The Courtship of Animals

by W. P. Pycraft

In the Locustidæ the ear is placed on the side of the front leg and the rim surrounding it may either take an oval shape or close up to form a slit. The air necessary for the efficient action of the acoustic apparatus is admitted through a gaping hole in the side of the body, above the base of the leg, an arrangement not met with among any other insects. The musical apparatus of these insects differs from that of the Acridiidæ, for it is formed only by the wing-cases, and not by the wing case and the leg. One of the wings bears a file on its inner surface, the other, the right wing, is furnished with a sharp edge placed on a prominent part of its inner margin. By slightly tilting the fore-wings, or wing-cases, and vibrating them rapidly, the edge passes under the file and a musical sound is produced. By this means one of our native long-horned Grasshoppers (Locusta viridissima) produces a shrill, but not unpleasant, sound, capable of being sustained continuously for a quarter of an hour. But a species encountered by Bates during his travels in the Amazons is a much more efficient performer. Known by the name of Tanana by the natives, it is so much admired by them for its singing powers that it is kept in little cages as we keep Canaries. That these organs are of importance to the species may be gathered from the case of a Bulgarian long-horned or Green Grasshopper (Poecilimon affinis), wherein the wings have so degenerated as to be useless in flight, but in the male they have been retained solely as musical instruments. In some species both sexes have a music-producing apparatus, but as a rule this is present only in the male.

That these curious and complex stridulating organs do indeed primarily act as aphrodisiacs seems to have been clearly demonstrated by the naturalist Bates, who, in speaking of the European Field Cricket remarks: “The male has been observed to place himself at the entrance to his burrow, and stridulate until a female approaches, when the louder notes are succeeded by a more subdued tone, whilst the successful musician caresses with his antennæ the mate he has won.”

Among the most efficient and most celebrated performers of all on these instruments of percussion are the “Katydids” of North America. The sounds they produce are said to form the words “Katy-did, O-she-did, Katy-did-she-did.” The first of these extraordinary concerts is heard about mid-July; by mid-August they are in full song. By others the sounds have been likened to those produced by the slow turning of a child’s rattle, ending in a sudden jerk; and this prolonged rattling, which is peculiar to the male, is always 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.

Pride of place, however, among insect performances of this kind must surely be awarded to the Cicadas, which are notoriously the noisiest members of the Insect world, far eclipsing the shrill calls of the Grasshoppers and even of the Crickets. Darwin remarks that the noise they made could “be plainly heard on board the Beagle when anchored off Brazil at a quarter of a mile from the shore; and Captain Hancock says it can be heard at a distance of a mile. The Greeks formerly kept, and the Chinese now keep, these insects in cages for the sake of their song, so that it must be pleasing to the ears of some men.” Only the males sing, the females being completely dumb, and this prompted the Greek poet Xenarchus to make the now famous remark, “Happy the Cicadas’ lives, for they have voiceless wives.” Another naturalist, Riley, says: “The general noise, on approaching the infested woods, is a combination of that of a distant threshing-machine and a distant frog-pond.” Another species, Tympanoterpes gigas, also Brazilian, is said to make a noise equal to the whistle of a locomotive: recalling that of a nest of young snakes, or young birds, when disturbed—a sort of scream. They can also produce a chirp like that of a Cricket and a very loud, shrill screech prolonged for fifteen or twenty seconds, gradually increasing and decreasing in force.

Curiously enough, no special auditory organs have yet been discovered, and it has been suggested that these insects do not hear in our sense of the word, but feel rhythmical vibrations. But whether the males “sing in rivalry,” as Dr. David Sharp suggests, is another matter. The purpose of the “song” in the first place is no doubt intended as a guide to the females seeking mates. But it is quite conceivable that the call of one male may stimulate every other male in the neighbourhood. Darwin, commenting on this aspect of the music, gives a quotation from Dr. Hertman, who says: “The drums are now ... heard in all directions. This I believe to be the marital summons from the males. Standing in thick chestnut sprouts about as high as my head, where hundreds were around me, I observed the females coming around the drumming males.... This season a dwarf pear-tree in my garden produced about fifty larvæ of Cicada pruinosa; and I several times noticed the females alight near a male while he was uttering his clanging notes.”

The structures, he remarks, from which these sounds proceed, “must be ranked amongst the most remarkable voice-organs in the animal kingdom. They are totally different from the stridulating organs that are found in many other insects.... Some difference of opinion has existed as to the manner in which the structures act, but the account given by Carlet ... will, we believe, be found to be essentially correct.” They are partly thoracic and partly abdominal. On examining a male Cicada there will be seen, on the under surface, two plates, meeting in the middle line of the body and overlapping the base of the abdomen. They can be slightly moved away from the abdomen, and thereby a wide fissure will be produced, displaying the mechanism beneath. If the whole operculum be removed, three membranes will be seen, an external, called the “timbal,” an anterior folded and soft membrane, and a posterior “mirror.” This last is a very beautiful object, tensely stretched and pellucid, yet reflecting all the hues of the rainbow. The sound is primarily produced by the vibrations of the timbal, to which a muscle is attached; the other membranes are probably also set vibrating, and the whole skeleton helps to increase or modify the sound, which is probably also influenced by the position of the operculum, which varies in different species. A further control of the tension of the air is exerted by “stigmata” or pores, and certain air-chambers connected therewith.

Throughout these pages comment has been made on the apparently “fortuitous” character of complex patterns and structures. The “musical-box” of the Cicada affords yet another instance. Nevertheless there is an impressive harmony between the several parts; an interdependence which is not fortuitous. There is obviously a nexus of growth-controlling factors preserving harmony between each separate part which as yet has escaped all endeavour to discover.

While it is difficult to picture the initial stages of growth of so complex an organ as that of the Cicada, the beginnings of simpler structures such as the stridulating organs of Beetles and Grasshoppers are more easily discernible. “It is probable,” remarks Darwin, “that the two sexes of many kinds of Beetles were at first enabled to find each other by the slight shuffling noise produced by the rubbing together of the adjoining hard parts of their bodies; and that as those males or females which made the greatest noise succeeded best in finding partners, rugosities on various parts of their bodies were gradually developed by means of sexual selection into true stridulating organs.”

Structures to which we can ascribe a use are commonly supposed to have been evolved for the purpose which we assign to them. The “horns” of Beetles afford a case in point; but there are many other equally remarkable and extravagant developments among the insects which seem to defy explanation. And they will continue to do so until it is realized that they are but exaggerations of the normal processes of growth, which is not limited to definite areas but may produce extensions and excrescences of an almost infinitely varied character. The only controlling factor is that imposed by Natural Selection when these growth-changes tend to impair the well-being of the organism as a whole. Often such changes confer benefits, giving rise to new organs, and in this case Natural Selection encourages the new departure. Nothing, indeed, “succeeds like success.” New departures in one direction may be promptly suppressed, in another they spell fortune: there is no “Socialism” in Nature. Often these “new departures” neither help nor hinder, and instances of this kind are commonly afforded by “ornaments.” One of the most singular illustrations of this kind is furnished by that extraordinary Long-horned Grasshopper of India (Schizodactylus monstrosus), wherein the wings, when at rest, have their tips coiled up like a watch-spring, while the appendages to the legs are scarcely less remarkable. It is a burrower, driving long tunnels in the banks of rivers. But little is known of its habits, save that it does not emerge from its burrow till night, when it takes long flights. This being so, the bizarre character of its wings and legs is the more difficult to explain on any Sexual Selection theory. But regarded as spontaneous variations which have not fallen under the ban of Natural Selection, they are somewhat less puzzling, though, having regard to the extraordinary transformation which the burrowing Mole-cricket and the allied Cylindrodes have undergone, in adaptation to fossorial habits, the legs of this insect are remarkable indeed.

While there can be no doubt that the musical performances of the Crickets and Locusts play an important part in courtship, in some of the Long-horned Locusts, at any rate, the males fight furiously when mate-hunting, and to this end the head and jaws are greatly enlarged. During the progress of the duel the wings are extended and held erect, which is hardly what one would have expected, since in this position they would seem to be more exposed to danger.