Fig. 194.—Diagram of arrangement of parts of the ear as seen in transverse section of the tibia of a Locustid. A, J, V, H, outer, inner, anterior, posterior aspects of leg; a, d, thin part of integument forming anterior tympanum; b, c, thicker portion of same; f, g, posterior tympanum; a, f, and d, h, g, thick portions of integument; i, k, internal protuberances of same; l, m, n, o, walls of the anterior tracheal vesicle, vTr; p, q, s, r, walls of the posterior tracheal vesicle, hTr; o″, projection of tympanal orifice of prothorax; tr-n, tracheal nerve-end organ, crista acustica; st, rod; de, curtain-membrane; hn, e, supra-tympanal, nerve-end organ; hn, ganglion cells; st′, rods; e, point of integumental fixation of nerve endings. (After Graber.)
The chief features of the acoustic apparatus of the legs of Locustidae will be gathered from the accompanying diagrammatic transverse section through the tibia. In this figure the deep black parts indicate the outer wall of the tibia and its prolongations, the white spaces indicate the parts filled with air, while the dotted portions are occupied by blood or some of the body organs;[[248]] the circular space o″ is not part of the actual structure, but represents the area of the external acoustic orifice of the prothorax; it is not, however, so large as it should be.
Although the tibial ears of Locustidae are very perfect organs, there is great difficulty in deciding on the exact nature of their functions. They would appear to be admirably adapted to determine the precise locality from which a sound proceeds, especially in those cases—and they are the highest forms—in which the tympanum is placed in a cavity the external orifice of which is a slit (Fig. 193, B); for the legs can be moved in the freest manner in every direction, so as to bring the drum into the most direct line of the vibrations. But as to what kinds of vibrations may be perceived, and the manner in which they may be transmitted to the nerves, there is but little evidence. On reference to the diagram it will be noticed that the tympanum, the tympanal vesicles, and the nervous apparatus are not in close connexion, so that even the mode by which the impulses are transmitted is obscure.
The musical organs of the Locustidae are different from those of the Acridiidae, and are invariably situate on the basal part of the tegmina. They are found, in the great majority of cases, only in the male; in the tribes Ephippigerides and Callimenides they exist in each sex. One of the wings bears a file on its inner surface, while the other—on the right side of the body—is provided with a sharp edge placed on a prominent part of its inner margin. By slightly tilting the tegmina and vibrating them rapidly, the edge passes under the file, and a musical sound is produced. These structures are limited to the small anal area of the wing, and when the tegmina are very greatly reduced in size, it is this part that still remains. There is much variety in the details of the structure. The nervures of this part of the tegmina are different in the male from what they are in the female, and, moreover, the two wing-covers of the male differ from one another. It is apparently the vibrations of the right tegmen that produce the sound, and this part usually bears a space of a glassy nature, which probably improves the character of the sound produced. Our chief British songster of this group, Locusta viridissima, is only provided with phonetic organs (Fig. 195) of a somewhat imperfect character, but in the genus Mecopoda there is great perfection of the structures. The anal areas of the two tegmina are in this case very different; that of the left one, which bears the file, being similar in texture to the rest of the wing-cover, while the corresponding part of the other tegmen is rigid and transparent, and greatly distorted, so as to create a cavity which, no doubt, improves the sound; the scraper too is very perfectly formed. The difference between this form of musical organ and that of L. viridissima is curious, inasmuch as in the better instrument the important modifications are confined to one tegmen, while in the other form both tegmina are largely changed. The difference appears to be that in Locusta the left tegmen, as well as the right one, acts as a sounding-board, while in Mecopoda it does not do so, but when the wings are closed quite covers and conceals the musical instrument.
The Locustidae, notwithstanding the fact that their alar organs are generally more ample than those of the Acridiidae, seem to be, as a rule, of more sedentary habits, and more nocturnal in their activity. The musical powers of the different species are very varied. Locusta viridissima produces a shrill and monotonous but not disagreeable, sound, and is capable of sustaining it for a quarter of an hour without any intermission, except a break for the sake of starting again immediately with greater force, like a performer on a flute. It occasionally chirps in the day, but the act is then very brief. Bates informs us that one of these singing grasshoppers, called Tananá by the natives of the Amazon valley, is much admired for its singing, and is kept in little cages. The Amazonian naturalist thought the music of this species superior to that of any other Orthopterous Insect he had heard. The name of this grasshopper is Thliboscelus camellifolius. It is very similar in appearance to Cyrtophyllus crepitans, the Insect we have represented in Fig. 187.
Fig. 195.—Inner face of base of tegmina of Locusta viridissima: A, the two wing-covers separated; B, in natural position with mesonotum connecting them, showing file and edge scraping it; a, the stridulating file; b, the rudimentary file on other tegmen.
The most notorious of the musical Locustids are the Katydids of North America. There are several species of them—they belong, indeed, to more than one genus,—but it seems that sounds somewhat resembling the words Katy-did are perceptible in most of their performances. These sounds are frequently repeated with slight variations—Katy-did, O-she-did, Katy-did-she-did. Riley describes the music of the Katydid we represent in Fig. 196 as follows:[[249]] "The first notes from this Katydid are heard about the middle of July, and the species is in full song by the first of August. The wing-covers are partially opened by a sudden jerk, and the notes produced by the gradual closing of the same. The song consists of a series of from twenty-five to thirty raspings, as of a stiff quill drawn across a coarse file. There are about five of these raspings or trills per second, all alike, and with equal intervals, except the last two or three, which, with the closing of the wing-covers, run into each other. The whole strongly recalls the slow turning of a child's wooden rattle, ending by a sudden jerk of the same; and this prolonged rattling, which is peculiar to the male, is invariably and instantly answered by a single sharp 'chirp' or 'tschick' from one or more females, who produce the sound by a sudden upward jerk of the wings."
Fig. 196.—Katydid, Microcentrum retinerve. N. America. (After Riley.)