Social Life in the Insect World

by Jean-Henri Fabre

THE SONG OF THE CIGALE

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.