Those of other countries, however, have been held in less estimation for their powers of song; or rather have been execrated for the deafening din that they produce. Virgil accuses those of Italy of bursting the very shrubs with their noise[670]; and Sir J. E. Smith observes that this species, which is very common, makes a most disagreeable dull chirping[671]. Another, Cicada septendecim—which fortunately, as its name imports, appears only once in seventeen years—makes such a continual din from morning to evening that people cannot hear each other speak. They appear in Pennsylvania in incredible numbers in the middle of May[672].—"In the hotter months of summer," says Dr. Shaw, "especially from midday to the middle of the afternoon, the Cicada, τεττιξ, or grasshopper, as we falsely translate it, is perpetually stunning our ears with its most excessively shrill and ungrateful noise. It is in this respect the most troublesome and impertinent of insects, perching upon a twig and squalling sometimes two or three hours without ceasing; thereby too often disturbing the studies, or short repose that is frequently indulged, in these hot climates, at those hours. The τεττιξ of the Greeks must have had a quite different voice, more soft surely and melodious; otherwise the fine orators of Homer, who are compared to it, can be looked upon no better than loud loquacious scolds[673]."—An insect of this tribe, and I am told a very noisy one, has been found by Mr. Daniel Bydder, before mentioned, in the New Forest, Hampshire. Previously to this it was not thought that any of these insect musicians were natives of the British Isles.—Captain Hancock informs me that the Brazilian Cicadæ sing so loud as to be heard at the distance of a mile. This is as if a man of ordinary stature, supposing his powers of voice increased in the ratio of his size, could be heard all over the world. So that Stentor himself becomes a mute when compared with these insects.
You feel very curious, doubtless, to know by what means these little animals are enabled to emit such prodigious sounds. I have lately mentioned to you the drum of certain grasshoppers; this, however, appears to be an organ of a very simple structure; but since it is essential to the economy of the Cicadæ that their males should so much exceed all other insects in the loudness of their tones, they are furnished with a much more complex, and indeed most wonderful, apparatus, which I shall now describe. If you look at the underside of the body of a male, the first thing that will strike you is a pair of large plates of an irregular form—in some semi-oval, in others triangular, in others again a segment of a circle of greater or less diameter—covering the anterior part of the belly, and fixed to the trunk between the abdomen and the hind legs[674]. These are the drum-covers or opercula, from beneath which the sound issues. At the base of the posterior legs, just above each operculum, there is a small pointed triangular process (pessellum)[675], the object of which, as Reaumur supposes, is to prevent them from being too much elevated. When an operculum is removed, beneath it you will find on the exterior side a hollow cavity, with a mouth somewhat linear, which seems to open into the interior of the abdomen[676]: next to this, on the inner side, is another large cavity of an irregular shape, the bottom of which is divided into three portions; of these the posterior is lined obliquely with a beautiful membrane, which is very tense—in some species semi-opake, and in others transparent—and reflects all the colours of the rainbow. This mirror is not the real organ of sound, but is supposed to modulate it[677]. The middle portion is occupied by a plate of a horny substance, placed horizontally and forming the bottom of the cavity. On its inner side this plate terminates in a carina or elevated ridge, common to both drums[678]. Between the plate and the after-breast (postpectus) another membrane, folded transversely, fills an oblique, oblong, or semi-lunar cavity[679]. In some species I have seen this membrane in tension—probably the insect can stretch or relax it at its pleasure. But even all this apparatus is insufficient to produce the sound of these animals;—one still more important and curious yet remains to be described. This organ can only be discovered by dissection. A portion of the first and second segments being removed from that side of the back of the abdomen which answers to the drums, two bundles of muscles meeting each other in an acute angle, attached to a place opposite to the point of the mucro of the first ventral segment of the abdomen, will appear[680]. In Reaumur's specimens these bundles of muscles seem to have been cylindrical; but in one I dissected (Cicada capensis) they were tubiform, the end to which the true drum is attached being dilated[681]. These bundles consist of a prodigious number of muscular fibres applied to each other, but easily separable. Whilst Reaumur was examining one of these, pulling it from its place with a pin, he let it go again, and immediately, though the animal had been long dead, the usual sound was emitted. On each side of the drum-cavities, when the opercula are removed, another cavity of a lunulate shape, opening into the interior of the abdomen, is observable[682]. In this is the true drum, the principal organ of sound, and its aperture is to the Cicada what our larynx is to us. If these creatures are unable themselves to modulate their sounds, here are parts enough to do it for them: for the mirrors, the membranes, and the central portions, with their cavities, all assist in it. In the cavity last described, if you remove the lateral part of the first dorsal segment of the abdomen, you will discover a semi-opaque and nearly semicircular concavo-convex membrane with transverse folds—this is the drum[683]. Each bundle of muscles, before mentioned, is terminated by a tendinous plate nearly circular, from which issue several little tendons that, forming a thread, pass through an aperture in the horny piece that supports the drum, and are attached to its under or concave surface. Thus the bundle of muscles being alternately and briskly relaxed and contracted, will by its play draw in and let out the drum: so that its convex surface being thus rendered concave when pulled in, when let out a sound will be produced by the effort to recover its convexity; which, striking upon the mirror and other membranes before it escapes from under the operculum, will be modulated and augmented by them[684]. I should imagine that the muscular bundles are extended and contracted by the alternate approach and recession of the trunk and abdomen to and from each other.
And now, my friend, what adorable wisdom, what consummate art and skill are displayed in the admirable contrivance and complex structure of this wonderful, this unparalleled apparatus! The Great Creator has placed in these insects an organ for producing and emitting sounds, which in the intricacy of its construction seems to resemble that which he has given to man, and the larger animals, for receiving them. Here is a cochlea; a meatus; and, as it should seem, more than one tympanum.
I am, &c.
[LETTER XXV.]
ON LUMINOUS INSECTS.
We boast of our candles, our wax-lights, and our Argand lamps, and pity our fellow-men who, ignorant of our methods of producing artificial light, are condemned to pass their nights in darkness. We regard these inventions as the results of a great exertion of human intellect, and never conceive it possible that other animals are able to avail themselves of modes of illumination equally efficient; and are furnished with the means of guiding their nocturnal evolutions by actual lights, similar in their effect to those which we make use of. Yet many insects are thus provided. Some are forced to content themselves with a single candle, not more vivid than the rush-light which glimmers in the peasant's cottage; others exhibit two or four, which cast a stronger radiance; and a few can display a lamp little inferior in brilliancy to some of ours. Not that these insects are actually possessed of candles and lamps. You are aware that I am speaking figuratively. But Providence has supplied them with an effectual substitute—a luminous preparation or secretion, which has all the advantages of our lamps and candles without their inconveniences; which gives light sufficient to direct their motions, while it is incapable of burning; and whose lustre is maintained without needing fresh supplies of oil or the application of the snuffers.
Of the insects thus singularly provided, the common glow-worm (Lampyris noctiluca) is the most familiar instance. Who that has ever enjoyed the luxury of a summer evening's walk in the country, in the southern parts of our island, but has viewed with admiration these "stars of the earth and diamonds of the night?" And if, living like me in a district where it is rarely met with, the first time you saw this insect, chanced to be, as it was in my case, one of those delightful evenings which an English summer seldom yields, when not a breeze disturbs the balmy air, and "every sense is joy," and hundreds of these radiant worms, studding their mossy couch with mild effulgence, were presented to your wondering eye in the course of a quarter of a mile,—you could not help associating with the name of glow-worm the most pleasing recollections. No wonder that an insect, which chiefly exhibits itself on occasions so interesting, and whose economy is so remarkable, should have afforded exquisite images and illustrations to those poets who have cultivated Natural History.