Whether the innumerable locust armies, to which I have so often called your attention, make any noise in their flight, I have not been able to ascertain; the mere impulse of the wings of myriads and myriads of these creatures upon the air, must, one would think, produce some sound. In the symbolical locusts mentioned in the Apocalypse[605], this is compared to the sound of chariots rushing to battle: an illustration which the inspired author of that book would scarcely have had recourse to, if the real locusts winged their way in silence.
Amongst the Hemiptera, I know only a single species that is of noisy flight; though doubtless, were the attention of entomologists directed to that object, others would be found exhibiting the same peculiarity. The insect I allude to (Coreus marginatus) is one of the numerous tribe of bugs; when flying, especially when hovering together in a sunny sheltered spot, they emit a hum as loud as that of the hive-bee.
From the magnitude and strength of their wings, it might be supposed that many lepidopterous insects would not be silent in their flight;—and indeed many of the hawk-moths (Sphinx, F.), and some of the larger moths (Bombyx, F.), are not so; Cossus ligniperda, for instance, is said to emulate the booming of beetles by means of its large stiff wings; whence in Germany it is called the humming-bird (Brumm-Vogel).—But the great body of these numerous tribes, even those that fan the air with "sail-broad vans," produce little or no sound by their motion. I must therefore leave them, as well as the Trichoptera and Neuroptera, which are equally barren of insects of sounding wing—and proceed to an order, the Hymenoptera, in which the insects that compose it are, many of them, of more fame for this property.
The indefatigable hive-bee, as she flies from flower to flower, amuses the observer with her hum, which, though monotonous, pleases by exciting the idea of happy industry, that wiles the toils of labour with a song. When she alights upon a flower, and is engaged in collecting its sweets, her hum ceases; but it is resumed again the moment that she leaves it.—The wasp and hornet also are strenuous hummers; and when they enter our apartments, their hum often brings terror with it. But the most sonorous fliers of this order are the larger humble-bees, whose bombination, booming, or bombing, may be heard from a considerable distance, gradually increasing as the animal approaches you, and when, in its wheeling flight, it rudely passes close to your ear, almost stunning you by its sharp, shrill, and deafening sound. Many genera, however, of this order fly silently.
But the noisiest wings belong to insects of the dipterous order, a majority of which, probably, give notice of their approach by the sound of their trumpets. Most of those, however, that have a slender body,—the gnat genus (Culex) excepted,—explore the air in silence. Of this description are the Tipulariæ, the Asilidæ, the genus Empis, and their affinities. The rest are more or less insects of a humming flight; and with respect to many of them, their hum is a sound of terror and dismay to those who hear it. To man, the trumpet of the gnat or mosquito; and to beasts, that of the gad-fly; of various kinds of horse-flies; and of the Ethiopian zimb, as I have before related at large[606], is the signal of intolerable annoyance. Homer, in his Batrachomyomachia, long ago celebrated the first of these as a trumpeter—
"For their sonorous trumpets far renown'd,
Of battle the dire charge mosquitos sound."
Mr. Pope, in his translation, with his usual inaccuracy, thinking no doubt to improve upon his author, has turned the old bard's gnats into hornets. In Guiana these animals are distinguished by a name still more tremendous, being called the devil's trumpeters[607]. I have observed that early in the spring, before their thirst for blood seizes them, gnats when flying emit no sound. At this moment (Feb. 18) two females are flying about my windows in perfect silence.
After this short account of insects that give notice when they are upon the wing by the sounds that precede them, I must inquire by what means these sounds are produced. Ordinarily, except perhaps in the case of the gnat, they seem perfectly independent of the will of the animal; and in almost every instance, the sole instruments that cause the noise of flying insects are their wings, or some parts near to them, which, by their friction against the trunk, occasion a vibration—as the fingers upon the strings of a guitar—yielding a sound more or less acute in proportion to the rapidity of their flight—the action of the air perhaps upon these organs giving it some modifications. Whether, in the beetles that fly with noise, the elytra contribute more or less to produce it, seems not to have been clearly ascertained: yet, since they fly with force as well as velocity, the action of the air may cause some motion in them, enough to occasion friction. With respect to Diptera, Latreille contends that the noise of flies on the wing cannot be the result of friction, because their wings are then expanded; but though to us flies seem to sail through the air without moving these organs, yet they are doubtless all the while in motion, though too rapid for the eye to perceive it. When the aphidivorous flies are hovering, the vertical play of their wings, though very rapid, is easily seen; but when they fly off it is no longer visible. Repeated experiments have been tried to ascertain the cause of sound in this tribe, but it should seem with different results. De Geer, whose observations were made upon one of the flies just mentioned, appears to have proved that, in the insect he examined, the sounds were produced by the friction of the root or base of the wings against the sides of the cavity in which they are inserted. To be convinced of this, he affirms, the observer has nothing to do but to hold each wing with the finger and thumb, and stretching them out, taking care not to hurt the animal, in opposite directions, thus to prevent their motion,—and immediately all sound will cease. For further satisfaction he made the following experiment. He first cut off the wings of one of these flies very near the base; but finding that it still continued to buz as before, he thought that the winglets and poisers, which he remarked were in a constant vibration, might occasion the sound. Upon this, cutting both off, he examined the mutilated fly with a microscope, and found that the remaining fragments of the wings were in constant motion all the time that the buzzing continued; but that upon pulling them up by the roots all sound ceased[608]. Shelver's experiments, noticed in my last letter, go to prove, with respect to the insects that he examined, that the winglets are more particularly concerned with the buzzing. Upon cutting off the wings of a fly—but he does not state that he pulled them up by the roots—he found the sound continued. He next cut off the poisers—the buzzing went on. This experiment was repeated eighteen times with the same result. Lastly, when he took off the winglets, either wholly or partially, the buzzing ceased. This, however, if correct, can only be a cause of this noise in the insects that have winglets. Numbers have them not. He next, therefore, cut off the poisers of a crane-fly (Tipula crocata), and found that it buzzed when it moved the wing. He cut off half the latter, yet still the sound continued; but when he had cut off the whole of these organs the sound entirely ceased[609].
Aristophanes in his Clouds, deriding Socrates, introduces Chærephon as asking that philosopher whether gnats made their buz with their mouth or their tail[610]. Upon which Mouffet very gravely observes, that the sound of one of these insects approaching is much more acute than that of one retiring; from whence he very sapiently concludes, that not the tail but the mouth must be their organ of sound[611]. But after all, the friction of the base of the wings against the thorax seems to be the sole cause of the alarming buz of the gnat as well as that of other Diptera. The warmer the weather, the greater is their thirst for blood, the more forcible their flight, the motion of their wings more rapid, and the sound produced by that motion more intense. In the night—but perhaps this may arise from the universal stillness that then reigns—their hum appears louder than in the day: whence its tones may seem to be modified by the will of the animal.
Sounds also are sometimes emitted by insects when they are feeding or otherwise employed. The action of the jaws of a large number of cockchafers produces a noise resembling the sawing of timber; that of the locusts has been compared to the crackling of a flame of fire driven by the wind; indeed the collision at the same instant of myriads of millions of their powerful jaws must be attended by a considerable sound. The timber-borers also—the Buprestes; the stag-horn beetles; and particularly the capricorn-beetles—the mandibles of whose larvæ resemble a pair of mill-stones[612]—most probably do not feed in silence. A little wood-louse (Atropos pulsatoria)—which on that account has been confounded with the death-watch—is said also, when so engaged, to emit a ticking noise.—Certain two-winged flies seen in spring, distinguished by a very long proboscis (Bombylius), hum all the time that they suck the honey from the flowers; as do also many hawk-moths, particularly that called from this circumstance the humming-bird (Macroglossa Stellatarum), which, while it hovers over them, unfolding its long tongue, pilfers their sweets without interrupting its song.—The giant cock-roach (Blatta gigantea), which abounds in old timber houses in the warmer parts of the world, makes a noise when the family are asleep like a pretty smart rapping with the knuckles—three or four sometimes appearing to answer each other.—On this account in the West Indies it is called the Drummer; and they sometimes beat such a reveille, that only good sleepers can rest for them[613]. As the animals of this genus generally come forth in the night for the purpose of feeding, this noise is probably connected with that subject.