The Story Book of Science by Jean Henri Fabre
A Numerous Family
PLANT-LOUSE, we will suppose," resumed Uncle Paul, "has
just established itself on the tender shoot of a rosebush.
It is alone, all alone. A few days after, young plant-lice
surround it: they are its sons. How many are there? Ten,
twenty, a hundred? Let us say ten. Is that enough to assure
the preservation of the species? Don't laugh at my question.
I know well that if the plant-lice were missing from the
rosebushes, the order of things would not be sensibly
"The ants would be the most to be pitied," said Emile.
"The round earth would continue to turn just the same, even
when the last plant-louse was dying on its leaf; but it is
not, in truth, an idle question to ask if ten plant-lice
suffice to preserve the race; for science has no higher
object than the quest of providential means for maintaining
everything in a just measure of prosperity.
"Well, ten plant-lice coming from one would be far too many
if we did not have to take account of destructive agencies.
One replacing one, the population remains the same; ten
replacing one, in a short time the number increases beyond
all possible limits. Think of the dervish's grain of wheat
sixty-four times, so that it becomes a bed of wheat
of a finger's depth over the whole earth. What would it be
if it had been multiplied ten times instead of doubled! In
like manner, after a few years, the descendants of a first
plant-louse, continually multiplied tenfold, would be in
straitened circumstances in this world. But there is the
great reaper, death, which puts an invincible obstacle to
overcrowding, counterbalances life in its overgrowing
fecundity, and, in partnership with it, keeps all things in
a perpetual youth. On a rosebush apparently most peaceful
there is death every minute. But the small, the humble, and
weak, are the habitual pasture, the daily bread, of the
large eaters. To how many dangers is not the plant-louse
exposed, so tiny, so weak, and without any means of defense!
No sooner does a little bird, hardly out of the shell,
discover with its piercing eyes a spot haunted by the
plant-lice, than, merely as an appetizer, it will swallow
hundreds. And if a worm, far more rapacious, a horrible worm
expressly created and put into the world to eat you alive,
joins in, ah! my poor plant-lice, may God, the good God of
little creatures, protect you; for your race is indeed in
"This devourer is of a delicate green with a white stripe on
its back. It is tapering in front, swollen at the back. When
it doubles itself up it takes the shape of a tear-drop. They
call it the ants' lion because of the ravages it makes in
the stupid herd. It establishes itself among them. With its
pointed mouth, it seizes one, the biggest, the plumpest; it
sucks it and throws away the skin, which is too hard
Its pointed head is lowered again, a second plant-louse
seized, raised from the leaf, and sucked. Then another and
another, a twentieth, a hundredth. The foolish herd, whose
ranks are thinning, do not even seem to perceive what is
going on. The trapped plant-louse kicks between the lion's
fangs; the others, as if nothing were happening, continue to
feed peacefully. It would take a good deal more than that to
spoil their appetite! They eat while they are waiting to be
eaten. The lion has had enough. He squats amidst the herd to
digest at his ease. But digestion is soon over and already
the greedy worm has its eye on those that he will soon
crunch. After two weeks of continual feasting, after having
browsed as it were on whole herds of plant-lice, the worm
turns into an elegant little dragon-fly with eyes as bright
as gold, and known as the hemerobius.
(a) larva (b) pupa (c) first joint of larva
"Is that all? Oh, no. Here is the lady-bug, the good God's
bug. It is round and red, with black spots. It is very
pleasing; it has an innocent air. Who would take it also to
be a devourer, filling its
stomach with plant-lice! Look at
it closely on the rosebush, and you will see it at its
ferocious feasting. It is very pretty and innocent-looking;
but it is a glutton, there is no denying the fact, so fond
is it of plant-lice.
"Is that all? Oh, no. Those poor plant-lice are manna, the
regular diet of all sorts of ravagers. Young birds eat them,
the hemerobius eats them, lady-birds eat them, gluttons of
all kinds eat them; and still there are always plant-lice.
Ah! that is where, in the fight between fecundity which
repairs and the rough battle of life which destroys, the
weak excel by opposing legions and legions to the chances of
annihilation. In vain the devourers come from all sides and
pounce upon their prey; the devoured survive by sacrificing
a million to preserve one. The weaker they are, the more
fruitful they are.
"The herring, cod, and sardine are given over as pasturage
for the devourers of the sea, earth and sky. When they
undertake long voyages to graze in favorable spots, their
extermination is imminent. The hungry ones of the sea
surround the school of fish; the famished ones of the sky
hover over their route; those of the earth await them on the
shore. Man hastens to lend a strong hand to the killing and
to take his share of the sea food. He equips fleets, goes to
the fish with naval armies in which all nations are
represented; he dries in the sun, salts, smokes, packs. But
there is no perceptible diminution in the supply; for him
the weak are infinite in number. One cod lays nine million
are the devourers that will see the end of such
"Nine million eggs!" exclaimed Emile. "Is that a great many?"
"Just to count them, one by one, would take nearly a year of
ten working hours each day."
"Whoever counted them had lots of patience," was Emile's
"They are not counted," replied Uncle Paul; "they are
weighed, which is quickly done; and from the weight the
number is deduced.
"Like the cod in the sea, the plant-lice are exposed on
their rosebushes and alders to numerous chances of
destruction. I have told you that they are the daily bread
of a multitude of eaters. So, to increase their legions,
they have rapid means that are not found in other insects.
Instead of laying eggs, very slow in developing, they bring
forth living plant-lice, which all, absolutely all, in two
weeks have obtained their growth and begin to produce
another generation. This is repeated all through the season,
that is to say at least half the year, so that the number of
generations succeeding one another during this period cannot
be less than a dozen. Let us say that one plant-louse
produces ten, which is certainly below the actual number.
Each of these ten plant-lice borne by the first one bears
ten more, making one hundred in all; each of these hundred
bears ten, in all one thousand; each of the thousand bears
ten, in all ten thousand; and so on, multiplying always by
ten, eleven times. Here is the same calculation as the
dervish's grain of wheat, which grew with such
rapidity when they multiplied it by two. For the family of
the plant-lice the increase is much more rapid, as the
multiplication is made by ten. It is true that the
calculation stops at the twelfth instead of going on to the
sixty-fourth. No matter, the result would stupefy you; it is
equal to a hundred thousand millions. To count a cod's eggs,
one by one, would take nearly a year; to count the
descendants of one plant-louse for six months would take ten
thousand years! Where are the devourers that would see the
end of the miserable louse? Guess how much space these
plant-lice would cover, as closely packed as they are on the
"Perhaps as large a place as our garden," suggested Claire.
"More than that; the garden is a hundred meters long and the
same in width. Well, the family of that one plant-louse
would cover a surface ten times larger; that is to say, ten
hectares. What do you say to that? Is it not necessary that
the young birds, little lady-bugs, and the dragon-fly with
the golden eyes should work hard in the extermination of the
louse, which if unhindered would in a few years overrun the
"In spite of the hungry ones which devour them, the
plant-lice seriously alarm mankind.
Winged plant-lice have
been seen flying in clouds thick enough to obscure the
daylight. Their black legions went from one canton to
another, alighted on the fruit trees, and ravaged them. Ah!
when God wishes to try us, the elements are not always
unchained. He sends against us in our pride the paltriest of
creatures. The invisible mower, the feeble plant-louse,
comes, and man is filled with fear; for the good things of
the earth are in great peril.
"Man, so powerful, can do nothing against these little
creatures, invincible in their multitude."
Uncle Paul finished the story of the ants and their cows.
Several times since, Emile, Jules, and Claire have talked of
the prodigious families of the plant-louse and the cod, but
rather lost themselves in the millions and thousand
millions. Their uncle was right: his stories interested them
much more than Mother Ambroisine's tales.