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Chapter XI
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 Muller 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 Muller 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.
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 beau
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