THE PEA-WEEVIL: THE LARVA

The Life of the Weevil by Jean-Henri Fabre and Alexander Teixeira de Mattos, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE PEA-WEEVIL: THE LARVA

Chapter XII. 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.[200]

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 [201]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 [202]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. [203]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, [204]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. [205]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 [206]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. [207]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 [208]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, [209]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 [210]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 [211]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 [212]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!

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