BEETLES THAT “BLUFF”
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 creatures, 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 Prionidæ, offer an 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 Prionidæ 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 rôle 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 that 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 among 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 _Phanœus
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 different 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 Copridæ, 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, reptiles 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 and 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 of 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 antennæ of the male have a few of the
middle joints dilated and their under surfaces furnished with 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 antennæ 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 one 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 (_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.”
[Illustration: 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.
[Illustration: 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 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.
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 they 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 antennæ; 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
æsthetic 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 Phasmidæ 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
of 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 larvæ 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 span 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 this 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 and 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 remarkable
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?
So 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 (_Sialidæ_), 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 Perlidæ, or Stone-flies, which, like the Sialidæ, 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 flight 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 end of the abdomen.
Having regard to the fact that three thousand species of Perlidæ 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.