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THE PEA-WEEVIL - BRUCHUS PISI




Peas are held in high esteem by mankind. From remote ages man has
endeavoured, by careful culture, to produce larger, tenderer, and
sweeter varieties. Of an adaptable character, under careful treatment
the plant has evolved in a docile fashion, and has ended by giving us
what the ambition of the gardener desired. To-day we have gone far
beyond the yield of the Varrons and Columelles, and further still beyond
the original pea; from the wild seeds confided to the soil by the first
man who thought to scratch up the surface of the earth, perhaps with the
half-jaw of a cave-bear, whose powerful canine tooth would serve him as
a ploughshare!

Where is it, this original pea, in the world of spontaneous vegetation?
Our own country has nothing resembling it. Is it to be found elsewhere?
On this point botany is silent, or replies only with vague
probabilities.

We find the same ignorance elsewhere on the subject of the majority of
our alimentary vegetables. Whence comes wheat, the blessed grain which
gives us bread? No one knows. You will not find it here, except in the
care of man; nor will you find it abroad. In the East, the birthplace
of agriculture, no botanist has ever encountered the sacred ear growing
of itself on unbroken soil.

Barley, oats, and rye, the turnip and the beet, the beetroot, the
carrot, the pumpkin, and so many other vegetable products, leave us in
the same perplexity; their point of departure is unknown to us, or at
most suspected behind the impenetrable cloud of the centuries. Nature
delivered them to us in the full vigour of the thing untamed, when their
value as food was indifferent, as to-day she offers us the sloe, the
bullace, the blackberry, the crab; she gave them to us in the state of
imperfect sketches, for us to fill out and complete; it was for our
skill and our labour patiently to induce the nourishing pulp which was
the earliest form of capital, whose interest is always increasing in the
primordial bank of the tiller of the soil.

As storehouses of food the cereal and the vegetable are, for the greater
part, the work of man. The fundamental species, a poor resource in their
original state, we borrowed as they were from the natural treasury of
the vegetable world; the perfected race, rich in alimentary materials,
is the result of our art.

If wheat, peas, and all the rest are indispensable to us, our care, by a
just return, is absolutely necessary to them. Such as our needs have
made them, incapable of resistance in the bitter struggle for survival,
these vegetables, left to themselves without culture, would rapidly
disappear, despite the numerical abundance of their seeds, as the
foolish sheep would disappear were there no more sheep-folds.

They are our work, but not always our exclusive property. Wherever food
is amassed, the consumers collect from the four corners of the sky; they
invite themselves to the feast of abundance, and the richer the food the
greater their numbers. Man, who alone is capable of inducing agrarian
abundance, is by that very fact the giver of an immense banquet at which
legions of feasters take their place. By creating more juicy and more
generous fruits he calls to his enclosures, despite himself, thousands
and thousands of hungry creatures, against whose appetites his
prohibitions are helpless. The more he produces, the larger is the
tribute demanded of him. Wholesale agriculture and vegetable abundance
favour our rival the insect.

This is the immanent law. Nature, with an equal zeal, offers her mighty
breast to all her nurslings alike; to those who live by the goods of
others no less than to the producers. For us, who plough, sow, and reap,
and weary ourselves with labour, she ripens the wheat; she ripens it
also for the little Calender-beetle, which, although exempted from the
labour of the fields, enters our granaries none the less, and there,
with its pointed beak, nibbles our wheat, grain by grain, to the husk.

For us, who dig, weed, and water, bent with fatigue and burned by the
sun, she swells the pods of the pea; she swells them also for the
weevil, which does no gardener's work, yet takes its share of the
harvest at its own hour, when the earth is joyful with the new life of
spring.

Let us follow the manoeuvres of this insect which takes its tithe of
the green pea. I, a benevolent ratepayer, will allow it to take its
dues; it is precisely to benefit it that I have sown a few rows of the
beloved plant in a corner of my garden. Without other invitation on my
part than this modest expenditure of seed-peas it arrives punctually
during the month of May. It has learned that this stony soil, rebellious
to the culture of the kitchen-gardener, is bearing peas for the first
time. In all haste therefore it has hurried, an agent of the
entomological revenue system, to demand its dues.

Whence does it come? It is impossible to say precisely. It has come from
some shelter, somewhere, in which it has passed the winter in a state of
torpor. The plane-tree, which sheds its rind during the heats of the
summer, furnishes an excellent refuge for homeless insects under its
partly detached sheets of bark.

I have often found our weevil in such a winter refuge. Sheltered under
the dead covering of the plane, or otherwise protected while the winter
lasts, it awakens from its torpor at the first touch of a kindly sun.
The almanack of the instincts has aroused it; it knows as well as the
gardener when the pea-vines are in flower, and seeks its favourite
plant, journeying thither from every side, running with quick, short
steps, or nimbly flying.

A small head, a fine snout, a costume of ashen grey sprinkled with
brown, flattened wing-covers, a dumpy, compact body, with two large
black dots on the rear segment--such is the summary portrait of my
visitor. The middle of May approaches, and with it the van of the
invasion.

They settle on the flowers, which are not unlike white-winged
butterflies. I see them at the base of the blossom or inside the cavity
of the "keel" of the flower, but the majority explore the petals and
take possession of them. The time for laying the eggs has not yet
arrived. The morning is mild; the sun is warm without being oppressive.
It is the moment of nuptial flights; the time of rejoicing in the
splendour of the sunshine. Everywhere are creatures rejoicing to be
alive. Couples come together, part, and re-form. When towards noon the
heat becomes too great, the weevils retire into the shadow, taking
refuge singly in the folds of the flowers whose secret corners they know
so well. To-morrow will be another day of festival, and the next day
also, until the pods, emerging from the shelter of the "keel" of the
flower, are plainly visible, enlarging from day to day.

A few gravid females, more pressed for time than the others, confide
their eggs to the growing pod, flat and meagre as it issues from its
floral sheath. These hastily laid batches of eggs, expelled perhaps by
the exigencies of an ovary incapable of further delay, seem to me in
serious danger; for the seed in which the grub must establish itself is
as yet no more than a tender speck of green, without firmness and
without any farinaceous tissue. No larva could possible find sufficient
nourishment there, unless it waited for the pea to mature.

But is the grub capable of fasting for any length of time when once
hatched? It is doubtful. The little I have seen tells me that the
new-born grub must establish itself in the midst of its food as quickly
as possible, and that it perishes unless it can do so. I am therefore of
opinion that such eggs as are deposited in immature pods are lost.
However, the race will hardly suffer by such a loss, so fertile is the
little beetle. We shall see directly how prodigal the female is of her
eggs, the majority of which are destined to perish.

The important part of the maternal task is completed by the end of May,
when the shells are swollen by the expanding peas, which have reached
their final growth, or are but little short of it. I was anxious to see
the female Bruchus at work in her quality of Curculionid, as our
classification declares her.[8] The other weevils are Rhyncophora,
beaked insects, armed with a drill with which to prepare the hole in
which the egg is laid. The Bruchus possesses only a short snout or
muzzle, excellently adapted for eating soft tissues, but valueless as a
drill.

The method of installing the family is consequently absolutely
different. There are no industrious preparations as with the Balinidae,
the Larinidae, and the Rhynchitides. Not being equipped with a long
oviscapt, the mother sows her eggs in the open, with no protection
against the heat of the sun and the variations of temperature. Nothing
could be simpler, and nothing more perilous to the eggs, in the absence
of special characteristics which would enable them to resist the
alternate trials of heat and cold, moisture and drought.

In the caressing sunlight of ten o'clock in the morning the mother runs
up and down the chosen pod, first on one side, then on the other, with a
jerky, capricious, unmethodical gait. She repeatedly extrudes a short
oviduct, which oscillates right and left as though to graze the skin of
the pod. An egg follows, which is abandoned as soon as laid.

A hasty touch of the oviduct, first here, then there, on the green skin
of the pea-pod, and that is all. The egg is left there, unprotected, in
the full sunlight. No choice of position is made such as might assist
the grub when it seeks to penetrate its larder. Some eggs are laid on
the swellings created by the peas beneath; others in the barren valleys
which separate them. The first are close to the peas, the second at some
distance from them. In short, the eggs of the Bruchus are laid at
random, as though on the wing.

We observe a still more serious vice: the number of eggs is out of all
proportion to the number of peas in the pod. Let us note at the outset
that each grub requires one pea; it is the necessary ration, and is
largely sufficient to one larva, but is not enough for several, nor even
for two. One pea to each grub, neither more nor less, is the
unchangeable rule.

We should expect to find signs of a procreative economy which would
impel the female to take into account the number of peas contained in
the pod which she has just explored; we might expect her to set a
numerical limit on her eggs in conformity with that of the peas
available. But no such limit is observed. The rule of one pea to one
grub is always contradicted by the multiplicity of consumers.

My observations are unanimous on this point. The number of eggs
deposited on one pod always exceeds the number of peas available, and
often to a scandalous degree. However meagre the contents of the pod
there is a superabundance of consumers. Dividing the sum of the eggs
upon such or such a pod by that of the peas contained therein, I find
there are five to eight claimants for each pea; I have found ten, and
there is no reason why this prodigality should not go still further.
Many are called, but few are chosen! What is to become of all these
supernumeraries, perforce excluded from the banquet for want of space?

The eggs are of a fairly bright amber yellow, cylindrical in form,
smooth, and rounded at the ends. Their length is at most a twenty-fifth
of an inch. Each is affixed to the pod by means of a slight network of
threads of coagulated albumen. Neither wind nor rain can loosen their
hold.

The mother not infrequently emits them two at a time, one above the
other; not infrequently, also, the uppermost of the two eggs hatches
before the other, while the latter fades and perishes. What was lacking
to this egg, that it should fail to produce a grub? Perhaps a bath of
sunlight; the incubating heat of which the outer egg has robbed it.
Whether on account of the fact that it is shadowed by the other egg, or
for other reasons, the elder of the eggs in a group of two rarely
follows the normal course, but perishes on the pod, dead without having
lived.

There are exceptions to this premature end; sometimes the two eggs
develop equally well; but such cases are exceptional, so that the
Bruchid family would be reduced to about half its dimensions if the
binary system were the rule. To the detriment of our peas and to the
advantage of the beetle, the eggs are commonly laid one by one and in
isolation.

A recent emergence is shown by a little sinuous ribbon-like mark, pale
or whitish, where the skin of the pod is raised and withered, which
starts from the egg and is the work of the new-born larva; a
sub-epidermic tunnel along which the grub works its way, while seeking a
point from which it can escape into a pea. This point once attained, the
larva, which is scarcely a twenty-fifth of an inch in length, and is
white with a black head, perforates the envelope and plunges into the
capacious hollow of the pod.

It has reached the peas and crawls upon the nearest. I have observed it
with the magnifier. Having explored the green globe, its new world, it
begins to sink a well perpendicularly into the sphere. I have often seen
it half-way in, wriggling its tail in the effort to work the quicker. In
a short time the grub disappears and is at home. The point of entry,
minute, but always easily recognisable by its brown coloration on the
pale green background of the pea, has no fixed location; it may be at
almost any point on the surface of the pea, but an exception is usually
made of the lower half; that is, the hemisphere whose pole is formed by
the supporting stem.

It is precisely in this portion that the germ is found, which will not
be eaten by the larva, and will remain capable of developing into a
plant, in spite of the large aperture made by the emergence of the adult
insect. Why is this particular portion left untouched? What are the
motives that safeguard the germ?

It goes without saying that the Bruchus is not considering the gardener.
The pea is meant for it and for no one else. In refusing the few bites
that would lead to the death of the seed, it has no intention of
limiting its destruction. It abstains from other motives.

Let us remark that the peas touch laterally, and are pressed one
against the other, so that the grub, when searching for a point of
attack, cannot circulate at will. Let us also note that the lower pole
expands into the umbilical excrescence, which is less easy of
perforation than those parts protected by the skin alone. It is even
possible that the umbilicum, whose organisation differs from that of the
rest of the pea, contains a peculiar sap that is distasteful to the
little grub.

Such, doubtless, is the reason why the peas exploited by the Bruchus are
still able to germinate. They are damaged, but not dead, because the
invasion was conducted from the free hemisphere, a portion less
vulnerable and more easy of access. Moreover, as the pea in its entirety
is too large for a single grub to consume, the consumption is limited to
the portion preferred by the consumer, and this portion is not the
essential portion of the pea.

With other conditions, with very much smaller or very much larger seeds,
we shall observe very different results. If too small, the germ will
perish, gnawed like the rest by the insufficiently provisioned inmate;
if too large, the abundance of food will permit of several inmates.
Exploited in the absence of the pea, the cultivated vetch and the broad
bean afford us an excellent example; the smaller seed, of which all but
the skin is devoured, is left incapable of germination; but the large
bean, even though it may have held a number of grubs, is still capable
of sprouting.

Knowing that the pod always exhibits a number of eggs greatly in excess
of the enclosed peas, and that each pea is the exclusive property of one
grub, we naturally ask what becomes of the superfluous grubs. Do they
perish outside when the more precocious have one by one taken their
places in their vegetable larder? or do they succumb to the intolerant
teeth of the first occupants? Neither explanation is correct. Let us
relate the facts.

On all old peas--they are at this stage dry--from which the adult
Bruchus has emerged, leaving a large round hole of exit, the
magnifying-glass will show a variable number of fine reddish
punctuations, perforated in the centre. What are these spots, of which I
count five, six, and even more on a single pea? It is impossible to be
mistaken: they are the points of entry of as many grubs. Several grubs
have entered the pea, but of the whole group only one has survived,
fattened, and attained the adult age. And the others? We shall see.

At the end of May, and in June, the period of egg-laying, let us inspect
the still green and tender peas. Nearly all the peas invaded show us the
multiple perforations already observed on the dry peas abandoned by the
weevils. Does this actually mean that there are several grubs in the
pea? Yes. Skin the peas in question, separate the cotyledons, and break
them up as may be necessary. We shall discover several grubs, extremely
youthful, curled up comma-wise, fat and lively, each in a little round
niche in the body of the pea.

Peace and welfare seem to reign in the little community. There is no
quarrelling, no jealousy between neighbours. The feast has commenced;
food is abundant, and the feasters are separated one from another by the
walls of uneaten substance. With this isolation in separate cells no
conflicts need be feared; no sudden bite of the mandibles, whether
intentional or accidental. All the occupants enjoy the same rights of
property, the same appetite, and the same strength. How does this
communal feast terminate?

Having first opened them, I place a number of peas which are found to be
well peopled in a glass test-tube. I open others daily. In this way I
keep myself informed as to the progress of the various larvae. At first
nothing noteworthy is to be seen. Isolated in its narrow chamber, each
grub nibbles the substance around it, peacefully and parsimoniously. It
is still very small; a mere speck of food is a feast; but the contents
of one pea will not suffice the whole number to the end. Famine is
ahead, and all but one must perish.

Soon, indeed, the aspect of things is entirely changed. One of the
grubs--that which occupies the central position in the pea--begins to
grow more quickly than the others. Scarcely has it surpassed the others
in size when the latter cease to eat, and no longer attempt to burrow
forwards. They lie motionless and resigned; they die that gentle death
which comes to unconscious lives. Henceforth the entire pea belongs to
the sole survivor. Now what has happened that these lives around the
privileged one should be thus annihilated? In default of a satisfactory
reply, I will propose a suggestion.

In the centre of the pea, less ripened than the rest of the seed by the
chemistry of the sun, may there not be a softer pulp, of a quality
better adapted to the infantile digestion of the grub? There, perhaps,
being nourished by tenderer, sweeter, and perhaps more tasty tissues,
the stomach becomes more vigorous, until it is fit to undertake less
easily digested food. A nursling is fed on milk before proceeding to
bread and broth. May not the central portion of the pea be the
feeding-bottle of the Bruchid?

With equal rights, fired by an equal ambition, all the occupants of the
pea bore their way towards the delicious morsel. The journey is
laborious, and the grubs must rest frequently in their provisional
niches. They rest; while resting they frugally gnaw the riper tissues
surrounding them; they gnaw rather to open a way than to fill their
stomachs.

Finally one of the excavators, favoured by the direction taken, attains
the central portion. It establishes itself there, and all is over; the
others have only to die. How are they warned that the place is taken? Do
they hear their brother gnawing at the walls of his lodging? can they
feel the vibration set up by his nibbling mandibles? Something of the
kind must happen, for from that moment they make no attempt to burrow
further. Without struggling against the fortunate winner, without
seeking to dislodge him, those which are beaten in the race give
themselves up to death. I admire this candid resignation on the part of
the departed.

Another condition--that of space--is also present as a factor. The
pea-weevil is the largest of our Bruchidae. When it attains the adult
stage it requires a certain amplitude of lodging, which the other
weevils do not require in the same degree. A pea provides it with a
sufficiently spacious cell; nevertheless, the cohabitation of two in one
pea would be impossible; there would be no room, even were the two to
put up with a certain discomfort. Hence the necessity of an inevitable
decimation, which will suppress all the competitors save one.

Now the superior volume of the broad bean, which is almost as much
beloved by the weevil as the pea, can lodge a considerable community,
and the solitary can live as a cenobite. Without encroaching on the
domain of their neighbours, five or six or more can find room in the one
bean.

Moreover, each grub can find its infant diet; that is, that layer which,
remote from the surface, hardens only gradually and remains full of sap
until a comparatively late period. This inner layer represents the crumb
of a loaf, the rest of the bean being the crust.

In the pea, a sphere of much less capacity, it occupies the central
portion; a limited point at which the grub develops, and lacking which
it perishes; but in the bean it lines the wide adjoining faces of the
two flattened cotyledons. No matter where the point of attack is made,
the grub has only to bore straight down when it quickly reaches the
softer tissues. What is the result? I have counted the eggs adhering to
a bean-pod and the beans included in the pod, and comparing the two
figures I find that there is plenty of room for the whole family at the
rate of five or six dwellers in each bean. No superfluous larvae perish
of hunger when barely issued from the egg; all have their share of the
ample provision; all live and prosper. The abundance of food balances
the prodigal fertility of the mother.

If the Bruchus were always to adopt the broad bean for the establishment
of her family I could well understand the exuberant allowance of eggs to
one pod; a rich food-stuff easily obtained evokes a large batch of
eggs. But the case of the pea perplexes me. By what aberration does the
mother abandon her children to starvation on this totally insufficient
vegetable? Why so many grubs to each pea when one pea is sufficient only
for one grub?

Matters are not so arranged in the general balance-sheet of life. A
certain foresight seems to rule over the ovary so that the number of
mouths is in proportion to the abundance or scarcity of the food
consumed. The Scarabaeus, the Sphex, the Necrophorus, and other insects
which prepare and preserve alimentary provision for their families, are
all of a narrowly limited fertility, because the balls of dung, the dead
or paralysed insects, or the buried corpses of animals on which their
offspring are nourished are provided only at the cost of laborious
efforts.

The ordinary bluebottle, on the contrary, which lays her eggs upon
butcher's meat or carrion, lays them in enormous batches. Trusting in
the inexhaustible riches represented by the corpse, she is prodigal of
offspring, and takes no account of numbers. In other cases the provision
is acquired by audacious brigandage, which exposes the newly born
offspring to a thousand mortal accidents. In such cases the mother
balances the chances of destruction by an exaggerated flux of eggs. Such
is the case with the Meloides, which, stealing the goods of others under
conditions of the greatest peril, are accordingly endowed with a
prodigious fertility.

The Bruchus knows neither the fatigues of the laborious, obliged to
limit the size of her family, nor the misfortunes of the parasite,
obliged to produce an exaggerated number of offspring. Without painful
search, entirely at her ease, merely moving in the sunshine over her
favourite plant, she can ensure a sufficient provision for each of her
offspring; she can do so, yet is foolish enough to over-populate the pod
of the pea; a nursery insufficiently provided, in which the great
majority will perish of starvation. This ineptitude is a thing I cannot
understand: it clashes too completely with the habitual foresight of the
maternal instinct.

I am inclined to believe that the pea is not the original food plant of
the Bruchus. The original plant must rather have been the bean, one seed
of which is capable of supporting half a dozen or more larvae. With the
larger cotyledon the crying disproportion between the number of eggs and
the available provision disappears.

Moreover, it is indubitable that the bean is of earlier date than the
pea. Its exceptional size and its agreeable flavour would certainly have
attracted the attention of man from the remotest periods. The bean is a
ready-made mouthful, and would be of the greatest value to the hungry
tribe. Primitive man would at an early date have sown it beside his
wattled hut. Coming from Central Asia by long stages, their wagons drawn
by shaggy oxen and rolling on the circular discs cut from the trunks of
trees, the early immigrants would have brought to our virgin land, first
the bean, then the pea, and finally the cereal, that best of safeguards
against famine. They taught us the care of herds, and the use of bronze,
the material of the first metal implement. Thus the dawn of civilisation
arose over France. With the bean did those ancient teachers also
involuntarily bring us the insect which to-day disputes it with us? It
is doubtful; the Bruchidae seem to be indigenous. At all events, I find
them levying tribute from various indigenous plants, wild vegetables
which have never tempted the appetite of man. They abound in particular
upon the great forest vetch (_Lathyrus latifolius_), with its
magnificent heads of flowers and long handsome pods. The seeds are not
large, being indeed smaller than the garden pea; but eaten to the very
skin, as they invariably are, each is sufficient to the needs of its
grub.

We must not fail to note their number. I have counted more than twenty
in a single pod, a number unknown in the case of the pea, even in the
most prolific varieties. Consequently this superb vetch is in general
able to nourish without much loss the family confided to its pod.

Where the forest vetch is lacking, the Bruchus, none the less, bestows
its habitual prodigality of eggs upon another vegetable of similar
flavour, but incapable of nourishing all the grubs: for the example, the
travelling vetch (_Vicia peregrina_) or the cultivated vetch (_Vicia
sativa_). The number of eggs remains high even upon insufficient pods,
because the original food-plant offered a copious provision, both in the
multiplicity and the size of the seeds. If the Bruchus is really a
stranger, let us regard the bean as the original food-plant; if
indigenous, the large vetch.

Sometime in the remote past we received the pea, growing it at first in
the prehistoric vegetable garden which already supplied the bean. It was
found a better article of diet than the broad bean, which to-day, after
such good service, is comparatively neglected. The weevil was of the
same opinion as man, and without entirely forgetting the bean and the
vetch it established the greater part of its tribe upon the pea, which
from century to century was more widely cultivated. To-day we have to
share our peas: the Bruchidae take what they need, and bestow their
leavings on us.

This prosperity of the insect which is the offspring of the abundance
and quality of our garden products is from another point of view
equivalent to decadence. For the weevil, as for ourselves, progress in
matters of food and drink is not always beneficial. The race would
profit better if it remained frugal. On the bean and the vetch the
Bruchus founded colonies in which the infant mortality was low. There
was room for all. On the pea-vine, delicious though its fruits may be,
the greater part of its offspring die of starvation. The rations are
few, and the hungry mouths are multitudinous.

We will linger over this problem no longer. Let us observe the grub
which has now become the sole tenant of the pea by the death of its
brothers. It has had no part in their death; chance has favoured it,
that is all. In the centre of the pea, a wealthy solitude, it performs
the duty of a grub; the sole duty of eating. It nibbles the walls
enclosing it, enlarging its lodgment, which is always entirely filled by
its corpulent body. It is well shaped, fat, and shining with health. If
I disturb it, it turns gently in its niche and sways its head. This is
its manner of complaining of my importunities. Let us leave it in peace.

It profits so greatly and so swiftly by its position that by the time
the dog-days have come it is already preparing for its approaching
liberation. The adult is not sufficiently well equipped to open for
itself a way out through the pea, which is now completely hardened. The
larva knows of this future helplessness, and with consummate art
provides for its release. With its powerful mandibles it bores a channel
of exit, exactly round, with extremely clean-cut sides. The most skilful
ivory-carver could do no better.

To prepare the door of exit in advance is not enough; the grub must also
provide for the tranquillity essential to the delicate processes of
nymphosis. An intruder might enter by the open door and injure the
helpless nymph. This passage must therefore remain closed. But how?

As the grub bores the passage of exit it consumes the farinaceous matter
without leaving a crumb. Having come to the skin of the pea it stops
short. This membrane, semi-translucid, is the door to the chamber of
metamorphosis, its protection against the evil intentions of external
creatures.

It is also the only obstacle which the adult will encounter at the
moment of exit. To lessen the difficulty of opening it the grub takes
the precaution of gnawing at the inner side of the skin, all round the
circumference, so as to make a line of least resistance. The perfect
insect will only have to heave with its shoulder and strike a few blows
with its head in order to raise the circular door and knock it off like
the lid of a box. The passage of exit shows through the diaphanous skin
of the pea as a large circular spot, which is darkened by the obscurity
of the interior. What passes behind it is invisible, hidden as it is
behind a sort of ground glass window.

A pretty invention, this little closed porthole, this barricade against
the invader, this trap-door raised by a push when the time has come for
the hermit to enter the world. Shall we credit it to the Bruchus? Did
the ingenious insect conceive the undertaking? Did it think out a plan
and work out a scheme of its own devising? This would be no small
triumph for the brain of a weevil. Before coming to a conclusion let us
try an experiment.

I deprive certain occupied peas of their skin, and I dry them with
abnormal rapidity, placing them in glass test-tubes. The grubs prosper
as well as in the intact peas. At the proper time the preparations for
emergence are made.

If the grub acts on its own inspiration, if it ceases to prolong its
boring directly it recognises that the outer coating, auscultated from
time to time, is sufficiently thin, what will it do under the conditions
of the present test? Feeling itself at the requisite distance from the
surface it will stop boring; it will respect the outer layer of the bare
pea, and will thus obtain the indispensable protecting screen.

Nothing of the kind occurs. In every case the passage is completely
excavated; the entrance gapes wide open, as large and as carefully
executed as though the skin of the pea were in its place. Reasons of
security have failed to modify the usual method of work. This open
lodging has no defence against the enemy; but the grub exhibits no
anxiety on this score.

Neither is it thinking of the outer enemy when it bores down to the skin
when the pea is intact, and then stops short. It suddenly stops because
the innutritious skin is not to its taste. We ourselves remove the
parchment-like skins from a mess of pease-pudding, as from a culinary
point of view they are so much waste matter. The larva of the Bruchus,
like ourselves, dislikes the skin of the pea. It stops short at the
horny covering, simply because it is checked by an uneatable substance.
From this aversion a little miracle arises; but the insect has no sense
of logic; it is passively obedient to the superior logic of facts. It
obeys its instinct, as unconscious of its act as is a crystal when it
assembles, in exquisite order, its battalions of atoms.

Sooner or later during the month of August we see a shadowy circle form
on each inhabited pea; but only one on each seed. These circles of
shadow mark the doors of exit. Most of them open in September. The lid,
as though cut out with a punch, detaches itself cleanly and falls to the
ground, leaving the orifice free. The Bruchus emerges, freshly clad, in
its final form.

The weather is delightful. Flowers are abundant, awakened by the summer
showers; and the weevils visit them in the lovely autumn weather. Then,
when the cold sets in, they take up their winter quarters in any
suitable retreat. Others, still numerous, are less hasty in quitting the
native seed. They remain within during the whole winter, sheltered
behind the trap-door, which they take care not to touch. The door of the
cell will not open on its hinges, or, to be exact, will not yield along
the line of least resistance, until the warm days return. Then the late
arrivals will leave their shelter and rejoin the more impatient, and
both will be ready for work when the pea-vines are in flower.

To take a general view of the instincts in their inexhaustible variety
is, for the observer, the great attraction of the entomological world;
for nowhere do we gain a clearer sight of the wonderful way in which the
processes of life are ordered. Thus regarded entomology is not, I know,
to the taste of everybody; the simple creature absorbed in the doings
and habits of insects is held in low esteem. To the terrible
utilitarian, a bushel of peas preserved from the weevil is of more
importance than a volume of observations which bring no immediate
profit.

Yet who has told you, O man of little faith, that what is useless to-day
will not be useful to-morrow? If we learn the customs of insects or
animals we shall understand better how to protect our goods. Do not
despise disinterested knowledge, or you may rue the day. It is by the
accumulation of ideas, whether immediately applicable or otherwise, that
humanity has done, and will continue to do, better to-day than
yesterday, and better to-morrow than to-day. If we live on peas and
beans, which we dispute with the weevil, we also live by knowledge, that
mighty kneading-trough in which the bread of progress is mixed and
leavened. Knowledge is well worth a few beans.

Among other things, knowledge tells us: "The seedsman need not go to the
expense of waging war upon the weevil. When the peas arrive in the
granary, the harm is already done; it is irreparable, but not
transmissible. The untouched peas have nothing to fear from the
neighbourhood of those which have been attacked, however long the
mixture is left. From the latter the weevils will issue when their time
has come; they will fly away from the storehouse if escape is possible;
if not, they will perish without in any way attacking the sound peas. No
eggs, no new generation will ever be seen upon or within the dried peas
in the storehouse; there the adult weevil can work no further mischief."

The Bruchus is not a sedentary inhabitant of granaries: it requires the
open air, the sun, the liberty of the fields. Frugal in everything, it
absolutely disdains the hard tissues of the vegetable; its tiny mouth is
content with a few honeyed mouthfuls, enjoyed upon the flowers. The
larvae, on the other hand, require the tender tissues of the green pea
growing in the pod. For these reasons the granary knows no final
multiplication on the part of the despoiler.

The origin of the evil is in the kitchen-garden. It is there that we
ought to keep a watch on the misdeeds of the Bruchus, were it not for
the fact that we are nearly always weaponless when it comes to fighting
an insect. Indestructible by reason of its numbers, its small size, and
its cunning, the little creature laughs at the anger of man. The
gardener curses it, but the weevil is not disturbed: it imperturbably
continues its trade of levying tribute. Happily we have assistants more
patient and more clear-sighted than ourselves.

During the first week of August, when the mature Bruchus begins to
emerge, I notice a little Chalcidian, the protector of our peas. In my
rearing-cages it issues under my eyes in abundance from the peas
infested by the grub of the weevil. The female has a reddish head and
thorax; the abdomen is black, with a long augur-like oviscapt. The male,
a little smaller, is black. Both sexes have reddish claws and
thread-like antennae.

In order to escape from the pea the slayer of the weevil makes an
opening in the centre of the circular trap-door which the grub of the
weevil prepared in view of its future deliverance. The slain has
prepared the way for the slayer. After this detail the rest may be
divined.

When the preliminaries to the metamorphosis are completed, when the
passage of escape is bored and furnished with its lid of superficial
membrane, the female Chalcidian arrives in a busy mood. She inspects the
peas, still on the vine, and enclosed in their pods; she auscultates
them with her antennae; she discovers, hidden under the general envelope,
the weak points in the epidermic covering of the peas. Then, applying
her oviscapt, she thrusts it through the side of the pod and perforates
the circular trap-door. However far withdrawn into the centre of the
pea, the Bruchus, whether larvae or nymph, is reached by the long
oviduct. It receives an egg in its tender flesh, and the thing is done.
Without possibility of defence, since it is by now a somnolent grub or a
helpless pupa, the embryo weevil is eaten until nothing but skin
remains. What a pity that we cannot at will assist the multiplication of
this eager exterminator! Alas! our assistants have got us in a vicious
circle, for if we wished to obtain the help of any great number of
Chalcidians we should be obliged in the first place to breed a
multiplicity of Bruchidae.





Next: AN INVADER - THE HARICOT - WEEVIL

Previous: THE ELEPHANT - BEETLE



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