THE PEA-WEEVIL: THE LARVA
Another condition, that of space, is present as a factor. The
Pea-weevil is the largest of our Bruchi. When she attains the adult age
she requires a bigger lodging than is demanded by the other
seed-destroyers. A pea provides her with a very adequate cell;
nevertheless, cohabitation in twos would be impossible: there would be
no room, even if the occupants accepted the discomfort. And so the
inexorable need returns for reducing the numbers and, in the seed
invaded, doing away with all the competitors save one.
On the other hand, the broad bean, which is almost as great a favourite
of the Bruchus as the pea, is able to house a whole community. The grub
that was but now a solitary becomes a cenobite. There is room for five
or six more, without encroaching on the neighbours’ domain. Moreover,
each grub finds infant-food within its reach, that is to say, the layer
which, being at some distance from the surface, hardens slowly and
retains the dainty juices for a greater length of time. This inner
layer may be regarded as the crumb of an otherwise crusty loaf.
In the pea, which is a small sphere, it occupies the central part, a
limited area which the grub has to reach or perish; in the bean, a
generous muffin, it includes the large joint of the two flat
seed-lobes. No matter where the big seed is tackled, each larva need
but bore straight ahead and it quickly reaches the coveted food.
Then what happens? I add up the eggs adhering to a bean-pod, I count
the seeds inside, and on comparing the two totals, I find that there is
plenty of room for the whole family, at the rate of five or six to each
bean. Here we have no surplus larvæ dying of starvation almost as soon
as they leave the egg: all have their share of the ample portion, all
live and prosper. The abundance of the provisions counterbalances the
mother’s extravagance.
If the Bruchus always adopted the broad bean as the establishment of
her family, I could very well explain her exuberant emission of germs
on a single pod: a rich supply of food, easily acquired, invites a
large colony. The pea, on the other hand, puzzles me. What vagary makes
the mother abandon her offspring to starvation on this insufficient
legumen? Why so many boarders gathered around a seed which forms the
ration of one alone?
It is not thus that matters are arranged in life’s general
balance-sheet. A certain foresight rules the ovaries and makes them
adjust the number of eaters to the abundance or scarcity of the thing
eaten. The Sacred Beetle, the Sphex-wasp, the Burying-beetle and the
other manufacturers of preserved provisions for the family set close
limits to their fertility, because the soft loaves of their baking, the
baskets containing their game and the contents of their sepulchral
retting-vat are all obtained at the cost of laborious and often
unproductive efforts.
The Bluebottle, on the contrary, heaps her eggs in bundles. Trusting in
the inexhaustible wealth of a corpse, she lavishes her maggots without
counting the number. At other times, the provision is obtained by
crafty brigandage, exposing the new-born offspring to a thousand fatal
accidents. Then the mother makes up for the chances of destruction by
an excessive outpouring of eggs. This is the case with the Oil-beetles,
who, stealing the property of others under very parlous conditions, are
for that reason endowed with prodigious fertility.
The Bruchus knows neither the fatigues of the hard worker, obliged to
restrict her family, nor the woes of the parasite, obliged to go to the
other extreme. Without costly researches, entirely at her ease, merely
by strolling in the sun over her favourite plant, she can ensure an
adequate provision for each of her children; she can do this, and yet
the mad creature takes it into her head to over-populate the pea-pod, a
niggardly baby-farm in which the great majority will die of starvation.
This folly passes my understanding: it clashes so utterly with the
usual perspicacity of the maternal instinct.
I am therefore inclined to believe that the pea was not the Bruchus’
original share in the distribution of the earth’s gifts. It must rather
have been the bean, one seed of which is capable of entertaining half a
dozen visitors and more. With a seed of this size, the startling
disproportion between the number of the insect’s eggs and the
foodstuffs available disappears.
Besides, there is not a doubt that, of our various culinary
acquisitions, the broad bean is the earliest in date. Its exceptional
dimensions and its pleasant flavour have certainly attracted man’s
attention since the most remote times. It is a ready-made mouthful, of
great value to the hungry tribe, which would have hastened to secure
its increase by sowing it in the patch of garden beside the house, a
hut of wattled branches plastered with mud. This was the beginning of
agriculture.
Travelling by long stages, with their waggons drawn by shaggy Oxen and
rolling on solid wheels cut out of the trunks of trees, the emigrants
from Central Asia brought to our uncultivated tracts first the bean,
then the pea and finally the cereal, that eminent stand-by against
hunger. They taught us the care of herds and the use of bronze, of
which the first metal implements were made. Thus did the dawn of
civilization rise over Europe.
With the bean did those ancient pioneers bring us, involuntarily, the
insect which disputes its possession with us to-day? There is room for
doubt; the Bruchus seems to be a native. I find her at least levying
tribute on divers Leguminosæ of the country, spontaneous plants which
have never tempted man’s appetite. She abounds in particular on the
great broad-leaved everlasting pea (Lathyrus latifolius), with its
magnificent clusters of flowers and its long and handsome pods. Its
seeds are not large, are much smaller than those of our peas; but,
gnawed to the very skin, as they always are by their occupants, they
are each sufficient to the welfare of its grub.
Note also their considerable number: I have counted more than twenty to
the pod, a wealth unknown to the garden pea, even in its most prolific
state. Thus the superb perennial is generally able, without much loss,
to feed the family entrusted to its pod.
Where the everlasting pea is lacking, the Bruchus none the less
continues her habitual flux of germs on another legumen, of similar
flavour but incapable of nourishing all the grubs, as for instance on
the broad-podded vetch (Vicia peregrina) or the common vetch (V.
sativa). The number of eggs remains high even on these insufficient
pods, because the original plant offered a copious provender, whether
by the multiplicity or by the large size of the seeds. If the Bruchus
is really a foreigner, we may accept the bean as her first victim; if
the insect is a native, let us accept the everlasting pea.
Some time in the remote past the pea reached us, gathered at first in
the same prehistoric garden-patch which already supplied the bean. Man
found it a better food than the horse-bean, which is very much
neglected to-day after doing such good service. The Weevil was of the
same opinion and, without quite forgetting her broad bean and her
everlasting pea, generally pitched her camp on the garden pea, which
became more widely cultivated from century to century. To-day we have
to go shares: the Bruchus takes what she wants and lets us have her
leavings.
The insect’s prosperity, born of the abundance and quality of our
products, from another point of view spells decadence. For the Weevil
as for ourselves, progress in the matter of food and drink does not
always mean improvement. The race fares better by remaining frugal. On
her horse-bean, on her everlasting pea, the Bruchus founded colonies in
which the infant mortality was low. There was room for all. On the pea,
a delectable sweetmeat, the greater part of the guests die of
starvation. The rations are few and the claimants legion.
We will linger over this problem no longer. Let us inquire into the
grub which has become the sole owner of the pea through the death of
its brothers. It has had no part in that decease; chance has favoured
it, that is all. In the centre of the pea, a luxurious solitude, it
performs a grub’s duty, the one and only duty of eating. It gnaws the
walls around and enlarges its cell, which it always fills completely
with its fair round belly. It is a plump and shapely creature,
glistening with health. If I tease it, it turns lazily in its cell and
wags its head. This is its way of complaining of my rudeness. Let us
leave it in peace.
The anchorite thrives so well and so fast that, by the dog-days, it is
already making ready for its coming liberation. The adult has not the
necessary tools to open for herself her way out of the pea, which is
now quite hard. The larva knows of this future helplessness and
provides against it with consummate art. With its strong jaws it bores
an exit-shaft, absolutely circular, with very clean-cut sides. Our best
ivory-carvers could produce nothing neater.
To prepare the door of escape in advance is not enough; we must also
think of the tranquillity essential to the delicate work of the
nymphosis. An intruder might enter through the open door and work
mischief upon the defenceless nymph. This opening must therefore be
kept shut. And how? Here is the device.
The grub boring the exit-hole eats the floury matter without leaving a
single crumb. On reaching the skin of the seed, suddenly it stops
short. This semitranslucent membrane is the screen protecting the
chamber in which the metamorphosis takes place, the door that defends
the cabin against ill-intentioned intruders. It is also the only
obstacle which the adult will encounter at the time of moving. To
lessen the difficulty of forcing it out, the grub takes the precaution
of carving a groove of least resistance inside the skin, all around the
circumference. The perfect insect will only have to heave with its
shoulders, to strike a blow or two with its head, in order to raise the
lid and knock it off, like the lid of a box. The exit-hole shows
through the transparent skin of the pea in the shape of a large
circular spot, darkened by the obscurity within. What happens below
cannot be seen, hidden as it is behind a sort of ground-glass window.
A pretty invention, this little port-hole, this barricade against the
invader, this trap-door lifted with a push of the hermit’s shoulder
when the time has come. Shall we give the Bruchus the credit of it?
Could the ingenious insect imagine the enterprise, ponder a plan and
work upon a scheme of its own devising? This would be a fine triumph
for the Weevil’s brain. Before deciding, let us hear what experiment
has to tell us.
I skin some inhabited peas; I save them from drying too quickly by
placing them in glass tubes. The grubs do as well here as in the intact
peas. The preparations for the deliverance are made at the proper time.
If the miner acts on its own inspiration, if it ceases to prolong its
shaft as soon as it perceives, by sounding it now and again, that the
ceiling is thin enough, what ought to happen under the present
conditions? Feeling that it is as near the surface as it wishes to be,
the grub will stop boring; it will respect the last layer of the bare
pea and will thus obtain the indispensable defensive screen.
Nothing of the kind takes place. The well is excavated entirely; its
mouth is open to the outside, as wide, as carefully finished as though
the skin of the pea were still protecting it. Reasons of safety have in
no way modified the usual work. The foe can enter this open lodging;
the grub gives the matter not a thought.
Nor has it this in mind when it refrains from boring right through the
pea still clad in its skin. It stops suddenly, because it does not like
the non-farinaceous skin. We remove the skins before making our peas
into soup: they have no culinary value; they are not good. The larva of
the Bruchus appears to be like ourselves: it hates the tough outside of
the pea. Warned by the unpleasant taste, it stops at the skin; and this
aversion causes a little miracle. The insect has no logical sense of
its own. It passively obeys a higher logic; it obeys, but is as
unconscious of its art as crystals are when assembling their battalions
of atoms in exquisite order.
Sooner or later, in August, dark circles form on the peas, always one
to each seed, with no exception. These mark the exit-hatches. Most of
them open in September. The lid, which looks as though cut out with a
punch, comes off very neatly and falls, leaving the opening of the cell
free. The Bruchus issues, freshly clad, in her final form.
The weather is delightful. Flowers abound, awakened by the showers; the
emigrants from the peas visit them in autumnal revelry. Then, when the
cold sets in, they take up their winter-quarters in some retreat or
other. Others, quite as numerous, are less eager to quit the native
seed. They stay there, motionless, all through the frosty season,
sheltered behind the trap which they are careful not to touch. The door
of the cell will not open on its hinges, that is to say, along its line
of least resistance, until the hot weather returns. Then the laggards
leave their homes and rejoin the more forward; and all are ready for
work when the peas come into flower.
The great attraction of the insect world for the observer is that he
can obtain a more or less general survey of the instincts, in their
inexhaustible variety; for nowhere do we see the wonderful order of
life’s details more clearly revealed. Entomology, I know, does not
appeal to everybody from this point of view: people have a poor opinion
of the artless person absorbed in the behaviour of insects. To the
terrible utilitarian, a measure of peas saved from the Weevil is of
more importance than any number of observations which bring no
immediate profit.
And who has told you, O man of little faith, that what is useless
to-day may not be useful to-morrow? If we learn the habits of animals,
we shall be better able to protect our property. Do not despise
disinterested ideas, lest you live to rue the day. It is by
accumulating ideas, whether immediately applicable or not, that mankind
has done and will continue to do better to-day than yesterday, better
in the future than in the present. If we live by peas and horse-beans,
which the Weevil disputes with us, we also live by knowledge, that
mighty kneading-trough in which the dough of progress is mixed and
fermented. Science is well worth a bean or two. Among other things, it
tells us:
‘The corn-chandler need not trouble to wage war upon the Weevil. By the
time that the peas are stored, the harm is done; it is irreparable, but
not transmissible. The untouched seeds have nothing to fear from the
proximity of the seeds attacked, however long they may remain together.
The Bruchus will issue from the latter when her time comes; she will
fly out of the granary, if escape be possible; if not, she will die
without in any way infesting the seeds that are still sound. No eggs,
no new generation will ever be seen on the dried peas in our
storehouse; nor will any damage be caused by the feeding of the adult.’
Our Bruchus is not a sedentary inhabitant of the granaries: she needs
the open air, the sunshine, the freedom of the fields. Very frugal on
her own behalf, she absolutely disdains the hardness of the legumen;
all that her slender snout requires is a few honeyed mouthfuls sipped
from the flowers. The larva, on the other hand, demands the soft bread
of the green pea still growing inside the pod. For these reasons, the
storehouse knows no further multiplication on the part of the ravager
introduced at the beginning.
The origin of the mischief lies out of doors. It is here more than
elsewhere that we ought to keep a watch on the Weevil’s misdeeds, were
it not that we are nearly always unarmed when it comes to fighting
against insects. Indestructible because of their numbers, their small
size, their sly cunning, the little creatures laugh at man’s anger. The
gardener fumes and curses; the Weevil remains unconcerned:
imperturbably she continues to levy her tithe.
Fortunately, we have assistants, more patient and more clear-sighted
than ourselves. In the first week of August, when the adult Bruchus is
beginning to move away, I make the acquaintance of a little Chalcis,
the protector of our peas. In my rearing-jars a number of her comes out
of the Weevil’s home before my eyes. The female has a red head and
thorax and a black abdomen, with a long boring-tool. The male, a little
smaller, is clad in black. Both sexes have dull-red legs and
thread-like antennæ.
In order to leave the pea, the exterminator of the Bruchus opens
herself a window in the centre of the disk which the Weevil’s grub has
bored in the skin with a view to its future deliverance. The devoured
has prepared the way out for the devourer. This detail enables us to
guess the rest.
When the preliminaries of the metamorphosis are finished, when the
exit-hole is bored, furnished with its lid, a surface cuticle, the
Chalcis comes bustling along. She inspects the peas, still on the
plant, in their pods; she tries them with her antennæ; she discovers,
hidden under the general outer wrapper of the pod, the weak points in
the ceiling formed by the skin. Then, raising her sounding-rod, she
thrusts it through the pod and pierces the thin lid. However deeply
secreted in the centre of the pea, the Weevil, whether larva or nymph,
is reached by the long implement. It receives an egg in its tender
flesh; and the trick is done. Without any chance of defence, for it is
by now either a torpid grub or else a nymph, the corpulent infant will
be drained to the skin.
What a pity that we are not able at will to promote the multiplication
of this zealous exterminator! Alas, our agricultural auxiliaries have
us in a disappointing vicious circle: if we wish to obtain the
assistance of large numbers of the Chalcids that bore holes in peas, we
must first have large numbers of Pea-weevils!