are called by the general name of
ichneumons. This morning Emile caught one, which I will show you.”
“I found it on a flower,” said Emile, “and I wrapped a handkerchief
round my hand for fear of being stung. The thing it has sticking out at
the end of its body looks rather dangerous.”
“The precaution was needless,” his uncle assured him. “No ichneumon,
however long its terebra may be, can sting your hand. The hymenopters
that are to be feared have their sting hidden, and they bring it out
only at the moment of attack.”
“Those three thread-like things as long as the insect’s body—what are
they for?” asked Jules.
“The two lateral ones unite and form a scabbard for holding and
protecting the middle one, the most important of the three, for it is
used to deposit the eggs at the point chosen as suitable by the
insect.”
“I’ve seen ichneumons very much like this one,” said Louis, “with the
terebra stuck right into the thick bark of a poplar. They must have
been laying their eggs in the wood under the bark.”
“No, they were doing something better than that. The larvæ of
ichneumons live in the bodies of other larvæ, which they devour little
by little without killing them until the very last. They are
carnivorous larvæ: they must have fresh meat that is renewed about as
fast as they eat it. The ichneumons Louis speaks of were engaged in
depositing their eggs in the bodies of plump worms that live in the
wood of the tree and turn into beetles.”
“But these beetle-worms,” Jules pointed out, “were not in sight. They
were under the bark and perhaps in the wood itself.”
“The ichneumon does not need to see them to know where they are.”
“Does it hear them then?”
“No more than it sees them. The worm stays quietly in its little
tunnel, being careful not to make any noise that will attract the
attention of its enemy.”
“Then the ichneumon must at least smell them.”
“That is very doubtful. A live larva has no odor. And, besides, the
most difficult part is not to find out whether or not a suitably plump
larva is there under the bark, at such and such a depth; it is also
necessary to ascertain whether another ichneumon egg has not already
been laid in the body of the coveted worm, as one larva would be
insufficient for two nurslings. An egg deposited in some fat worm is
not seen, heard, or felt; that is very evident. Nevertheless the
ichneumon never plunges its terebra through the bark into a larva
already occupied. What guides it? I do not know; nobody knows. Instinct
has ways and means unsuspected by our wisdom. An ichneumon alights on
the trunk of a tree. The perfectly healthy bark betrays to the sharpest
eyes no sign of what the hymenopter is after. Never mind; the insect
soon satisfies itself as to whether or not the place is a good one for
its purpose. It makes a careful examination, tapping with its antennæ
and keeping up a continual vibratory movement. A spot is chosen. The
insect plants itself firmly on its legs, draws up its abdomen, and,
holding its terebra in a vertical position, plunges the point of it
into an imperceptible fissure in the bark. The little auger pierces the
bark, though not without effort and occasional pauses, for there are
difficulties to be overcome; nevertheless it goes as far as its length
permits. The end is attained; the point of the instrument penetrates
the flesh of the worm hidden under the bark. The egg being deposited in
the incision thus made, the insect withdraws its blade carefully, so as
not to break it, and proceeds to lay eggs in the bodies of other
larvæ.”
“Then the great length of this blade,” Jules remarked, “though at first
it seems unhandy for the creature, is on the contrary perfectly fitted
for the work to be done. With too short a terebra the ichneumon
couldn’t reach the larvæ under the bark and even in the wood itself.”
“From the length of the terebra one can estimate at what depth, as a
rule, the eggs are laid. Ichneumons with a long terebra lay their eggs
in larvæ protected by a thick layer of bark or wood or earth or other
material; those with a short terebra seek larvæ living in the open air,
such as caterpillars for example. However, if the caterpillar is
covered with long hairs, thus keeping the ichneumon at a distance from
the skin of its victim, a long implement is still necessary to convey
the eggs into the flesh. But for smooth-skinned caterpillars without
any defense the ichneumon is furnished with a very short terebra, often
invisible when not in action, so that the end of the insect’s abdomen
must be pressed in order to make the egg-conveyer show itself, be it
lancet, saw, fine blade, or other instrument of that sort.
“The ichneumon on the hunt is one of the most curious sights
imaginable. Caterpillars are peacefully browsing on leaves. An
ichneumon comes along, flies about over them, selects the ones that
look best for its purpose, and never fails to exclude those that
already have eggs in their bodies. At the sound of their enemy’s wings
the terrified caterpillars stop eating and begin to move their heads in
a startled manner from side to side, doubtless in the hope of
frightening off the foe. But the latter pays no attention to these vain
threats; it alights on the caterpillar of its choice and, so quickly
that you hardly have time to see the operation, darts its terebra and
lays an egg in the wound.”
“And doesn’t the caterpillar make any resistance?” asked Emile.
“It performs some lively antics, but that is all. The poor creature
cannot defend itself against an enemy in the air, a winged enemy that
can always keep at a safe distance and is always ready to fly away. The
other eligible caterpillars in the group are attacked in the same
manner, one by one, until the ichneumon has laid all its eggs.”
“Each caterpillar gets only one egg?” asked Jules.
“That depends on the size of the egg-layer. If it is large it lays but
one egg in each caterpillar, so that the larvæ may each have enough
food; if small, it lays several.”
“And then what happens?” Emile inquired.
“The ichneumon gone, the pricked caterpillars soon recover their
composure and resume their eating. The prick, not being poisoned with
venom, causes very little pain; besides, it would take a good deal more
than that to affect their appetites. All goes well for a few days, as
long as the eggs are unhatched.”
“Do those eggs hatch in the caterpillar’s body?”
“Yes.”
“And as soon as they have hatched do the little larvæ set to work
devouring the caterpillar’s inside?”
“That’s the way of it.”
“What an awful stomach-ache for the poor caterpillars!”
“Nevertheless, even with such pain as may result from the gradual
consumption of their flesh, the caterpillars continue to eat as if
nothing had happened, the satisfaction of their appetite making them
forget their sufferings, so imperious is their need of food. And
furthermore, for a reason that I will explain, the parasitic worms
observe a certain caution in their ravages.
“In the body of every animal are certain organs more indispensable than
others for the maintenance of life, and if they are injured, even
though slightly, death follows. Such are the heart and brain in the
higher animals. In a caterpillar’s body there are, it is true, no heart
or brain like those of animals higher in the scale; but there are
analogous organs just as necessary to the ongoing of the vital
functions. If the ichneumon’s larvæ, in eating their victim’s flesh,
were to injure these vital organs, the caterpillar would quickly die,
and the larvæ would perish too, for they must have fresh meat, not
decayed flesh. It is a question of life or death to them whether the
grubs avoid biting in the wrong place or not. The caterpillar must live
in order that they may live; it must prolong its miserable existence
until they are ready for their metamorphosis. So the little parasites
scrupulously respect any organ indispensable to the maintenance of life
and feed on the rest; guided by instinct, they distinguish admirably
between what they may attack and what they must let alone. A time
comes, however, when because of their approaching transformation they
do not need to exercise further self-restraint, and they accordingly
devour the parts until then left intact. Thereupon the caterpillar
dies, being reduced to an empty skin which the larvæ promptly abandon
in order to spin their cocoons and turn into nymphs and finally into
ichneumons.
“Sometimes the caterpillar is spared until it has shut itself up in a
case and turned into a chrysalis, with the result that the larvæ
inhabiting the caterpillar find themselves, without any labor on their
part, provided with snug quarters for the winter. Out of every such
wormy chrysalis, consumed to the skin, there emerges in the spring, not
a butterfly, but a swarm of ichneumons.”
“Last autumn,” said Jules, “I found in the garden a large brown cocoon,
and I hoped to see a beautiful butterfly come out of it; but this
spring, to my great astonishment, out came a swarm of little flies.”
“What you took for flies was a brood of ichneumons. Yet there are
flies, real flies, that lay their eggs in the bodies of caterpillars,
just as the hymenopterous butterflies do with their terebra.”
“With their strange way of living on other creatures,” observed Louis,
“ichneumons must destroy lots of caterpillars.”
“They destroy so many that often, if you take a hundred caterpillars
haphazard from cabbages or other growing vegetables, you will find not
more than two or three that are sound and in a condition to undergo
metamorphosis.”
“Can those that have eggs in them be told from the rest?” asked Louis.
“Easily. The point pierced by the ichneumon’s terebra is surrounded by
a little black spot. When you are gathering caterpillars it is well not
to crush those that you see are pricked, nor yet those that look
diseased and have a loose skin. They are ichneumon-feeders, and their
preservation means so many more swarms of ichneumons the next summer
for the destruction of caterpillars.”