Читать книгу Social Life in the Insect World - Fabre Jean-Henri - Страница 11

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Where I live I can capture five species of Cigale, the two principal species being the common Cigale and the variety which lives on the flowering ash. Both of these are widely distributed and are the only species known to the country folk. The larger of the two is the common Cigale. Let me briefly describe the mechanism with which it produces its familiar note.

On the under side of the body of the male, immediately behind the posterior limbs, are two wide semicircular plates which slightly overlap one another, the right hand lying over the left hand plate. These are the shutters, the lids, the dampers of the musical-box. Let us remove them. To the right and left lie two spacious cavities which are known in Provençal as the chapels (li capello). Together they form the church (la glèiso). Their forward limit is formed by a creamy yellow membrane, soft and thin; the hinder limit by a dry membrane coloured like a soap bubble and known in Provençal as the mirror (mirau).

The church, the mirrors, and the dampers are commonly regarded as the organs which produce the cry of the Cigale. Of a singer out of breath one says that he has broken his mirrors (a li mirau creba). The same phrase is used of a poet without inspiration. Acoustics give the lie to the popular belief. You may break the mirrors, remove the covers with a snip of the scissors, and tear the yellow anterior membrane, but these mutilations do not silence the song of the Cigale; they merely change its quality and weaken it. The chapels are resonators; they do not produce the sound, but merely reinforce it by the vibration of their anterior and posterior membranes; while the sound is modified by the dampers as they are opened more or less widely.

The actual source of the sound is elsewhere, and is somewhat difficult for a novice to find. On the outer wall of either chapel, at the ridge formed by the junction of back and belly, is a tiny aperture with a horny circumference masked by the overlapping damper. We will call this the window. This opening gives access to a cavity or sound-chamber, deeper than the "chapels," but of much smaller capacity. Immediately behind the attachment of the posterior wings is a slight protuberance, almost egg-shaped, which is distinguishable, on account of its dull black colour, from the neighbouring integuments, which are covered with a silvery down. This protuberance is the outer wall of the sound-chamber.

Let us cut it boldly away. We shall then lay bare the mechanism which produces the sound, the cymbal. This is a small dry, white membrane, oval in shape, convex on the outer side, and crossed along its larger diameter by a bundle of three or four brown nervures, which give it elasticity. Its entire circumference is rigidly fixed. Let us suppose that this convex scale is pulled out of shape from the interior, so that it is slightly flattened and as quickly released; it will immediately regain its original convexity owing to the elasticity of the nervures. From this oscillation a ticking sound will result.

Twenty years ago all Paris was buying a silly toy, called, I think, the cricket or cri-cri. It was a short slip of steel fixed by one end to a metallic base. Pressed out of shape by the thumb and released, it yielded a very distressing, tinkling click. Nothing else was needed to take the popular mind by storm. The "cricket" had its day of glory. Oblivion has executed justice upon it so effectually that I fear I shall not be understood when I recall this celebrated device.

The membranous cymbal and the steel cricket are analogous instruments. Both produce a sound by reason of the rapid deformation and recovery of an elastic substance—in one case a convex membrane; in the other a slip of steel. The "cricket" was bent out of shape by the thumb. How is the convexity of the cymbals altered? Let us return to the "church" and break down the yellow curtain which closes the front of each chapel. Two thick muscular pillars are visible, of a pale orange colour; they join at an angle, forming a V, of which the point lies on the median line of the insect, against the lower face of the thorax. Each of these pillars of flesh terminates suddenly at its upper extremity, as though cut short, and from the truncated portion rises a short, slender tendon, which is attached laterally to the corresponding cymbal.

There is the whole mechanism, no less simple than that of the steel "cricket." The two muscular columns contract and relax, shorten and lengthen. By means of its terminal thread each sounds its cymbal, by depressing it and immediately releasing it, when its own elasticity makes it spring back into shape. These two vibrating scales are the source of the Cigale's cry.

Do you wish to convince yourself of the efficiency of this mechanism? Take a Cigale but newly dead and make it sing. Nothing is simpler. Seize one of these muscular columns with the forceps and pull it in a series of careful jerks. The extinct cri-cri comes to life again; at each jerk there is a clash of the cymbal. The sound is feeble, to be sure, deprived of the amplitude which the living performer is able to give it by means of his resonating chambers; none the less, the fundamental element of the song is produced by this anatomist's trick.

Would you, on the other hand, silence a living Cigale?—that obstinate melomaniac, who, seized in the fingers, deplores his misfortune as loquaciously as ever he sang the joys of freedom in his tree? It is useless to violate his chapels, to break his mirrors; the atrocious mutilation would not quiet him. But introduce a needle by the lateral aperture which we have named the "window" and prick the cymbal at the bottom of the sound-box. A little touch and the perforated cymbal is silent. A similar operation on the other side of the insect and the insect is dumb, though otherwise as vigorous as before and without any perceptible wound. Any one not in the secret would be amazed at the result of my pin-prick, when the destruction of the mirrors and the other dependencies of the "church" do not cause silence. A tiny perforation of no importance to the insect is more effectual than evisceration.

The dampers, which are rigid and solidly built, are motionless. It is the abdomen itself which, by rising and falling, opens or closes the doors of the "church." When the abdomen is lowered the dampers exactly cover the chapels as well as the windows of the sound-boxes. The sound is then muted, muffled, diminished. When the abdomen rises the chapels are open, the windows unobstructed, and the sound acquires its full volume. The rapid oscillations of the abdomen, synchronising with the contractions of the motor muscles of the cymbals, determine the changing volume of the sound, which seems to be caused by rapidly repeated strokes of a fiddlestick.

If the weather is calm and hot, towards mid-day the song of the Cigale is divided into strophes of several seconds' duration, which are separated by brief intervals of silence. The strophe begins suddenly. In a rapid crescendo, the abdomen oscillating with increasing rapidity, it acquires its maximum volume; it remains for a few seconds at the same degree of intensity, then becomes weaker by degrees, and degenerates into a shake, which decreases as the abdomen returns to rest. With the last pulsations of the belly comes silence; the length of the silent interval varies according to the state of the atmosphere. Then, of a sudden, begins a new strophe, a monotonous repetition of the first; and so on indefinitely.

It often happens, especially during the hours of the sultry afternoons, that the insect, intoxicated with sunlight, shortens and even suppresses the intervals of silence. The song is then continuous, but always with an alternation of crescendo and diminuendo. The first notes are heard about seven or eight o'clock in the morning, and the orchestra ceases only when the twilight fails, about eight o'clock at night. The concert lasts a whole round of the clock. But if the sky is grey and the wind chilly the Cigale is silent.

The second species, only half the size of the common Cigale, is known in Provence as the Cacan; the name, being a fairly exact imitation of the sound emitted by the insect. This is the Cigale of the flowering ash, far more alert and far more suspicious than the common species. Its harsh, loud song consists of a series of cries—can! can! can! can!—with no intervals of silence subdividing the poem into stanzas. Thanks to its monotony and its harsh shrillness, it is a most odious sound, especially when the orchestra consists of hundreds of performers, as is often the case in my two plane-trees during the dog-days. It is as though a heap of dry walnuts were being shaken up in a bag until the shells broke. This painful concert, which is a real torment, offers only one compensation: the Cigale of the flowering ash does not begin his song so early as the common Cigale, and does not sing so late in the evening.

Although constructed on the same fundamental principles, the vocal organs exhibit a number of peculiarities which give the song its special character. The sound-box is lacking, which suppresses the entrance to it, or the window. The cymbal is uncovered, and is visible just behind the attachment of the hinder wing. It is, as before, a dry white scale, convex on the outside, and crossed by a bundle of fine reddish-brown nervures.

1. THE ADULT CIGALE, FROM BELOW. 2. THE ADULT CIGALE, FROM BELOW. 3. THE CIGALE OF THE FLOWERING ASH, MALE AND FEMALE.

From the forward side of the first segment of the abdomen project two short, wide, tongue-shaped projections, the free extremities of which rest on the cymbals. These tongues may be compared to the blade of a watchman's rattle, only instead of engaging with the teeth of a rotating wheel they touch the nervures of the vibrating cymbal. From this fact, I imagine, results the harsh, grating quality of the cry. It is hardly possible to verify the fact by holding the insect in the fingers; the terrified Cacan does not go on singing his usual song.

The dampers do not overlap; on the contrary, they are separated by a fairly wide interval. With the rigid tongues, appendages of the abdomen, they half shelter the cymbals, half of which is completely bare. Under the pressure of the finger the abdomen opens a little at its articulation with the thorax. But the insect is motionless when it sings; there is nothing of the rapid vibrations of the belly which modulate the song of the common Cigale. The chapels are very small; almost negligible as resonators. There are mirrors, as in the common Cigale, but they are very small; scarcely a twenty-fifth of an inch in diameter. In short, the resonating mechanism, so highly developed in the common Cigale, is here extremely rudimentary. How then is the feeble vibration of the cymbals re-enforced until it becomes intolerable?

This species of Cigale is a ventriloquist. If we examine the abdomen by transmitted light, we shall see that the anterior two-thirds of the abdomen are translucent. With a snip of the scissors we will cut off the posterior third, to which are relegated, reduced to the strictly indispensable, the organs necessary to the propagation of the species and the preservation of the individual. The rest of the abdomen presents a spacious cavity, and consists simply of the integuments of the walls, except on the dorsal side, which is lined with a thin muscular layer, and supports a fine digestive canal, almost a thread. This large cavity, equal to nearly half the total volume of the insect, is thus almost absolutely empty. At the back are seen the two motor muscles of the cymbals, two muscular columns arranged like the limbs of a V. To right and left of the point of this V shine the tiny mirrors; and between the two branches of muscle the empty cavity is prolonged into the depths of the thorax.

This empty abdomen with its thoracic annex forms an enormous resonator, such as no other performer in our countryside can boast of. If I close with my finger the orifice of the truncated abdomen the sound becomes flatter, in conformity with the laws affecting musical resonators; if I fit into the aperture of the open body a tube or trumpet of paper the sound grows louder as well as deeper. With a paper cone corresponding to the pitch of the note, with its large end held in the mouth of a test-tube acting as a resonator, we have no longer the cry of the Cigale, but almost the bellowing of a bull. My little children, coming up to me by chance at the moment of this acoustic experiment, fled in terror.

The grating quality of the sound appears to be due to the little tongues which press on the nervures of the vibrating cymbals; the cause of its intensity is of course the ample resonator in the abdomen. We must admit that one must truly have a real passion for song before one would empty one's chest and stomach in order to make room for a musical-box. The necessary vital organs are extremely small, confined to a mere corner of the body, in order to increase the amplitude of the resonating cavity. Song comes first of all; other matters take the second rank.

It is lucky that the Cacan does not follow the laws of evolution. If, more enthusiastic in each generation, it could acquire, in the course of progress, a ventral resonator comparable to my paper trumpets, the South of France would sooner or later become uninhabitable, and the Cacan would have Provence to itself.

After the details already given concerning the common Cigale it is hardly needful to tell you how the insupportable Cacan can be reduced to silence. The cymbals are plainly visible on the exterior. Pierce them with the point of a needle, and immediately you have perfect silence. If only there were, in my plane-trees, among the insects which carry gimlets, some friends of silence like myself, who would devote themselves to such a task! But no: a note would be lacking in the majestic symphony of harvest-tide.

We are now familiar with the structure of the musical organ of the Cigale. Now the question arises: What is the object of these musical orgies? The reply seems obvious: they are the call of the males inviting their mates; they constitute a lovers' cantata.

I am going to consider this reply, which is certainly a very natural one. For thirty years the common Cigale and his unmusical friend the Cacan have thrust their society upon me. For two months every summer I have them under my eyes, and their voice in my ears. If I do not listen to them very willingly I observe them with considerable zeal. I see them ranged in rows on the smooth rind of the plane-trees, all with their heads uppermost, the two sexes mingled, and only a few inches apart.

The proboscis thrust into the bark, they drink, motionless. As the sun moves, and with it the shadow, they also move round the branch with slow lateral steps, so as to keep upon that side which is most brilliantly illuminated, most fiercely heated. Whether the proboscis is at work or not the song is never interrupted.

Now are we to take their interminable chant for a passionate love-song? I hesitate. In this gathering the two sexes are side by side. One does not spend months in calling a person who is at one's elbow. Moreover, I have never seen a female rush into the midst of even the most deafening orchestra. Sight is a sufficient prelude to marriage, for their sight is excellent. There is no need for the lover to make an everlasting declaration, for his mistress is his next-door neighbour.

Is the song a means of charming, of touching the hard of heart? I doubt it. I observe no sign of satisfaction in the females; I have never seen them tremble or sway upon their feet, though their lovers have clashed their cymbals with the most deafening vigour.

My neighbours the peasants say that at harvest-time the Cigale sings to them: Sego, sego, sego! (Reap, reap, reap!) to encourage them in their work. Harvesters of ideas and of ears of grain, we follow the same calling; the latter produce food for the stomach, the former food for the mind. Thus I understand their explanation and welcome it as an example of gracious simplicity.

Science asks for a better explanation, but finds in the insect a world which is closed to us. There is no possibility of foreseeing, or even of suggesting the impression produced by this clashing of cymbals upon those who inspire it. The most I can say is that their impassive exterior seems to denote a complete indifference. I do not insist that this is so; the intimate feelings of the insect are an insoluble mystery.

Another reason for doubt is this: all creatures affected by song have acute hearing, and this sense of hearing, a vigilant sentinel, should give warning of danger at the slightest sound. The birds have an exquisite delicacy of hearing. If a leaf stirs among the branches, if two passers-by exchange a word, they are suddenly silent, anxious, and on their guard. But the Cigale is far from sharing in such emotions. It has excellent sight. Its great faceted eyes inform it of all that happens to right and left; its three stemmata, like little ruby telescopes, explore the sky above its head. If it sees us coming it is silent at once, and flies away. But let us get behind the branch on which it is singing; let us manœuvre so as to avoid the five centres of vision, and then let us speak, whistle, clap the hands, beat two stones together. For far less a bird which could not see you would stop its song and fly away terrified. The Cigale imperturbably continues to sing as if nothing had occurred.

Of my experiences of this kind I will mention only one, the most remarkable of many.

I borrowed the municipal artillery; that is, the iron boxes which are charged with gunpowder on the day of the patron saint. The artilleryman was delighted to load them for the benefit of the Cigales, and to fire them off for me before my house. There were two of these boxes stuffed full of powder as though for the most solemn rejoicing. Never was politician making his electoral progress favoured with a bigger charge. To prevent damage to my windows the sashes were all left open. The two engines of detonation were placed at the foot of the plane-trees before my door, no precautions being taken to mask them. The Cigales singing in the branches above could not see what was happening below.

There were six of us, spectators and auditors. We waited for a moment of relative quiet. The number of singers was counted by each of us, as well as the volume and rhythm of the song. We stood ready, our ears attentive to the aerial orchestra. The box exploded with a clap of thunder.

No disturbance ensued above. The number of performers was the same, the rhythm the same, the volume the same. The six witnesses were unanimous: the loud explosion had not modified the song of the Cigales in the least. The second box gave an identical result.

What are we to conclude from this persistence of the orchestra, its lack of surprise or alarm at the firing of a charge? Shall we conclude that the Cigale is deaf? I am not going to venture so far as that; but if any one bolder than myself were to make the assertion I really do not know what reasons I could invoke to disprove it. I should at least be forced to admit that it is very hard of hearing, and that we may well apply to it the homely and familiar phrase: to shout like a deaf man.

When the blue-winged cricket, basking on the pebbles of some country footpath, grows deliciously intoxicated with the heat of the sun and rubs its great posterior thighs against the roughened edge of its wing-covers; when the green tree-frog swells its throat in the foliage of the bushes, distending it to form a resonant cavity when the rain is imminent, is it calling to its absent mate? By no means. The efforts of the former produce a scarcely perceptible stridulation; the palpitating throat of the latter is as ineffectual; and the desired one does not come.

Does the insect really require to emit these resounding effusions, these vociferous avowals, in order to declare its passion? Consult the immense majority whom the conjunction of the sexes leaves silent. In the violin of the grasshopper, the bagpipe of the tree-frog, and the cymbals of the Cacan I see only their peculiar means of expressing the joy of living, the universal joy which every species of animal expresses after its kind.

If you were to tell me that the Cigales play on their noisy instruments careless of the sound produced, and merely for the pleasure of feeling themselves alive, just as we rub our hands in a moment of satisfaction, I should not be particularly shocked. That there is a secondary object in their conceit, in which the silent sex is interested, is very possible and very natural, but it is not as yet proven.[1]