WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration--Warning colours
among insects--Butterflies--Caterpillars--Mimicry--How mimicry
has been produced--Heliconidae--Perfection of the
imitation--Other cases of mimicry among Lepidoptera--Mimicry
among protected groups--Its explanation--Extension of the
principle--Mimicry in other orders of insects--Mimicry among the
vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among
birds--Objections to the theory of mimicry--Concluding remarks
on warning colours and mimicry.
We have now to deal with a class of colours which are the very opposite
of those we have hitherto considered, since, instead of serving to
conceal the animals that possess them or as recognition marks to their
associates, they are developed for the express purpose of rendering the
species conspicuous. The reason of this is that the animals in question
are either the possessors of some deadly weapons, as stings or poison
fangs, or they are uneatable, and are thus so disagreeable to the usual
enemies of their kind that they are never attacked when their peculiar
powers or properties are known. It is, therefore, important that they
should not be mistaken for defenceless or eatable species of the same
class or order, since in that case they might suffer injury, or even
death, before their enemies discovered the danger or the uselessness of
the attack. They require some signal or danger-flag which shall serve as
a warning to would-be enemies not to attack them, and they have usually
obtained this in the form of conspicuous or brilliant coloration, very
distinct from the protective tints of the defenceless animals allied to
them.
_The Skunk as illustrating Warning Coloration._
While staying a few days, in July 1887, at the Summit Hotel on the
Central Pacific Railway, I strolled out one evening after dinner, and on
the road, not fifty yards from the house, I saw a pretty little white
and black animal with a bushy tail coming towards me. As it came on at a
slow pace and without any fear, although it evidently saw me, I thought
at first that it must be some tame creature, when it suddenly occurred
to me that it was a skunk. It came on till within five or six yards of
me, then quietly climbed over a dwarf wall and disappeared under a small
outhouse, in search of chickens, as the landlord afterwards told me.
This animal possesses, as is well known, a most offensive secretion,
which it has the power of ejecting over its enemies, and which
effectually protects it from attack. The odour of this substance is so
penetrating that it taints, and renders useless, everything it touches,
or in its vicinity. Provisions near it become uneatable, and clothes
saturated with it will retain the smell for several weeks, even though
they are repeatedly washed and dried. A drop of the liquid in the eyes
will cause blindness, and Indians are said not unfrequently to lose
their sight from this cause. Owing to this remarkable power of offence
the skunk is rarely attacked by other animals, and its black and white
fur, and the bushy white tail carried erect when disturbed, form the
danger-signals by which it is easily distinguished in the twilight or
moonlight from unprotected animals. Its consciousness that it needs only
to be seen to be avoided gives it that slowness of motion and
fearlessness of aspect which are, as we shall see, characteristic of
most creatures so protected.
_Warning Colours among Insects._
It is among insects that warning colours are best developed, and most
abundant. We all know how well marked and conspicuous are the colours
and forms of the stinging wasps and bees, no one of which in any part of
the world is known to be protectively coloured like the majority of
defenceless insects. Most of the great tribe of Malacoderms among
beetles are distasteful to insect-eating animals. Our red and black
Telephoridae, commonly called "soldiers and sailors," were found, by Mr.
Jenner Weir, to be refused by small birds. These and the allied
Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by
Mr. Belt's tame monkey and by his fowls, though most other insects were
greedily eaten by them. The Coccinellidae or lady-birds are another
uneatable group, and their conspicuous and singularly spotted bodies
serve to distinguish them at a glance from all other beetles.
These uneatable insects are probably more numerous than is supposed,
although we already know immense numbers that are so protected. The most
remarkable are the three families of butterflies--Heliconidae, Danaidae,
and Acraeidae--comprising more than a thousand species, and
characteristic respectively of the three great tropical regions--South
America, Southern Asia, and Africa. All these butterflies have
peculiarities which serve to distinguish them from every other group in
their respective regions. They all have ample but rather weak wings, and
fly slowly; they are always very abundant; and they all have conspicuous
colours or markings, so distinct from those of other families that, in
conjunction with their peculiar outline and mode of flight, they can
usually be recognised at a glance. Other distinctive features are, that
their colours are always nearly the same on the under surface of their
wings as on the upper; they never try to conceal themselves, but rest on
the upper surfaces of leaves or flowers; and, lastly, they all have
juices which exhale a powerful scent, so that when one kills them by
pinching the body, the liquid that exudes stains the fingers yellow, and
leaves an odour that can only be removed by repeated washings.
Now, there is much direct evidence to show that this odour, though not
very offensive to us, is so to most insect-eating creatures. Mr. Bates
observed that, when set out to dry, specimens of Heliconidae were less
subject to the attacks of vermin; while both he and I noticed that they
were not attacked by insect-eating birds or dragonflies, and that their
wings were not found in the forest paths among the numerous wings of
other butterflies whose bodies had been devoured. Mr. Belt once observed
a pair of birds capturing insects for their young; and although the
Heliconidae swarmed in the vicinity, and from their slow flight could
have been easily caught, not one was ever pursued, although other
butterflies did not escape. His tame monkey also, which would greedily
munch up other butterflies, would never eat the Heliconidae. It would
sometimes smell them, but always rolled them up in its hand and then
dropped them.
We have also some corresponding evidence as to the distastefulness of
the Eastern Danaidae. The Hon. Mr. Justice Newton, who assiduously
collected and took notes upon the Lepidoptera of Bombay, informed Mr.
Butler of the British Museum that the large and swift-flying butterfly
Charaxes psaphon, was continually persecuted by the bulbul, so that he
rarely caught a specimen of this species which had not a piece snipped
out of the hind wings. He offered one to a bulbul which he had in a
cage, and it was greedily devoured, whilst it was only by repeated
persecution that he succeeded in inducing the bird to touch a
Danais.[92]
Besides these three families of butterflies, there are certain groups of
the great genus Papilio--the true swallow-tailed butterflies--which have
all the characteristics of uneatable insects. They have a special
coloration, usually red and black (at least in the females), they fly
slowly, they are very abundant, and they possess a peculiar odour
somewhat like that of the Heliconidae. One of these groups is common in
tropical America, another in tropical Asia, and it is curious that,
although not very closely allied, they have each the same red and black
colours, and are very distinct from all the other butterflies of their
respective countries. There is reason to believe also that many of the
brilliantly coloured and weak-flying diurnal moths, like the fine
tropical Agaristidae and burnet-moths, are similarly protected, and that
their conspicuous colours serve as a warning of inedibility. The common
burnet-moth (Anthrocera filipendula) and the equally conspicuous
ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to
insect-eating creatures.
The most interesting and most conclusive example of warning coloration
is, however, furnished by caterpillars, because in this case the facts
have been carefully ascertained experimentally by competent observers.
In the year 1866, when Mr. Darwin was collecting evidence as to the
supposed effect of sexual selection in bringing about the brilliant
coloration of the higher animals, he was struck by the fact that many
caterpillars have brilliant and conspicuous colours, in the production
of which sexual selection could have no place. We have numbers of such
caterpillars in this country, and they are characterised not only by
their gay colours but by not concealing themselves. Such are the mullein
and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of
the buff-tip, and many others. Some of these caterpillars are
wonderfully conspicuous, as in the case of that noticed by Mr. Bates in
South America, which was four inches long, banded across with black and
yellow, and with bright red head, legs, and tail. Hence it caught the
eye of any one who passed by, even at the distance of many yards.
Mr. Darwin asked me to try and suggest some explanation of this
coloration; and, having been recently interested in the question of the
warning coloration of butterflies, I suggested that this was probably a
similar case,--that these conspicuous caterpillars were distasteful to
birds and other insect-eating creatures, and that their bright
non-protective colours and habit of exposing themselves to view, enabled
their enemies to distinguish them at a glance from the edible kinds and
thus learn not to touch them; for it must be remembered that the bodies
of caterpillars while growing are so delicate, that a wound from a
bird's beak would be perhaps as fatal as if they were devoured.[93] At
this time not a single experiment or observation had been made on the
subject, but after I had brought the matter before the Entomological
Society, two gentlemen, who kept birds and other tame animals, undertook
to make experiments with a variety of caterpillars.
Mr. Jenner Weir was the first to experiment with ten species of small
birds in his aviary, and he found that none of them would eat the
following smooth-skinned conspicuous caterpillars--Abraxas
grossulariata, Diloba caeruleocephala, Anthrocera filipendula, and
Cucullia verbasci. He also found that they would not touch any hairy or
spiny larvae, and he was satisfied that it was not the hairs or the
spines, but the unpleasant taste that caused them to be rejected,
because in one case a young smooth larva of a hairy species, and in
another case the pupa of a spiny larva, were equally rejected. On the
other hand, all green or brown caterpillars as well as those that
resemble twigs were greedily devoured.[94]
Mr. A.G. Butler also made experiments with some green lizards (Lacerta
viridis), which greedily ate all kinds of food, including flies of many
kinds, spiders, bees, butterflies, and green caterpillars; but they
would not touch the caterpillar of the gooseberry-moth (Abraxas
grossulariata), or the imago of the burnet-moth (Anthrocera
filipendula). The same thing happened with frogs. When the gooseberry
caterpillars were first given to them, "they sprang forward and licked
them eagerly into their mouths; no sooner, however, had they done so,
than they seemed to become aware of the mistake that they had made, and
sat with gaping mouths, rolling their tongues about, until they had got
quit of the nauseous morsels, which seemed perfectly uninjured, and
walked off as briskly as ever." Spiders seemed equally to dislike them.
This and another conspicuous caterpillar (Halia wavaria) were rejected
by two species--the geometrical garden spider (Epeira diadema) and a
hunting spider.[95]
Some further experiments with lizards were made by Professor Weismann,
quite confirming the previous observations; and in 1886 Mr. E.B. Poulton
of Oxford undertook a considerable series of experiments, with many
other species of larvae and fresh kinds of lizards and frogs. Mr.
Poulton then reviewed the whole subject, incorporating all recorded
facts, as well as some additional observations made by Mr. Jenner Weir
in 1886. More than a hundred species of larvae or of perfect insects of
various orders have now been made the subject of experiment, and the
results completely confirm my original suggestion. In almost every case
the protectively coloured larvae have been greedily eaten by all kinds
of insectivorous animals, while, in the immense majority of cases, the
conspicuous, hairy, or brightly coloured larvae have been rejected by
some or all of them. In some instances the inedibility of the larvae
extends to the perfect insect, but not in others. In the former cases
the perfect insect is usually adorned with conspicuous colours, as the
burnet and ragwort moths; but in the case of the buff-tip, the moth
resembles a broken piece of rotten stick, yet it is partly inedible,
being refused by lizards. It is, however, very doubtful whether these
are its chief enemies, and its protective form and colour may be needed
against insectivorous birds or mammals.
Mr. Samuel H. Scudder, who has largely bred North American butterflies,
has found so many of the eggs and larvae destroyed by hymenopterous and
dipterous parasites that he thinks at least nine-tenths, perhaps a
greater proportion, never reach maturity. Yet he has never found any
evidence that such parasites attack either the egg or the larva of the
inedible Danais archippus, so that in this case the insect is
distasteful to its most dangerous foes in all the stages of its
existence, a fact which serves to explain its great abundance and its
extension over almost the whole world.[96]
One case has been found of a protectively coloured larva,--one,
moreover, which in all its habits shows that it trusts to concealment to
escape its enemies--which was yet always rejected by lizards after they
had seized it, evidently under the impression that from its colour it
would be eatable. This is the caterpillar of the very common moth Mania
typica; and Mr. Poulton thinks that, in this case, the unpleasant taste
is an incidental result of some physiological processes in the organism,
and is itself a merely useless character. It is evident that the insect
would not conceal itself so carefully as it does if it had not some
enemies, and these are probably birds or small mammals, as its
food-plants are said to be dock and willow-herb, not suggestive of
places frequented by lizards; and it has been found by experiment that
lizards and birds have not always the same likes and dislikes. The case
is interesting, because it shows that nauseous fluids sometimes occur
sporadically, and may thus be intensified by natural selection when
required for the purpose of protection. Another exceptional case is
that of the very conspicuous caterpillar of the spurge hawk-moth
(Deilephila euphorbiae), which was at once eaten by a lizard, although,
as it exposes itself on its food-plant in the daytime and is very
abundant in some localities, it must almost certainly be disliked by
birds or by some animals who would otherwise devour it. If disturbed
while feeding it is said to turn round with fury and eject a quantity of
green liquid, of an acid and disagreeable smell similar to that of the
spurge milk, only worse.[97]
These facts, and Mr. Poulton's evidence that some larvae rejected by
lizards at first will be eaten if the lizards are very hungry, show that
there are differences in the amount of the distastefulness, and render
it probable that if other food were wanting many of these conspicuous
insects would be eaten. It is the abundance of the eatable kinds that
gives value to the inedibility of the smaller number; and this is
probably the reason why so many insects rely on protective colouring
rather than on the acquisition of any kind of defensive weapons. In the
long run the powers of attack and defence must balance each other. Hence
we see that even the powerful stings of bees and wasps only protect them
against some enemies, since a tribe of birds, the bee-eaters, have been
developed which feed upon them, and some frogs and lizards do so
occasionally.
The preceding outline will sufficiently explain the characteristics of
"warning coloration" and the end it serves in nature. There are many
other curious modifications of it, but these will be best appreciated
after we have discussed the remarkable phenomenon of "mimicry," which is
bound up with and altogether depends upon "warning colour," and is in
some cases the chief indication we have of the possession of some
offensive weapon to secure the safety of the species imitated.
_Mimicry._
This term has been given to a form of protective resemblance, in which
one species so closely resembles another in external form and colouring
as to be mistaken for it, although the two may not be really allied and
often belong to distinct families or orders. One creature seems
disguised in order to be made like another; hence the terms "mimic" and
mimicry, which imply no voluntary action on the part of the imitator. It
has long been known that such resemblances do occur, as, for example,
the clear-winged moths of the families Sesiidae and Aegeriidae, many of
which resemble bees, wasps, ichneumons, or saw-flies, and have received
names expressive of the resemblance; and the parasitic flies (Volucella)
which closely resemble bees, on whose larvae the larvae of the flies
feed.
The great bulk of such cases remained, however, unnoticed, and the
subject was looked upon as one of the inexplicable curiosities of
nature, till Mr. Bates studied the phenomenon among the butterflies of
the Amazon, and, on his return home, gave the first rational explanation
of it.[98] The facts are, briefly, these. Everywhere in that fertile
region for the entomologist the brilliantly coloured Heliconidae abound,
with all the characteristics which I have already referred to when
describing them as illustrative of "warning coloration." But along with
them other butterflies were occasionally captured, which, though often
mistaken for them, on account of their close resemblance in form,
colour, and mode of flight, were found on examination to belong to a
very distinct family, the Pieridae. Mr. Bates notices fifteen distinct
species of Pieridae, belonging to the genera Leptalis and Euterpe, each
of which closely imitates some one species of Heliconidae, inhabiting
the same region and frequenting the same localities. It must be
remembered that the two families are altogether distinct in structure.
The larvae of the Heliconidae are tubercled or spined, the pupae
suspended head downwards, and the imago has imperfect forelegs in the
male; while the larvae of the Pieridae are smooth, the pupae are
suspended with a brace to keep the head erect, and the forefeet are
fully developed in both sexes. These differences are as large and as
important as those between pigs and sheep, or between swallows and
sparrows; while English entomologists will best understand the case by
supposing that a species of Pieris in this country was coloured and
shaped like a small tortoise-shell, while another species on the
Continent was equally like a Camberwell beauty--so like in both cases
as to be mistaken when on the wing, and the difference only to be
detected by close examination. As an example of the resemblance,
woodcuts are given of one pair in which the colours are simple, being
olive, yellow, and black, while the very distinct neuration of the wings
and form of the head and body can be easily seen.
[Illustration: FIG. 23.--Methona psidii (Heliconidae). Leptalis orise
(Pieridae).]
Besides these Pieridae, Mr. Bates found four true Papilios, seven
Erycinidae, three Castnias (a genus of day-flying moths), and fourteen
species of diurnal Bombycidae, all imitating some species of Heliconidae
which inhabited the same district; and it is to be especially noted that
none of these insects were so abundant as the Heliconidae they
resembled, generally they were far less common, so that Mr. Bates
estimated the proportion in some cases as not one to a thousand. Before
giving an account of the numerous remarkable cases of mimicry in other
parts of the world, and between various groups of insects and of higher
animals, it will be well to explain briefly the use and purport of the
phenomenon, and also the mode by which it has been brought about.
_How Mimicry has been Produced._
The fact has been now established that the Heliconidae possess an
offensive odour and taste, which lead to their being almost entirely
free from attack by insectivorous creatures; they possess a peculiar
form and mode of flight, and do not seek concealment; while their
colours--although very varied, ranging from deep blue-black, with white,
yellow, or vivid red bands and spots, to the most delicate
semitransparent wings adorned with pale brown or yellow markings--are
yet always very distinctive, and unlike those of all the other families
of butterflies in the same country. It is, therefore, clear that if any
other butterflies in the same region, which are eatable and suffer great
persecution from insectivorous animals, should come to resemble any of
these uneatable species so closely as to be mistaken for them by their
enemies, they will obtain thereby immunity from persecution. This is the
obvious and sufficient reason why the imitation is useful, and therefore
why it occurs in nature. We have now to explain how it has probably been
brought about, and also why a still larger number of persecuted groups
have not availed themselves of this simple means of protection.
From the great abundance of the Heliconidae[99] all over tropical
America, the vast number of their genera and species, and their marked
distinctions from all other butterflies, it follows that they constitute
a group of high antiquity, which in the course of ages has become more
and more specialised, and owing to its peculiar advantages has now
become a dominant and aggressive race. But when they first arose from
some ancestral species or group which, owing to the food of the larvae
or some other cause, possessed disagreeable juices that caused them to
be disliked by the usual enemies of their kind, they were in all
probability not very different either in form or coloration from many
other butterflies. They would at that time be subject to repeated
attacks by insect-eaters, and, even if finally rejected, would often
receive a fatal injury. Hence arose the necessity for some
distinguishing mark, by which the devourers of butterflies in general
might learn that these particular butterflies were uneatable; and every
variation leading to such distinction, whether by form, colour, or mode
of flight, was preserved and accumulated by natural selection, till the
ancestral Heliconoids became well distinguished from eatable
butterflies, and thenceforth comparatively free from persecution. Then
they had a good time of it. They acquired lazy habits, and flew about
slowly. They increased abundantly and spread all over the country, their
larvae feeding on many plants and acquiring different habits; while the
butterflies themselves varied greatly, and colour being useful rather
than injurious to them, gradually diverged into the many coloured and
beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridae,
inhabiting the same district, happened to be sufficiently like some of
the Heliconidae to be occasionally mistaken for them. These, of course,
survived while their companions were devoured. Those among their
descendants that were still more like Heliconidae again survived, and at
length the imitation would become tolerably perfect. Thereafter, as the
protected group diverged into distinct species of many different
colours, the imitative group would occasionally be able to follow it
with similar variations,--a process that is going on now, for Mr. Bates
informs us that in each fresh district he visited he found closely
allied representative species or varieties of Heliconidae, and along
with them species of Leptalis (Pieridae), which had varied in the same
way so as still to be exact imitations. But this process of imitation
would be subject to check by the increasing acuteness of birds and other
animals which, whenever the eatable Leptalis became numerous, would
surely find them out, and would then probably attack both these and
their friends the Heliconidae in order to devour the former and reject
the latter. The Pieridae would, however, usually be less numerous,
because their larvae are often protectively coloured and therefore
edible, while the larvae of the Heliconidae are adorned with warning
colours, spines, or tubercles, and are uneatable. It seems probable that
the larvae and pupae of the Heliconidae were the first to acquire the
protective distastefulness, both because in this stage they are more
defenceless and more liable to fatal injury, and also because we now
find many instances in which the larvae are distasteful while the
perfect insects are eatable, but I believe none in which the reverse is
the case. The larvae of the Pieridae are now beginning to acquire
offensive juices, but have not yet obtained the corresponding
conspicuous colours; while the perfect insects remain eatable, except
perhaps in some Eastern groups, the under sides of whose wings are
brilliantly coloured although this is the part which is exposed when at
rest.
It is clear that if a large majority of the larvae of Lepidoptera, as
well as the perfect insects, acquired these distasteful properties, so
as seriously to diminish the food supply of insectivorous and nestling
birds, these latter would be forced by necessity to acquire
corresponding tastes, and to eat with pleasure what some of them now eat
only under pressure of hunger; and variation and natural selection would
soon bring about this change.
Many writers have denied the possibility of such wonderful resemblances
being produced by the accumulation of fortuitous variations, but if the
reader will call to mind the large amount of variability that has been
shown to exist in all organisms, the exceptional power of rapid increase
possessed by insects, and the tremendous struggle for existence always
going on, the difficulty will vanish, especially when we remember that
nature has the same fundamental groundwork to act upon in the two
groups, general similarity of forms, wings of similar texture and
outline, and probably some original similarity of colour and marking.
Yet there is evidently considerable difficulty in the process, or with
these great resources at her command nature would have produced more of
these mimicking forms than she has done. One reason of this deficiency
probably is, that the imitators, being always fewer in number, have not
been able to keep pace with the variations of the much more numerous
imitated form; another reason may be the ever-increasing acuteness of
the enemies, which have again and again detected the imposture and
exterminated the feeble race before it has had time to become further
modified. The result of this growing acuteness of enemies has been, that
those mimics that now survive exhibit, as Mr. Bates well remarks, "a
palpably intentional likeness that is perfectly staggering," and also
"that those features of the portrait are most attended to by nature
which produce the most effective deception when the insects are seen in
nature." No one, in fact, can understand the perfection of the imitation
who has not seen these species in their native wilds. So complete is it
in general effect that in almost every box of butterflies, brought from
tropical America by amateurs, are to be found some species of the
mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae,
placed together under the impression that they are the same species. Yet
more extraordinary, it sometimes deceives the very insects themselves.
Mr. Trimen states that the male Danais chrysippus is sometimes deceived
by the female Diadema bolina which mimics that species. Dr. Fritz
Müller, writing from Brazil to Professor Meldola, says, "One of the most
interesting of our mimicking butterflies is Leptalis melite. The female
alone of this species imitates one of our common white Pieridae, which
she copies so well that even her own male is often deceived; for I have
repeatedly seen the male pursuing the mimicked species, till, after
closely approaching and becoming aware of his error, he suddenly
returned."[100] This is evidently not a case of true mimicry, since the
species imitated is not protected; but it may be that the less abundant
Leptalis is able to mingle with the female Pieridae and thus obtain
partial immunity from attack. Mr. Kirby of the insect department of the
British Museum informs me that there are several species of South
American Pieridae which the female Leptalis melite very nearly
resembles. The case, however, is interesting as showing that the
butterflies are themselves deceived by a resemblance which is not so
great as that of some mimicking species.
_Other Examples of Mimicry among Lepidoptera._
In tropical Asia, and eastward to the Pacific Islands, the Danaidae take
the place of the Heliconidae of America, in their abundance, their
conspicuousness, their slow flight, and their being the subjects of
mimicry. They exist under three principal forms or genera. The genus
Euploea is the most abundant both in species and individuals, and
consists of fine broad-winged butterflies of a glossy or metallic
blue-black colour, adorned with pure white, or rich blue, or dusky
markings situated round the margins of the wings. Danais has generally
more lengthened wings, of a semitransparent greenish or a rich brown
colour, with radial or marginal pale spots; while the fine Hestias are
of enormous size, of a papery or semitransparent white colour, with
dusky or black spots and markings. Each of these groups is mimicked by
various species of the genus Papilio, usually with such accuracy that it
is impossible to distinguish them on the wing.[101] Several species of
Diadema, a genus of butterflies allied to our Vanessas, also mimic
species of Danais, but in this case the females only are affected, a
subject which will be discussed in another chapter.
Another protected group in the Eastern tropics is that of the beautiful
day-flying moths forming the family Agaristidae. These are usually
adorned with the most brilliant colours or conspicuous markings, they
fly slowly in forests among the butterflies and other diurnal insects,
and their great abundance sufficiently indicates their possession of
some distastefulness which saves them from attack. Under these
conditions we may expect to find other moths which are not so protected
imitating them, and this is the case. One of the common and wide-ranging
species (Opthalmis lincea), found in the islands from Amboyna to New
Ireland, is mimicked in a wonderful manner by one of the Liparidae (the
family to which our common "tussock" and "vapourer" moths belong). This
is a new species collected at Amboyna during the voyage of the
_Challenger_, and has been named Artaxa simulans. Both insects are
black, with the apex of the fore wings ochre coloured, and the outer
half of the hind wings bright orange. The accompanying woodcuts (for the
use of which I am indebted to Mr. John Murray of the _Challenger_
Office) well exhibit their striking resemblance to each other.
[Illustration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans
(Liparidae).]
In Africa exactly similar phenomena recur, species of Papilio and of
Diadema mimicking Danaidae or Acraeidae with the most curious accuracy.
Mr. Trimen, who studied this subject in South Africa, has recorded eight
species or varieties of Diadema, and eight of Papilio, which each mimic
some species of Danais; while eight species or varieties of Panopaea
(another genus of Nymphalidae), three of Melanitis (Eurytelidae), and
two of Papilio, resemble with equal accuracy some species of
Acraea.[102] He has also independently observed the main facts on which
the explanation of the phenomenon rests,--the unpleasant odour of the
Danais and Acraea, extending to their larvae and pupae; their great
abundance, slow flight, and disregard of concealment; and he states that
while lizards, mantidae, and dragonflies all hunt butterflies, and the
rejected wings are to be found abundantly at some of their
feeding-places, those of the two genera Danais and Acraea were never
among them.
The two groups of the great genus Papilio (the true swallow-tailed
butterflies) which have been already referred to as having the special
characteristics of uneatable insects, have also their imitators in other
groups; and thus, the belief in their inedibility--derived mainly from
their style of warning coloration and their peculiar habits--is
confirmed. In South America, several species of the "Aeneas" group of
these butterflies are mimicked by Pieridae and by day-flying moths of
the genera Castnia and Pericopis. In the East, Papilio hector, P.
diphilus, and P. liris, all belonging to the inedible group, are
mimicked by the females of other species of Papilio belonging to very
distinct groups; while in Northern India and China, many fine day-flying
moths (Epicopeia) have acquired the strange forms and peculiar colours
of some of the large inedible Papilios of the same regions.
In North America, the large and handsome Danais archippus, with rich
reddish-brown wings, is very common; and it is closely imitated by
Limenitis misippus, a butterfly allied to our "white admiral," but which
has acquired a colour quite distinct from that of the great bulk of its
allies. In the same country there is a still more interesting case. The
beautiful dark bronzy green butterfly, Papilio philenor, is inedible
both in larva and perfect insect, and it is mimicked by the equally dark
Limenitis ursula. There is also in the Southern and Western States a
dark female form of the yellow Papilio turnus, which in all probability
obtains protection from its general resemblance to P. philenor. Mr. W.H.
Edwards has found, by extensive experiment, that both the dark and
yellow females produce their own kinds, with very few exceptions; and he
thinks that the dark form has the advantage in the more open regions and
in the prairies, where insectivorous birds abound. But in open country
the dark form would be quite as conspicuous as the yellow form, if not
more so, so that the resemblance to an inedible species would be there
more needed.[103]
The only probable case of mimicry in this country is that of the moth,
Diaphora mendica, whose female only is white, while the larva is of
protective colours, and therefore almost certainly edible. A much more
abundant moth, of about the same size and appearing about the same time,
is Spilosoma menthrasti, also white, but in this case both it and its
larva have been proved to be inedible. The white colour of the female
Diaphora, although it must be very conspicuous at night, may, therefore,
have been acquired in order to resemble the uneatable Spilosoma, and
thus gain some protection.[104]
_Mimicry among Protected (Uneatable) Genera._
Before giving some account of the numerous other cases of warning
colours and of mimicry that occur in the animal kingdom, it will be well
to notice a curious phenomenon which long puzzled entomologists, but
which has at length received a satisfactory explanation.
We have hitherto considered, that mimicry could only occur when a
comparatively scarce and much persecuted species obtained protection by
its close external resemblance to a much more abundant uneatable species
inhabiting its own district; and this rule undoubtedly prevails among
the great majority of mimicking species all over the world. But Mr.
Bates also found a number of pairs of species of different genera of
Heliconidae, which resembled each other quite as closely as did the
other mimicking species he has described; and since all these insects
appear to be equally protected by their inedibility, and to be equally
free from persecution, it was not easy to see why this curious
resemblance existed, or how it had been brought about. That it is not
due to close affinity is shown by the fact that the resemblance occurs
most frequently between the two distinct sub-families into which (as Mr.
Bates first pointed out) the Heliconidae are naturally divided on
account of very important structural differences. One of these
sub-families (the true Heliconinae) consists of two genera only,
Heliconius and Eueides, the other (the Danaoid Heliconinae) of no less
than sixteen genera; and, in the instances of mimicry we are now
discussing, one of the pairs or triplets that resemble each other is
usually a species of the large and handsome genus Heliconius, the others
being species of the genera Mechanitis, Melinaea, or Tithorea, though
several species of other Danaoid genera also imitate each other. The
following lists will give some idea of the number of these curious
imitative forms, and of their presence in every part of the Neotropical
area. The bracketed species are those that resemble each other so
closely that the difference is not perceptible when they are on the
wing.
In the Lower Amazon region are found--
{ Heliconius sylvana.
{ Melinaea egina.
{ Heliconius numata.
{ Melinaea mneme.
{ Tithorea harmonia.
{ Methona psidii.
{ Thyridia ino.
{ Ceratina ninonia.
{ Melinaea mnasias.
In Central America are found--
{ Heliconius zuleika.
Nicaragua { Melinaea hezia.
{ Mechanitis sp.
{ Heliconius formosus.
{ Tithorea penthias.
Guatemala { Heliconius telchina.
{ Melinaea imitata.
In the Upper Amazon region--
{ Heliconius pardalinus.
{ Melinaea pardalis.
{ Heliconius aurora.
{ Melinaea lucifer.
In New Grenada--
{ Heliconius ismenius.
{ Melinaea messatis.
{ Heliconius messene.
{ Melinaea mesenina.
{ (?) Mechanitis sp.
{ Heliconius hecalesia.
{ Tithorea hecalesina.
{ Heliconius hecuba.
{ Tithorea bonplandi.
In Eastern Peru and Bolivia--
{ Heliconius aristona.
{ Melinaea cydippe.
{ (?) Mechanitis mothone.
In Pernambuco--
{ Heliconius ethra.
{ Mechanitis nesaea.
In Rio Janeiro--
{ Helieonius eucrate.
{ Mechanitis lysimnia.
In South Brazil--
{ Thyridia megisto.
{ Ituna ilione.
{ Acraea thalia.
{ Eueides pavana.
Besides these, a number of species of Ithomia and Napeogenes, and of
Napeogenes and Mechanitis, resemble each other with equal accuracy, so
that they are liable to be mistaken for each other when on the wing; and
no doubt many other equally remarkable cases are yet unnoticed.
[Illustration: FIG. 25.--Wings of Ituna Ilione, female. Wings of
Thyridia megisto, female.]
The figures above of the fore and hind wings of two of these mimicking
species, from Dr. Fritz Müller's original paper in _Kosmos_, will serve
to show the considerable amount of difference, in the important
character of the neuration of the wings, between these butterflies,
which really belong to very distinct and not at all closely allied
genera. Other important characters are--(1) The existence of a small
basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)
the division of the cell between the veins 1_b_ and 2 of the hind wings
in the former genus, while it is undivided in the latter; and (3) the
existence in Thyridia of scent-producing tufts of hair on the upper edge
of the hind wing, while in Ituna these are wanting; but in place of them
are extensible processes at the end of the abdomen, also emitting a
powerful scent. These differences characterise two marked subdivisions
of the Danaoid Heliconinae, each containing several distinct genera; and
these subdivisions are further distinguished by very different forms of
larvae, that to which Ituna belongs having from two to four long
threadlike tentacles on the back, while in that containing Thyridia
these are always absent. The former usually feed on Asclepiadeae, the
latter on Solanaceae or Scrophulariaceae.
The two species figured, though belonging to such distinct and even
remote genera, have acquired almost identical tints and markings so as
to be deceptively alike. The surface of the wings is, in both,
transparent yellowish, with black transverse bands and white marginal
spots, while both have similar black-and white-marked bodies and long
yellow antennae. Dr. Müller states that they both show a preference for
the same flowers growing on the edges of the forest paths.[105]
We will now proceed to give the explanation of these curious
similarities, which have remained a complete puzzle for twenty years.
Mr. Bates, when first describing them, suggested that they might be due
to some form of parallel variation dependent on climatic influences; and
I myself adduced other cases of coincident local modifications of
colour, which did not appear to be explicable by any form of
mimicry.[106] But we neither of us hit upon the simple explanation given
by Dr. Fritz Müller in 1879.
His theory is founded on the assumed, but probable, fact, that
insect-eating birds only learn by experience to distinguish the edible
from the inedible butterflies, and in doing so necessarily sacrifice a
certain number of the latter. The quantity of insectivorous birds in
tropical America is enormous; and the number of young birds which every
year have to learn wisdom by experience, as regards the species of
butterflies to be caught or to be avoided, is so great that the
sacrifice of life of the inedible species must be considerable, and, to
a comparatively weak or scarce species, of vital importance. The number
thus sacrificed will be fixed by the quantity of young birds, and by the
number of experiences requisite to cause them to avoid the inedible
species for the future, and not at all by the numbers of individuals of
which each species consists. Hence, if two species are so much alike as
to be mistaken for one another, the fixed number annually sacrificed by
inexperienced birds will be divided between them, and both will benefit.
But if the two species are very unequal in numbers, the benefit will be
comparatively slight for the more abundant species, but very great for
the rare one. To the latter it may make all the difference between
safety and destruction.
To give a rough numerical example. Let us suppose that in a given
limited district there are two species of Heliconidae, one consisting of
only 1000, the other of 100,000 individuals, and that the quota required
annually in the same district for the instruction of young insectivorous
birds is 500. By the larger species this loss will be hardly felt; to
the smaller it will mean the most dreadful persecution resulting in a
loss of half the total population. But, let the two species become
superficially alike, so that the birds see no difference between them.
The quota of 500 will now be taken from a combined population of 101,000
butterflies, and if proportionate numbers of each suffer, then the weak
species will only lose five individuals instead of 500 as it did before.
Now we know that the different species of Heliconidae are not equally
abundant, some being quite rare; so that the benefit to be derived in
these latter cases would be very important. A slight inferiority in
rapidity of flight or in powers of eluding attack might also be a cause
of danger to an inedible species of scanty numbers, and in this case too
the being merged in another much more abundant species, by similarity
of external appearance, would be an advantage.
The question of fact remains. Do young birds pursue and capture these
distasteful butterflies till they have learned by bitter experience what
species to avoid? On this point Dr. Müller has fortunately been able to
obtain some direct evidence, by capturing several Acraeas and
Heliconidae which had evidently been seized by birds but had afterwards
escaped, as they had pieces torn out of the wing, sometimes
symmetrically out of both wings, showing that the insect had been seized
when at rest and with the two pairs of wings in contact. There is,
however, a general impression that this knowledge is hereditary, and
does not need to be acquired by young birds; in support of which view
Mr. Jenner Weir states that his birds always disregarded inedible
caterpillars. When, day by day, he threw into his aviary various larvae,
those which were edible were eaten immediately, those which were
inedible were no more noticed than if a pebble had been thrown before
the birds.
The cases, however, are not strictly comparable. The birds were not
young birds of the first year; and, what is more important, edible
larvae have a comparatively simple coloration, being always brown or
green and smooth. Uneatable larvae, on the other hand, comprise all that
are of conspicuous colours and are hairy or spiny. But with butterflies
there is no such simplicity of contrast. The eatable butterflies
comprise not only brown or white species, but hundreds of Nymphalidae,
Papilionidae, Lycaenidae, etc., which are gaily coloured and of an
immense variety of patterns. The colours and patterns of the inedible
kinds are also greatly varied, while they are often equally gay; and it
is quite impossible to suppose that any amount of instinct or inherited
habit (if such a thing exists) could enable young insectivorous birds to
distinguish all the species of one kind from all those of the other.
There is also some evidence to show that animals do learn by experience
what to eat and what to avoid. Mr. Poulton was assured by Rev. G.J.
Bursch that very young chickens peck at insects which they afterwards
avoid. Lizards, too, often seized larvae which they were unable to eat
and ultimately rejected.
Although the Heliconidae present, on the whole, many varieties of
coloration and pattern, yet, in proportion to the number of distinct
species in each district, the types of coloration are few and very well
marked, and thus it becomes easier for a bird or other animal to learn
that all belonging to such types are uneatable. This must be a decided
advantage to the family in question, because, not only do fewer
individuals of each species need to be sacrificed in order that their
enemies may learn the lesson of their inedibility, but they are more
easily recognised at a distance, and thus escape even pursuit. There is
thus a kind of mimicry between closely allied species as well as between
species of distinct genera, all tending to the same beneficial end. This
may be seen in the four or five distinct species of the genus Heliconius
which all have the same peculiar type of coloration--a yellow band
across the upper wings and radiating red stripes on the lower,--and are
all found in the same forests of the Lower Amazon; in the numerous very
similar species of Ithomia with transparent wings, found in every
locality of the same region; and in the very numerous species of Papilio
of the "Aeneas" group, all having a similar style of marking, the
resemblance being especially close in the females. The very uniform type
of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of
the same character.[107] In all these cases the similarity of the allied
species is so great, that, when they are on the wing at some distance
off, it is difficult to distinguish one species from another. But this
close external resemblance is not always a sign of very near affinity;
for minute examination detects differences in the form and scalloping of
the wings, in the markings on the body, and in those on the under
surface of the wings, which do not usually characterise the closest
allies. It is to be further noted, that the presence of groups of very
similar species of the same genus, in one locality, is not at all a
common phenomenon among unprotected groups. Usually the species of a
genus found in one locality are each well marked and belong to somewhat
distinct types, while the closely allied forms--those that require
minute examination to discriminate them as distinct species--are most
generally found in separate areas, and are what are termed
representative forms.
The extension we have now given to the theory of mimicry is important,
since it enables us to explain a much wider range of colour phenomena
than those which were first imputed to mimicry. It is in the richest
butterfly region in the world--the Amazon valley--that we find the most
abundant evidence of the three distinct sets of facts, all depending on
the same general principle. The form of mimicry first elucidated by Mr.
Bates is characterised by the presence in each locality of certain
butterflies, or other insects, themselves edible and belonging to edible
groups, which derived protection from having acquired a deceptive
resemblance to some of the inedible butterflies in the same localities,
which latter were believed to be wholly free from the attacks of
insectivorous birds. Then came the extension of the principle, by Dr. F.
Müller, to the case of species of distinct genera of the inedible
butterflies resembling each other quite as closely as in the former
cases, and like them always found in the same localities. They derive
mutual benefit from becoming, in appearance, one species, from which a
certain toll is taken annually to teach the young insectivorous birds
that they are uneatable. Even when the two or more species are
approximately equal in numbers, they each derive a considerable benefit
from thus combining their forces; but when one of the species is scarce
or verging on extinction, the benefit becomes exceedingly great, being,
in fact, exactly apportioned to the need of the species.
The third extension of the same principle explains the grouping of
allied species of the same genera of inedible butterflies into sets,
each having a distinct type of coloration, and each consisting of a
number of species which can hardly be distinguished on the wing. This
must be useful exactly in the same way as in the last case, since it
divides the inevitable toll to insectivorous birds and other animals
among a number of species. It also explains the fact of the great
similarity of many species of inedible insects in the same locality--a
similarity which does not obtain to anything like the same extent among
the edible species. The explanation of the various phenomena of
resemblance and mimicry, presented by the distasteful butterflies, may
now be considered tolerably complete.
_Mimicry in other Orders of Insects._
A very brief sketch of these phenomena will be given, chiefly to show
that the same principle prevails throughout nature, and that, wherever a
rather extensive group is protected, either by distastefulness or
offensive weapons, there are usually some species of edible and
inoffensive groups that gain protection by imitating them. It has been
already stated that the Telephoridae, Lampyridae, and other families of
soft-winged beetles, are distasteful; and as they abound in all parts of
the world, and especially in the tropics, it is not surprising that
insects of many other groups should imitate them. This is especially the
case with the longicorn beetles, which are much persecuted by
insectivorous birds; and everywhere in tropical regions some of these
are to be found so completely disguised as to be mistaken for species of
the protected groups. Numbers of these imitations have been already
recorded by Mr. Bates and myself, but I will here refer to a few others.
In the recently published volumes on the Longicorn and Malacoderm
beetles of Central America[108] there are numbers of beautifully
coloured figures of the new species; and on looking over them we are
struck by the curious resemblance of some of the Longicorns to species
of the Malacoderm group. In some cases we discover perfect mimics, and
on turning to the descriptions we always find these pairs to come from
the same locality. Thus the Otheostethus melanurus, one of the
Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar
coloration, and size, and both are found at Chontales in Nicaragua, the
species mimicked having, however, as is usual, a wider range. The
curious and very rare little longicorn, Tethlimmena aliena, quite unlike
its nearest allies in the same country, is an exact copy on a somewhat
smaller scale of a malacoderm, Lygistopterus amabilis, both found at
Chontales. The pretty longicorn, Callia albicornis, closely resembles
two species of malacoderms (Silis chalybeipennis and Colyphus
signaticollis), all being small beetles with red head and thorax and
bright blue elytra, and all three have been found at Panama. Many other
species of Callia also resemble other malacoderms; and the longicorn
genus Lycidola has been named from its resemblance to various species of
the Lycidae, one of the species here figured (Lycidola belti) being a
good mimic of Calopteron corrugatum and of several other allied species,
all being of about the same size and found at Chontales. In these cases,
and in most others, the longicorn beetles have lost the general form and
aspect of their allies to take on the appearance of a distinct tribe.
Some other groups of beetles, as the Elateridae and Eucnemidae, also
deceptively mimic malacoderms.
Wasps and bees are often closely imitated by insects of other orders.
Many longicorn beetles in the tropics exactly mimic wasps, bees, or
ants. In Borneo a large black wasp, whose wings have a broad white patch
near the apex (Mygnimia aviculus), is closely imitated by a heteromerous
beetle (Coloborhombus fasciatipennis), which, contrary to the general
habit of beetles, keeps its wings expanded in order to show the white
patch on their apex, the wing-coverts being reduced to small oval
scales, as shown in the figure. This is a most remarkable instance of
mimicry, because the beetle has had to acquire so many characters which
are unknown among its allies (except in another species from Java)--the
expanded wings, the white band on them, and the oval scale-like
elytra.[109] Another remarkable case has been noted by Mr. Neville
Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly
imitated in colour, size, shape, attitude when at rest, and mode of
flight, by a beetle of the genus Laphria.[110]
The tiger-beetles (Cicindelidae) are also the subjects of mimicry by
more harmless insects. In the Malay Islands I found a heteromerous
beetle which exactly resembled a Therates, both being found running on
the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics
Collyris, another genus of the same family; while in the Philippine
Islands there is a cricket (Condylodeira tricondyloides), which so
closely resembles a tiger-beetle of the genus Tricondyla that the
experienced entomologist, Professor Westwood, at first placed it in his
cabinet among those beetles.
[Illustration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus
fasciatipennis (Beetle).]
[Illustration: FIG. 27.
a. Doliops sp. (Longicorn)
mimics Pachyrhynchus orbifae, (b) (a hard curculio).
c. Doliops curculionoides mimics (d) Pachyrhynchus sp.
e. Scepastus pachyrhynchoides (a grasshopper),
mimics (f) Apocyrtus sp. (a hard curculio).
g. Doliops sp. mimics (h) Pachyrhynchus sp.
i. Phoraspis (grasshopper) mimics (k) a Coccinella.
All the above are from the Philippines. The exact correspondence of the
colours of the insects themselves renders the mimicry much more complete
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.]