Social Life in the Insect World Part 22

But it has reached us without the company of its licensed consumer; for there must a.s.suredly be a weevil in its native country which levies tribute on its nourishing tissues. Our native bean-eaters have mistaken the stranger; they have not had time as yet to grow familiar with it, or to appreciate its merits; they have prudently abstained from touching the _ayacot_, whose novelty awoke suspicion. Until our own days the Mexican bean remained untouched: unlike our other leguminous seeds, which are all eagerly exploited by the weevil.

This state of affairs could not last. If our own fields do not contain the insect amateur of the haricot the New World knows it well enough. By the road of commercial exchange, sooner or later some worm-eaten sack of haricots must bring it to Europe. The invasion is inevitable.

According to doc.u.ments now before me, indeed, it has already taken place. Three or four years ago I received from Maillane, in the Bouches-du-Rhone, what I sought in vain in my own neighbourhood, although I questioned many a farmer and housewife, and astonished them by my questions. No one had ever seen the pest of the haricot; no one had ever heard of it. Friends who knew of my inquiries sent me from Maillane, as I have said, information that gave great satisfaction to my naturalist's curiosity. It was accompanied by a measure of haricots which were utterly and outrageously spoiled; every bean was riddled with holes, changed into a kind of sponge. Within them swarmed innumerable weevils, which recalled, by their diminutive size, the lentil-weevil, _Bruchus lenti_.

The senders told me of the loss experienced at Maillane. The odious little creature, they said, had destroyed the greater portion of the harvest. A veritable plague, such as had never before been known, had fallen upon the haricots, leaving the housewife barely a handful to put in the saucepan. Of the habits of the creature and its way of going to work nothing was known. It was for me to discover them by means of experiment.

Quick, then, let us experiment! The circ.u.mstances favour me. We are in the middle of June, and in my garden there is a bed of early haricots; the black Belgian haricots, sown for use in the kitchen. Since I must sacrifice the toothsome vegetable, let us loose the terrible destroyer on the ma.s.s of verdure. The development of the plant is at the requisite stage, if I may go by what the _Bruchus pisi_ has already taught me; the flowers are abundant, and the pods are equally so; still green, and of all sizes.

I place on a plate two or three handfuls of the infested haricots, and set the populous heap in the full sunlight by the edge of my bed of beans. I can imagine what will happen. Those insects which are already free, and those which the stimulus of the sunshine will presently liberate, will emerge and take to their wings. Finding the maternal haricot close at hand they will take possession of the vines. I shall see them exploring pods and flowers, and before very long they will lay their eggs. That is how the pea-weevil would behave under similar conditions.

But no: to my surprise and confusion, matters do not fall out as I foresaw. For a few minutes the insects bustle about in the sunlight, opening and closing their wing-covers to ease the mechanism of flight; then one by one they fly away, mounting in the luminous air; they grow smaller and smaller to the sight, and are quickly lost to view. My persevering attentions have not met with the slightest success; not one of the weevils has settled on my haricots.

When the joys of liberty have been tasted will they return--to-night, to-morrow, or later? No, they do not return. All that week, at favourable hours, I inspect the rows of beans pod by pod, flower by flower; but never a Bruchus do I see, nor even an egg. Yet the season is propitious, for at this very moment the mothers imprisoned in my jars lay a profusion of eggs upon the dry haricots.

Next season I try again. I have at my disposal two other beds, which I have sown with the late haricot, the red haricot; partly for the use of the household, but princ.i.p.ally for the benefit of the weevil. Arranged in convenient rows, the two crops will be ready, one in August and one in September or later.

With the red haricot I repeat the experiment already essayed with the black haricot. On several occasions, in suitable weather, I release large numbers of weevils from my gla.s.s jars, the general headquarters of the tribe. On each occasion the result is plainly negative. All through the season, until both crops are exhausted, I repeat my search almost daily; but I can never discover a single pod infested, nor even a single weevil perching on leaf or flower.

Certainly the inspection has not been at fault. The household is warned to respect certain rows of beans which I have reserved for myself. It is also requested to keep a look-out for eggs on all the pods gathered. I myself examine with a magnifying-gla.s.s all the haricots coming from my own or from neighbouring gardens before handing them over to the housewife to be sh.e.l.led. All my trouble is wasted: there is not an egg to be seen.

To these experiments in the open air I add others performed under gla.s.s.

I place, in some tall, narrow bottles, fresh haricot pods hanging from their stems; some green, others mottled with crimson, and containing seeds not far from mature. Each bottle is finally given a population of weevils. This time I obtain some eggs, but I am no further advanced; they are laid on the sides of the bottles, but not on the pods.

Nevertheless, they hatch. For a few days I see the grubs wandering about, exploring the pods and the gla.s.s with equal zeal. Finally one and all perish without touching the food provided.

The conclusion to be drawn from these facts is obvious: the young and tender haricot is not the proper diet. Unlike the _Bruchus pisi_, the female of the haricot-weevil refuses to trust her family to beans that are not hardened by age and desiccation; she refused to settle on my bean-patch because the food she required was not to be found there. What does she require? Evidently the mature, dry, hard haricot, which falls to earth with the sound of a small pebble. I hasten to satisfy her. I place in the bottles some very mature, h.o.r.n.y pods, thoroughly desiccated by exposure to the sun. This time the family prospers, the grubs perforate the dry sh.e.l.l, reach the beans, penetrate them, and henceforth all goes well.

To judge by appearances, then, the weevil invades the granary. The beans are left standing in the fields until both plants and pods, shrivelled by the sun, are completely desiccated. The process of beating the pods to loosen and separate the beans is thus greatly facilitated. It is then that the weevil, finding matters to suit her, commences to lay her eggs.

By storing his crop a little late the peasant stores the pest as well.

But the weevil more especially attacks the haricot when warehoused. Like the Calander-beetle, which nibbles the wheat in our granaries but despises the cereal while still on the stalk, it abhors the bean while tender, and prefers to establish itself in the peace and darkness of the storehouse. It is a formidable enemy to the merchant rather than to the peasant.

What a fury of destruction once the ravager is installed in the vegetable treasure-house! My bottles give abundant evidence of this. One single haricot bean shelters a numerous family; often as many as twenty members. And not one generation only exploits the bean, but three or four in the year. So long as the skin of the bean contains any edible matter, so long do new consumers establish themselves within it, so that the haricot finally becomes a mere sh.e.l.l stuffed with excreta. The skin, despised by the grubs, is a mere sac, pierced with holes as many as the inhabitants that have deserted it; the ruin is complete.

The _Bruchus pisi_, a solitary hermit, consumes only so much of the pea as will leave a cell for the nymph; the rest remains intact, so that the pea may be sown, or it will even serve as food, if we can overcome our repugnance. The American insect knows nothing of these limitations; it empties the haricot completely and leaves a skinful of filth that I have seen the pigs refuse. America is anything but considerate when she sends us her entomological pests. We owe the Phylloxera to America; the Phylloxera, that calamitous insect against which our vine-growers wage incessant war: and to-day she is sending us the haricot-weevil, which threatens to be a plague of the future. A few experiments gave me some idea of the peril of such an invasion.

For nearly three years there have stood, on my laboratory table, some dozens of jars and bottles covered with pieces of gauze which prevent escape while permitting of a constant ventilation. These are the cages of my menagerie. In them I rear the haricot-weevil, varying the system of education at will. Amongst other things I have learned that this insect, far from being exclusive in its choice, will accommodate itself to most of our leguminous foods.

All the haricots suit it, black and white, red and variegated, large and small; those of the latest crop and those which have been many years in stock and are almost completely refractory to boiling water. The loose beans are attacked by preference, as being easier to invade, but when the loose beans are not available those in the natural shelter of their pods are attacked with equal zest. However dry and parchment-like the pods, the grubs have no difficulty in attaining the seeds. When attacked in the field or garden, the bean is attacked in this way through the pod. The bean known in Provence as the blind haricot--_lou faiou borgne_--a bean with a long pod, which is marked with a black spot at the navel, which has the look of a closed and blackened eye, is also greatly appreciated; indeed, I fancy my little guests show an obvious preference for this particular bean.

So far, nothing abnormal; the Bruchus does not wander beyond the limits of the botanical family _Phaseolus_. But here is a characteristic that increases the peril, and shows us this lover of beans in an unexpected light. Without the slightest hesitation it accepts the dry pea, the bean, the vetch, the tare, and the chick-pea; it goes from one to the other, always satisfied; its offspring live and prosper in all these seeds as well as in the haricot. Only the lentil is refused, perhaps on account of its insufficient volume. The American weevil is a formidable experimentalist.

The peril would be much greater did the insect pa.s.s from leguminous seeds to cereals, as at first I feared it might. But it does not do so; imprisoned in my bottles together with a handful of wheat, barley, rice, or maize, the Bruchus invariably perished and left no offspring. The result was the same with oleaginous seeds: such as castor-oil and sunflower. Nothing outside the bean family is of any use to the Bruchus.

Thus limited, its portion is none the less considerable, and it uses and abuses it with the utmost energy. The eggs are white, slender, and cylindrical. There is no method in their distribution, no choice in their deposition. The mother lays them singly or in little groups, on the walls of the jar as well as on the haricots. In her negligence she will even lay them on maize, coffee, castor-oil seeds, and other seeds, on which the newly born grubs will promptly perish, not finding them to their taste. What place has maternal foresight here? Abandoned no matter where in the heap of seeds, the eggs are always in place, as it is left to the grub to search and to find the points of invasion.

In five days at most the egg is hatched. A little white creature with a red-brown head emerges. It is a mere speck of a creature, just visible to the naked eye. Its body is thickened forward, to give more strength to its implements--its mandibles--which have to perforate the hard substance of the dry bean, which is as tough as wood. The larvae of the Buprestis and the Capricornis, which burrow in the trunks of trees, are similarly shaped. Directly it issues from the egg the wriggling creature makes off at random with an activity we should hardly expect in one so young. It wanders. .h.i.ther and thither, eager to find food and shelter as soon as possible.

Within twenty-four hours it has usually attained both. I see the tiny grub perforate the h.o.r.n.y skin that covers the cotyledons; I watch its efforts; I surprise it sunk half-way in the commencement of a burrow, at the mouth of which is a white floury powder, the waste from the mandibles. It works its way inward and buries itself in the heart of the seed. It will emerge in the adult form in the course of about five weeks, so rapid is its evolution.

This hasty development allows of several generations in the year. I have recorded four. On the other hand, one isolated couple has furnished me with a family of eighty. Consider only the half of this number--supposing the s.e.xes to be equal in number--and at the end of a year the couples issued from this original pair would be represented by the fortieth power of forty; in larvae they would represent the frightful total of more than five millions. What a mountain of haricots would be ravaged by such a legion!

The industry of the larvae reminds us at every point what we have learned from the _Bruchus pisi_. Each grub excavates a lodging in the ma.s.s of the bean, respecting the epidermis, and preparing a circular trap-door which the adult can easily open with a push at the moment of emergence.

At the termination of the larval phase the lodgements are betrayed on the surface of the bean by so many shadowy circles. Finally the lid falls, the insect leaves its cell, and the haricot remains pierced by as many holes as it has nourished grubs.

Extremely frugal, satisfied with a little farinaceous powder, the adults seem by no means anxious to abandon the native heap or bin so long as there are beans untouched. They mate in the interstices of the heap; the mothers sow their eggs at random; the young larvae establish themselves some in beans that are so far intact, some in beans which are perforated but not yet exhausted; and all through the summer the operations of breeding are repeated once in every five weeks. The last generation of the year--that of September or October--sleeps in its cells until the warm weather returns.

If the haricot pest were ever to threaten us seriously it would not be very difficult to wage a war of extermination against it. Its habits teach us what tactics we ought to follow. It exploits the dried and gathered crop in the granary or the storehouse. If it is difficult to attack it in the open it would also be useless. The greater part of its affairs are managed elsewhere, in our storehouses. The enemy establishes itself under our roof and is ready to our hand. By means of insecticides defence should be relatively easy.

CHAPTER XX

THE GREY LOCUST

I have just witnessed a moving spectacle: the last moult of a locust; the emergence of the adult from its larval envelope. It was magnificent.

I am speaking of the Grey Locust, the colossus among our acridians,[10]

which is often seen among the vines in September when the grapes are gathered. By its size--and it grows as long as a man's finger--it lends itself to observation better than any other of its tribe.

The larva, disgustingly fat, like a rude sketch of the perfect insect, is commonly of a tender green; but it is sometimes of a bluish green, a dirty yellow, or a ruddy brown, or even an ashen grey, like the grey of the adult cricket. The corselet is strongly keeled and indented, and is sprinkled with fine white spots. As powerful as in the adult insect, the hind-leg has a corpulent haunch, streaked with red, and a long shin like a two-edged saw.

The elytra, which in a few days will extend far beyond the tip of the abdomen, are at present too small triangular wing-like appendages, touching along their upper edges, and continuing and emphasising the keel or ridge of the corselet. Their free ends stick up like the gable of a house. They remind one of the skirts of a coat, the maker of which has been ludicrously stingy with the cloth, as they merely cover the creature's nakedness at the small of the back. Underneath there are two narrow appendages, the germs of the wings, which are even smaller than the elytra. The sumptuous, elegant sails of to-morrow are now mere rags, so miserly in their dimensions as to be absolutely grotesque. What will emerge from these miserable coverings? A miracle of grace and amplitude.

Let us observe the whole process in detail. Feeling itself ripe for transformation, the insect climbs up the wire-gauze cover by means of its hinder and intermediate limbs. The fore-limbs are folded and crossed on the breast, and are not employed in supporting the insect, which hangs in a reversed position, the back downwards. The triangular winglets, the sheaths of the elytra, open along their line of juncture and separate laterally; the two narrow blades, which contain the wings, rise in the centre of the interval and slightly diverge. The proper position for the process of moulting has now been a.s.sumed and the proper stability a.s.sured.

The first thing to do is to burst the old skin. Behind the corselet, under the pointed roof of the prothorax, a series of pulsations is produced by alternate inflation and deflation. A similar state of affairs is visible in front of the neck, and probably under the entire surface of the yielding carapace. The fineness of the membrane at the articulations enables us to perceive it at these unarmoured points, but the cuira.s.s of the corselet conceals it in the central portion.

At these points the circulatory reserves of the insect are pulsing in tidal onsets. Their gradual increase is betrayed by pulsations like those of a hydraulic ram. Distended by this rush of humours, by this injection in which the organism concentrates all its forces, the outer skin finally splits along the line of least resistance which the subtle previsions of life have prepared. The fissure extends the whole length of the corselet, opening precisely along the ridge of the keel, as though the two symmetrical halves had been soldered together.

Unbreakable elsewhere, the envelope has yielded at this median point, which had remained weaker than the rest of the sheath. The fissure runs back a little way until it reaches a point between the attachments of the wings; on the head it runs forward as far as the base of the antennae, when it sends a short ramification right and left.

Through this breach the back is seen; quite soft, and very pale, with scarcely a tinge of grey. Slowly it curves upwards and becomes more and more strongly hunched; at last it is free.

The head follows, withdrawing itself from its mask, which remains in place, intact in the smallest detail, but looking very strange with its great unseeing gla.s.sy eyes. The sheaths of the antennae, without a wrinkle, without the least derangement, and in their natural place, hang over this dead, translucid face.

In emerging from their narrow sheaths, which clasped them so tightly and precisely, the thread-like antennae have evidently met with no resistance, or the sheaths would have been turned inside out, or crumpled out of shape, or wrinkled at least. Without harming the jointed or knotted covers, the contents, of equal volume and equally knotty, have slipped out as easily as though they were smooth, slippery objects sliding out of a loose sheath. The method of extraction is still more astonishing in the case of the hind-legs.

It is now, however, the turn of the front and intermediate pairs of legs. They pull out of their gauntlets and leggings without the least hitch; nothing is torn, nothing buckled; the outer skin is not even crumpled, and all the tissues remain in their natural position. The insect is now hanging from the dome of the cover solely by the claws of the long hind-legs. It hangs in an almost vertical position, the head downwards, swinging like a pendulum if I touch the cover. Four tiny, steely claws are its only support. If they gave or unclasped themselves the insect would be lost, as it is as yet unable to unfurl its enormous wings; but even had the wings emerged they could not grip the air in time to save the creature from the consequences of a fall. But the four claws hold fast; life, before withdrawing from them, left them rigidly contracted, so that they should support without yielding the struggles and withdrawals to follow.

Now the wing-covers and wings emerge. These are four narrow strips, vaguely seamed and furrowed, like strings of rolled tissue-paper. They are barely a quarter of their final length.

They are so soft that they bend under their own weight, and hang down the creature's sides in the reverse of their normal position. The free extremities, which normally point backwards, are now pointing towards the cricket's head as it hangs reversed. The organs of future flight are like four leaves of withered foliage shattered by a terrific rainstorm.

A profound transformation is necessary to bring the wings to their final perfection. The inner changes are already at work; liquids are solidifying; alb.u.minous secretions are bringing order out of chaos; but so far no outward sign betrays what is happening in the mysterious laboratory of the organism. All seems inert and lifeless.

In the meantime the posterior limbs disengage themselves. The great haunches become visible, streaked on the inner faces with a pale rose, which rapidly turns to a vivid crimson. Emergence is easy, the thick and muscular upper portion of the haunch preparing the way for the narrower part of the limb.

It is otherwise with the shank. This, in the adult insect, is armed along its whole length by a double series of stiff, steely spines.

Moreover, the lower extremity is terminated by four strong spurs. The shank forms a veritable saw, but with two parallel sets of teeth; and it is so strongly made that it may well be compared, the question of size apart, to the great saw of a quarry-man.