It is, indeed, a matter of great difficulty to decide as to the exact function of these ear-like acoustic organs, which, we may remind the reader, are peculiar to the saltatorial Orthoptera, and we must refer for a full discussion of the subject to Graber's masterly works,[[218]] contenting ourselves with a brief outline, which we may commence by saying that the Orthoptera with ears are believed to be sensitive to sounds by means other than these organs. This suggests that the latter exist for some purpose of perception of special sound. But if so what can this be? Only the males possess, so far as we know, effective sound-producing organs, but both sexes have the special ears; moreover, these structures are present in numerous species where we do not know of the existence of phonetic organs in either sex. Thus it appears at present impossible to accept these organs as being certainly special structures for the perception of the music of the species. It is generally thought that the females are charmed by the music of the males, and that these are stimulated to rivalry by the production of the sounds; and Dufour[[219]] has suggested that this process reacts on the physiological processes of the individual. There has not been a sufficient amount of observation to justify us in accepting these views, and they do not in any way dispose of the difficulty arising from the existence of the acoustic organs in species that do not, so far as we know, produce special sounds. It is possible that the solution of the difficulty may be found in the fact that these apparently dumb species do really produce some sound, though we are quite ignorant as to their doing so. It is well known that sounds inaudible to some human ears are perfectly distinct to others. Tyndall, in his work on Sound, has illustrated this by a fact that is of special interest from our present point of view. "Crossing the Wengern Alp with a friend," he says, "the grass on each side of the path swarmed with Insects which to me rent the air with their shrill chirruping. My friend heard nothing of this, the Insect world lying beyond his limit of audition." If human ears are so different in their capacities for perceiving vibrations, it of course becomes more probable that auditory organs so differently constituted as are those of Insects from our own may hear sounds when the best human ear can detect nothing audible. On the whole, therefore, it would appear most probable that the Orthoptera provided with acoustic organs, and which we consider dumb, are not really so, but produce sounds we cannot hear, and do so in some manner unknown to us. If this be the case it is probable that these ears are special organs for hearing particular sounds.
Scudder, who has given considerable attention to the subject of Orthopteran music, says that in N. America "the uniformity with which each species of Stenobothrus plays its own song is quite remarkable. One kind, Stenobothrus curtipennis, produces about six notes per second, and continues them from one and a half to two and a half seconds; another, S. melanopleurus, makes from nine to twelve notes in about three seconds. In both cases the notes follow each other uniformly, and are slower in the shade than in the sun."
Some of the species of Acridiidae, it should be noticed, produce a noise during their flights through the air, due to the friction of the wings; whether this has a definite importance, or whether it may be entirely incidental, has scarcely yet been considered.
Information of a satisfactory kind as to the post-embryonic development of the Acridiidae is but scanty. We have represented in Fig. 84, A, the condition in which a migratory locust, Schistocerca peregrina, leaves the egg, and we will here complete the account of its growth; following Brongniart,[[220]] whose statement is confirmed by Lestage and other naturalists. Immediately on leaving the egg the young locust casts its skin, and is then of a clear green colour, but it rapidly becomes brown, and in twelve hours is black. At this early age the gregarious instinct, possessed by this and some other species of Acridiidae, becomes evident. In six days the individual undergoes a second moult, after which it is black, spotted and banded with white, and with a rose-coloured streak on each side of the hind body. The third ecdysis occurs in six or eight days after the second; the rose colour becomes more distinct, and the head is of a brown tint instead of black. After eight days the fourth ecdysis occurs; the creature is then about 35 millimètres long; its colour has much changed, the position of the markings is the same, but the rose colour is replaced by citron yellow, the line of the spiracles is marked with white, and at this time the creature has the "first rudiments of wings," and is very voracious. In ten days another ecdysis takes place, the yellow colour is more vivid, the prothorax is definitely speckled with white, and the hind body is increasing much in size. In fifteen or twenty days the sixth moult occurs, and the Insect appears in its perfect form; the large tegmina now present are marked with black in the manner so well known, and the surface generally is variegated with bluish and rosy marks. Although this is the colour in Algeria, yet apparently it is not so farther south; the Insects that arrive thence in the French colony are on some occasions of a different colour, viz. reddish or yellowish, those of this latter tint being, it is believed, older specimens of the reddish kind. M. Brongniart points out that some Phasmidae—of the Phyllium group—undergo an analogous series of colour-changes in the course of the individual development, though other species do not.
Fig. 174.—Development of wings in Caloptenus spretus: the upper row gives a lateral view of the thoracic segments, and the lower row a dorsal view of these segments; 1, second instar; 2, third instar; 3, fourth instar; 4, fifth instar. (After Riley.) t, tegmen; w, wing.
Fig. 175.—Caloptenus spretus. North America. A, Newly hatched, much magnified; B, adult, natural size. (After Riley.)
Riley and Packard have given an account[[221]] of some parts of the post-embryonic development of the Rocky Mountain Locust, which enables us to form a satisfactory conception of the stages of development of the wings. Fig. 175, A, represents the first instar, the young locust, just emerged from the egg and colourless. Fig. 174 shows some of the subsequent stages of development of the wings, the upper line of figures giving a profile view of the thoracic segments, and the lower line showing their dorsal aspects; 1 shows the condition of the parts in the second instar, the chief difference from the first instar being the development of colour; in the third instar there is an evident slight development of the future alar organs, exhibited chiefly in the outgrowth and lobing of the free posterior angles of the meso- and metanota, as shown in Fig. 174, 2. After the third moult there is a great difference; the instar then disclosed—the fourth—has undergone a considerable change in the position of the meso- and metathoraces, which are thrust forward under the pronotum; this has become more enlarged and hood-like (Fig. 174, 3); at the same time the wing-rudiments have become free and detached, the metathoracic pair being the larger, and overlapping the other pair. The fifth instar (Fig. 174, 4) differs but little from the fourth, except in the larger size of the pronotum and wing-rudiments. The sixth—shown in Fig. 175, B—is the perfect Insect, with the alar organs free and large, the prothorax much changed in form, the colour different. From the above it will be seen that the chief changes occurred at the third and fifth ecdyses, after each of which a considerable difference in the form of the Insect was revealed. In the first three instars the sexes can scarcely be distinguished, in the fourth they are quite distinct, and in the fifth coupling is possible, though usually it does not occur till the final stage is attained.
The discovery that Orthoptera change their colours in the course of their development, and even after they have become adult, is important, not only from a physiological point of view, but because it throws some light on the questions as to the number of species and the geographical distribution of the migratory locusts, as to which there has existed a great confusion.