in nature than it appears in the above figures.]
One of the characters by which some beetles are protected is excessive
hardness of the elytra and integuments. Several genera of weevils
(Curculionidae) are thus saved from attack, and these are often mimicked
by species of softer and more eatable groups. In South America, the
genus Heilipus is one of these hard groups, and both Mr. Bates and M.
Roelofs, a Belgian entomologist, have noticed that species of other
genera exactly mimic them. So, in the Philippines, there is a group of
Curculionidae, forming the genus Pachyrhynchus, in which all the species
are adorned with the most brilliant metallic colours, banded and spotted
in a curious manner, and are very smooth and hard. Other genera of
Curculionidae (Desmidophorus, Alcides), which are usually very
differently coloured, have species in the Philippines which mimic the
Pachyrhynchi; and there are also several longicorn beetles (Aprophata,
Doliops, Acronia, and Agnia), which also mimic them. Besides these,
there are some longicorns and cetonias which reproduce the same colours
and markings; and there is even a cricket (Scepastus pachyrhynchoides),
which has taken on the form and peculiar coloration of these beetles in
order to escape from enemies, which then avoid them as uneatable.[111]
The figures on the opposite page exhibit several other examples of these
mimicking insects.
Innumerable other cases of mimicry occur among tropical insects; but we
must now pass on to consider a few of the very remarkable, but much
rarer instances, that are found among the higher animals.
_Mimicry among the Vertebrata._
Perhaps the most remarkable cases yet known are those of certain
harmless snakes which mimic poisonous species. The genus Elaps, in
tropical America, consists of poisonous snakes which do not belong to
the viper family (in which are included the rattlesnakes and most of
those which are poisonous), and which do not possess the broad
triangular head which characterises the latter. They have a peculiar
style of coloration, consisting of alternate rings of red and black, or
red, black, and yellow, of different widths and grouped in various ways
in the different species; and it is a style of coloration which does not
occur in any other group of snakes in the world. But in the same regions
are found three genera of harmless snakes, belonging to other families,
some few species of which mimic the poisonous Elaps, often so exactly
that it is with difficulty one can be distinguished from the other. Thus
Elaps fulvius in Guatemala is imitated by the harmless Pliocerus
equalis; Elaps corallinus in Mexico is mimicked by the harmless
Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by
Oxyrhopus trigeminus; while in other parts of South America similar
cases of mimicry occur, sometimes two harmless species imitating the
same poisonous snake.
A few other instances of mimicry in this group have been recorded. There
is in South Africa an egg-eating snake (Dasypeltis scaber), which has
neither fangs nor teeth, yet it is very like the Berg adder (Clothos
atropos), and when alarmed renders itself still more like by flattening
out its head and darting forward with a hiss as if to strike a foe.[112]
Dr. A.B. Meyer has also discovered that, while some species of the genus
Callophis (belonging to the same family as the American Elaps) have
large poison fangs, other species of the same genus have none; and that
one of the latter (C. gracilis) resembles a poisonous species (C.
intestinalis) so closely, that only an exact comparison will discover
the difference of colour and marking. A similar kind of resemblance is
said to exist between another harmless snake, Megaerophis flaviceps, and
the poisonous Callophis bivirgatus; and in both these cases the harmless
snake is less abundant than the poisonous one, as occurs in all examples
of true mimicry.[113]
In the genus Elaps, above referred to, the very peculiar style of colour
and marking is evidently a "warning colour" for the purpose of
indicating to snake-eating birds and mammals that these species are
poisonous; and this throws light on the long-disputed question of the
use of the rattle of the rattlesnake. This reptile is really both
sluggish and timid, and is very easily captured by those who know its
habits. If gently tapped on the head with a stick, it will coil itself
up and lie still, only raising its tail and rattling. It may then be
easily caught. This shows that the rattle is a warning to its enemies
that it is dangerous to proceed to extremities; and the creature has
probably acquired this structure and habit because it frequents open or
rocky districts where protective colour is needful to save it from being
pounced upon by buzzards or other snake-eaters. Quite parallel in
function is the expanded hood of the Indian cobra, a poisonous snake
which belongs also to the Elapidae. This is, no doubt, a warning to its
foes, not an attempt to terrify its prey; and the hood has been
acquired, as in the case of the rattlesnake, because, protective
coloration being on the whole useful, some mark was required to
distinguish it from other protectively coloured, but harmless, snakes.
Both these species feed on active creatures capable of escaping if their
enemy were visible at a moderate distance.
_Mimicry among Birds._
The varied forms and habits of birds do not favour the production among
them of the phenomena of warning colours or of mimicry; and the extreme
development of their instincts and reasoning powers, as well as their
activity and their power of flight, usually afford them other means of
evading their enemies. Yet there are a few imperfect, and one or two
very perfect cases of true mimicry to be found among them. The less
perfect examples are those presented by several species of cuckoos, an
exceedingly weak and defenceless group of birds. Our own cuckoo is, in
colour and markings, very like a sparrow-hawk. In the East, several of
the small black cuckoos closely resemble the aggressive drongo-shrikes
of the same country, and the small metallic cuckoos are like glossy
starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus)
resembles one of the fine pheasants (Euplocamus) of the same country,
both in form and in its rich metallic colours.
More perfect cases of mimicry occur between some of the dull-coloured
orioles in the Malay Archipelago and a genus of large honey-suckers - the
Tropidorhynchi or "Friar-birds." These latter are powerful and noisy
birds which go in small flocks. They have long, curved, and sharp beaks,
and powerful grasping claws; and they are quite able to defend
themselves, often driving away crows and hawks which venture to approach
them too nearly. The orioles, on the other hand, are weak and timid
birds, and trust chiefly to concealment and to their retiring habits to
escape persecution. In each of the great islands of the Austro-Malayan
region there is a distinct species of Tropidorhynchus, and there is
always along with it an oriole that exactly mimics it. All the
Tropidorhynchi have a patch of bare black skin round the eyes, and a
ruff of curious pale recurved feathers on the nape, whence their name of
Friar-birds, the ruff being supposed to resemble the cowl of a friar.
These peculiarities are imitated in the orioles by patches of feathers
of corresponding colours; while the different tints of the two species
in each island are exactly the same. Thus in Bouru both are earthy
brown; in Ceram they are both washed with yellow ochre; in Timor the
under surface is pale and the throat nearly white, and Mr. H.O. Forbes
has recently discovered another pair in the island of Timor Laut. The
close resemblance of these several pairs of birds, of widely different
families, is quite comparable with that of many of the insects already
described. It is so close that the preserved specimens have even
deceived naturalists; for, in the great French work, _Voyage de
l'Astrolabe_, the oriole of Bouru is actually described and figured as a
honey-sucker; and Mr. Forbes tells us that, when his birds were
submitted to Dr. Sclater for description, the oriole and the
honey-sucker were, previous to close examination, considered to be the
same species.
_Objections to the Theory of Mimicry._
To set forth adequately the varied and surprising facts of mimicry would
need a large and copiously illustrated volume; and no more interesting
subject could be taken up by a naturalist who has access to our great
collections and can devote the necessary time to search out the many
examples of mimicry that lie hidden in our museums. The brief sketch of
the subject that has been here given will, however, serve to indicate
its nature, and to show the weakness of the objections that were at
first made to it. It was urged that the action of "like conditions,"
with "accidental resemblances" and "reversion to ancestral types," would
account for the facts. If, however, we consider the actual phenomena as
here set forth, and the very constant conditions under which they occur,
we shall see how utterly inadequate are these causes, either singly or
combined. These constant conditions are -
1. That the imitative species occur in the same area and occupy
the very same station as the imitated.
2. That the imitators are always the more defenceless.
3. That the imitators are always less numerous in individuals.
4. That the imitators differ from the bulk of their allies.
5. That the imitation, however minute, is _external_ and
_visible_ only, never extending to internal characters or to
such as do not affect the external appearance.
These five characteristic features of mimicry show us that it is really
an exceptional form of protective resemblance. Different species in the
same group of organisms may obtain protection in different ways: some by
a general resemblance to their environment; some by more exactly
imitating the objects that surround them - bark, or leaf, or flower;
while others again gain an equal protection by resembling some species
which, from whatever cause, is almost as free from attack as if it were
a leaf or a flower. This immunity may depend on its being uneatable, or
dangerous, or merely strong; and it is the resemblance to such creatures
for the purpose of sharing in their safety that constitutes mimicry.
_Concluding Remarks on Warning Colours and Mimicry._
Colours which have been acquired for the purpose of serving as a warning
of inedibility, or of the possession of dangerous offensive weapons, are
probably more numerous than have been hitherto supposed; and, if so, we
shall be able to explain a considerable amount of colour in nature for
which no use has hitherto been conjectured. The brilliant and varied
colours of sea-anemones and of many coral animals will probably come
under this head, since we know that many of them possess the power of
ejecting stinging threads from various parts of their bodies which
render them quite uneatable to most animals. Mr. Gosse describes how, on
putting an Anthea into a tank containing a half-grown bullhead (Cottus
bubalis) which had not been fed for some time, the fish opened his mouth
and sucked in the morsel, but instantly shot it out again. He then
seized it a second time, and after rolling it about in his mouth for a
moment shot it out again, and then darted away to hide himself in a
hole. Some tropical fishes, however, of the genera Tetrodon,
Pseudoscarus, Astracion, and a few others, seem to have acquired the
power of feeding on corals and medusae; and the beautiful bands and
spots and bright colours with which they are frequently adorned, may be
either protective when feeding in the submarine coral groves, or may, in
some cases, be warning colours to show that they themselves are
poisonous and uneatable.
A remarkable illustration of the wide extension of warning colours, and
their very definite purpose in nature, is afforded by what may now be
termed "Mr. Belt's frog." Frogs in all parts of the world are, usually,
protectively coloured with greens or browns; and the little tree-frogs
are either green like the leaves they rest upon, or curiously mottled to
imitate bark or dead leaves. But there are a certain number of very
gaily coloured frogs, and these do not conceal themselves as frogs
usually do. Such was the small toad found by Darwin at Bahia Blanca,
which was intense black and bright vermilion, and crawled about in the
sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt
found a little frog gorgeously dressed in a livery of red and blue,
which did not attempt concealment and was very abundant, a combination
of characters which convinced him that it was uneatable. He, therefore,
took a few specimens home with him and gave them to his fowls and ducks,
but none would touch them. At last, by throwing down pieces of meat, for
which there was a great competition among the poultry, he managed to
entice a young duck into snatching up one of the little frogs. Instead
of swallowing it, however, the duck instantly threw it out of its mouth,
and went about jerking its head as if trying to get rid of some
unpleasant taste.[114]
The power of predicting what will happen in a given case is always
considered to be a crucial test of a true theory, and if so, the theory
of warning colours, and with it that of mimicry, must be held to be well
established. Among the creatures which probably have warning colours as
a sign of inedibility are, the brilliantly coloured nudibranchiate
molluscs, those curious annelids the Nereis and the Aphrodite or
sea-mouse, and many other marine animals. The brilliant colours of the
scallops (Pecten) and some other bivalve shells are perhaps an
indication of their hardness and consequent inedibility, as in the case
of the hard beetles; and it is not improbable that some of the
phosphorescent fishes and other marine organisms may, like the
glow-worm, hold out their lamp as a warning to enemies.[115] In
Queensland there is an exceedingly poisonous spider, whose bite will
kill a dog, and cause severe illness with excruciating pain in man. It
is black, with a bright vermilion patch on the middle of the body; and
it is so well recognised by this conspicuous coloration that even the
spider-hunting wasps avoid it.[116]
Locusts and grasshoppers are generally of green protective tints, but
there are many tropical species most gaudily decorated with red, blue,
and black colours. On the same general grounds as those by which Mr.
Belt predicted the inedibility of his conspicuous frog, we might safely
predict the same for these insects; but we have fortunately a proof that
they are so protected, since Mr. Charles Home states that one of the
bright coloured Indian locusts was invariably rejected when offered to
birds and lizards.[117]
* * * * *
The examples now given lead us to the conclusion that colours acquired
for the purpose of serving as a danger-signal to enemies are very
widespread in nature, and, with the corresponding colours of the species
which mimic them, furnish us with a rational explanation of a
considerable portion of the coloration of animals which is outside the
limits of those colours that have been acquired for either protection or
recognition. There remains, however, another set of colours, chiefly
among the higher animals, which, being connected with some of the most
interesting and most disputed questions in natural history, must be
discussed in a separate chapter.
FOOTNOTES:
[Footnote 92: _Nature_, vol. iii. p. 165. Professor Meldola observed
that specimens of Danais and Euplaea in collections were less subject to
the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was
corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.]
[Footnote 93: See Darwin's _Descent of Man_, p. 325.]
[Footnote 94: _Transactions of the Entomological Society of London_,
1869, p. 21.]
[Footnote 95: _Ibid._, p. 27.]
[Footnote 96: _Nature_, vol. iii. p. 147.]
[Footnote 97: Stainton's _Manual of Butterflies and Moths_, vol. i. p.
93; E.B. Poulton, _Proceedings of the Zool. Soc. of London_, 1887, pp.
191-274.]
[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp.
495-566, coloured plates.]
[Footnote 99: These butterflies are now divided into two sub-families,
one of which is placed with the Danaidae; but to avoid confusion I shall
always speak of the American genera under the old term Heliconidae.]
[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878,
p. 158.]
[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation
of Malayan Papilionidae; and, Wallace's _Contributions to Natural
Selection_ chaps. iii. and iv., where full details are given.]
[Footnote 102: See _Trans. Linn. Soc._, vol. xxvi., with two coloured
plates illustrating cases of mimicry.]
[Footnote 103: Edwards's _Butterflies of North America_, second series,
part vi.]
[Footnote 104: Professor Meldola informs me that he has recorded another
case of mimicry among British moths, in which Acidalia subsericata
imitates Asthena candidata. See _Ent. Mo. Mag._, vol. iv. p. 163.]
[Footnote 105: From Professor Meldola's translation of Dr. F. Müller's
paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]
[Footnote 106: _Island Life_, p. 255.]
[Footnote 107: This extension of the theory of mimicry was pointed out
by Professor Meldola in the paper already referred to; and he has
answered the objections to Dr. F. Müller's theory with great force in
the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]
[Footnote 108: Godman and Salvin's _Biologia Centrali-Americana,
Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]
[Footnote 109: _Trans. Ent. Soc._, 1885, p. 369.]
[Footnote 110: _Proc. Cambridge Phil. Soc._, vol. iii. part ii., 1877.]
[Footnote 111: _Compte-Rendu de la Société Entomologique de Belgaue_,
series ii., No. 59, 1878.]
[Footnote 112: _Nature_, vol. xxxiv. p. 547.]
[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]
[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]
[Footnote 115: Mr. Belt first suggested this use of the light of the
Lampyridae (fireflies and glow-worms) - _Naturalist in Nicaragua_, p.
320. Mr. Verrill and Professor Meldola made the same suggestion in the
case of medusae and other phosphorescent marine organisms (_Nature_,
vol. xxx. pp. 281, 289).]
[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]
[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]
CHAPTER X
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
Sex colours in the mollusca and crustacea - In insects - In
butterflies and moths - Probable causes of these colours - Sexual
selection as a supposed cause - Sexual coloration of birds - Cause
of dull colours of female birds - Relation of sex colour to
nesting habits - Sexual colours of other vertebrates - Sexual
selection by the struggles of males - Sexual characters due to
natural selection - Decorative plumage of males and its effect on
the females - Display of decorative plumage by the males - A
theory of animal coloration - The origin of accessory
plumes - Development of accessory plumes and their display - The
effect of female preference will be neutralised by natural
selection - General laws of animal coloration - Concluding
remarks.
In the preceding chapters we have dealt chiefly with the coloration of
animals as distinctive of the several species; and we have seen that, in
an enormous number of cases, the colours can be shown to have a definite
purpose, and to be useful either as a means of protection or
concealment, of warning to enemies, or of recognition by their own kind.
We have now to consider a subordinate but very widespread
phenomenon - -the differences of colour or of ornamental appendages in
the two sexes. These differences are found to have special relations
with the three classes of coloration above referred to, in many cases
confirming the explanation already given of their purport and use, and
furnishing us with important aid in formulating a general theory of
animal coloration.
In comparing the colours of the two sexes we find a perfect gradation,
from absolute identity of colour up to such extreme difference that it
is difficult to believe that the two forms can belong to the same
species; and this diversity in the colours of the sexes does not bear
any constant relation to affinity or systematic position. In both
insects and birds we find examples of complete identity and extreme
diversity of the sexes; and these differences occur sometimes in the
same tribe or family, and sometimes even in the same genus.
It is only among the higher and more active animals that sexual
differences of colour acquire any prominence. In the mollusca the two
sexes, when separated, are always alike in colour, and only very rarely
present slight differences in the form of the shell. In the extensive
group of crustacea the two sexes as a rule are identical in colour,
though there are often differences in the form of the prehensile organs;
but in a very few cases there are differences of colour also. Thus, in a
Brazilian species of shore-crab (Gelasimus) the female is grayish-brown,
while in the male the posterior part of the cephalo-thorax is pure
white, with the anterior part of a rich green. This colour is only
acquired by the males when they become mature, and is liable to rapid
change in a few minutes to dusky tints.[118] In some of the freshwater
fleas (Daphnoidae) the males are ornamented with red and blue spots,
while in others similar colours occur in both sexes. In spiders also,
though as a rule the two sexes are alike in colour, there are a few
exceptions, the males being ornamented with brilliant colours on the
abdomen, while the female is dull coloured.
_Sexual Coloration in Insects._
It is only when we come to the winged insects that we find any large
amount of peculiarity in sexual coloration, and even here it is only
developed in certain orders. Flies (Diptera), field-bugs (Hemiptera),
cicadas (Homoptera), and the grasshoppers, locusts, and crickets
(Orthoptera) present very few and unimportant sexual differences of
colour; but the last two groups have special musical organs very fully
developed in the males of some of the species, and these no doubt enable
the sexes to discover and recognise each other. In some cases, however,
when the female is protectively coloured, as in the well-known
leaf-insects already referred to (p. 207), the male is smaller and much
less protectively formed and coloured. In the bees and wasps
(Hymenoptera) it is also the rule that the sexes are alike in colour,
though there are several cases among solitary bees where they differ;
the female being black, and the male brown in Anthophora retusa, while
in Andraena fulva the female is more brightly coloured than the male. Of
the great order of beetles (Coleoptera) the same thing may be said.
Though often so rich and varied in their colours the sexes are usually
alike, and Mr. Darwin was only able to find about a dozen cases in which
there was any conspicuous difference between them.[119] They exhibit,
however, numerous sexual characters, in the length of the antennae, and
in horns, legs, or jaws remarkably enlarged or curiously modified in the
male sex.
It is in the family of dragonflies (order Neuroptera) that we first meet
with numerous cases of distinctive sexual coloration. In some of the
Agrionidae the males have the bodies rich blue and the wings black,
while the females have the bodies green and the wings transparent. In
the North American genus Hetaerina the males alone have a carmine spot
at the base of each wing; but in some other genera the sexes hardly
differ at all.
The great order of Lepidoptera, including the butterflies and moths,
affords us the most numerous and striking examples of diversity of
sexual colouring. Among the moths the difference is usually but slight,
being manifested in a greater intensity of the colour of the smaller
winged male; but in a few cases there is a decided difference, as in the
ghost-moth (Hepialus humuli), in which the male is pure white, while the
female is yellow with darker markings. This may be a recognition colour,
enabling the female more readily to discover her mate; and this view
receives some support from the fact that in the Shetland Islands the
male is almost as yellow as the female, since it has been suggested that