The Coloration, and other Forms of Ornament in Beetles, and the Significance thereof in regard to the Sexual Selection Theory—The Courtship of Grasshoppers and their Kin—The Remarkable Ears of Locusts and Grasshoppers—The Field-cricket and the Katydid as Troubadours—The Wonderful Performances of the Cicadas—The Duels of Long-horned Locusts—Dragon-flies—The May-flies’ “Dance of Death”—The Jaws of the Giant Alder-fly and their Strange Use—Some Curious Facts about Stone-flies. In these pages it is contended that neither brilliant coloration nor any other form of ornamentation is to be ascribed to the direct action of “Sexual Selection.” That is to say, such conspicuous features have not been dependent on the action of female choice for their survival and development, but are rather the “expression points” of the internal, inherent growth variations, which, not being inimical to the welfare of the species, have been free to pursue their development in any direction which apparent chance may dictate.
The Butterflies and Moths well illustrate this in regard to coloration, for scent, not colour, would seem to be their principal source of information as to the outer world. The Beetles are no less instructive; for these 209creatures, though they contain numerous highly-coloured and some exquisitely beautiful species, are more remarkable for their bizarre shapes, and it seems impossible to regard these as the products of sexual selection. Yet this is the interpretation of their origin which, in the judgment of Darwin, we must adopt. He evidently had misgivings as to the correctness of this view; but it must be remembered that in reviewing the facts relating to these lower orders of Creation he was biased by the evidence which he had brought together in regard to the behaviour of the higher groups under the stimulus of sexual emotion. Convinced that female choice obtained here, he was but following the logical result of such conclusions in postulating the same factor wherever it could conceivably be applied. The most formidable critic of the Darwinian theory of Sexual Selection was Darwin himself. The dominant ambition in all his work was to explain his facts, not to establish his theory; and he was convinced that his theory of Sexual Selection did achieve that end; though there were cases where the evidence he was analysing seemed less clear than in others. That the Beetles presented difficulties is evident from his comments thereon. He was puzzled by the vivid coloration which some species present. “They may serve,” he remarks, “as a warning or means of recognition ... as with Beetles the colours of the two sexes are generally alike, we have no evidence that they have been gained through sexual selection; but this is at least possible, for they may have been developed in one sex and then transferred to the other; and this view is even in some degree probable in those groups which possess other well-marked secondary sexual characters....
“Some Longicorns, especially certain Prionid?, offer 210an exception to the rule that the sexes of Beetles do not differ in colour. Most of these insects are large and splendidly coloured. The males of the genus Pyrodes ... are generally redder but rather duller than the females, the latter being coloured of a more or less splendid golden-green. On the other hand, in one species the male is golden-green, the female being tinted with red and purple. In the genus Esmeralda the sexes differ so greatly in colour that they have been ranked as distinct species: in one species both are of a beautiful shining green, but the male has a red thorax. On the whole, as far as I could judge, the females of those Prionid? in which the sexes differ are coloured more richly than the males, and this does not accord with the common rule in regard to colour when acquired through sexual selection.”
While there is nothing very remarkable in the two sexes being coloured alike, it is certainly strange to find the female more brilliantly coloured than the male. And this because among the higher vertebrates, as among the birds, the female exceeds in brilliance only where she also plays the r?le of wooer instead of wooed; leaving to the male the whole responsibility of rearing the family. With the Beetles the family has to rear itself, parental care being limited to the right disposal of the eggs. By some change in the character of the germ-plasm the females may have, in these cases, acquired more “maleness,” more of the qualities which are answerable for the secondary sexual characters of the male, or, what seems rather to be the case here, a result like that which has been reached in certain of the Pigeons and Parrots has been arrived at. That is to say, the tendency to intensification of pigment in the female struck out a new line, instead of following 211that of the male. This rather rare form of sexual dimorphism is also met with, it will be remembered, among the Butterflies and Moths.
While brilliant colour is the more usual form of ornament among the Beetles, there are many species wherein the males have developed enormous horns, or have greatly exaggerated the length of the jaws; and these outgrowths give the impression of a formidable armature, but so far as the evidence goes this is by no means the case. They must therefore be relegated to the category of “ornaments,” though the term “excrescences” would more fittingly apply to them, for they are “ornaments” only from a human standpoint. At any rate, there is no evidence whatever that they serve to enhance their possessors in the eyes of the females.
In relation to the Sexual Selection theory these excrescences are of quite exceptional interest, for they throw a strong light on the meaning of ornament, such as obtains among birds, which seem to show a consciousness of its existence and effectiveness. Darwin argued from the birds to the Beetles. Convinced that the gorgeous crests and trains and vivid colours were appreciated by the females of the former, he was impelled to believe that the ornaments of the latter had developed in like case by the fostering influences of the females. Similarly, from the evidence as to the use of horns in the case of mammals, and spurs in the case of birds, he was induced to believe that the horn-like outgrowths of Beetles had been attained by like influences. But in both kinds of cases, he could only infer their action, for he could discover no really decisive instances of conquest either by display or by battle, such as he was able to produce in the case of the higher animals. Had chance directed his attention in the beginning either to the Warblers 212among the birds, or the beetles among the insects, his interpretation of the action of sexual selection, it is more than probable, would have been materially different from that developed in the “Descent of Man.” No additions of any importance have been added to the facts he so laboriously collected.
As touching the “horns” it should be remarked that these may arise either from the head or from the thorax, or from both, and sometimes even from the under surface of the body.
One of the most remarkable instances of these singular outgrowths is that of the Hercules Beetle (Dynastes hercules) of the West Indies and tropical America. Herein the roof of the head is prolonged into a great upturned beam bearing tooth-like prominences, and the top of this is opposed to a still more massive beam, whose base covers the whole roof of the thorax, and whose tip extends far beyond that projecting from the head. A pair of “teeth” point downwards from the middle of this beam, whose under surface is thickly covered with short chestnut-coloured hairs forming a brush-like surface. In another, Copris isidis, the head bears two short, rhinoceros-like horns, and the thorax a short, triangular overhanging ledge: in Phan?us jaunus there is a single horn on the head, and the thorax bears two short, forwardly-projecting blades, one on each side; while in Onthophagus rangifer—the Reindeer Beetle—the head bears a pair of horns curiously like the antlers of a deer. One might cite many such instances, all varying in detail, but these will suffice.
Darwin, in commenting on these structures, remarked: “The extraordinary size of the horns and their widely 213different structure in closely-allied forms indicate that they have been formed for some purpose; but their excessive variability in the males of the same species leads to the inference that this purpose cannot be of a definite nature. The horns do not show marks of friction, as if used for any ordinary work. Some authors suppose that as the males wander about much more than the females, they require horns as a defence against their enemies; but as the horns are often blunt, they do not seem well adapted for defence. The most obvious conjecture is that they are used by the males for fighting together, but the males have never been observed to fight, nor could Mr. Bates, after a careful examination of numerous species, find any sufficient evidence, in their mutilated or broken condition, of their having been thus used. If the males had been habitual fighters, the size of their bodies would probably have been increased through sexual selection, so as to have exceeded that of the females; but Mr. Bates, after comparing the two sexes in above a hundred species of the Coprid?, did not find any marked difference in this respect amongst well-developed individuals. In Lethrus, moreover, a Beetle belonging to the same great division of the Lamellicorns, the males are known to fight, but are not provided with horns, though their mandibles are much larger than those of the female.
“The conclusion that horns have been acquired as ornaments is that which best agrees with the fact of their having been so immensely, yet not fixedly, developed—as shown by their extreme variability in the same species This view will at first appear extremely improbable, but we shall ... find with many animals standing much higher in the scale, namely, fishes, amphibians, 214reptiles and birds, that various kinds of crests, knobs, horns and combs have been developed apparently for this sole purpose.”
The assumption that these “animals standing much higher in the scale” owe their weapons to the selective action of the females forms the crux of the whole Sexual Selection theory in regard to the significance of ornament. The evidence that the intensification of pigment and the eccentricities of growth in the shape of crests and frills have a fascinating effect on the female is more than under suspicion; it is discredited by the facts which have come to light in regard to behaviour during the periods of sexual exaltation. And there is a growing conviction that this is so. No better proof could be found that “ornaments” can, and do, exist in spite of, rather than because of, the action of “sexual selection.” They are the accidents of this selection, not a part of its machinery.
Incipient horns are found in not a few cases among the females of these insects, while in others, as in the case of the Reindeer Beetle, they are almost as well developed as in the males. This is what one would expect to find if these outgrowths were the result of inherent variations restrained as to their size by natural selection, which eliminates only when this growth penalizes, by increasing the struggle for existence.
As to the actual behaviour of Beetles when sexually excited but very little information is obtainable; but there are records of species the males of which fight with rivals for the possession of females. Wallace saw two males of Leptorhynchus augustatus, a Beetle with no name in common speech and a long beak, “fighting for a female, who stood close by busy at her boring. They pushed at each other with their rostra, and clawed 215and thumped in the greatest rage.” The smaller male, however, “soon ran away, acknowledging himself vanquished.” In this case, it is to be noted, the combatants lacked weapons. With the Stag Beetle it is otherwise, and this species is said to engage in fierce conflicts. Darwin cites an instance where two males were enclosed with one female in a box, when the larger severely pinched the smaller one, until he resigned his pretensions. This being so, it is curious to find that the female, which makes no display of pugnacity, has the stronger jaws. The fact that there are so few records of fighting among male Beetles, and the absence of injury to the highly-polished surfaces of the horns or jaws where these are conspicuously large, seem to indicate that at most no more than a semblance of fighting ever takes place. In a North American Stag Beetle (Lucanus elaphus) the jaws, which are greatly developed, are used, Darwin tells us, for seizing the female, but they do not appear to be employed for this purpose in any other species. It might be held that they play the part of terrifying agents, as the eye-spots of Caterpillars and adult Lepidoptera are believed to do. At any rate, they seem to be so used in the case of a Beetle of South Chile (Chiasmognathus grantii) wherein the jaws are of great size and have their inner edges toothed. When threatened “he faces round, opens his great jaws, and at the same time stridulates freely.” But this parade of force is evidently no more than “bluffing,” for Darwin, who describes this behaviour, remarks, “the mandibles were not strong enough to pinch my finger so as to cause actual pain.” In the female, it may be remarked, the jaws are quite small.
That too much stress has been laid upon the significance 216of the enlarged jaws and other hypertrophied developments in the Beetles seems to be shown by the case of certain carnivorous Beetles, of which one species (Taphroderes distortus) may serve as an example. Herein the left jaw takes the form of a long, crooked strap-shaped outgrowth, whose purpose cannot even be conjectured. And in this connection one may cite the case of certain species of Homoptera—Bugs—which occur in tropical South America. Here, in both sexes, as may be seen in Plate 40, Fig. 4, the neck-shield is produced backwards far beyond the body, to form a most elaborate superstructure which appears to confound the most ingenious attempts at interpretation.
It is to be noted that wherever special structures are necessary for the performance of specific acts such as are of vital importance to the well-being of the race, they are developed to perfection: there is little or no variation in their size, and no doubt as to their purpose. Thus in many species means are necessary to enable the male to seize and hold the female during the sexual embrace. In the Water Beetle of our ponds and ditches (Dytiscus marginalis) the male bears a very remarkable sucker on each fore-leg, the adhesive surface of which, under the microscope, reveals an extraordinary complexity and wondrous beauty. This sucker forms a very conspicuous “secondary sexual character,” and is used in embracing his mate, whose back is deeply grooved to enhance the hold of the suckers. In some species punctures take the place of grooves. Suckers, like those of Dytiscus are met with again in a Wasp (Crabro cribrarius). In another genus of Beetles (Penthe) cited by Darwin, the antenn? of the male have a few of the middle joints dilated and their under surfaces furnished 217with a cushion of hairs to aid in the sexual embrace.
Beetles are creatures of solitary habits; how, then, do they find their mates when, by the insistence of the reproductive desires, they are driven forth to begin the search? Though we have no direct evidence, it seems more than probable that, as with the Butterflies and Moths, scent furnishes their most reliable guide. At any rate, in a large number of species, as among the Lamellicornia, the antenn? bear leaf-like plates, which are much more developed in the males, in which they probably serve as scent-detecting organs.
In some species stridulating organs occur such as are met with in even greater perfection among the Crickets and Grasshoppers, and among the Spiders and Scorpions. That these “musical-boxes” provide a means of communication between the sexes there can be no doubt, even if, as some contend, they are commonly used only to frighten enemies. This purpose may well be the explanation of their presence in the larval Stag Beetle, for it cannot be claimed that they have any relation to the acts of courtship at this stage of development.
Stridulating organs, wherever they are met with, are fashioned on the same principle. The mechanism for sound-production differs conspicuously from that which produces the voice in the vertebrates. For where there are no lungs or breathing apparatus, comparable to that of birds and beasts, there can be no internal voice-mechanism. Instead, the skeleton which in these creatures forms the external surface of the body—that is to say, it encloses the muscles, whereas in the vertebrates it is internal and overlain by the muscles—produces the necessary sounds. And this by means of rubbing two opposed surfaces against 218one another, one of which is ridged, the other toothed. In the details both of position and structure a wonderful variety will be discovered when all the known types are surveyed, and it is possible that they perform different functions in different groups.
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 (Acridiid?) 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 219females 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 220huge, 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.
Plate 33.
 
CRICKETS AND MAY FLIES.
1 and 2 afford illustrations of the excessive development of “ornament “; fig. 3 of devices for seizing the female; figs. 4 and 5 of unaccountable differences in the development of wings.
1. The Pneumatic Cricket (Pneumora scutellaris), showing the strange markings on the female.
2. The Cleft-footed Burrowing Cricket (Schizodactylus monstrosus).
3. The Giant Alder-fly (Corydalis crassicornis), with its huge jaws for grasping the female.
4. The Stone-fly (Perla maxima), the large-winged Continental form.
5. Loch Tanna Stone-fly (Isogenus nubecula), male, with vistigial wings.
[Face page 220.
In the Locustid? 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 Acridiid?, for it is formed only by the wing-cases, and not by the wing 221case 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 Grass............