INSECTS, continued. ORDER LEPIDOPTERA. (BUTTERFLIES AND MOTHS.)
Courtship of butterflies—Battles—Ticking noise—Colours common to both
sexes, or more brilliant in the males—Examples—Not due to the direct
action of the conditions of life—Colours adapted for protection—Colours
of moths—Display—Perceptive powers of the
Lepidoptera—Variability—Causes of the difference in colour between the
males and females—Mimicry, female butterflies more brilliantly coloured
than the males—Bright colours of caterpillars—Summary and concluding
remarks on the secondary sexual characters of insects—Birds and insects
compared.
In this great Order the most interesting points for us are the
differences in colour between the sexes of the same species, and
between the distinct species of the same genus. Nearly the whole of the
following chapter will be devoted to this subject; but I will first
make a few remarks on one or two other points. Several males may often
be seen pursuing and crowding round the same female. Their courtship
appears to be a prolonged affair, for I have frequently watched one or
more males pirouetting round a female until I was tired, without seeing
the end of the courtship. Mr. A.G. Butler also informs me that he has
several times watched a male courting a female for a full quarter of an
hour; but she pertinaciously refused him, and at last settled on the
ground and closed her wings, so as to escape from his addresses.
Although butterflies are weak and fragile creatures, they are
pugnacious, and an emperor butterfly (1. Apatura Iris: ‘The
Entomologist’s Weekly Intelligence,’ 1859, p. 139. For the Bornean
Butterflies, see C. Collingwood, ‘Rambles of a Naturalist,’ 1868, p.
183.) has been captured with the tips of its wings broken from a
conflict with another male. Mr. Collingwood, in speaking of the
frequent battles between the butterflies of Borneo, says, “They whirl
round each other with the greatest rapidity, and appear to be incited
by the greatest ferocity.”
The Ageronia feronia makes a noise like that produced by a toothed
wheel passing under a spring catch, and which can be heard at the
distance of several yards: I noticed this sound at Rio de Janeiro, only
when two of these butterflies were chasing each other in an irregular
course, so that it is probably made during the courtship of the sexes.
(2. See my ‘Journal of Researches,’ 1845, p. 33. Mr. Doubleday has
detected (‘Proc. Ent. Soc.’ March 3, 1845, p. 123) a peculiar
membranous sac at the base of the front wings, which is probably
connected with the production of the sound. For the case of Thecophora,
see ‘Zoological Record,’ 1869, p. 401. For Mr. Buchanan White’s
observations, the Scottish Naturalist, July 1872, p. 214.)
Some moths also produce sounds; for instance, the males Theocophora
fovea. On two occasions Mr. F. Buchanan White (3. ‘The Scottish
Naturalist,’ July 1872, p. 213.) heard a sharp quick noise made by the
male of Hylophila prasinana, and which he believes to be produced, as
in Cicada, by an elastic membrane, furnished with a muscle. He quotes,
also, Guenee, that Setina produces a sound like the ticking of a watch,
apparently by the aid of “two large tympaniform vesicles, situated in
the pectoral region”; and these “are much more developed in the male
than in the female.” Hence the sound-producing organs in the
Lepidoptera appear to stand in some relation with the sexual functions.
I have not alluded to the well-known noise made by the Death’s Head
Sphinx, for it is generally heard soon after the moth has emerged from
its cocoon.
Giard has always observed that the musky odour, which is emitted by two
species of Sphinx moths, is peculiar to the males (4. ‘Zoological
Record,’ 1869, p. 347.); and in the higher classes we shall meet with
many instances of the males alone being odoriferous.
Every one must have admired the extreme beauty of many butterflies and
of some moths; and it may be asked, are their colours and diversified
patterns the result of the direct action of the physical conditions to
which these insects have been exposed, without any benefit being thus
derived? Or have successive variations been accumulated and determined
as a protection, or for some unknown purpose, or that one sex may be
attractive to the other? And, again, what is the meaning of the colours
being widely different in the males and females of certain species, and
alike in the two sexes of other species of the same genus? Before
attempting to answer these questions a body of facts must be given.
With our beautiful English butterflies, the admiral, peacock, and
painted lady (Vanessae), as well as many others, the sexes are alike.
This is also the case with the magnificent Heliconidae, and most of the
Danaidae in the tropics. But in certain other tropical groups, and in
some of our English butterflies, as the purple emperor, orange-tip,
etc. (Apatura Iris and Anthocharis cardamines), the sexes differ either
greatly or slightly in colour. No language suffices to describe the
splendour of the males of some tropical species. Even within the same
genus we often find species presenting extraordinary differences
between the sexes, whilst others have their sexes closely alike. Thus
in the South American genus Epicalia, Mr. Bates, to whom I am indebted
for most of the following facts, and for looking over this whole
discussion, informs me that he knows twelve species, the two sexes of
which haunt the same stations (and this is not always the case with
butterflies), and which, therefore, cannot have been differently
affected by external conditions. (5. See also Mr. Bates’s paper in
‘Proc. Ent. Soc. of Philadelphia,’ 1865, p. 206. Also Mr. Wallace on
the same subject, in regard to Diadema, in ‘Transactions, Entomological
Society of London,’ 1869, p. 278.) In nine of these twelve species the
males rank amongst the most brilliant of all butterflies, and differ so
greatly from the comparatively plain females that they were formerly
placed in distinct genera. The females of these nine species resemble
each other in their general type of coloration; and they likewise
resemble both sexes of the species in several allied genera found in
various parts of the world. Hence we may infer that these nine species,
and probably all the others of the genus, are descended from an
ancestral form which was coloured in nearly the same manner. In the
tenth species the female still retains the same general colouring, but
the male resembles her, so that he is coloured in a much less gaudy and
contrasted manner than the males of the previous species. In the
eleventh and twelfth species, the females depart from the usual type,
for they are gaily decorated almost like the males, but in a somewhat
less degree. Hence in these two latter species the bright colours of
the males seem to have been transferred to the females; whilst in the
tenth species the male has either retained or recovered the plain
colours of the female, as well as of the parent-form of the genus. The
sexes in these three cases have thus been rendered nearly alike, though
in an opposite manner. In the allied genus Eubagis, both sexes of some
of the species are plain-coloured and nearly alike; whilst with the
greater number the males are decorated with beautiful metallic tints in
a diversified manner, and differ much from their females. The females
throughout the genus retain the same general style of colouring, so
that they resemble one another much more closely than they resemble
their own males.
In the genus Papilio, all the species of the Aeneas group are
remarkable for their conspicuous and strongly contrasted colours, and
they illustrate the frequent tendency to gradation in the amount of
difference between the sexes. In a few species, for instance in P.
ascanius, the males and females are alike; in others the males are
either a little brighter, or very much more superb than the females.
The genus Junonia, allied to our Vanessae, offers a nearly parallel
case, for although the sexes of most of the species resemble each
other, and are destitute of rich colours, yet in certain species, as in
J. oenone, the male is rather more bright-coloured than the female, and
in a few (for instance J. andremiaja) the male is so different from the
female that he might be mistaken for an entirely distinct species.
Another striking case was pointed out to me in the British Museum by
Mr. A. Butler, namely, one of the tropical American Theclae, in which
both sexes are nearly alike and wonderfully splendid; in another
species the male is coloured in a similarly gorgeous manner, whilst the
whole upper surface of the female is of a dull uniform brown. Our
common little English blue butterflies of the genus Lycaena, illustrate
the various differences in colour between the sexes, almost as well,
though not in so striking a manner, as the above exotic genera. In
Lycaena agestis both sexes have wings of a brown colour, bordered with
small ocellated orange spots, and are thus alike. In L. oegon the wings
of the males are of a fine blue, bordered with black, whilst those of
the female are brown, with a similar border, closely resembling the
wings of L. agestis. Lastly, in L. arion both sexes are of a blue
colour and are very like, though in the female the edges of the wings
are rather duskier, with the black spots plainer; and in a bright blue
Indian species both sexes are still more alike.
I have given the foregoing details in order to shew, in the first
place, that when the sexes of butterflies differ, the male as a general
rule is the more beautiful, and departs more from the usual type of
colouring of the group to which the species belongs. Hence in most
groups the females of the several species resemble each other much more
closely than do the males. In some cases, however, to which I shall
hereafter allude, the females are coloured more splendidly than the
males. In the second place, these details have been given to bring
clearly before the mind that within the same genus, the two sexes
frequently present every gradation from no difference in colour, to so
great a difference that it was long before the two were placed by
entomologists in the same genus. In the third place, we have seen that
when the sexes nearly resemble each other, this appears due either to
the male having transferred his colours to the female, or to the male
having retained, or perhaps recovered, the primordial colours of the
group. It also deserves notice that in those groups in which the sexes
differ, the females usually somewhat resemble the males, so that when
the males are beautiful to an extraordinary degree, the females almost
invariably exhibit some degree of beauty. From the many cases of
gradation in the amount of difference between the sexes, and from the
prevalence of the same general type of coloration throughout the whole
of the same group, we may conclude that the causes have generally been
the same which have determined the brilliant colouring of the males
alone of some species, and of both sexes of other species.
As so many gorgeous butterflies inhabit the tropics, it has often been
supposed that they owe their colours to the great heat and moisture of
these zones; but Mr. Bates (6. ‘The Naturalist on the Amazons,’ vol. i.
1863, p. 19.) has shown by the comparison of various closely-allied
groups of insects from the temperate and tropical regions, that this
view cannot be maintained; and the evidence becomes conclusive when
brilliantly-coloured males and plain-coloured females of the same
species inhabit the same district, feed on the same food, and follow
exactly the same habits of life. Even when the sexes resemble each
other, we can hardly believe that their brilliant and
beautifully-arranged colours are the purposeless result of the nature
of the tissues and of the action of the surrounding conditions.
With animals of all kinds, whenever colour has been modified for some
special purpose, this has been, as far as we can judge, either for
direct or indirect protection, or as an attraction between the sexes.
With many species of butterflies the upper surfaces of the wings are
obscure; and this in all probability leads to their escaping
observation and danger. But butterflies would be particularly liable to
be attacked by their enemies when at rest; and most kinds whilst
resting raise their wings vertically over their backs, so that the
lower surface alone is exposed to view. Hence it is this side which is
often coloured so as to imitate the objects on which these insects
commonly rest. Dr. Rossler, I believe, first noticed the similarity of
the closed wings of certain Vanessae and other butterflies to the bark
of trees. Many analogous and striking facts could be given. The most
interesting one is that recorded by Mr. Wallace (7. See the interesting
article in the ‘Westminster Review,’ July 1867, p. 10. A woodcut of the
Kallima is given by Mr. Wallace in ‘Hardwicke’s Science Gossip,’
September 1867, p. 196.) of a common Indian and Sumatran butterfly
(Kallima) which disappears like magic when it settles on a bush; for it
hides its head and antennae between its closed wings, which, in form,
colour and veining, cannot be distinguished from a withered leaf with
its footstalk. In some other cases the lower surfaces of the wings are
brilliantly coloured, and yet are protective; thus in Thecla rubi the
wings when closed are of an emerald green, and resemble the young
leaves of the bramble, on which in spring this butterfly may often be
seen seated. It is also remarkable that in very many species in which
the sexes differ greatly in colour on their upper surface, the lower
surface is closely similar or identical in both sexes, and serves as a
protection. (8. Mr. G. Fraser, in ‘Nature,’ April 1871, p. 489.)
Although the obscure tints both of the upper and under sides of many
butterflies no doubt serve to conceal them, yet we cannot extend this
view to the brilliant and conspicuous colours on the upper surface of
such species as our admiral and peacock Vanessae, our white
cabbage-butterflies (Pieris), or the great swallow-tail Papilio which
haunts the open fens—for these butterflies are thus rendered visible to
every living creature. In these species both sexes are alike; but in
the common brimstone butterfly (Gonepteryx rhamni), the male is of an
intense yellow, whilst the female is much paler; and in the orange-tip
(Anthocharis cardamines) the males alone have their wings tipped with
bright orange. Both the males and females in these cases are
conspicuous, and it is not credible that their difference in colour
should stand in any relation to ordinary protection. Prof. Weismann
remarks (9. ‘Einfluss der Isolirung auf die Artbildung,’ 1872, p. 58.),
that the female of one of the Lycaenae expands her brown wings when she
settles on the ground, and is then almost invisible; the male, on the
other hand, as if aware of the danger incurred from the bright blue of
the upper surface of his wings, rests with them closed; and this shows
that the blue colour cannot be in any way protective. Nevertheless, it
is probable that conspicuous colours are indirectly beneficial to many
species, as a warning that they are unpalatable. For in certain other
cases, beauty has been gained through the imitation of other beautiful
species, which inhabit the same district and enjoy an immunity from
attack by being in some way offensive to their enemies; but then we
have to account for the beauty of the imitated species.
As Mr. Walsh has remarked to me, the females of our orange-tip
butterfly, above referred to, and of an American species (Anth.
genutia) probably shew us the primordial colours of the parent-species
of the genus; for both sexes of four or five widely-distributed species
are coloured in nearly the same manner. As in several previous cases,
we may here infer that it is the males of Anth. cardamines and genutia
which have departed from the usual type of the genus. In the Anth. sara
from California, the orange-tips to the wings have been partially
developed in the female; but they are paler than in the male, and
slightly different in some other respects. In an allied Indian form,
the Iphias glaucippe, the orange-tips are fully developed in both
sexes. In this Iphias, as pointed out to me by Mr. A. Butler, the under
surface of the wings marvellously resembles a pale-coloured leaf; and
in our English orange-tip, the under surface resembles the flower-head
of the wild parsley, on which the butterfly often rests at night. (10.
See the interesting observations by T.W. Wood, ‘The Student,’ Sept.
1868, p. 81.) The same reason which compels us to believe that the
lower surfaces have here been coloured for the sake of protection,
leads us to deny that the wings have been tipped with bright orange for
the same purpose, especially when this character is confined to the
males.
Most Moths rest motionless during the whole or greater part of the day
with their wings depressed; and the whole upper surface is often shaded
and coloured in an admirable manner, as Mr. Wallace has remarked, for
escaping detection. The front-wings of the Bombycidae and Noctuidae
(11. Mr. Wallace in ‘Hardwicke’s Science Gossip,’ September 1867, p.
193.), when at rest, generally overlap and conceal the hind-wings; so
that the latter might be brightly coloured without much risk; and they
are in fact often thus coloured. During flight, moths would often be
able to escape from their enemies; nevertheless, as the hind-wings are
then fully exposed to view, their bright colours must generally have
been acquired at some little risk. But the following fact shews how
cautious we ought to be in drawing conclusions on this head. The common
Yellow Under-wings (Triphaena) often fly about during the day or early
evening, and are then conspicuous from the colour of their hind-wings.
It would naturally be thought that this would be a source of danger;
but Mr. J. Jenner Weir believes that it actually serves them as a means
of escape, for birds strike at these brightly coloured and fragile
surfaces, instead of at the body. For instance, Mr. Weir turned into
his aviary a vigorous specimen of Triphaena pronuba, which was
instantly pursued by a robin; but the bird’s attention being caught by
the coloured wings, the moth was not captured until after about fifty
attempts, and small portions of the wings were repeatedly broken off.
He tried the same experiment, in the open air, with a swallow and T.
fimbria; but the large size of this moth probably interfered with its
capture. (12. See also, on this subject, Mr. Weir’s paper in
‘Transactions, Entomological Society,’ 1869, p. 23.) We are thus
reminded of a statement made by Mr. Wallace (13. ‘Westminster Review,’
July 1867, p. 16.), namely, that in the Brazilian forests and Malayan
islands, many common and highly-decorated butterflies are weak flyers,
though furnished with a broad expanse of wing; and they “are often
captured with pierced and broken wings, as if they had been seized by
birds, from which they had escaped: if the wings had been much smaller
in proportion to the body, it seems probable that the insect would more
frequently have been struck or pierced in a vital part, and thus the
increased expanse of the wings may have been indirectly beneficial.”
DISPLAY.
The bright colours of many butterflies and of some moths are specially
arranged for display, so that they may be readily seen. During the
night colours are not visible, and there can be no doubt that the
nocturnal moths, taken as a body, are much less gaily decorated than
butterflies, all of which are diurnal in their habits. But the moths of
certain families, such as the Zygaenidae, several Sphingidae,
Uraniidae, some Arctiidae and Saturniidae, fly about during the day or
early evening, and many of these are extremely beautiful, being far
brighter coloured than the strictly nocturnal kinds. A few exceptional
cases, however, of bright-coloured nocturnal species have been
recorded. (14. For instance, Lithosia; but Prof. Westwood (‘Modern
Class. of Insects,’ vol. ii. p. 390) seems surprised at this case. On
the relative colours of diurnal and nocturnal Lepidoptera, see ibid.
pp. 333 and 392; also Harris, ‘Treatise on the Insects of New England,’
1842, p. 315.)
There is evidence of another kind in regard to display. Butterflies, as
before remarked, elevate their wings when at rest, but whilst basking
in the sunshine often alternately raise and depress them, thus exposing
both surfaces to full view; and although the lower surface is often
coloured in an obscure manner as a protection, yet in many species it
is as highly decorated as the upper surface, and sometimes in a very
different manner. In some tropical species the lower surface is even
more brilliantly coloured than the upper. (15. Such differences between
the upper and lower surfaces of the wings of several species of Papilio
may be seen in the beautiful plates to Mr. Wallace’s ‘Memoir on the
Papilionidae of the Malayan Region,’ in ‘Transactions of the Linnean
Society,’ vol. xxv. part i. 1865.) In the English fritillaries
(Argynnis) the lower surface alone is ornamented with shining silver.
Nevertheless, as a general rule, the upper surface, which is probably
more fully exposed, is coloured more brightly and diversely than the
lower. Hence the lower surface generally affords to entomologists the
more useful character for detecting the affinities of the various
species. Fritz Müller informs me that three species of Castnia are
found near his house in S. Brazil: of two of them the hind-wings are
obscure, and are always covered by the front-wings when these
butterflies are at rest; but the third species has black hind-wings,
beautifully spotted with red and white, and these are fully expanded
and displayed whenever the butterfly rests. Other such cases could be
added.
If we now turn to the enormous group of moths, which, as I hear from
Mr. Stainton, do not habitually expose the under surface of their wings
to full view, we find this side very rarely coloured with a brightness
greater than, or even equal to, that of the upper side. Some exceptions
to the rule, either real or apparent, must be noticed, as the case of
Hypopyra. (16. See Mr. Wormald on this moth: ‘Proceedings of the
Entomological Society,’ March 2, 1868.) Mr. Trimen informs me that in
Guenee’s great work, three moths are figured, in which the under
surface is much the more brilliant. For instance, in the Australian
Gastrophora the upper surface of the fore-wing is pale
greyish-ochreous, while the lower surface is magnificently ornamented
by an ocellus of cobalt-blue, placed in the midst of a black mark,
surrounded by orange-yellow, and this by bluish-white. But the habits
of these three moths are unknown; so that no explanation can be given
of their unusual style of colouring. Mr. Trimen also informs me that
the lower surface of the wings in certain other Geometrae (17. See also
an account of the S. American genus Erateina (one of the Geometrae) in
‘Transactions, Ent. Soc.’ new series, vol. v. pl. xv. and xvi.) and
quadrifid Noctuae are either more variegated or more brightly-coloured
than the upper surface; but some of these species have the habit of
“holding their wings quite erect over their backs, retaining them in
this position for a considerable time,” and thus exposing the under
surface to view. Other species, when settled on the ground or herbage,
now and then suddenly and slightly lift up their wings. Hence the lower
surface of the wings being brighter than the upper surface in certain
moths is not so anomalous as it at first appears. The Saturniidae
include some of the most beautiful of all moths, their wings being
decorated, as in our British Emperor moth, with fine ocelli; and Mr.
T.W. Wood (18. ‘Proc Ent. Soc. of London,’ July 6, 1868, p. xxvii.)
observes that they resemble butterflies in some of their movements;
“for instance, in the gentle waving up and down of the wings as if for
display, which is more characteristic of diurnal than of nocturnal
Lepidoptera.”
It is a singular fact that no British moths which are brilliantly
coloured, and, as far as I can discover, hardly any foreign species,
differ much in colour according to sex; though this is the case with
many brilliant butterflies. The male, however, of one American moth,
the Saturnia Io, is described as having its fore-wings deep yellow,
curiously marked with purplish-red spots; whilst the wings of the
female are purple-brown, marked with grey lines. (19. Harris,
‘Treatise,’ etc., edited by Flint, 1862, p. 395.) The British moths
which differ sexually in colour are all brown, or of various dull
yellow tints, or nearly white. In several species the males are much
darker than the females (20. For instance, I observe in my son’s
cabinet that the males are darker than the females in the Lasiocampa
quercus, Odonestis potatoria, Hypogymna dispar, Dasychira pudibunda,
and Cycnia mendica. In this latter species the difference in colour
between the two sexes is strongly marked; and Mr. Wallace informs me
that we here have, as he believes, an instance of protective mimicry
confined to one sex, as will hereafter be more fully explained. The
white female of the Cycnia resembles the very common Spilosoma
menthrasti, both sexes of which are white; and Mr. Stainton observed
that this latter moth was rejected with utter disgust by a whole brood
of young turkeys, which were fond of eating other moths; so that if the
Cycnia was commonly mistaken by British birds for the Spilosoma, it
would escape being devoured, and its white deceptive colour would thus
be highly beneficial.), and these belong to groups which generally fly
about during the afternoon. On the other hand, in many genera, as Mr.
Stainton informs me, the males have the hind-wings whiter than those of
the female—of which fact Agrotis exclamationis offers a good instance.
In the Ghost Moth (Hepialus humuli) the difference is more strongly
marked; the males being white, and the females yellow with darker
markings. (21. It is remarkable, that in the Shetland Islands the male
of this moth, instead of differing widely from the female, frequently
resembles her closely in colour (see Mr. MacLachlan, ‘Transactions,
Entomological Society,’ vol. ii. 1866, p. 459). Mr. G. Fraser suggests
(‘Nature,’ April 1871, p. 489) that at the season of the year when the
ghost-moth appears in these northern islands, the whiteness of the
males would not be needed to render them visible to the females in the
twilight night.) It is probable that in these cases the males are thus
rendered more conspicuous, and more easily seen by the females whilst
flying about in the dusk.
From the several foregoing facts it is impossible to admit that the
brilliant colours of butterflies, and of some few moths, have commonly
been acquired for the sake of protection. We have seen that their
colours and elegant patterns are arranged and exhibited as if for
display. Hence I am led to believe that the females prefer or are most
excited by the more brilliant males; for on any other supposition the
males would, as far as we can see, be ornamented to no purpose. We know
that ants and certain Lamellicorn beetles are capable of feeling an
attachment for each other, and that ants recognise their fellows after
an interval of several months. Hence there is no abstract improbability
in the Lepidoptera, which probably stand nearly or quite as high in the
scale as these insects, having sufficient mental capacity to admire
bright colours. They certainly discover flowers by colour. The
Humming-bird Sphinx may often be seen to swoop down from a distance on
a bunch of flowers in the midst of green foliage; and I have been
assured by two persons abroad, that these moths repeatedly visit
flowers painted on the walls of a room, and vainly endeavour to insert
their proboscis into them. Fritz Müller informs me that several kinds
of butterflies in S. Brazil shew an unmistakable preference for certain
colours over others: he observed that they very often visited the
brilliant red flowers of five or six genera of plants, but never the
white or yellow flowering species of the same and other genera, growing
in the same garden; and I have received other accounts to the same
effect. As I hear from Mr. Doubleday, the common white butterfly often
flies down to a bit of paper on the ground, no doubt mistaking it for
one of its own species. Mr. Collingwood (22. ‘Rambles of a Naturalist
in the Chinese Seas,’ 1868, p. 182.) in speaking of the difficulty in
collecting certain butterflies in the Malay Archipelago, states that “a
dead specimen pinned upon a conspicuous twig will often arrest an
insect of the same species in its headlong flight, and bring it down
within easy reach of the net, especially if it be of the opposite sex.”
The courtship of butterflies is, as before remarked, a prolonged
affair. The males sometimes fight together in rivalry; and many may be
seen pursuing or crowding round the same female. Unless, then, the
females prefer one male to another, the pairing must be left to mere
chance, and this does not appear probable. If, on the other band, the
females habitually, or even occasionally, prefer the more beautiful
males, the colours of the latter will have been rendered brighter by
degrees, and will have been transmitted to both sexes or to one sex,
according to the law of inheritance which has prevailed. The process of
sexual selection will have been much facilitated, if the conclusion can
be trusted, arrived at from various kinds of evidence in the supplement
to the ninth chapter; namely, that the males of many Lepidoptera, at
least in the imago state, greatly exceed the females in number.
Some facts, however, are opposed to the belief that female butterflies
prefer the more beautiful males; thus, as I have been assured by
several collectors, fresh females may frequently be seen paired with
battered, faded, or dingy males; but this is a circumstance which could
hardly fail often to follow from the males emerging from their cocoons
earlier than the females. With moths of the family of the Bombycidae,
the sexes pair immediately after assuming the imago state; for they
cannot feed, owing to the rudimentary condition of their mouths. The
females, as several entomologists have remarked to me, lie in an almost
torpid state, and appear not to evince the least choice in regard to
their partners. This is the case with the common silk-moth (B. mori),
as I have been told by some continental and English breeders. Dr.
Wallace, who has had great experience in breeding Bombyx cynthia, is
convinced that the females evince no choice or preference. He has kept
above 300 of these moths together, and has often found the most
vigorous females mated with stunted males. The reverse appears to occur
seldom; for, as he believes, the more vigorous males pass over the
weakly females, and are attracted by those endowed with most vitality.
Nevertheless, the Bombycidae, though obscurely-coloured, are often
beautiful to our eyes from their elegant and mottled shades.
I have as yet only referred to the species in which the males are
brighter coloured than the females, and I have attributed their beauty
to the females for many generations having chosen and paired with the
more attractive males. But converse cases occur, though rarely, in
which the females are more brilliant than the males; and here, as I
believe, the males have selected the more beautiful females, and have
thus slowly added to their beauty. We do not know why in various
classes of animals the males of some few species have selected the more
beautiful females instead of having gladly accepted any female, as
seems to be the general rule in the animal kingdom: but if, contrary to
what generally occurs with the Lepidoptera, the females were much more
numerous than the males, the latter would be likely to pick out the
more beautiful females. Mr. Butler shewed me several species of
Callidryas in the British Museum, in some of which the females
equalled, and in others greatly surpassed the males in beauty; for the
females alone have the borders of their wings suffused with crimson and
orange, and spotted with black. The plainer males of these species
closely resemble each other, shewing that here the females have been
modified; whereas in those cases, where the males are the more ornate,
it is these which have been modified, the females remaining closely
alike.
In England we have some analogous cases, though not so marked. The
females alone of two species of Thecla have a bright-purple or orange
patch on their fore-wings. In Hipparchia the sexes do not differ much;
but it is the female of H. janira which has a conspicuous light-brown
patch on her wings; and the females of some of the other species are
brighter coloured than their males. Again, the females of Colias edusa
and hyale have “orange or yellow spots on the black marginal border,
represented in the males only by thin streaks”; and in Pieris it is the
females which “are ornamented with black spots on the fore-wings, and
these are only partially present in the males.” Now the males of many
butterflies are known to support the females during their marriage
flight; but in the species just named it is the females which support
the males; so that the part which the two sexes play is reversed, as is
their relative beauty. Throughout the animal kingdom the males commonly
take the more active share in wooing, and their beauty seems to have
been increased by the females having accepted the more attractive
individuals; but with these butterflies, the females take the more
active part in the final marriage ceremony, so that we may suppose that
they likewise do so in the wooing; and in this case we can understand
how it is that they have been rendered the more beautiful. Mr. Meldola,
from whom the foregoing statements have been taken, says in conclusion:
“Though I am not convinced of the action of sexual selection in
producing the colours of insects, it cannot be denied that these facts
are strikingly corroborative of Mr. Darwin’s views.” (23. ‘Nature,’
April 27, 1871, p. 508. Mr. Meldola quotes Donzel, in ‘Soc. Ent. de
France,’ 1837, p. 77, on the flight of butterflies whilst pairing. See
also Mr. G. Fraser, in ‘Nature,’ April 20, 1871, p. 489, on the sexual
differences of several British butterflies.)
As sexual selection primarily depends on variability, a few words must
be added on this subject. In respect to colour there is no difficulty,
for any number of highly variable Lepidoptera could be named. One good
instance will suffice. Mr. Bates shewed me a whole series of specimens
of Papilio sesostris and P. childrenae; in the latter the males varied
much in the extent of the beautifully enamelled green patch on the
fore-wings, and in the size of the white mark, and of the splendid
crimson stripe on the hind-wings; so that there was a great contrast
amongst the males between the most and the least gaudy. The male of
Papilio sesostris is much less beautiful than of P. childrenae; and it
likewise varies a little in the size of the green patch on the
fore-wings, and in the occasional appearance of the small crimson
stripe on the hind-wings, borrowed, as it would seem, from its own
female; for the females of this and of many other species in the Aeneas
group possess this crimson stripe. Hence between the brightest
specimens of P. sesostris and the dullest of P. childrenae, there was
but a small interval; and it was evident that as far as mere
variability is concerned, there would be no difficulty in permanently
increasing the beauty of either species by means of selection. The
variability is here almost confined to the male sex; but Mr. Wallace
and Mr. Bates have shewn (24. Wallace on the Papilionidae of the
Malayan Region, in ‘Transact. Linn. Soc.’ vol. xxv. 1865, pp. 8, 36. A
striking case of a rare variety, strictly intermediate between two
other well-marked female varieties, is given by Mr. Wallace. See also
Mr. Bates, in ‘Proc. Entomolog. Soc.’ Nov. 19, 1866, p. xl.) that the
females of some species are extremely variable, the males being nearly
constant. In a future chapter I shall have occasion to shew that the
beautiful eye-like spots, or ocelli, found on the wings of many
Lepidoptera, are eminently variable. I may here add that these ocelli
offer a difficulty on the theory of sexual selection; for though
appearing to us so ornamental, they are never present in one sex and
absent in the other, nor do they ever differ much in the two sexes.
(25. Mr. Bates was so kind as to lay this subject before the
Entomological Society, and I have received answers to this effect from
several entomologists.) This fact is at present inexplicable; but if it
should hereafter be found that the formation of an ocellus is due to
some change in the tissues of the wings, for instance, occurring at a
very early period of development, we might expect, from what we know of
the laws of inheritance, that it would be transmitted to both sexes,
though arising and perfected in one sex alone.
On the whole, although many serious objections may be urged, it seems
probable that most of the brilliantly-coloured species of Lepidoptera
owe their colours to sexual selection, excepting in certain cases,
presently to be mentioned, in which conspicuous colours have been
gained through mimicry as a protection. From the ardour of the male
throughout the animal kingdom, he is generally willing to accept any
female; and it is the female which usually exerts a choice. Hence, if
sexual selection has been efficient with the Lepidoptera, the male,
when the sexes differ, ought to be the more brilliantly coloured, and
this undoubtedly is the case. When both sexes are brilliantly coloured
and resemble each other, the characters acquired by the males appear to
have been transmitted to both. We are led to this conclusion by cases,
even within the same genus, of gradation from an extraordinary amount
of difference to identity in colour between the two sexes.
But it may be asked whether the difference in colour between the sexes
may not be accounted for by other means besides sexual selection. Thus
the males and females of the same species of butterfly are in several
cases known (26. H.W. Bates, ‘The Naturalist on the Amazons,’ vol. ii.
1863, p. 228. A.R. Wallace, in ‘Transactions, Linnean Society,’ vol.
xxv. 1865, p. 10.) to inhabit different stations, the former commonly
basking in the sunshine, the latter haunting gloomy forests. It is
therefore possible that different conditions of life may have acted
directly on the two sexes; but this is not probable (27. On this whole
subject see ‘The Variation of Animals and Plants under Domestication,’
1868, vol. ii. chap. xxiii.) as in the adult state they are exposed to
different conditions during a very short period; and the larvae of both
are exposed to the same conditions. Mr. Wallace believes that the
difference between the sexes is due not so much to the males having
been modified, as to the females having in all or almost all cases
acquired dull colours for the sake of protection. It seems to me, on
the contrary, far more probable that it is the males which have been
chiefly modified through sexual selection, the females having been
comparatively little changed. We can thus understand how it is that the
females of allied species generally resemble one another so much more
closely than do the males. They thus shew us approximately the
primordial colouring of the parent-species of the group to which they
belong. They have, however, almost always been somewhat modified by the
transfer to them of some of the successive variations, through the
accumulation of which the males were rendered beautiful. But I do not
wish to deny that the females alone of some species may have been
specially modified for protection. In most cases the males and females
of distinct species will have been exposed during their prolonged
larval state to different conditions, and may have been thus affected;
though with the males any slight change of colour thus caused will
generally have been masked by the brilliant tints gained through sexual
selection. When we treat of Birds, I shall have to discuss the whole
question, as to how far the differences in colour between the sexes are
due to the males having been modified through sexual selection for
ornamental purposes, or to the females having been modified through
natural selection for the sake of protection, so that I will here say
but little on the subject.
In all the cases in which the more common form of equal inheritance by
both sexes has prevailed, the selection of bright-coloured males would
tend to make the females bright-coloured; and the selection of
dull-coloured females would tend to make the males dull. If both
processes were carried on simultaneously, they would tend to counteract
each other; and the final result would depend on whether a greater
number of females from being well protected by obscure colours, or a
greater number of males by being brightly-coloured and thus finding
partners, succeeded in leaving more numerous offspring.
In order to account for the frequent transmission of characters to one
sex alone, Mr. Wallace expresses his belief that the more common form
of equal inheritance by both sexes can be changed through natural
selection into inheritance by one sex alone, but in favour of this view
I can discover no evidence. We know from what occurs under
domestication that new characters often appear, which from the first
are transmitted to one sex alone; and by the selection of such
variations there would not be the slightest difficulty in giving bright
colours to the males alone, and at the same time or subsequently, dull
colours to the females alone. In this manner the females of some
butterflies and moths have, it is probable, been rendered inconspicuous
for the sake of protection, and widely different from their males.
I am, however, unwilling without distinct evidence to admit that two
complex processes of selection, each requiring the transference of new
characters to one sex alone, have been carried on with a multitude of
species,—that the males have been rendered more brilliant by beating
their rivals, and the females more dull-coloured by having escaped from
their enemies. The male, for instance, of the common brimstone
butterfly (Gonepteryx), is of a far more intense yellow than the
female, though she is equally conspicuous; and it does not seem
probable that she specially acquired her pale tints as a protection,
though it is probable that the male acquired his bright colours as a
sexual attraction. The female of Anthocharis cardamines does not
possess the beautiful orange wing-tips of the male; consequently she
closely resembles the white butterflies (Pieris) so common in our
gardens; but we have no evidence that this resemblance is beneficial to
her. As, on the other hand, she resembles both sexes of several other
species of the genus inhabiting various quarters of the world, it is
probable that she has simply retained to a large extent her primordial
colours.
Finally, as we have seen, various considerations lead to the conclusion
that with the greater number of brilliantly-coloured Lepidoptera it is
the male which has been chiefly modified through sexual selection; the
amount of difference between the sexes mostly depending on the form of
inheritance which has prevailed. Inheritance is governed by so many
unknown laws or conditions, that it seems to us to act in a capricious
manner (28. The ‘Variation of Animals and Plants under Domestication,’
vol. ii. chap. xii. p. 17.); and we can thus, to a certain extent,
understand how it is that with closely allied species the sexes either
differ to an astonishing degree, or are identical in colour. As all the
successive steps in the process of variation are necessarily
transmitted through the female, a greater or less number of such steps
might readily become developed in her; and thus we can understand the
frequent gradations from an extreme difference to none at all between
the sexes of allied species. These cases of gradation, it may be added,
are much too common to favour the supposition that we here see females
actually undergoing the process of transition and losing their
brightness for the sake of protection; for we have every reason to
conclude that at any one time the greater number of species are in a
fixed condition.
MIMICRY.
This principle was first made clear in an admirable paper by Mr. Bates
(29. ‘Transact. Linn. Soc.’ vol. xxiii. 1862, p. 495.), who thus threw
a flood of light on many obscure problems. It had previously been
observed that certain butterflies in S. America belonging to quite
distinct families, resembled the Heliconidae so closely in every stripe
and shade of colour, that they could not be distinguished save by an
experienced entomologist. As the Heliconidae are coloured in their
usual manner, whilst the others depart from the usual colouring of the
groups to which they belong, it is clear that the latter are the
imitators, and the Heliconidae the imitated. Mr. Bates further observed
that the imitating species are comparatively rare, whilst the imitated
abound, and that the two sets live mingled together. From the fact of
the Heliconidae being conspicuous and beautiful insects, yet so
numerous in individuals and species, he concluded that they must be
protected from the attacks of enemies by some secretion or odour; and
this conclusion has now been amply confirmed (30. ‘Proc. Entomological
Soc.’ Dec. 3, 1866, p. xlv.), especially by Mr. Belt. Hence Mr. Bates
inferred that the butterflies which imitate the protected species have
acquired their present marvellously deceptive appearance through
variation and natural selection, in order to be mistaken for the
protected kinds, and thus to escape being devoured. No explanation is
here attempted of the brilliant colours of the imitated, but only of
the imitating butterflies. We must account for the colours of the
former in the same general manner, as in the cases previously discussed
in this chapter. Since the publication of Mr. Bates’ paper, similar and
equally striking facts have been observed by Mr. Wallace in the Malayan
region, by Mr. Trimen in South Africa, and by Mr. Riley in the United
States. (31. Wallace, ‘Transact. Linn. Soc.’ vol. xxv. 1865 p. i.;
also, ‘Transact. Ent. Soc.’ vol. iv. (3rd series), 1867, p. 301.
Trimen, ‘Linn. Transact.’ vol. xxvi. 1869, p. 497. Riley, ‘Third Annual
Report on the Noxious Insects of Missouri,’ 1871, pp. 163-168. This
latter essay is valuable, as Mr. Riley here discusses all the
objections which have been raised against Mr. Bates’s theory.)
As some writers have felt much difficulty in understanding how the
first steps in the process of mimicry could have been effected through
natural selection, it may be well to remark that the process probably
commenced long ago between forms not widely dissimilar in colour. In
this case even a slight variation would be beneficial, if it rendered
the one species more like the other; and afterwards the imitated
species might be modified to an extreme degree through sexual selection
or other means, and if the changes were gradual, the imitators might
easily be led along the same track, until they differed to an equally
extreme degree from their original condition; and they would thus
ultimately assume an appearance or colouring wholly unlike that of the
other members of the group to which they belonged. It should also be
remembered that many species of Lepidoptera are liable to considerable
and abrupt variations in colour. A few instances have been given in
this chapter; and many more may be found in the papers of Mr. Bates and
Mr. Wallace.
With several species the sexes are alike, and imitate the two sexes of
another species. But Mr. Trimen gives, in the paper already referred
to, three cases in which the sexes of the imitated form differ from
each other in colour, and the sexes of the imitating form differ in a
like manner. Several cases have also been recorded where the females
alone imitate brilliantly-coloured and protected species, the males
retaining “the normal aspect of their immediate congeners.” It is here
obvious that the successive variations by which the female has been
modified have been transmitted to her alone. It is, however, probable
that some of the many successive variations would have been transmitted
to, and developed in, the males had not such males been eliminated by
being thus rendered less attractive to the females; so that only those
variations were preserved which were from the first strictly limited in
their transmission to the female sex. We have a partial illustration of
these remarks in a statement by Mr. Belt (32. ‘The Naturalist in
Nicaragua,’ 1874, p. 385.); that the males of some of the Leptalides,
which imitate protected species, still retain in a concealed manner
some of their original characters. Thus in the males “the upper half of
the lower wing is of a pure white, whilst all the rest of the wings is
barred and spotted with black, red and yellow, like the species they
mimic. The females have not this white patch, and the males usually
conceal it by covering it with the upper wing, so that I cannot imagine
its being of any other use to them than as an attraction in courtship,
when they exhibit it to the females, and thus gratify their deep-seated
preference for the normal colour of the Order to which the Leptalides
belong.”
BRIGHT COLOURS OF CATERPILLARS.
Whilst reflecting on the beauty of many butterflies, it occurred to me
that some caterpillars were splendidly coloured; and as sexual
selection could not possibly have here acted, it appeared rash to
attribute the beauty of the mature insect to this agency, unless the
bright colours of their larvae could be somehow explained. In the first
place, it may be observed that the colours of caterpillars do not stand
in any close correlation with those of the mature insect. Secondly,
their bright colours do not serve in any ordinary manner as a
protection. Mr. Bates informs me, as an instance of this, that the most
conspicuous caterpillar which he ever beheld (that of a Sphinx) lived
on the large green leaves of a tree on the open llanos of South
America; it was about four inches in length, transversely banded with
black and yellow, and with its head, legs, and tail of a bright red.
Hence it caught the eye of any one who passed by, even at the distance
of many yards, and no doubt that of every passing bird.
I then applied to Mr. Wallace, who has an innate genius for solving
difficulties. After some consideration he replied: “Most caterpillars
require protection, as may be inferred from some kinds being furnished
with spines or irritating hairs, and from many being coloured green
like the leaves on which they feed, or being curiously like the twigs
of the trees on which they live.” Another instance of protection,
furnished me by Mr. J. Mansel Weale, may be added, namely, that there
is a caterpillar of a moth which lives on the mimosas in South Africa,
and fabricates for itself a case quite indistinguishable from the
surrounding thorns. From such considerations Mr. Wallace thought it
probable that conspicuously coloured caterpillars were protected by
having a nauseous taste; but as their skin is extremely tender, and as
their intestines readily protrude from a wound, a slight peck from the
beak of a bird would be as fatal to them as if they had been devoured.
Hence, as Mr. Wallace remarks, “distastefulness alone would be
insufficient to protect a caterpillar unless some outward sign
indicated to its would-be destroyer that its prey was a disgusting
morsel.” Under these circumstances it would be highly advantageous to a
caterpillar to be instantaneously and certainly recognised as
unpalatable by all birds and other animals. Thus the most gaudy colours
would be serviceable, and might have been gained by variation and the
survival of the most easily-recognised individuals.
This hypothesis appears at first sight very bold, but when it was
brought before the Entomological Society (33. ‘Proceedings,
Entomological Society,’ Dec. 3, 1866, p. xlv. and March 4, 1867, p.
lxxx.) it was supported by various statements; and Mr. J. Jenner Weir,
who keeps a large number of birds in an aviary, informs me that he has
made many trials, and finds no exception to the rule, that all
caterpillars of nocturnal and retiring habits with smooth skins, all of
a green colour, and all which imitate twigs, are greedily devoured by
his birds. The hairy and spinose kinds are invariably rejected, as were
four conspicuously-coloured species. When the birds rejected a
caterpillar, they plainly shewed, by shaking their heads, and cleansing
their beaks, that they were disgusted by the taste. (34. See Mr. J.
Jenner Weir’s paper on Insects and Insectivorous Birds, in ‘Transact.
Ent. Soc.’ 1869, p. 21; also Mr. Butler’s paper, ibid. p. 27. Mr. Riley
has given analogous facts in the ‘Third Annual Report on the Noxious
Insects of Missouri,’ 1871, p. 148. Some opposed cases are, however,
given by Dr. Wallace and M. H. d’Orville; see ‘Zoological Record,’
1869, p. 349.) Three conspicuous kinds of caterpillars and moths were
also given to some lizards and frogs, by Mr. A. Butler, and were
rejected, though other kinds were eagerly eaten. Thus the probability
of Mr. Wallace’s view is confirmed, namely, that certain caterpillars
have been made conspicuous for their own good, so as to be easily
recognised by their enemies, on nearly the same principle that poisons
are sold in coloured bottles by druggists for the good of man. We
cannot, however, at present thus explain the elegant diversity in the
colours of many caterpillars; but any species which had at some former
period acquired a dull, mottled, or striped appearance, either in
imitation of surrounding objects, or from the direct action of climate,
etc., almost certainly would not become uniform in colour, when its
tints were rendered intense and bright; for in order to make a
caterpillar merely conspicuous, there would be no selection in any
definite direction.
A SUMMARY AND CONCLUDING REMARKS ON INSECTS.
Looking back to the several Orders, we see that the sexes often differ
in various characters, the meaning of which is not in the least
understood. The sexes, also, often differ in their organs of sense and
means of locomotion, so that the males may quickly discover and reach
the females. They differ still oftener in the males possessing
diversified contrivances for retaining the females when found. We are,
however, here concerned only in a secondary degree with sexual
differences of these kinds.
In almost all the Orders, the males of some species, even of weak and
delicate kinds, are known to be highly pugnacious; and some few are
furnished with special weapons for fighting with their rivals. But the
law of battle does not prevail nearly so widely with insects as with
the higher animals. Hence it probably arises, that it is in only a few
cases that the males have been rendered larger and stronger than the
females. On the contrary, they are usually smaller, so that they may be
developed within a shorter time, to be ready in large numbers for the
emergence of the females.
In two families of the Homoptera and in three of the Orthoptera, the
males alone possess sound-producing organs in an efficient state. These
are used incessantly during the breeding-season, not only for calling
the females, but apparently for charming or exciting them in rivalry
with other males. No one who admits the agency of selection of any
kind, will, after reading the above discussion, dispute that these
musical instruments have been acquired through sexual selection. In
four other Orders the members of one sex, or more commonly of both
sexes, are provided with organs for producing various sounds, which
apparently serve merely as call-notes. When both sexes are thus
provided, the individuals which were able to make the loudest or most
continuous noise would gain partners before those which were less
noisy, so that their organs have probably been gained through sexual
selection. It is instructive to reflect on the wonderful diversity of
the means for producing sound, possessed by the males alone, or by both
sexes, in no less than six Orders. We thus learn how effectual sexual
selection has been in leading to modifications which sometimes, as with
the Homoptera, relate to important parts of the organisation.
From the reasons assigned in the last chapter, it is probable that the
great horns possessed by the males of many Lamellicorn, and some other
beetles, have been acquired as ornaments. From the small size of
insects, we are apt to undervalue their appearance. If we could imagine
a male Chalcosoma (Fig. 16), with its polished bronzed coat of mail,
and its vast complex horns, magnified to the size of a horse, or even
of a dog, it would be one of the most imposing animals in the world.
The colouring of insects is a complex and obscure subject. When the
male differs slightly from the female, and neither are
brilliantly-coloured, it is probable that the sexes have varied in a
slightly different manner, and that the variations have been
transmitted by each sex to the same without any benefit or evil thus
accruing. When the male is brilliantly-coloured and differs
conspicuously from the female, as with some dragon-flies and many
butterflies, it is probable that he owes his colours to sexual
selection; whilst the female has retained a primordial or very ancient
type of colouring, slightly modified by the agencies before explained.
But in some cases the female has apparently been made obscure by
variations transmitted to her alone, as a means of direct protection;
and it is almost certain that she has sometimes been made brilliant, so
as to imitate other protected species inhabiting the same district.
When the sexes resemble each other and both are obscurely coloured,
there is no doubt that they have been in a multitude of cases so
coloured for the sake of protection. So it is in some instances when
both are brightly-coloured, for they thus imitate protected species, or
resemble surrounding objects such as flowers; or they give notice to
their enemies that they are unpalatable. In other cases in which the
sexes resemble each other and are both brilliant, especially when the
colours are arranged for display, we may conclude that they have been
gained by the male sex as an attraction, and have been transferred to
the female. We are more especially led to this conclusion whenever the
same type of coloration prevails throughout a whole group, and we find
that the males of some species differ widely in colour from the
females, whilst others differ slightly or not at all with intermediate
gradations connecting these extreme states.
In the same manner as bright colours have often been partially
transferred from the males to the females, so it has been with the
extraordinary horns of many Lamellicorn and some other beetles. So
again, the sound-producing organs proper to the males of the Homoptera
and Orthoptera have generally been transferred in a rudimentary, or
even in a nearly perfect condition, to the females; yet not
sufficiently perfect to be of any use. It is also an interesting fact,
as bearing on sexual selection, that the stridulating organs of certain
male Orthoptera are not fully developed until the last moult; and that
the colours of certain male dragon-flies are not fully developed until
some little time after their emergence from the pupal state, and when
they are ready to breed.
Sexual selection implies that the more attractive individuals are
preferred by the opposite sex; and as with insects, when the sexes
differ, it is the male which, with some rare exceptions, is the more
ornamented, and departs more from the type to which the species
belongs;—and as it is the male which searches eagerly for the female,
we must suppose that the females habitually or occasionally prefer the
more beautiful males, and that these have thus acquired their beauty.
That the females in most or all the Orders would have the power of
rejecting any particular male, is probable from the many singular
contrivances possessed by the males, such as great jaws, adhesive
cushions, spines, elongated legs, etc., for seizing the female; for
these contrivances show that there is some difficulty in the act, so
that her concurrence would seem necessary. Judging from what we know of
the perceptive powers and affections of various insects, there is no
antecedent improbability in sexual selection having come largely into
play; but we have as yet no direct evidence on this head, and some
facts are opposed to the belief. Nevertheless, when we see many males
pursuing the same female, we can hardly believe that the pairing is
left to blind chance—that the female exerts no choice, and is not
influenced by the gorgeous colours or other ornaments with which the
male is decorated.
If we admit that the females of the Homoptera and Orthoptera appreciate
the musical tones of their male partners, and that the various
instruments have been perfected through sexual selection, there is
little improbability in the females of other insects appreciating
beauty in form or colour, and consequently in such characters having
been thus gained by the males. But from the circumstance of colour
being so variable, and from its having been so often modified for the
sake of protection, it is difficult to decide in how large a proportion
of cases sexual selection has played a part. This is more especially
difficult in those Orders, such as Orthoptera, Hymenoptera, and
Coleoptera, in which the two sexes rarely differ much in colour; for we
are then left to mere analogy. With the Coleoptera, however, as before
remarked, it is in the great Lamellicorn group, placed by some authors
at the head of the Order, and in which we sometimes see a mutual
attachment between the sexes, that we find the males of some species
possessing weapons for sexual strife, others furnished with wonderful
horns, many with stridulating organs, and others ornamented with
splendid metallic tints. Hence it seems probable that all these
characters have been gained through the same means, namely sexual
selection. With butterflies we have the best evidence, as the males
sometimes take pains to display their beautiful colours; and we cannot
believe that they would act thus, unless the display was of use to them
in their courtship.
When we treat of Birds, we shall see that they present in their
secondary sexual characters the closest analogy with insects. Thus,
many male birds are highly pugnacious, and some are furnished with
special weapons for fighting with their rivals. They possess organs
which are used during the breeding-season for producing vocal and
instrumental music. They are frequently ornamented with combs, horns,
wattles and plumes of the most diversified kinds, and are decorated
with beautiful colours, all evidently for the sake of display. We shall
find that, as with insects, both sexes in certain groups are equally
beautiful, and are equally provided with ornaments which are usually
confined to the male sex. In other groups both sexes are equally
plain-coloured and unornamented. Lastly, in some few anomalous cases,
the females are more beautiful than the males. We shall often find, in
the same group of birds, every gradation from no difference between the
sexes, to an extreme difference. We shall see that female birds, like
female insects, often possess more or less plain traces or rudiments of
characters which properly belong to the males and are of use only to
them. The analogy, indeed, in all these respects between birds and
insects is curiously close. Whatever explanation applies to the one
class probably applies to the other; and this explanation, as we shall
hereafter attempt to shew in further detail, is sexual selection.