THE PEA-WEEVIL: THE EGGS
Man holds the pea in high esteem. Ever since the days of antiquity, he
has tried, by devoting greater and greater attention to its
cultivation, to make it produce larger, tenderer, and sweeter
varieties. The adaptable plant, gently entreated, has complied with his
desires and has ended by giving us what the gardener’s ambition aimed
at obtaining. How far we moderns have progressed beyond the crop of the
Varros [53] and Columellas, [54] how far, above all, beyond the
original peas, beyond the wild seeds confided to the soil by the first
man who thought of scraping the earth, maybe with a jaw-bone of the
Cave-bear, [55] whose mighty canine did duty as a ploughshare!
Where is this plant, the first source of the pea, in the world of
spontaneous vegetation? Our regions possess nothing like it. Is it to
be found elsewhere? On this point botany is silent, or replies only
with vague probabilities.
For that matter, the same ignorance prevails on the subject of most of
our edible plants. Whence comes wheat, the blessed grain that gives us
bread? No one knows. Except in the fields tilled by man, you need not
look for it in this country. You need not look for it abroad either. In
the East, where agriculture had its birth, no botanist ever came across
the sacred ear increasing of its own accord on ground not broken by the
plough.
Barley, oats and rye, the turnip and the radish, the beet, the carrot,
the pumpkin leave us in a like uncertainty: their origin is unknown, or
at most suspected behind the impenetrable mist of the ages. Nature
delivered them to us in the full vigour of things untamed, when they
were of little value as food, as she nowadays offers us the wild
blackberry and the sloe; she gave them to us in a rudimentary and
incomplete state; and it was for our husbandry and ingenuity patiently
to hoard the nutritive pulp, that earliest form of capital, with
dividends always increasing in the most excellent bank of the tiller of
the soil.
As storehouses of provisions, the cereal and the garden vegetable are,
for the most part, the work of man. The founders of the species, a poor
resource in their original condition, we borrowed as we found them from
nature’s green treasury; the improved race, rich in nourishing matter,
is the result of our art.
But, if wheat, peas and the rest are indispensable to us, our care, in
fair exchange, is absolutely necessary to their maintenance. Such as
our needs have made them, incapable of resistance in the savage
conflict of living things, these plants, if left to themselves, without
cultivation, would rapidly disappear, despite the numerical immensity
of their seeds, even as the silly Sheep would shortly disappear were
there no sheepfolds.
They are our work, but not always our exclusive property. Wherever food
is amassed, consumers flock from the four corners of the sky; they
invite themselves to the copious feast; and, the richer the victuals,
the greater their numbers. Man, who alone is capable of provoking
agrarian luxuriance, becomes by this very fact the giver of an immense
banquet whereat legions of guests take their places. By creating more
palatable and more generous victuals, he willy-nilly summons to his
granaries thousands and thousands of famished creatures against whose
teeth his prohibitions battle in vain. The more he produces, the larger
tribute he has to pay. Big crops and sumptuous hoards favour the
insects, our rivals as consumers.
It is the prevailing law. Nature offers her mighty breast with equal
zeal to all her children, to those who live by others’ goods no less
than to the producers. For us who plough and sow and reap, wearing
ourselves out with toil, she ripens the wheat; she ripens it also for
the little Corn-weevil, who, though exempted from the labour of the
fields, will nevertheless settle in our granaries and with her pointed
beak nibble the heap of corn, grain by grain, to the husk. For us who
dig and weed and water, bent with fatigue and burnt by the heat of the
day, nature swells the pea-pods; she swells them also for the
Pea-weevil, who, doing no gardener’s work, will all the same take her
share of the crop at her own time, when the earth is joyful with the
new life of spring.
Let us watch the actions of this zealous tax-collector, who levies her
tithes in green peas. I, a well-meaning rate-payer, will let her have
her way: it is precisely for her benefit that I have sown a few rows of
the beloved plant in my enclosure. With no other invitation from me
than this modest seed-plot, she arrives punctually in the course of
May. She has learnt that in this stony soil, unfitted for
market-gardening, peas are flourishing for the first time. And she has
hastened thither to exercise her privileges as an entomological
revenue-officer.
Whence does she come? It is impossible to say exactly. She has come
from some refuge or other where she has spent the winter in a state of
torpor. The plane-tree, which strips itself of its own initiative
during the heat of summer, furnishes excellent shelters for homeless
paupers under its patches of loose-hanging bark. I have often found our
Pea-thief in one of these winter sanctuaries. Sheltered under the dead
covering of the plane, or otherwise protected while the winter raged,
she woke from her slumbers at the first kisses of a kindly sun. The
almanac of the instincts has taught her; she knows as well as the
gardener when the peas are in flower, and she comes to her plant more
or less from every direction, ambling at a slow pace, but swift in
flight.
A small head, a slender snout, a dress of ashen grey sprinkled with
brown, flat wing-cases, a squat thick-set figure, with two large black
dots on the flat of the tail: there you have a rough sketch of my
visitor. The vanguard arrives by the end of the first fortnight in May.
The Weevils settle on the flowers, which are like so many white
Butterflies’ wings: I see some installed at the foot of the upper
petal, I see some hidden in the casket of the keel. Others, more
numerous these, explore the blossoms and take possession of them. The
laying-time has not yet come. It is a mild morning; the sun is hot
without being oppressive. This is the moment for nuptial exploits and
for raptures amid the splendour of the light. Life therefore is enjoyed
for a little while. Couples form, soon part and soon come together
again. When the heat grows too great, towards the middle of the day,
each Jack and Jill retire into the shade, in a fold of the flower whose
secret recesses they know so well. To-morrow they will resume the
festival and the next day too, until the pod, splitting the sheath of
its keel, appears outside, more and more swollen from day to day.
A few pregnant mothers, harder-pressed than the rest, confide their
eggs to the growing pod, as it issues flat and tiny from its floral
scabbard. These eggs laid prematurely, pushed out perhaps through the
exigencies of an ovary which can wait no longer, seem to me in serious
danger. The seed in which the grub is to make its home is as yet but a
feeble granule, without substance and without floury contents. No
Weevil-larva would ever find an adequate meal there, unless by biding
its time until the seed ripened.
But is the grub, once hatched, capable of long fasting? It is doubtful.
The little that I have seen tells me that the new-born larva begins
eating with all speed and, if it cannot do so, dies. I therefore regard
as lost the eggs laid upon immature pods. The prosperity of the race
will hardly suffer, thanks to the Weevil’s fertility. Moreover, we
shall see presently with what reckless prodigality she scatters her
germs, most of which are doomed to perish.
The bulk of the mother’s work is finished by the end of May, when the
pods begin to bulge with protuberances revealing the pressure of the
peas, which have now attained their final size, or very nearly. I was
anxious to see the Bruchus at work, in her quality of a Curculio, which
is how she is classified. [56] The other Weevils are Rhynchophoræ,
beak-wearers, armed with a rod that prepares the hollow in which the
egg is laid. Our friend possesses only a short snout, which does
capitally for sipping a few sweet mouthfuls, but which is of no value
as a boring-tool.
Therefore the method of installing the family is quite different. Here
we see no ingenious preparations, such as the Balanini, the Larini and
the Rhynchites showed us. Having no probe among her tools, the mother
scatters her eggs in the open, with no protection against the heat of
the sun or the inclemencies of the weather. Nothing could be simpler
and nothing more dangerous to the germs, in the absence of a special
constitution made to withstand the alternate trials of heat and cold,
drought and wet.
In the mild sunshine of ten o’clock in the morning, the mother, with a
jerky, capricious, unmethodical step, runs up and down the chosen pod,
first on one and then on the other surface. She protrudes at every
instant a short oviscapt, which swings right and left as though to
scrape the skin. An egg follows and is abandoned as soon as laid.
A hasty touch of the oviscapt, first here, then there, on the green
skin of the pea-pod; and that is all. The germ is left there,
unprotected, right in the sun. Nor is any choice of site made, to
assist the coming grub and shorten its quest when it has to make its
way unaided into the larder. There are eggs placed on the swellings
created by the peas; there are just as many in the barren dividing
valleys. It is for the grub to take its bearings accordingly. In short,
the Bruchus’ eggs are laid anyhow, as though sown on the wing.
A more serious flaw: the number of eggs confided to one pod is not in
proportion to that of the peas contained in it. Let us first realize
that each grub needs a ration of one pea, an obligatory ration, amply
sufficient for the welfare of one larva, but not big enough for several
consumers, nor even for two. A pea for each grub, no more and no less,
is the invariable rule.
Procreative economy would therefore demand that the mother, familiar
with the pod which she has just explored, should, when emitting her
germs, more or less limit their number to that of the peas which it
contains. Now there is no limit. To a single ration the impetuous
ovaries always offer a multiplicity of consumers.
My notes are unanimous on this point. The number of eggs laid on a pod
always exceeds, and often in a scandalous fashion, the number of peas
available. However scanty the food-wallet may be, the guests are
superabundant. Dividing the number of eggs perceived on a given pod by
that of the peas inside it, I find from five to eight claimants for
each pea; I find as many as ten; and there is nothing to tell me that
the prodigality does not go farther still. Many are called, but few are
chosen! Why all these supernumeraries, who are necessarily excluded
from the banquet for want of space?
The eggs are a fairly bright amber-yellow, cylindrical in form, smooth
and rounded at both ends. They are a millimetre long at most. [57] Each
of them is fixed to the pod by a thin network of threads of coagulated
albumen. Neither the rain nor the wind can loosen their hold.
The mother often emits them two at a time, one above the other; often
also the uppermost of the pair succeeds in hatching, whereas the lower
fades and perishes. What did this latter lack, to produce a grub? A
sun-bath, perhaps, the gentle incubation of which the upper egg robs
it. Whether through the effect of the untimely screen that overshadows
it, or for some other reason, the elder of the eggs in a group of two
rarely follows the normal course. It withers on the pod, dead before it
has come to life.
There are exceptions to this premature end. Sometimes the twin eggs
develop equally well; but these instances are so rare that the family
of the Bruchus would be reduced by nearly one-half if the binary system
were a fixed rule. To the detriment of the peas and to the Weevil’s
advantage there is one thing that lessens this destructive factor: the
eggs are laid one by one and in separate places.
A recent hatching is marked by a whitish, winding little ribbon, which
raises and fades the skin of the pod near the sloughed egg-shell. It is
the work of the new-born larva and is a subcutaneous tunnel along which
the tiny creature wends its way in search of a point through which to
penetrate. When it has found this spot, the grub, measuring hardly a
millimetre and pale-bodied, with a black cap, pierces the outer wrapper
and dives into the capacious sheath of the pod.
It reaches the peas and perches on the nearest. I watch it through the
magnifying-glass, exploring its globe, its world. It sinks a well at
right angles to the sphere. I see some which, half-way down, wriggle
their tails to stimulate their efforts. After a short spell of work,
the miner disappears and is at home.
The entrance-hole is minute, but is easily recognized at any time by
its brown colouring against the pale-green or yellow-green background
of the pea. It has no fixed site; we see it more or less anywhere on
the surface of the pea, excepting generally on the lower half, that is
to say, the hemisphere whose pole is formed by the base of the
funicular cord.
It is precisely in this part that the germ is found which will not be
consumed and will remain capable of developing into an embryo plant, in
spite of the large hole made by the adult insect in leaving. Why is
this portion left unscathed? What are the reasons that safeguard the
germ of the exploited seed?
It goes without saying that the Bruchus does not consider the gardener.
The pea is meant for it and none other. In refusing to take the few
bites which would entail the death of the seed, it has no intention of
reducing the damage. It abstains from other motives.
Remark that the peas touch at the sides, where they are pressed one
against the other. The grub seeking the point of attack cannot move
about at its ease. Remark also that the lower pole rests upon the
umbilical excrescence and opposes to any attempt at boring difficulties
which do not exist in the parts protected by the skin alone. It is even
possible that this umbilicus, which is differently organized, contains
special juices distasteful to the little larva.
This, beyond a doubt, is the secret of the peas exploited by the
Bruchus and yet remaining fit to sprout. They are injured but not dead,
because they are invaded in the free hemisphere, the part which is at
the same time easier to enter and less easy to wound. Moreover, as the
whole pea is too much for a single grub, the loss of substance is
reduced to the piece preferred by the consumer; and this piece is not
the essential part of the pea.
Given other conditions, with seeds either very small or exceedingly
large, we should see the results changing entirely. In the first case,
the germ would be gnawed like the remainder and would perish by the
tooth of the too niggardly served grub; in the second case, the
abundant food would allow of several guests. The common vetch and the
broad bean, exploited in the absence of the pea, tell us something in
this connection; the smaller seed, devoured all but the skin, is a ruin
whose germination we may expect in vain; the larger, on the contrary,
despite the Weevil’s numerous cells, is still capable of sprouting.
Admitting that the number of eggs on the pod is always much greater
than that of the peas contained, and that, on the other hand, each pea
is the exclusive property of one grub, we wonder what becomes of the
surplus. Do these larvæ perish outside, when the more precocious have
taken their places one by one in the leguminous larder? Do they succumb
to the intolerant teeth of the early occupants? They do neither. Let us
set forth the facts.
On all old peas, now dry, from which the adult Weevil has issued,
leaving a gaping hole, the magnifying-glass reveals a varying number of
fine, reddish-brown dots, perforated at the centre. What are these
spots, of which I count five or six or even more on a single pea? There
is no mistake possible: they are the entrance-points of so many grubs.
Several workers have therefore penetrated into the seed; and of the
whole gang only one has survived, waxed big and fat and attained the
adult age. And the others? We shall see.
At the end of May and in June, during the laying-season, inspect the
still green and tender peas. Nearly all the seeds invaded show us the
multiple dots which we already observed on the dry peas abandoned by
the Weevils. Does this actually mean an assembly of guests? Yes. Skin
the aforesaid seeds, separate the seed-lobes, subdivide them if
necessary. We discover several larvæ, very young, bent into a bow, fat
and wriggling, each in a little round hollow in the heart of the
victuals.
Peace and comfort seem to reign in the community. There is no
quarrelling, no jealous competition among neighbours. The eating has
begun, provisions are plentiful and the banqueters are separated from
one another by partitions formed by the as yet untouched portions of
the seed-lobes. With this isolation in separate cells, there is no fear
of squabbles; the guests will not bite one another, by accident or
intention. All the occupants enjoy the same rights of property, the
same appetite and the same strength. What will be the end of the
communal working?
I split some peas which I have found to be well-stocked and place them
in a glass tube. I add others daily. This method keeps me informed of
the boarders’ progress. At first there is nothing special. Isolated in
its narrow recess, each grub nibbles around itself and eats frugally
and peacefully. It is still quite small; a speck of food surfeits it.
Nevertheless, a dish consisting of one pea cannot satisfy so large a
number until the end. Famine threatens; all save one must die.
Soon indeed the aspect of things changes. One of the grubs, the one
occupying the central position in the pea, grows faster than the
others. He has hardly begun to be larger than his competitors when
these cease to eat and refrain from digging any farther. They lie
motionless and resigned; they die the gentle death which reaps
unconscious lives. They disappear, wasted away to nothing. They were so
tiny, the poor victims! Henceforth the whole pea belongs to the sole
survivor. But what has happened, to produce this desolation around the
privileged one? For lack of a relevant answer, I will propound a
suggestion.
In the centre of the pea, more gently stewed than the rest by the sun’s
chemistry, may there not be an infant-pap, a pulp of a quality
better-suited to the delicate organs of a grub? Here perhaps,
stimulated by tender, highly flavoured and sweeter food, the stomach
becomes more vigorous and fit to cope with food less easily digested. A
baby is fed on milk before it receives the basin of broth and the bread
of the able-bodied. Might not the central portion of the pea be the
Weevil-grub’s feeding-bottle?
Fired by one ambition and endowed with equal rights, all the occupants
of the seed set out towards the delicious morsel. It is a laborious
journey; and frequent halts are made in temporary recesses. The grubs
rest; pending better things to come, they frugally crunch the ripe
substance around them; they gnaw even more to open a way than to fill
their stomachs.
At last one of the excavators, favoured by the direction taken, reaches
the central dairy. It settles there and the thing is done: there is
nothing for the rest but to die. How do they come to know that the
place is taken? Do they hear their kinsman’s mandibles striking against
the wall of his cell? Can they feel the vibration of the nibbling at a
distance? Something of the sort must happen, for from that moment they
cease their attempts to burrow any farther. Without struggling with the
lucky winner, without seeking to dislodge him, those beaten in the race
allow themselves to die. I like this frank resignation on the part of
the late arrivals.