SOME OF THE “LOWER ORDERS”
Butterflies and Moths, and the Coloration of their Wings—Female
Choice and “Fine Feathers”—When Male Butterflies are Dominant—Sexual
Selection among Butterflies—Abortive Experiments—Wallace and the Sexual
Selection Theory—The Sense of Smell in Butterflies and Moths—Fragrant
Butterflies—Wingless Moths and their Lures to Lovers—Methods of Pairing
among Butterflies and Moths—More Experiments.
Not the least impressive feature met with in the study of animal
behaviour under the spell of the Sexual Instincts is its uniformity.
This fact becomes the more apparent as one turns to the lower grades
of life. Whether one starts with the vertebrates and works downwards,
or vice versa, the same problems arise and the same interpretation is
demanded. That is to say, the theory of “Sexual Selection” leads one to
the same conclusions whether it be tested by the evidence afforded by
the Butterflies and Moths, or that furnished by Birds or Mammals.
The accessory phenomena, the vehicles which give expression to these
internal fires, are in like manner curiously similar. These “vehicles”
are the “secondary sexual characters”—colour, and armature, and scent.
These very tangible signs are the phenomena in the Mystery Play of Sex
which first catch the attention of the investigator. To account for
these the theory of “Sexual Selection” was first devised.
After the birds, probably the group most conspicuous for its splendour
is that which contains the Scale-winged Insects or Lepidoptera, and it
has always been allowed that any explanation of the one must apply also
to the other. It seems impossible to avoid this conclusion. But before
going further it would be well to take note of one or two interesting
features in regard to coloration that have so far not been touched upon
in these pages.
The Coloration of Animals is generally regarded as a by no means
fortuitous feature, but one, on the contrary, controlled and
determined by various factors. Hence are recognized various kinds
of coloration: Obliterative or Protective-resemblance Coloration;
Warning Coloration; Mimetic Coloration; and Epigamic Coloration, or
the colours associated with courtship. These various types have been
subdivided and accorded technical labels by Professor E. B. Poulton,
in his “Colours of Animals,” but these need not be enlarged upon
here. Suffice it to say that it is generally held that all forms of
coloration can be explained, and all can be labelled, as to their
origin, with more or less certainty. There are those who doubt the
warranty for this classification. Commonly, it must be admitted, the
arguments of these sceptics are not impressive; they are sometimes even
stupid. That such coloration, however it be labelled, is subjected to
some control seems to be shown in the case of the Lepidoptera, for,
generally, in the Butterflies, the upper surface of the wings is much
more vividly coloured than the under surface, and this, apparently,
because when the creature is at rest the wings are brought up over
the back like the leaves of a book, so that the brightly-tinted, and
therefore conspicuous, area is concealed, as, for example, in the “Red
Admiral.” With the Moths the wings, while the creature is at rest, are
held horizontally, and it is the upper instead of the under surface
which is exposed, but the hind-wing is covered by the fore-wing. The
coloration is here very different; for while the exposed surfaces
of the fore-wings are commonly soberly tinted, the hind-wings may
be quite glaringly coloured. These bright colours are exposed only
during flight, or during moments of unusual excitement, as in the
case of the Eyed Hawk-moth. According to Weismann, this insect when
alarmed raises the fore-wings so as to expose the “eye-spots “on the
hind-wings, which, with the increased area of the wings, impart a
terrifying appearance to the body to would-be assailants. This is
as it may be, but for the moment the feature to be insisted upon is
that the bright colours are almost invariably hidden when the insect
is at rest, and by quite different means, determined, apparently, by
the different carriage of the wings. Now, according to some, bright
colours are begotten by strong light, but in the Moth and Butterfly the
surface area of the wing which is most exposed is the surface turned
to the light during rest, and this is the least coloured. The curious
relation between this coloration and the resting position is strikingly
illustrated by the case of one of the “small Blues” (_Lyccenæ_), cited
by Weismann. Herein the male, which has the upper surface of the wings
of a bright blue, rests in the position common to Butterflies—with the
wings raised and concealing the bright colour—while the female, which
has the upper surface of a dull brown, rests with the wings expanded.
As, however, the concealed under surface is not brightly coloured, it
is difficult to believe that these different postures and conspicuously
different colours can have been brought into existence solely by the
action of Natural Selection, which, it is generally contended, has
brought about the extinction of those individuals which neglected,
when resting, and therefore liable to be “caught napping,” to conceal
their arresting colours. There is, indeed, no apparent reason why the
female, which has nothing to conceal, should depart from the custom
common to Butterflies, of resting with the wings closed and raised,
this position effectively protecting the male. The facts seem to show
that the coloration of the exposed surfaces of the wings is determined
primarily by some physiological factor rather than by the incidence
of Natural Selection directly through external agencies. Thus, for
example, the action of light on the surface of the wings when in the
resting posture may well inhibit the production of vivid pigment owing
to some inherent physiological idiosyncrasy. But any individuals which
lack this inhibiting factor—as some species which, though resting, are
brightly coloured, appear to do—will be eliminated, if they live in an
environment harbouring eliminating factors, which the exceptions to the
rule we must suppose do not. But on this interpretation the fundamental
factor in the determination of the coloration is the action of light.
Selection imposes a bar only to certain types of coloration.
Some Butterflies and Moths, it has just been hinted, when resting
exhibit bright colours. Our “Swallow-tail” the under surface of the
wings is as brightly tinted as the upper. Among the Moths may be cited
many of the gorgeous Atlas Moths, the Hawk Moths, the beautiful Indian
_Dysphania militaris_—wherein the whole of the exposed surface is of a
beautiful and vivid violet and yellow—and the tropical members of the
Burnet Moths, belonging to the family Syntomidæ. In all these cases it
is not the under but the upper surface of the fore-wings which has thus
departed from the usual rule of the tribe. Not the least remarkable
feature of these insects is the fact that while the Atlas
and Hawk Moths are crepuscular in habits, the Dysphanias and Syntomids
and Burnet Moths are diurnal, and revel in the sunlight.
To revert for a moment to the factors to which these and other bright
and often conspicuous hues are due. That all highly-coloured animals
are descendants of dull-coloured ancestors there can be no room for
doubt. The vivid tints they now display are to be regarded as due to
some change in the metabolism, some clarifying process of the organism
whereby the various pigments became segregated, concentrated and
intensified. But many of the most vivid hues are not due to pigment at
all, but to changes in the surface structure of the coloured areas.
Such are the wonderful metallic colours which all kinds of animals
display. The iridescence is due to the breaking up of the light by
reflection from finely-grooved surfaces.
Whatever their nature, one still asks what is their origin, what
brought them into being. They cannot be regarded simply as adaptations
which have arisen to meet the demands of the environment, as are the
structural peculiarities of the skeleton for example; for in this
case both sexes, and all stages of growth, should display the same
hues, and this is rarely the case. Furthermore, we should not in this
case be left with a vast assemblage of forms which certainly cannot
be “pigeon-holed” as to the nature of their coloration. Such, for
example, as the marine types of birds.
The metallic and iridescent tints to which reference has just been
made, occur among animals to which they can be of but doubtful value,
as in the Golden Mole, for example, or the inside of the Oyster shell.
Their existence in such places well illustrates what we may call the
fortuitous, or apparently fortuitous, beginning of colour of whatever
kind, regarded from an analytical point of view. That is to say, we
are not concerned with the fact that animals are coloured—that is
inseparable from their existence; but with why this coloration should,
in some cases, assume so conspicuous a brilliancy and vividness—a
coloration varying in its character with every species, but apparently
unchanging among the individuals of that species.
No answer to this, likely to find general acceptance, seems to be
forthcoming at present. But it is significant to remark that all
coloration of the kind now under consideration has its origin, as
have most other structural characters, in the male. It is as true of
coloration as of, say, skeletal characters. One turns to the male
for what is new in the history of a species, to the female and young
for indications of past history. It is equally true that in their
coloration one finds the same sequence of development—the male first,
then the female, then the young, till both sexes, and all stages, are
once more alike in hue. And this rule seems to apply to coloration of
all kinds—Protective—Warning—Epigamic.
The tendency to develop brilliant colours is associated with some
physiological diathesis with which we are not yet acquainted. But
once having started, this tendency gathers force with each succeeding
generation and continues to exhibit an almost kaleidoscopic capacity
for change, unless, and until, checked by Natural Selection, whereby
its further progress in any given direction may be barred, or some
other element or aspect of the coloration may be introduced.
Given this controlling factor, all the various types of coloration
would seem to be interpretable. By almost common consent, however,
the resplendent coloration of the males among many species of birds,
a coloration often apparent only during the reproductive period, and
the more conspicuous ornamentation of the males of many other groups,
higher and lower in the scale of organization, are supposed to be
governed by an entirely different factor—female choice, or preference.
The exercise of this, it is contended, has gone on for countless
generations, and the tendency has ever been to heighten the intensity
of the ornament by the rejection of the less favoured suitors in favour
of their more resplendent rivals. Birds and Butterflies alike are
supposed to be swayed by the same irresistible desire to mate, and mate
only with what we may call the smartest and best—groomed of their many
suitors; and these, of course, being the most vigorous, most virile,
sustain the stamina of the race and so attain Nature’s end.
So long as attention was focused alone, or mainly, on birds conspicuous
for the highly ornamental character of their plumage, this theory
seemed reasonable and probable enough, for one may admit in their
courtships an element, at least, of intelligence and keenness of
perception. But it has now been abundantly demonstrated that the
animated displays so characteristic of these gaily-bedecked gallants,
are enacted with no less persistence and vim by species which exhibit
a Quaker-like soberness of dress. Thus, then, the champions of the
Sexual Selection theory have been dazzled by the tinsel, and have
missed the essential elements—the physical and psychological side of
the display—the contortions, prancings, and so on, and they have missed
the even more important element, the preliminary struggle for territory.
In this new light, the gaily-bedizened individuals of the Insect world
may be surveyed afresh. The explanation of such of their features as
are commonly attributed to Sexual Selection in terms of female choice,
whereby only the most favoured from among a crowd of suitors could
hope to succeed, may now be replaced by that which obtains also in
the case of the higher animals. It seems to fit the facts better. One
cannot understand, for example, how, on the interpretation of Sexual
Selection, the extraordinary disparity in numbers between the sexes
of some species of Butterflies came about. Thus in that marvellously
beautiful genus Ornithoptera there is one species (_O. brookiana_) in
which the females are excessively rare; so much so that the collector
Kunstler could only obtain fifteen females to one thousand males.
Though the males, among the Butterflies, are commonly much more
numerous than the females, the disparity is rarely so great as with
this species; but there are many in which the proportion of males to
females is as fifty to one. As with the higher vertebrates selection
affords no explanation of this curious disproportion. Though according
to Weismann it fulfills “the first postulate in ‘Sexual Selection’
namely, that there be an unequal number of individuals in the two
sexes.” But Sexual Selection here has a little over-reached itself,
for surely one hundred suitors seems an embarrassing number for an
inexperienced female to have to choose from! To say nothing of the
ninety-nine males doomed to perish without leaving offspring.
That the beauty of colour and form which the Lepidoptera, and
especially the diurnal Lepidoptera, or Butterflies, exhibit is due to
the choice by the females—albeit an unconscious choice—of the most
resplendent of her suitors, that is, in other words, that she yields at
last to the most ravishing member of the crowd—there is no evidence to
show. There would seem to be no possibility of a differential selection
from among a number of males, for there is no “display” comparable to
that, say, of birds. And what is more, it is unlikely that, if there
were, she would find anything to choose between them, for the range
of variation in, say, one hundred males of any given species is very
slight. Finally we have no trustworthy evidence to show that the eyes
of Butterflies and Moths are sufficiently good to enable them to make
nice distinctions between slightly different males. We have no evidence
that the eyes of Insects are capable of discriminating the details of
the often intricate patterns which their own wings, and those of their
suitors, exhibit.
In the matter of “Secondary Sexual Characters,” indeed, the Lepidoptera
exhibit very little difference between the sexes. As a rule the females
are larger, often strikingly so, but in the matter of coloration they
show far less disparity. But there are exceptions to every rule. A
striking illustration of this is afforded by the genus Ornithoptera.
The butterflies of this superb group are of huge size, and the females
are larger than their consorts, and commonly are extremely different
therefrom both in coloration and habits. In _Ornithoptera paradisea_
this disparity attains its maximum. The female, remarks Mr. David
Sharp, “is a large, sombre creature of black, white and grey colours,
but the male is brilliant with gold and green, and is made additionally
remarkable by a long tail of unusual form on each wing.” But a glance
at the two sexes will show that the female, though less gorgeously
arrayed, still disports a livery which is of a highly specialized
or elaborated character. How are we to account for her differences
in shape, size and coloration on the older interpretation of Sexual
Selection? The perceptual powers, the mentality, of a Butterfly are
surely of a far lower grade than those of a bird, or even a fish. Here,
therefore, we cannot attribute the same possibilities of response to
form and colour which we can ascribe with tolerable safety to the
vertebrates. Yet the Sexual Selection theory as generally understood
demands this.
So far so good. And now as to the part played by Sexual Selection among
the Lepidoptera. Darwin, in formulating this, found its application
to the Lepidoptera a very disconcerting problem, being naturally
disposed to regard the extraordinary wealth of colour which these
insects exhibit as the outcome of a process of female selection, in
every way comparable to that which he held to obtain among the birds.
He did not postulate a conscious, deliberate, selection; but a final
abandonment on the part of the female to the male which, by his beauty
and demonstrativeness, pleased her most. He assumed that at this
critical time she would always be surrounded by rival suitors, offering
varying if slight degrees of difference: and, indeed, in many cases she
is thus surrounded. He remarks, in discussing the case of Butterflies:
“The males sometimes fight together in rivalry, and many may be seen
pursuing or crowding round the same female.” But in the case of the
Silk-moths—and here is another illustration of the merciless criticism
to which he submitted his own theories—he remarks: “The females appear
not to evince the least choice in regard to their partners.” This fact,
which is certainly true in the case of both Butterflies and Moths, and
these gorgeous hues, disconcerted him, as is shown in the passage:
“Unless the female prefer one male to another, the pairing must be left
to mere chance, and this does not appear probable.”
The facts which have come to light in regard to the “Courtship” of
Butterflies since Darwin wrote are meagre enough, but such as have been
recorded give no support to the supposition that the females are really
influenced by, or even perceive the colours of, their mates. Just on
five-and-twenty years ago the naturalist Skertchly published some
observations on the Courtship of that magnificent Bornean Butterfly
_Ornithoptera brookiana_. He one day came on a male sipping honey from
the flowers of a tree, vibrating its wings with the rapidity of a
Hawk-moth, and the vivid green of the wings flashing in the sunlight,
though the crimson areas thereof were invisible. The female came “and
did all the wooing.” They circled about in flight with the female
above and somewhat behind, so that she could see, we are told, the
emerald markings; but there was no real evidence here that she was
really influenced by his coloration, and if this really were the case
then the coloration of the female equally demands an explanation,
for this, though less gorgeous than that of the male, is far from a
primitive type; on the contrary, it is of a highly differentiated
character. Furthermore, in this genus, as has already been remarked,
the males outnumber the females by, roughly, one hundred to one. Again,
Moseley, the naturalist on the Memorable Voyage of the Challenger in
1872, when in the Aru Islands, was once “lucky enough to find a flock
of about a dozen males fluttering round and mobbing a single female.
They were then hovering slowly, quite close to the ground, and were
easily caught.” But he was by no means convinced that any choice was
exerted. And he suggests “a series of experiments, in which, in the
case of highly-coloured and decorated Butterflies, the colours should
be rubbed off the wings of a few among a number of males, or painted
over of a black or brown colour. It might be tested whether the females
would always prefer the highly-coloured ones.” Such experiments are
foredoomed to failure, for the removal of the scales would remove the
only source of communication between the sexes.
Wallace, always a strenuous opponent of the Sexual Selection theory,
found in the behaviour of Butterflies and Moths when mate-hunting a
particularly powerful countervailing weapon. He assumes that Darwin
postulated a conscious selection on the part of the female, and with
some show of reason, though it is probable that Wallace was mistaken
in this. “The weakness of the evidence for conscious selection among
these insects,” he remarks, “is so palpable, that Mr. Darwin is obliged
to supplement it by the singularly inconclusive argument, ‘Unless
the female prefer one male to another the pairing must be left to
mere chance, and this does not appear probable’ But he has just said,
‘The males sometimes fight together in rivalry, and many may be seen
pursuing or crowding round the same female’ While in the case of the
Silk-moths—‘the females appear not to evince the least choice in
regard to their partners.’ Surely the plain inference from all this
is, that the males fight and struggle for the almost passive female,
and that the most vigorous and energetic, the strongest-winged or the
most persevering wins her. How can there be chance in this? Natural
Selection would here act, as in birds, in perpetuating the strongest
and most vigorous males; and as these would usually be the more highly
coloured of their race, the same results would be produced as regards
the intensification and variation of colour in the one case as the
other.”
Commenting on Darwin’s interpretation of those cases wherein the
females are more brilliantly coloured than the males, he insists that
on his (Darwin’s) theory “throughout the whole animal kingdom the males
are usually so ardent that they will accept any female, while the
females are coy, and choose the handsomest males, whence it is believed
the general brilliancy of males as compared with females has arisen.”
“Mr. Darwin admits,” he continues, “that these bright colours have
been acquired for protection [because they resemble those of species
which from their disagreeable taste are avoided by birds and other
insect-eating enemies]; but as there is no apparent cause for the
strict limitation of the colour to the female, he believes that it
has been kept down in the male by its being unattractive to her. This
appears to me to be a supposition opposed to the whole theory of
Sexual Selection itself. For this theory is, that minute variations
of colour in the male are attractive to the female, have always been
selected, and that thus the brilliant male colours have been produced.
But in this case he thinks that the female Butterfly had a constant
aversion to every trace of colour, even when we must suppose it was
constantly recurring during the successive variations which resulted
in such a marvellous change in herself. But the case admits of a
much more simple interpretation. For if we consider the fact that
the females frequent the forests where the Heliconidæ abound [the
distasteful species already referred to] while the males fly much in
the open and assemble in great numbers with other white and yellow
Butterflies on the banks of rivers, may it not be possible that the
appearance of orange-stripes or patches would be as injurious to the
male as it was useful to the female, by making him a more easy mark
for insectivorous birds among his white companions? This seems a more
probable supposition than the altogether hypothetical choice of the
female, sometimes exercised in favour of, and sometimes against, every
new variety of colour in her partner.”
Wallace’s arguments are not so crushing as he supposed them to be,
and they contribute nothing towards the solution of the problem to be
faced. But if colour played the part which Darwin believed, and colour
alone be concerned, it is curious that the males should recognize
their mates in a guise so unlike their own. How is it that they do not
pass them by as members of the totally different distasteful species?
Whenever, indeed, the female is more or less brightly liveried than
the male, how do the sexes recognize one another, and how, when they
live in environments so different as those referred to by Wallace,
do they find one another when possessed by the insistent demands of
the “sex-hunger” which is the all-essential stimulant to secure the
continuation of the race?
The factors which assure the satisfaction of this hunger differ in
some important features from those which obtain among the higher
animals—birds, for example. In the first place there is no necessity to
find and hold territory, which is an imperative necessity where there
are eggs to be brooded and young to be fed. In the second, the males,
as has just been remarked, must search for the females, often indeed,
in the case of many Moths, because they are wingless.
This search is conducted by the sense of smell. This fact, familiar
enough to-day to the entomologist and the student of Evolution,
was unknown to the earlier naturalists. Neither Darwin nor Wallace
suspected it. It would have been wonderful if they had, for there is
nothing in the general appearance of these insects which suggests an
organ of smell, nor is there anything in the structure of the nervous
system which would indicate this subtle sense. During recent years,
however, the number of workers engaged on the investigation of the
senses of animals has increased immensely, and great strides have been
made in perfecting instruments of research. To the efforts of these
workers we owe the discovery of the seat of the scent-detecting organs
and the source of the scent. The former are furnished by the antennæ,
which lodge also the senses of taste and touch.
Among the Lepidoptera these constitute important secondary sexual
characters, the antennæ, among the Moths at any rate, presenting
striking differences in male and female. The scent-producing organs
are very elusive structures, and so far have been definitely traced,
among Butterflies, only in the males, where they are formed by certain
peculiarly modified scales known as “androconia.” They may be either
irregularly scattered over the wing, or may form complex structures.
Sometimes they are arranged in the form of brightly-coloured,
bristle-like tufts on the hind-wings, sometimes in a fringe along the
edge of the hind-wing. In some of the Moths they are arranged to form
a thick, glistening white felt, which fills a folded-over portion
of the edge of the hind-wing, and in many cases “the perfume can be
retained,” Weismann remarks, “and then, by a sudden turning out of the
wing-fold, be allowed to stream forth.” In the Ghost-moth (_Hepialus
humuli_), the hind-legs of the male have become pressed into service
and have become transformed into scent-bottles, since they are swollen
and filled with glands for the manufacture of odorous matter.
The naturalist Fritz Müller discovered the fact that some of the
Butterflies which haunted his Brazilian garden exhaled a flower-like
fragrance. Anyone can test this curious trait for himself who will take
the trouble to brush his finger over the wing of a newly-caught male
Garden-White Butterfly (_Pieris napi_). The white powder which will
adhere to the finger will be found to be made up of the wing-scales,
which will exhale a delicate perfume of lemon or balsam! Among the
Moths the strong odour of musk is exhaled by the Convolvulus Hawk-moth
(_Sphinx convolvuli_).
It is, however, only in the males that these odours can be detected,
and, though palpable enough to human nostrils, their power of
diffusion is apparently extremely limited. They would seem to serve as
aphrodisiacs for the stimulation of the female, and, as a consequence,
there is no need that they should start into activity until the male
has arrived at the immediate neighbourhood of his prospective mate.
[Illustration: Plate 31.
BRIGHT COLOURS WHICH CANNOT UK ATTRIBUTED TO “SEXUAL SELECTION.”
1. Eyed Hawk-moth, under the influence of excitement.
2, A Butterfly, Zeuxidia horsfieldi, Feld, showing tufts of
scent-diffusing scales on the hind-wings.
Face page 200.]
With the females of the Moths, however, matters are otherwise. For the
most part Moths are nocturnal, and hence could not distinguish one
another when on the search for mates, and in many species the females
are wingless, and consequently are unable to move from the immediate
neighbourhood in which they emerged from the pupal stage. In either
case some means of informing the males of the presence of females is
an imperative necessity for the continuation of the race. This is
provided by means of a subtle odour exhaled by the females which,
though imperceptible to human nostrils, must possess an extraordinarily
penetrating power. Weismann gives an instance of this in the case of
the nocturnal Eyed Hawk-moth (_Smerinthus ocellatus_). He placed some
females, without any special intention, in a covered vessel near an
open window. “The very next morning several males had gathered, and
were sitting on the window-sill, or on the wall of the room close to
the vessel, and by continuing the experiment I caught, in the course of
nine nights, no fewer than forty-two males of this species, which I had
never believed to be so numerous in the gardens of the town....” To
this power of exhaling odours we may attribute the wingless condition
of many Moths, for otherwise the loss of flight would have brought
about extinction long before any perceptible reduction in the wings had
taken place. The odour which such prisoners emit seems to possess an
irresistible attractiveness, and this fact is commonly taken advantage
of by entomologists. The Common Vapourer Moth (_Orgyia antiqua_)
affords a good illustration of this. The female is wingless, and little
more than a pouch for eggs, but in certain seasons it is very abundant,
even in the midst of London. That experienced entomologist Prof. Selwyn
Image, in a letter to my friend Mr. John Cooke, remarks, on this theme,
that the Caterpillars may be seen crawling by hundreds in and around
the squares, while the males may be seen flying up and down New Oxford
Street or Tottenham Court Road. If a virgin female be put in a box
placed outside the window, within a very short space of time, often not
more than a few minutes, several males will be fluttering round her.
This device for attracting males is commonly known as “assembling.”
More striking is the case of the Oak-eggar Moth (_Lasiocampa quercus_).
Mr. Richard South, in his most useful “Moths of the British Isles,”
relates that on one occasion he had a number of pupæ in a cage in
a cottage on the edge of a moor near Lynton, North Devon, and these
attracted quite a number of males into the room containing the precious
casket, and he was enabled to capture several. The next day he placed a
female which had meanwhile emerged, in a “roomy chip-box, and carried
it, in a satchel, to the moor, where it was placed on the ground; the
males began to arrive soon afterwards, and some fine examples were
secured.” But the sequel is even more remarkable; for, he remarks:
“Although the female was taken on the moor only on one occasion, that
satchel continued to be an object of interest to the male Eggars
for several days afterwards.” That this scent is capable of being
transferred to foreign objects, and of retaining its power for several
days, is a striking proof of its pungency, yet it is quite impalpable
to human nostrils! The Kentish Glory Moth (_Endromis versicolor_)
affords yet another instance of this curious attraction by scent, the
effectiveness of which is not even lessened by exhalations of the
human body, for if a virgin female be placed in a box, and this be
placed in one’s pocket, the males will often swarm round one and even
endeavour to gain access to the box. In all such cases the females,
even when capable of flight—the female Vapourer is wingless—never fly
until after impregnation has taken place. Hence males with defective
scent—detecting powers inevitably fail to leave offspring.
Selection, then, here lies between males of the most active
scent-detecting powers, and not between those of the most brilliant
colours. Nevertheless, both males and females—where the females
are winged—exhibit a remarkably beautiful coloration, and this is
especially true of the Kentish Glory, wherein both sexes wear a
resplendent dress. That of the male—which is much smaller than the
female—differs in that the fore-wings are darker, but bear the same
pattern as in the female, while the hind-wings are chestnut-red instead
of cream colour as in the female. If this scent-factor has replaced
colour as an inciting agent to pairing, then these Moths should be of
sombre hues. That such is not the case seems sufficient to show that
the colour is not due to Sexual Selection, for it is highly improbable
that scent and colour are both of equal importance, and this being so,
one would expect to find the negligible factor eliminated.
The existence, then, of bright colours in this and other species
in like case, seems to show that it has nothing to do with Sexual
Selection, directly at any rate. The males having assembled, their
presence is probably communicated to the female by the characteristic
male odour, which is never of the same penetrating quality as that
of the female. There is no need that it should possess this, for the
females never seek their mates. The successful male, where several
rivals are competing, is probably not simply the strongest, but he
who also disperses the right odour necessary to provoke the pairing
response. These illustrations furnished by the scent-hunting,
scent-dispersing males and females are of the highest importance
to students of the Sexual Selection theory, for they seem to show
conclusively that coloration plays at any rate but a minor part
therein. The importance of the scent-detecting organs is shown in
the very different types of antennæ which obtain between male and
female Moths, those of the male taking the form of huge feather-like
structures, as in some Saturniidæ, and far exceeding those of the
female in size.
The methods of pairing which obtain among Butterflies and Moths, it is
not surprising to find, are very different; for whereas in the former
it takes place on the wing, in the latter the female is always in a
resting position. Where the females are winged, long flights are often
taken for the purpose of depositing and distributing the eggs: the
flightless forms make no such excursions. A few, as in the case of some
of the _Psychidæ_ are not only wingless, but limbless and maggot-like.
They never leave the chrysalis case, but deposit their eggs inside it.
Though there is undoubtedly much that is wonderful about the mating
of these scent-distributing species, the history of the Moths of the
genus Acentrophus is more wonderful and more mysterious still. For
the females are aquatic. The males may sometimes be found in crowds
fluttering over the surface of large but shallow sheets of water. The
females, which are wingless, come to the surface and, like sirens, draw
the males under water, where coupling takes place; after which they
probably immediately die. But how do they discover their submerged
mates? The escape from the water of any odour which the females may
possess seems well nigh impossible.
Whether display, such as birds appear to delight in ever takes place
among the Lepidoptera seems doubtful Nevertheless, something closely
akin thereto seems to have been found in the case of certain species
of Butterflies (_Heliconius melpomene_ and _H. rhea_), which have been
seen dancing in the air like gnats, and when some of them withdrew
others took their places. Again, having regard to the fact that birds,
when alarmed or excited, will perform the display which is more or less
characteristic of periods of sexual excitement, it is possible that the
position of alarm assumed by some of the Hawk Moths may also be used in
Courtship (Fig. 1, Plate 31). But we have no evidence on this point,
and from the part played by scent in the mating of Butterflies it seems
improbable that such displays take place.
A serious attempt to test the Sexual Selection theory by experiment—to
test the extent, if any, of female choice in mating—was made some
years ago by Mayer, an American naturalist, on the large Bombycid Moth
(_Callosamia promethea_). This species exhibits striking dissimilarity
between the sexes in regard to colour and pattern. “The females,”
remarks Professor Kellog, “are reddish brown in ground colour, while
the males are blackish, and in the two sexes the pattern is distinctly
different....” Mayer took four hundred and forty-nine pupæ, in
cocoons, of this moth and endeavoured to discover, first of all,
whether the males found the females by sight or smell. Enclosing
females in jars, some of which were covered and some of which were
uncovered, he found that males paid no attention to females enclosed in
transparent jars so closed as to prevent the escape of odours, while
such as were enclosed in boxes or wrapped in cotton-wool, so as to be
invisible, but yet capable of exhaling odour, were besieged by males.
To locate the organs of scent in the female he cut off the abdomen of
several and placed the abdomens and their late owners at some distance
apart. Males came to the abdomens and not to the thorax and wings.
Males whose antennæ were covered with shellac, photographic paste,
glue, paraffin, etc., showed no response to the female exhalations,
until the covering medium was removed.
Mayer next tested the selective action of the females. He began by
removing their wings and affixing to the stumps the wings of males.
The males mated with the females quite as readily as under normal
conditions, though the most conspicuous female characters had been
exchanged for those of the male. After this he affixed female wings
upon the males, but mating took place as usual. The females did not
seem to detect anything unusual in their suitors, nor did normal
males attempt to pair with males bearing female wings. Later he tried
the experiment of dyeing the wings of three hundred males scarlet
or green, and matched these against three hundred which were left
untouched. The disguised, dyed males succeeded in pairing as easily
as their normally-coloured brethren. The females exhibited no choice
whatever. Hence, then, we have further reason to believe that with
the Lepidoptera scent, not sight, is the channel by which mates are
found. So far as the evidence goes, it seems to show conclusively that
in all that concerns sexual relationships, scent is the guiding and
determining factor. By scent the females attract the males, and by
scent of another kind the males sharpen the procreative appetites of
the females.
If the interpretation adopted in these pages is correct, these
manifestations and emanations of colour and scent are readily
accounted for; for they are manifestations of inherent growth changes
which, having started, are free to go on increasing in amplitude
unless, and until, checked by natural selection. There is nothing
unreasonable or improbable in this interpretation; on the contrary,
it embraces also many other features hitherto ignored, but no less
demanding an explanation. Such, for example, as the infinite variety
of form and sculpture which the scales of the wings and the eggs
display. These are details visible only by the aid of the microscope,
but they demand explanation as much as the more obvious characters.
Moreover they have the advantage of belonging to a set of characters
which cannot in any way influence the choice, if choice there be, in
the selection of mates, nor are they of a nature likely to affect the
results of the struggle for existence. Of these characters, then—the
sculpturing of the egg-shell and of the scales, the “nervation” of
the wings, and coloration—we can say no more than that they are
idiosyncrasies of growth, free to develop in any direction unless, and
until, checked by natural selection, which will speedily eliminate
disharmonies with the environment.