Social Life in the Insect World Part 4

THE CIGALE. THE EGGS AND THEIR HATCHING

The Cigale confides its eggs to dry, slender twigs. All the branches examined by Reaumur which bore such eggs were branches of the mulberry: a proof that the person entrusted with the search for these eggs in the neighbourhood of Avignon did not bring much variety to his quest. I find these eggs not only on the mulberry-tree, but on the peach, the cherry, the willow, the j.a.panese privet, and other trees. But these are exceptions; what the Cigale really prefers is a slender twig of a thickness varying from that of a straw to that of a pencil. It should have a thin woody layer and plenty of pith. If these conditions are fulfilled the species matters little. I should pa.s.s in review all the semi-ligneous plants of the country were I to catalogue the various supports which are utilised by the gravid female.

Its chosen twig never lies along the ground; it is always in a more or less vertical position. It is usually growing in its natural position, but is sometimes detached; in the latter case it will by chance have fallen so that it retains its upright position. The insect prefers a long, smooth, regular twig which can receive the whole of its eggs. The best batches of eggs which I have found have been laid upon twigs of the _Spartium junceum_, which are like straws stuffed with pith, and especially on the upper twigs of the _Asphodelus cerasiferus_, which rises nearly a yard from the ground before ramifying.

It is essential that the support, no matter what its nature, should be dead and perfectly dry.

The first operation performed by the Cigale consists in making a series of slight lacerations, such as one might make with the point of a pin, which, if plunged obliquely downwards into the twig, would tear the woody fibres and would compress them so as to form a slight protuberance.

If the twig is irregular in shape, or if several Cigales have been working successively at the same point, the distribution of the punctures is confused; the eye wanders, incapable of recognising the order of their succession or the work of the individual. One characteristic is always present, namely, the oblique direction of the woody fragment which is raised by the perforation, showing that the Cigale always works in an upright position and plunges its rostrum downwards in the direction of the twig.

If the twig is regular, smooth, and conveniently long the perforations are almost equidistant and lie very nearly in a straight line. Their number varies; it is small when the mother, disturbed in her operations, has flown away to continue her work elsewhere; but they number thirty or forty, more or less, when they contain the whole of her eggs.

Each one of the perforations is the entrance to an oblique tunnel, which is bored in the medullary sheath of the twig. The aperture is not closed, except by the bunch of woody fibres, which, parted at the moment when the eggs are laid, recover themselves when the double saw of the oviduct is removed. Sometimes, but by no means always, you may see between the fibres a tiny glistening patch like a touch of dried white of egg. This is only an insignificant trace of some alb.u.minous secretion accompanying the egg or facilitating the work of the double saw of the oviduct.

Immediately below the aperture of the perforation is the egg chamber: a short, tunnel-shaped cavity which occupies almost the whole distance between one opening and that lying below it. Sometimes the separating part.i.tion is lacking, and the various chambers run into one another, so that the eggs, although introduced by the various apertures, are arranged in an uninterrupted row. This arrangement, however, is not the most usual.

The contents of the chambers vary greatly. I find in each from six to fifteen eggs. The average is ten. The total number of chambers varying from thirty to forty, it follows that the Cigale lays from three to four hundred eggs. Reaumur arrived at the same figures from an examination of the ovaries.

This is truly a fine family, capable by sheer force of numbers of surviving the most serious dangers. I do not see that the adult Cigale is exposed to greater dangers than any other insect: its eye is vigilant, its departure sudden, and its flight rapid; and it inhabits heights at which the prowling brigands of the turf are not to be feared.

The sparrow, it is true, will greedily devour it. From time to time he will deliberately and meditatively descend upon the plane-trees from the neighbouring roof and s.n.a.t.c.h up the singer, who squeaks despairingly. A few blows of the beak and the Cigale is cut into quarters, delicious morsels for the nestlings. But how often does the bird return without his prey! The Cigale, foreseeing his attack, empties its intestine in the eyes of its a.s.sailant and flies away.

But the Cigale has a far more terrible enemy than the sparrow. This is the green gra.s.shopper. It is late, and the Cigales are silent. Drowsy with light and heat, they have exhausted themselves in producing their symphonies all day long. Night has come, and with it repose; but a repose frequently troubled. In the thick foliage of the plane-trees there is a sudden sound like a cry of anguish, short and strident. It is the despairing lamentation of the Cigale surprised in the silence by the gra.s.shopper, that ardent hunter of the night, which leaps upon the Cigale, seizes it by the flank, tears it open, and devours the contents of the stomach. After the orgy of music comes night and a.s.sa.s.sination.

I obtained an insight into this tragedy in the following manner: I was walking up and down before my door at daybreak when something fell from the neighbouring plane-tree uttering shrill squeaks. I ran to see what it was. I found a green gra.s.shopper eviscerating a struggling Cigale. In vain did the latter squeak and gesticulate; the other never loosed its hold, but plunged its head into the entrails of the victim and removed them by little mouthfuls.

[Ill.u.s.tration: 1. THE CIGALE LAYING HER EGGS.

2. THE GREEN GRa.s.sHOPPER, THE FALSE CIGALE OF THE NORTH, DEVOURING THE TRUE CIGALE, A DWELLER IN THE SOUTH.]

This was instructive. The attack was delivered high up above my head, in the early morning, while the Cigale was resting; and the struggles of the unfortunate creature as it was dissected alive had resulted in the fall of a.s.sailant and a.s.sailed together. Since then I have often been the witness of similar a.s.sa.s.sinations.

I have even seen the gra.s.shopper, full of audacity, launch itself in pursuit of the Cigale, who fled in terror. So the sparrow-hawk pursues the skylark in the open sky. But the bird of prey is less ferocious than the insect; it pursues a creature smaller than itself. The locust, on the contrary, a.s.sails a colossus, far larger and far more vigorous than its enemy; yet the result is a foregone conclusion, in spite of this disproportion. With its powerful mandibles, like pincers of steel, the gra.s.shopper rarely fails to eviscerate its captive, which, being weaponless, can only shriek and struggle.

The Cigale is an easy prey during its hours of somnolence. Every Cigale encountered by the ferocious gra.s.shopper on its nocturnal round must miserably perish. Thus are explained those sudden squeaks of anguish which are sometimes heard in the boughs during the hours of the night and early morning, although the cymbals have long been silent. The sea-green bandit has fallen upon some slumbering Cigale. When I wished to rear some green gra.s.shoppers I had not far to seek for the diet of my pensioners; I fed them on Cigales, of which enormous numbers were consumed in my breeding-cages. It is therefore an established fact that the green gra.s.shopper, the false Cigale of the North, will eagerly devour the true Cigale, the inhabitant of the Midi.

But it is neither the sparrow nor the green gra.s.shopper that has forced the Cigale to produce such a vast number of offspring. The real danger is elsewhere, as we shall see. The risk is enormous at the moment of hatching and also when the egg is laid.

Two or three weeks after its escape from the earth--that is, about the middle of July--the Cigale begins to lay. In order to observe the process without trusting too much to chance, I took certain precautions which would, I felt sure, prove successful. The dry Asphodelus is the support preferred by the insect, as previous observations had a.s.sured me. It was also the plant which best lent itself to my experiments, on account of its long, smooth stems. Now, during the first years of my residence in the South I replaced the thistles in my paddock by other native plants of a less stubborn and p.r.i.c.kly species. Among the new occupants was the asphodel. This was precisely what I needed for my experiments. I left the dry stems of the preceding year in place, and when the breeding season arrived I inspected them daily.

I had not long to wait. As early as July 15th I found as many Cigales as I could wish on the stems of the asphodel, all in process of laying. The gravid female is always solitary. Each mother has her twig to herself, and is in no danger of being disturbed during the delicate operation of laying. When the first occupant has departed another may take her place, and so on indefinitely. There is abundance of room for all; but each prefers to be alone as her turn arrives. There is, however, no unpleasantness of any kind; everything pa.s.ses most peacefully. If a female Cigale finds a place which has been already taken she flies away and seeks another twig directly she discovers her mistake.

The gravid female always retains an upright position at this time, as indeed she does at other times. She is so absorbed in her task that she may readily be watched, even through a magnifying gla.s.s. The ovipositor, which is about four-tenths of an inch in length, is plunged obliquely and up to the hilt into the twig. So perfect is the tool that the operation is by no means troublesome. We see the Cigale tremble slightly, dilating and contracting the extremity of the abdomen in frequent palpitations. This is all that can be seen. The boring instrument, consisting of a double saw, alternately rises and sinks in the rind of the twig with a gentle, almost imperceptible movement.

Nothing in particular occurs during the process of laying the eggs. The insect is motionless, and hardly ten minutes elapse between the first cut of the ovipositor and the filling of the egg-chamber with eggs.

The ovipositor is then withdrawn with methodical deliberation, in order that it may not be strained or bent. The egg-chamber closes of its own accord as the woody fibres which have been displaced return to their position, and the Cigale climbs a little higher, moving upwards in a straight line, by about the length of its ovipositor. It then makes another puncture and a fresh chamber for another ten or twelve eggs. In this way it scales the twig from bottom to top.

These facts being understood, we are able to explain the remarkable arrangement of the eggs. The openings in the rind of the twig are practically equidistant, since each time the Cigale moves upward it is by a given length, namely, that of the ovipositor. Very rapid in flight, she is a very idle walker. At the most you may see her, on the living twig from which she is drinking, moving at a slow, almost solemn pace, to gain a more sunny point close at hand. On the dry twig in which she deposits her eggs she observes the same formal habits, and even exaggerates them, in view of the importance of the operation. She moves as little as possible, just so far as she must in order to avoid running two adjacent egg-chambers into one. The extent of each movement upwards is approximately determined by the depth of the perforation.

The apertures are arranged in a straight line when their number is not very large. Why, indeed, should the insect wander to right or to left upon a twig which presents the same surface all over? A lover of the sun, she chooses that side of the twig which is most exposed to it. So long as she feels the heat, her supreme joy, upon her back, she will take good care not to change the position which she finds so delightful for another in which the sun would fall upon her less directly.

The process of depositing the eggs is a lengthy one when it is carried out entirely on the same twig. Counting ten minutes for each egg-chamber, the full series of forty would represent a period of six or seven hours. The sun will of course move through a considerable distance before the Cigale can finish her work. In such cases the series of apertures follows a spiral curve. The insect turns round the stalk as the sun turns.

Very often as the Cigale is absorbed in her maternal task a diminutive fly, also full of eggs, busily exterminates the Cigale's eggs as fast as they are laid.

This insect was known to Reaumur. In nearly all the twigs examined he found its grub, the cause of a misunderstanding at the beginning of his researches. But he did not, could not see the audacious insect at work.

It is one of the Chalcididae, about one-fifth or one-sixth of an inch in length; entirely black, with knotty antennae, which are slightly thicker towards their extremities. The unsheathed ovipositor is implanted in the under portion of the abdomen, about the middle, and at right angles to the axis of the body, as in the case of the Leucospis, the pest of the apiary. Not having taken the precaution to capture it, I do not know what name the entomologists have bestowed upon it, or even if this dwarf exterminator of the Cigale has as yet been catalogued. What I am familiar with is its calm temerity, its impudent audacity in the presence of the colossus who could crush it with a foot. I have seen as many as three at once exploiting the unfortunate female. They keep close behind the Cigale, working busily with their probes, or waiting until their victim deposits her eggs.

The Cigale fills one of her egg-chambers and climbs a little higher in order to bore another hole. One of the bandits runs to the abandoned station, and there, almost under the claws of the giant, and without the least nervousness, as if it were accomplishing some meritorious action, it unsheathes its probe and thrusts it into the column of eggs, not by the open aperture, which is bristling with broken fibres, but by a lateral fissure. The probes works slowly, as the wood is almost intact.

The Cigale has time to fill the adjacent chamber.

As soon as she has finished one of these midges, the very same that has been performing its task below her, replaces her and introduces its disastrous egg. By the time the Cigale departs, her ovaries empty, the majority of the egg-chambers have thus received the alien egg which will work the destruction of their contents. A small, quick-hatching grub, richly nourished on a dozen eggs, will replace the family of the Cigale.

The experience of centuries has taught the Cigale nothing. With her excellent eyesight she must be able to perceive these terrible sappers as they hover about her, meditating their crime. Too peaceable giantess!

if you see them why do you not seize them in your talons, crush the pigmies at their work, so that you may proceed with your travail in security? But no, you will leave them untouched; you cannot modify your instincts, even to alleviate your maternal misfortunes.

The eggs of the common Cigale are of a shining ivory white. Conical at the ends, and elongated in form, they might be compared in shape to the weaver's shuttle. Their length is about one-tenth of an inch, their diameter about one-fiftieth. They are packed in a row, slightly overlapping one another. The eggs of the Cacan are slightly smaller, and are a.s.sembled in regular groups which remind one of microscopical bundles of cigars. We will consider the eggs of the common Cigale to the exclusion of the others, as their history is the history of all.

September is not yet over when the shining white as of ivory gives way to the yellow hue of cheese. During the first days of October you may see, at the forward end of the egg, two tiny points of chestnut brown, which are the eyes of the embryo in formation. These two shining eyes, which almost seem to gaze at one, and the cone-shaped head of the egg, give it the look of a tiny fish without fins--a fish for whom half a nut-sh.e.l.l would make a capacious aquarium.

About the same time I notice frequently, on the asphodels in the paddock and on those of the neighbouring hills, certain indications that the eggs have recently hatched out. There are certain cast-off articles of clothing, certain rags and tatters, left on the threshold of the egg-chamber by the new-born grubs as they leave it and hurry in search of a new lodging. We shall see in a moment what these vestiges mean.

But in spite of my visits, which were so a.s.siduous as to deserve success, I had never contrived to see the young Cigales emerge from their egg-chambers. My domestic researches had been pursued in vain. Two years running I had collected, in boxes, tubes, and bottles, a hundred twigs of every kind which were peopled by the eggs of the Cigale; but not one had shown me what I so desired to witness: the issue of the new-born Cigales.

Reaumur experienced the same disappointment. He tells us how all the eggs supplied by his friends were abortive, even when he placed them in a gla.s.s tube thrust under his armpit, in order to keep them at a high temperature. No, venerable master! neither the temperate shelter of our studies and laboratories, nor the incubating warmth of our bodies is sufficient here; we need the supreme stimulant, the kiss of the sun; after the cool of the mornings, which are already sharp, the sudden blaze of the superb autumn weather, the last endearments of summer.

It was under such circ.u.mstances, when a blazing sun followed a cold night, that I found the signs of completed incubation; but I always came too late; the young Cigales had departed. At most I sometimes found one hanging by a thread to its natal stem and struggling in the air. I supposed it to be caught in a thread of gossamer, or some shred of cobweb.

At last, on the 27th of October, despairing of success, I gathered some asphodels from the orchard, and the armful of dry twigs in which the Cigales had laid their eggs was taken up to my study. Before giving up all hope I proposed once more to examine the egg-chambers and their contents. The morning was cold, and the first fire of the season had been lit in my room. I placed my little bundle on a chair before the fire, but without any intention of testing the effect of the heat of the flames upon the concealed eggs. The twigs, which I was about to cut open, one by one, were placed there to be within easy reach of my hand, and for no other reason.

Then, while I was examining a split twig with my magnifying-gla.s.s, the phenomenon which I had given up all hope of observing took place under my eyes. My bundle of twigs was suddenly alive; scores and scores of the young larvae were emerging from their egg-chambers. Their numbers were such that my ambition as observer was amply satisfied. The eggs were ripe, on the point of hatching, and the warmth of the fire, bright and penetrating, had the effect of sunlight in the open. I was quick to profit by the unexpected piece of good fortune.

At the orifice of the egg-chamber, among the torn fibres of the bark, a little cone-shaped body is visible, with two black eye-spots; in appearance it is precisely like the fore portion of the b.u.t.ter-coloured egg; or, as I have said, like the fore portion of a tiny fish. You would think that an egg had been somehow displaced, had been removed from the bottom of the chamber to its aperture. An egg to move in this narrow pa.s.sage! a walking egg! No, that is impossible; eggs "do not do such things!" This is some mistake. We will break open the twig, and the mystery is unveiled. The actual eggs are where they always were, though they are slightly disarranged. They are empty, reduced to the condition of transparent skins, split wide open at the upper end. From them has issued the singular organism whose most notable characteristics are as follows:--

In its general form, the configuration of the head and the great black eyes, the creature, still more than the egg, has the appearance of an extremely minute fish. A simulacrum of a ventral fin increases the resemblance. This apparent fin in reality consists of the two fore-limbs, which, packed in a special sheath, are bent backwards, stretched out against one another in a straight line. Its small degree of mobility must enable the grub to escape from the egg-sh.e.l.l and, with greater difficulty, from the woody tunnel leading to the open air.

Moving outwards a little from the body, and then moving back again, this lever serves as a means of progression, its terminal hooks being already fairly strong. The four other feet are still covered by the common envelope, and are absolutely inert. It is the same with the antennae, which can scarcely be seen through the magnifying-gla.s.s. The organism which has issued from the egg is a boat-shaped body with a fin-shaped limb pointing backwards on the ventral face, formed by the junction of the two fore-limbs. The segmentation of the body is very clear, especially on the abdomen. The whole body is perfectly smooth, without the least suspicion of hair.

What name are we to give to this initial phase of the Cigale--a phase so strange, so unforeseen, and hitherto unsuspected? Must I amalgamate some more or less appropriate words of Greek and fabricate a portentous nomenclature? No, for I feel sure that barbarous alien phrases are only a hindrance to science. I will call it simply the _primary larva_, as I have done in the case of the Meloides, the Leucospis, and the Anthrax.

The form of the primary larva of the Cigale is eminently adapted to its conditions and facilitates its escape. The tunnel in which the egg is hatched is very narrow, leaving only just room for pa.s.sage. Moreover, the eggs are arranged in a row, not end to end, but partially overlapping. The larva escaping from the hinder ranks has to squeeze past the empty sh.e.l.ls, still in position, of the eggs which have already hatched, so that the narrowness of the pa.s.sage is increased by the empty egg-sh.e.l.ls. Under these conditions the larva as it will be presently, when it has torn its temporary wrappings, would be unable to effect the difficult pa.s.sage. With the enc.u.mbrance of antennae, with long limbs spreading far out from the axis of the body, with curved, pointed talons which hook themselves into their medium of support, everything would militate against a prompt liberation. The eggs in one chamber hatch almost simultaneously. It is therefore essential that the first-born larvae should hurry out of their shelter as quickly as possible, leaving the pa.s.sage free for those behind them. Hence the boat-like shape, the smooth hairless body without projections, which easily squeezes its way past obstructions. The primary larva, with its various appendages closely wrapped against its body by a common sheath, with its fish-like form and its single and only partially movable limb, is perfectly adapted to make the difficult pa.s.sage to the outer air.

This phase is of short duration. Here, for instance, a migrating larva shows its head, with its big black eyes, and raises the broken fibres of the entrance. It gradually works itself forward, but so slowly that the magnifying-gla.s.s scarcely reveals its progress. At the end of half an hour at the shortest we see the entire body of the creature; but the orifice by which it is escaping still holds it by the hinder end of the body.

Then, without further delay, the coat which it wears for this rough piece of work begins to split, and the larva skins itself, coming out of its wrappings head first. It is then the normal larva; the only form known to Reaumur. The rejected coat forms a suspensory thread, expanding at its free end to form a little cup. In this cup is inserted the end of the abdomen of the larva, which, before allowing itself to fall to earth, takes a sun-bath, grows harder, stretches itself, and tries its strength, lightly swinging at the end of its life-line.

This little flea, as Reaumur calls it, first white, then amber-coloured, is precisely the larva which will delve in the earth. The antennae, of fair length, are free and waving to and fro; the limbs are bending at their articulations; the fore-limbs, which are relatively powerful, open and shut their talons. I can scarcely think of any more curious spectacle than that of this tiny gymnast hanging by its tail, swinging to the faintest breath, and preparing in the air for its entry into the world. It hangs there for a variable period; some larvae let themselves fall at the end of half an hour; others spend hours in their long-stemmed cup; some even remain suspended until the following day.

Whether soon or late, the fall of the larva leaves suspended the thread by which it hung, the wrappings of the primary larva. When all the brood have disappeared, the aperture of the nest is thus hung with a branch of fine, short threads, twisted and knotted together, like dried white of egg. Each thread is expanded into a tiny cup at its free end. These are very delicate and ephemeral relics, which perish at a touch. The least wind quickly blows them away.