The Courtship of Animals

by W. P. Pycraft

The Locusts and Grasshoppers are among the finest performers of these “harpists,” and it would seem that in this group, at any rate, the music they make is of an erotic character. In one of our native Grasshoppers (Stenobothrus melanopterus) these high-pitched and somewhat strident notes are produced by rubbing the roughened inner surface of the hindmost thigh, which forms the base of the great leaping leg, against one of the libs of the wing-case which is specially enlarged and has a sharp edge. Thereby the wing is thrown into a state of vibration and the musical sound produced. The roughening of the inner surface of the thigh just referred to is produced by a row of bead-like projections whose appearance under the microscope is depicted in the adjoining illustration. This apparatus is well developed in the males, and but feebly, or not at all, in the females. That the music it produces is appreciated by the performers and their mates there can be no doubt, for they are provided with a special apparatus which fulfils the purpose of an ear. In the short-horned Grasshoppers (Acridiidæ) this is placed in the middle of the body just above the base of the great thigh. It differs in the details of its construction. In some cases it is formed by a delicate sheet of membrane surrounded by a rim, in others the membrane may be slightly depressed, and in some very much so, the rim closing up to form a broad slit. Such ears, it is to be noted, exist in both sexes, while the stridulating organs do not. That such sound-producing organs serve as stimulants to the sexual passions of the females is but a natural inference. Some authorities, however, regard this as doubtful, since there are species which appear to lack these stridulating instruments, though possessing ears. But closer observation will probably show that these apparently dumb species are not really so, as Dr. David Sharp, commenting on this fact remarks: “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 capacity for perceiving vibrations, it of course becomes more than 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 which 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.”

Plate 32.

STRIDULATING ORGANS. ETC.

1. The stridulating mechanism of the Red Ocypode Crab.

2. The stridulating apparatus of a Grasshopper—highly magnified.

3. The head of a Gnat with the compound eyes split up.

4. The “ear” of a Grasshopper.

[Face page 218.

Certain of the Grasshoppers of Africa, known to entomologists as Pneumorides, have undergone a most extraordinary transformation of their bodily shape, as if in response to the demands of these musical performances. They have entirely lost the power of leaping, and the abdomen, in the male, has become transformed into a huge, pellucid, inflated bag or bladder, apparently to serve the purpose of a resonator, increasing the volume of the sound produced by the stridulating organ, which consists of a series of ridges placed on each side. The noise which this mechanism produces is, as might be supposed, considerable. It is curious to remark that in this group the females are more vividly coloured than the males. In the case of one South African species—Pneumora scutellaris—this coloration is so extravagant that she has been said to look as if “got up” for a fancy-dress ball (Plate 33, Fig. 1). Her ground colour is of a light green, with pearly-white markings, surrounded by an edging of magenta; the white areas are very numerous. The face has magenta patches, and numerous, tiny, pearly-white tubercles, each of which, when placed on a green part, is surrounded by a ring of mauve. This scheme of coloration distinguishes her as one of the most remarkably coloured of insects. But to what are we to attribute these hues? Sexual Selection will not explain them, and it seems unreasonable to regard them either as affording a protective or a warning coloration. They may then, perhaps, be allowed to rank as another instance of unchecked variation in the direction of vivid colouring, such as has been already described as occurring both among birds and other animals lower in the scale.