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 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 antenn? the mate he has won.”
among the most efficient and most celebrated performers of all on these instruments of percussion are 222the “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 223chirp 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 larv? 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 224overlapping 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 225bodies; 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 226so, 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.
all the insects so far surveyed have been more or less conspicuous for their vivid hues, yet in none of these have elaborate “displays” been recorded. to find demonstrativeness of this kind one must turn apparently to a group of minute, lowly organized, dull-coloured, wingless insects with ugly misshapen heads and bodies. the sexes do not differ in appearance, but they are interesting on account of the sedulous court which the males pay to the females. the late lord avebury, in a communication to the linnean society, remarked of them: “it is very amusing to see these little creatures (smynthurus luteus) coquetting together. the male, which is much smaller than the female, runs round her, and 227they butt one another, standing face to face and moving backwards and forwards like two playful lambs. then the female pretends to run away and the male runs after her. with a queer appearance of anger, he gets in front and stands facing her again; then she turns coyly round, but he, quicker and more active, scuttles round too, and seems to whip her with his antenn?; then for a bit they stand face to face, play with their antennas and seem to be all in all to one another.”
the dragon-flies are among the most beautiful of insects; they are also relatively long-lived, and they are conspicuous. yet this beauty must be attributed to some inherent inward grace rather than to the ?sthetic instincts of the female. moreover, in the matter of size and beauty there is little to choose between the sexes; where any difference occurs the males have the advantage. though the mode of copulation is well known, nothing has been discovered as to the means whereby male and female discover one another. it is doubtful whether this can be done by sight, for with all the beauty of their shimmering suits of mail and gauzy wings, their vision is limited to a field of a few inches. possibly scent is their guide; at any rate, dead dragon-flies have a vile odour.
it is worth noting that there are no wingless dragon-flies, and that none have developed unnecessary ornament in the form of spines, horns, or frills of any kind, such as are so commonly met with among groups of more sedentary habits like the phasmid? and the beetles, for example. in other words, there is clearly a direct relation between ornament and the mode of life. it is also clear that some modes of subsistence are very inelastic, allowing of no more than very slight structural variations, for the dragon-flies are an extremely ancient group. fossil species of large size are known from the lower lias, and the remains 228of a giant measuring two feet in expanse of wings has been found in the carboniferous. this species, however, seems to have stood near the parting of the ways between the may-flies and the dragon-flies. but be this as it may, undoubtedly dragon-flies hovered over the backs of sleeping ichthyosaurs and furnished food for pterodactyles millions of years ago, as they now hover over lazy kine for the sake of the flies forgathered there, or dodge to avoid the stoop of the hobby, and in all this vast space of time they have not appreciably changed.
and what is true of the dragon-fly is true also of the may-fly, for it is clear that they are of the same stock. it is true at any rate in so far as the conformation of the body is concerned. the possibility that it may be equally true in regard to the details of their life-history almost staggers one, because these are, in many respects, of a quite remarkable character. as with the dragon-fly, there is a prolonged period of larval life, lasting from one to two years, which time is passed in streams and pools where a luxuriant vegetation ensures a plentiful supply of food. some are carnivorous, but in the majority of species minute plants only are eaten. more than forty species are to be reckoned as natives of the british islands, the commonest being the “green drake” and “grey drake,” beloved by the fisherman. these names are applied, it may be mentioned, to the phase known as the sub-imago which precedes the fully-adult stage, of ephemera vulgata and e. danica.
save that it is curious that while the larv? of some species are carnivorous those of others are vegetarians, there is nothing very remarkable about what may be called the infantile period. but when this is ended the 229span of life remaining to them as adults is brief indeed. instinctively realizing that the time of transfiguration is at hand, the erstwhile crawling grub rises to the surface of the stream, and almost in the twinkling of an eye it mounts into the air on gauzy wings, there for a brief space to execute an aerial dance which in its every phase is amazing. some species never see the sun. they emerge as the sky begins to redden, and as its glory fades they, too, expire. this brief space is all that nature has allowed them in which to fulfil her behest to all living things—to increase and multiply. and myriads die without even a chance to effect this consummation of existence.
the dance is a dance of death, and it is performed by a host so vast as to surpass the bounds of belief save to those who have had the good fortune to witness a scene so amazing.
d’albertis tells of a gathering which he witnessed on the fly river, new guinea—for these insects have a world-wide distribution—wherein countless myriads were assembled. “for miles the surface of the river, from side to side, was white with them as they hung over it on gauzy wings; at certain moments, obeying some mysterious signal, they would rise in the air and then sink down anew like a fall of snow.” and in this assemblage he estimated that there was but one female to every five or six thousand males. it is during this flight that the act of mating is performed. the fortunate male from the host of rivals, in this mid-air embrace is aided by the foremost pair of legs, which are especially curved to effect this purpose. the embrace is momentary. thereafter he dies; to the female a somewhat longer span of life remains, for she has yet to deposit her eggs, and 230this being done en masse, she, too, expires.
it is curious that these creatures, which in their winged state have never seen the sun, should be attracted by light. but such is the case. i well remember witnessing an instance of this years ago, while staying, one august, at bingen on the rhine. dinner was served in the open air, and just as the soup was served may-flies in myriads swarmed round the lamps and fell on the tables as thick as snow-flakes. some of these were in copula, and i succeeded in bottling a few specimens for the british museum, where they still remain to remind me of this amazing scene.
about three hundred species of may-flies are known, and some enjoy a somewhat longer span of life than others. in no case, however, do they emerge till just before sunset; but in some species it is believed life may be prolonged for as much as three or four days, or even longer, if the weather be cold and wet, so as to keep them in a state of enforced rest, which amounts to a state of coma.
that their hold on life during this final stage of existence is brief there can be no gainsaying, for it is passed fasting. jaws are wanting, and the whole alimentary canal has been transformed into one long air-chamber. its walls are now of extreme tenuity, and by changes in the interior of the tube, valves are formed which convert the stomach into a capacious air-sac.” when movements,” remarks dr. david sharp, “tend to increase the capacity of the body cavity then air enters into the stomachic sac by the mouth orifice, but when muscular contractions result in pressure on the sac they close the orifices of its extremities by the valve-like structures just referred to; the result is, that as the complex movements of the body are made the stomach becomes more 231and more distended by air.” it was known even to the old naturalists that the dancing may-fly is a sort of balloon, but they were not acquainted with the exact mode of inflation. palmen says that in addition to the valve-like arrangements we have described, the entry into the canal is controlled by a circular muscle with which are connected radiating muscles attached to the walls of the head. the canal thus strangely transformed performs the functions of a balloon, and at the same time aids the functions of the reproductive organs.
where vast numbers of individuals set out simultaneously to achieve their nuptials there would seem to be no need for special devices on the part of either sex to call attention to their whereabouts. nevertheless, it is highly probable that the female exhales some distinctive odour; otherwise, having regard to the fact that she is overwhelmingly outnumbered by suitors, her discovery in such a crowd would be impossible, and it is of vital importance that no time should be lost in effecting conjugation, for the time for its accomplishment is perilously short. but there is another possible means of discrimination—the males may distinguish the females by the very different appearance of the head in the latter. at any rate, this may be true of some species wherein the males have no less than seven eyes, and these of three different kinds! the compound eyes, characteristic of insects, are, in these, divided, one half being set upon the summit of a pillar raised high above the level of the head, the other part remaining in its normal place at the side of the head; and in front of these, on what may be called the forehead, are three separate simple eyes, or “ocelli.” a reference to plate 32, fig. 3, will make this clear.
that the history of the later life of the may-fly is 232remarkable no one will deny: in many respects it is unique. yet for all its strangeness it enables us to set our compasses, so to speak, in regard to the phenomena of sex in other groups. the extraordinary disparity in the proportions between male and female, for example, is full of significance, for it shows, as has been suggested more than once in these pages, that, in the case of polygamous species, we are probably in error in supposing that the excess of females is due to the reduction in the number of males by reason of the elimination of males by fighting. the excess of males, or females, as the case may be, is due to an inherent quality in the germ-plasm. the may-fly might be regarded as an excessively polyandrous species if the number of males in relation to females alone be regarded: but actually it is monogamous. after a prosaic infancy they are suddenly transformed into gay lovers, dancing a marriage-dance. but for them is no marriage feast, nor any later sequence of domesticity. one in ten thousand may find a mate, and only in this is he more fortunate than his neighbours, for, like them, he too must die before the dawn. theirs is not even a sleep and a forgetting, but “one splendid hour of life, and then—oblivion.” it may be urged that even these which might seem to have been fooled, have not really lived in vain, for hosts of animals feast upon their bodies. myriads, indeed, are snapped up by fishes even before they have opened their wings, while birds rudely invade the swarms as they dance in mid-air, feasting on these fasting ones. but this is, after all, an inglorious end, and leaves us still asking cui bono?
is this amazing life-history a thing of yesterday, a new phase, or an order of things as old as the origin of the species, dating back some millions of years?
233so far as one can profitably speculate on such a theme it would seem more likely to be a relatively recent innovation. the nearly-related alder-flies (sialid?), so well known to anglers, seem like to meet with a similar fate, for the female lives but for a few days only and the male has an even briefer existence as a winged insect. the family to which the alder-flies belong contains a few species which attain gigantic proportions, as, for example, in the case of the north american members of the genus corydalis, which are giants. the males thereof are remarkable for the fact that they are armed with enormous jaws, which may be likened to a pair of callipers whose limbs have been crossed. these weapons serve as claspers, enabling the males to seize and hold the females during the act of mating. but even here the same brief span of life has to suffice them, for death follows swiftly on the fulfilment of the nuptial rites.
the perlid?, or stone-flies, which, like the sialid?, are aquatic neuroptera, the larval stages being passed in streams, present very puzzling features in regard to the adult males which, so far, have baffled all attempts at solution; yet they seem to have a very important bearing on the all-important work of reproduction. they are among the earliest insects to appear in spring, and possess an extraordinary power of resisting cold. one species, capina vernalis, common in the albany river, in canada, frequently comes up through cracks in the ice and casts its skin there! another, nemoura glacialis, which appears at about the same time, actually performs the nuptial rites in crevices in the dissolving ice! happily reason is denied them, or they would find life a mockery indeed; for having attained their final development, when the joyous and exhilarating exercise of 234flight should be theirs, they are compelled forthwith to fulfil their reproductive functions and die—in an ice-chamber! the males have wings which are rarely or never unfurled; as a rule they present nothing more than a crumpled mass of gauzy tissue, as if glued together. such species as attain to flight are most indifferent performers, travelling but slowly, with laboured movements and settling after a few yards have been traversed.
as a rule, among insects, where there is a difference in the power of flight, it is the male which is superior. the case of nemoura, just referred to, affords an instance where the contrary is the case, and mr. j. j. lister records the case of one of these flies—isogenus nubecula—taken at loch tanna in arran, wherein the wings of the female were greatly reduced, while those of the male were so much so as to be mere useless vestiges. similar facts have been recorded of more than one species in scotland, but in all such cases the phenomenon seems to be associated with the appearance of the insect in very early spring. in another species—nemoura trifasciata—only the front wings are reduced, the hind pair being large enough to cover the body. in male specimens of perla maxima taken in scotland, the wings are so short as to be useless for the purposes of flight, yet, in the same species taken in central europe, they are of ample proportions.
these facts are puzzling indeed, but they seem to show that flight is not essential to attain the ends of reproduction. as to whether these flies secure their mates by any kind of “courtship,” or how they find one another, seems not to be known. but the female is remarkable for the fact that she carries her eggs about with her, to the number of five or six thousand, attached to the 235end of the abdomen.
having regard to the fact that three thousand species of perlid? are known, and that they have a wide distribution over the earth’s surface, one might have expected that more would be known of their singular life-history. they are, however, flies of very unattractive appearance and great frailty, hence, save to anglers, by whom they are esteemed as bait for trout, they attract but little attention.
