leaf-butterflies. I wished to make it clear that we have by no means
to do with a few exceptional cases, but with a great number, in all
of which resemblance to a leaf has been aimed at, although it has
been attained in varying degrees, and by very diverse ways. Whoever
surveys this wealth of fact must certainly receive the impression,
that, wherever it was advantageous to the existence of the species, the
evolution of such a deceptive resemblance has also been possible. In
any case one cannot but be convinced that it is not a case of chance
resemblance, as some naturalists have recently tried to maintain.
But I have not yet quite finished my outline-survey of the facts, for I
must not omit to mention that, in the evergreen tropical forests, there
are also large nocturnal Lepidoptera, which mimic leaves, sometimes
green ones, sometimes brown, dead ones.
[Illustration: FIG. 16. _Phyllodes ornata_, from Assam. Upper surface
with leaf-like marking only on the anterior wing, which is the only
part visible when at rest; ⅔ nat. size.]
Fig. 16 gives a good picture, reduced to two-thirds, of such a species,
_Phyllodes ornata_, from Assam. The posterior wings are conspicuously
coloured in deep black and yellow; in the resting position they are
covered by the anterior wings, and these are red-brown with black
markings which precisely and clearly mimic the ribs of a leaf. The
midrib begins near the tip of the anterior wing, but breaks off
half-way across the wing at two silvery white spots, similar to those
in many of the diurnal forms, which also mimic decaying leaves. Three
pairs of side veins go off backwards and forwards with remarkable
regularity from the midrib, almost at the same angle, and parallel
to one another, and three more are indicated by vague shading. Then
the midrib begins again in the internal half of the wing, though only
represented by a broad shading. The whole suggests two torn, rotten
leaves, one partly covering the other; and the deception will certainly
be perfect when the moth rests on the ground or among decaying leaves.
That all these extremely favourable protective colorations find their
explanation in the slow and gradually cumulative effects of natural
selection cannot be disputed; it is beyond doubt that they cannot be
explained, so far as we know, in any other way.
If, however, it were possible for a species of butterfly living in
the forest and among leaves to become, through natural selection,
in any degree, and in a continually increasing degree, like a leaf,
surely many insects living in the woods, and especially in the
tropical woods, would also have followed such an advantageous path of
variation--at least, so we should be inclined to think. And this is
indeed the case; numerous insects, of different orders, if they are
as large as a leaf, have taken on the colour, form, and usually also
the markings, of a leaf. Thus green and also decaying and dead leaves
are most realistically imitated by many tropical Locustidæ. Besides
_Tropidoderus_, figured on p. 79, a _Pterochroa_ of South Brazil
affords a particularly fine illustration of this, for not only does
the ground-colour, brown or green, harmonize with that of a dead or
fresh leaf, but, at the same time, all sorts of details are marked on
the insect, which help to heighten the deceptive impression. Even the
outline of the wings is leaf-like, and leaf-veins are marked on the
wing-covers with the most beautiful distinctness, and finally there
is, especially in the light-green individuals, a spot at the wing tip
which, by means of a mixture of brown, yellow, reddish, and violet
colour-tones, mimics a decaying spot with astonishing fidelity. Here,
again, the origin of this special adaptation can be clearly recognized,
for the vaguely concentric arrangement of the colours indicates
that, in the ancestors of the species, an eye-spot had occurred on
this area, of the same kind as we still see on the posterior wing,
which is covered in the resting position. Thus we can again look
back on the history of the species and conclude that the dissolution
and degeneration of the eye-spot began at the time when the leaf
resemblance was evolved, and this was probably caused by some change of
habitat, which we can now no longer guess at.
Many species of leaf-like Orthoptera, both in the Old and New World,
have tough, green, parchment-like wing-covers which bear a remarkable
resemblance to the thick Magnolia-like leaves of tropical plants. Along
with these we must also mention the 'walking leaf,' which has been well
known for centuries. In its case, not the wing-covers alone, but the
head and thorax, and even the legs, are of the colour and shape of a
leaf.
The stick-insects, too, must not remain unnoticed; those quaint
inhabitants of warm countries, whose elongated brown body looks like a
knotted twig, and whose long legs, likewise stick-like, are stretched
out irregularly at different angles to the body, and usually remain
motionless when the insect is resting. These creatures are vegetarian,
and generally keep so still, that even the naturalist who is on the
look-out for them may easily overlook them. Even such an experienced
student of insects as Alfred Russel Wallace was deceived, for a native
of the Phillipines once brought him a specimen as a 'walking-stick'
insect, which he rejected, saying that this time it was no animal
but really a twig, until the native showed him that it was an insect
whose likeness to a twig was increased by the fact that it bore on
its back a ragged green growth, which looked exactly like a liverwort
(_Jungermannia_), which occurs on the twigs of the trees in that region.
We must also notice here the thorn-bugs, which are numerous on the
prickly shrubs of tropical deserts and plateaux, especially in Mexico.
These bear on the relatively very small body two or three large spines,
which make them look like a part of the thorny bush on which they sit.
But this masking by mimicry of thorns is not confined to insects, it is
seen in lizards as well, notably in _Moloch horridus_, a lizard that
lives in the Australian bush, and is covered all over with thorn-like
scales.
These examples should be enough to show that mimicry of the usual
surroundings on the part of animals which are in need of protection,
or are wont to lurk on the watch for their prey, are not isolated
exceptions, chance resemblances, or, as they used to be called, 'freaks
of nature,' but that, on the contrary, they are the rule, depending on
natural causes, and always occurring when these causes are operative.
That such protective resemblances seem to be much more frequent in
warmer climates than with us is probably a fallacy due to the fact that
the number of species (especially of insects) is very much greater
there, and that many insect types have their representatives of
considerable size of body, which not only makes them more conspicuous
_to us_, but makes some protective device in relation to their enemies
or victims much more necessary.
But we must here take account of one more example which occurs in our
fauna in many modifications: the caterpillars of Geometridæ. Many
of these soft and easily injured caterpillars resemble closely, in
colour and shade, the bark of the tree or shrub on which they live
(Fig. 17). At the same time they have the habit, when at rest, of
stretching themselves out straight and stiff, so that they stand out
free, at an acute angle from the branch, thus seeming like one of its
lateral twigs. In many species the resemblance is heightened by the
extraordinary pose of the head (_K_) and of the claw-like feet (_F_),
which, partly pressed close to the head, partly standing out from it,
give the anterior end of the caterpillar the appearance of two terminal
buds, while various little pointed, knotlike warts, scattered over the
body, represent the sleeping buds of the little twig. Who has not at
one time or other taken such a caterpillar for a little branch, and not
inexpert observers only, but even trained naturalists? Many a time I
have not been able to make quite sure of what I had before me until I
touched it!
[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_, seated
on a birch twig. _K_, head. _F_, feet. _m_, tubercle, resembling a
'sleeping bud'; nat. size.]
LECTURE V
TRUE MIMICRY
Mimicry: its discovery by Bates--Heliconiidæ and
Pieridæ--Danaides--_Papilio merope_ and its five females--The females
lead the way--Species with mimicry in both sexes--Objections--Enemies
of butterflies--The immunity of the models--Poisonousness of the
food-plants of immune species--Several mimics of the same immune
species--Persecuted species of the same genus resemble quite different
models--_Elymnias_--Degree of resemblance--Differences between the
caterpillars of the model and the copy--The same resemblance arrived
at by different ways--Transparent-winged butterflies--The gradually
increasing resemblance points to causes operating mechanically--Rarity
of the mimetic species--Danger to the existence of the species not a
necessary condition of mimetic transformation--_Papilio meriones_ and
_Papilio merope_--Comparison with the dimorphic caterpillars--_Papilio
turnus_--'Mimicry rings' of immune species--_Danais erippus_ and
_Limenitis archippus_--Marked divergence of mimetic species from their
nearest relatives--Mimicry in other insects--Imitators of ants and bees.
LET us now turn to the most remarkable of all protective form- and
colour-adaptations, the so-called Mimicry, including all cases of the
imitation of one animal by another, which we came to know first through
Bates, and to a fuller understanding of which A. R. Wallace and Fritz
Müller have especially contributed.
While the English naturalist, Bates[2], was collecting and observing
on the banks of the Amazons--as he did for twelve years--it sometimes
occurred that, among a swarm of those gaily coloured, quaintly shaped
butterflies, the Heliconiidæ (Pl. II, Fig. 13), he caught one which,
on closer examination, proved to be essentially different from its
numerous companions. It was certainly like them both in colour and
form, but it belonged to quite a different family of butterflies,
that of the Pieridæ or Whites (Pl. II, Fig. 19). These whites with
the colours of the Heliconiidæ always occurred singly in swarms of
the latter form, and Bates found that, in the different districts of
the Amazon, they always resembled in a striking manner the species of
Heliconiidæ there prevalent. Many of them had been previously known
to entomologists, and because they diverged so far from the usual
type of the Pieridæ, especially in the form of the wing, the name
Dysmorphia, the 'mis-shapen,' had been given to them, although the
meaning of this 'mis-shapenness' long remained a mystery. The French
Lepidopterist, Boisduval, went a step further when he pointed out as
something remarkable that nature sometimes makes several species of
quite different families exactly alike, and called attention to three
African butterflies, of which we shall have to speak later in detail.
But even he was too much fettered by the old views of the immutability
of species to arrive at a correct interpretation. Thus it was reserved
for Bates to take the decisive step. Observing that the Heliconiidæ
occurred frequently, and usually in large swarms, he concluded
that they must have few enemies, and as he never saw the numerous
insectivorous birds and insects hunting them, he further concluded
that they must have something disagreeable which secured them from the
attacks of these predaceous forms. On the other hand, he found that the
heliconid-like Whites were always rare, and he took this as a sign that
they were much persecuted, and that they must, therefore, be palatable
tit-bits for the insectivores. If it were possible, then, that a
species of Whites with the usual white colour of the family should give
rise to variations, which would make them in any degree resemble the
Heliconiidæ, which are secure from persecution, and if, in addition,
those that exhibited the profitable variation attached themselves to
swarms of the mimicked form, then these variants would be to a certain
extent secured from attack, and more and more so in proportion as the
resemblance to the protected model increased. The great likeness of
these Whites to the Heliconiidæ, Bates further argued, would depend on
a process of selection, based on the fact that, in each generation,
those individuals would on the average survive for reproduction
which were a little more like the model than the rest, and thus the
resemblance, doubtless slight to begin with, would gradually reach its
present degree of perfection.
[2] _Contributions to an Insect Fauna of the Amazon Valley_, Trans.
Linn. Soc., Vol. XXIII, 1862.
Bates's hypotheses have been subsequently confirmed in the most
striking way. The Heliconiidæ do possess a disagreeable taste and
odour, and are utterly rejected by birds, lizards, and other animals.
It has been directly observed that puff-birds, species of _Trogon_,
and other insectivorous birds, looking down from the tops of trees in
search of food, allowed to pass unheeded the swarms of gaily coloured
Heliconiidæ which were fluttering among the leaves, and experiments
with various insectivorous animals yielded the same result: _the
Heliconiidæ are immune_. We can, therefore, not only understand that
it must be advantageous to resemble them, we can also appreciate many
of their peculiar characters, such as their gay coloration, which must
serve as a sign of their disagreeable taste, and their slow, fluttering
flight, as well as their habit of flocking together, which must make
it easier for the birds to recognize them as uneatable. Everything
which marks out these unpalatable morsels, and makes them more readily
recognizable, must be to their advantage, and therefore must have been
favoured by natural selection (Pl. II, Fig. 13).
In the same way, every increase of resemblance on the part of the
mimics would increase their chances of escaping notice, and any one
who is accustomed to observe butterflies in nature can well understand
that even very slight resemblances may have formed the beginning of
the selection process; perhaps even a mere variation in the manner
of flight, combined with the habit of associating with the swarms of
Heliconiidæ. I myself have many times been momentarily deceived in our
own woods by a White of unusually majestic flight, so that I took it
for an _Apatura_ or a _Limenitis_. If, therefore, individual Whites
occurred here and there in the Amazon valley, which flew somewhat after
the manner of the Heliconiidæ, and associated with them, they might
possibly have attained a certain degree of security through that alone,
and it would be greatly increased if at the same time they varied
somewhat in colour in the direction of their companions.
In any case there can be no doubt whatever that in these cases a real
transformation of the species in colour and marking, and perhaps often,
too, in form of wing, has taken place, and that within comparatively
modern times--let us say during the distribution of a species which
required protection over a large continent, or since the last breaking
up of an immune species into local species. Various facts prove this;
above all, the circumstance that it is often only the females which
exhibit this protective mimicry; and that one and the same species may
mimic a different immune species in different areas, but always the one
occurring abundantly in that area, and so on.
Definite examples will make this clearer, and I will only say in
advance that, since the discovery of Bates, numerous cases of mimicry
in butterflies have been found, not only in South America, but in all
tropical countries which have a rich Lepidopteran fauna. And it is not
only between the Heliconiidæ and the Pieridæ that such relations have
been evolved; many much-persecuted, unprotected species of different
families everywhere mimic species which are rejected on account of
their nauseous taste, and these, too, belonging to different families.
The Heliconiidæ are a purely American group, but in the Old World
and in Australia their place is taken by the three great families of
Danaides, Euplœides, and Acræides, since, as it seems, they all taste
unpleasantly, and are rejected by all, or at least by most, of the
insectivorous birds. Numerous species of the genus _Danais_ (Pl. I,
Fig. 8), _Amauris_ (Pl. I, Fig. 5), _Euplœa_ (Pl. III, Fig. 25, 27),
and _Acræa_ (Pl. II, Fig. 2), and also many species of _Papilio_ and
other genera, enjoy the advantage of unpleasant taste, if not even of
poisonousness; they are, therefore, secure from pursuit, and are, in
consequence, much mimicked by palatable butterflies.
As a further example, I now select a diurnal butterfly from Africa,
_Papilio merope_ Cramer[3], which was shown by Trimen in 1868 to be
mimetic. The species has a wide distribution, for, if we except slight
local differences in the marking of the male, its range extends over
the greater part of Africa, from Abyssinia to the Cape, and from East
Africa to the Gold Coast.
[3] The West African form of _Papilio merope_ has been quite recently
distinguished from the southern form and regarded as a distinct
species, the latter being now called _Papilio cenea_. The differences
in the males are very slight--somewhat shorter wings, shorter
wing-tail, and so on--differences which seem relatively unimportant in
comparison with the differences between the males and the females.
The male is a beautiful large butterfly, yellowish white, with a touch
of black, and with little tails to the posterior wings (Pl. I, Fig.
1), like our own swallowtail. A very nearly related species occurs
in Madagascar, and there the female is similarly coloured, though it
may be distinguished by having a little more black on the wing. On
the mainland of Africa, however, the females of _Papilio merope_ are
so different in colour and form of wing that it would be difficult to
believe them of the same species as the male had not both sexes more
than once been reared from the eggs of one mother. The females (Pl. I,
Fig. 6) in South Africa imitate a species of _Amauris_, _A. echeria_
(Pl. I, Fig. 7), of a dark ground-colour with white, or brownish-white,
mirrors and spots, and they resemble it most deceptively. But what
makes the case more interesting in its theoretical aspect is that
_Danais echeria_ of Cape Colony is markedly different from _Danais
echeria_ of Natal, and the female of _Papilio merope_ has followed
those two local varieties, and has likewise a Cape and a Natal local
form. Even this is not all, for in Cape Colony there are two other
females of _Papilio merope_. One of them has a yellow ground-colour,
and resembles _Danais chrysippus_, which is extremely abundant there
(Pl. I, Fig. 3); the other is entirely different (Pl. I, Fig. 4), for
it closely mimics another Danaid occurring in the same districts of
Africa, and also immune, _Amauris niavius_ (Pl. I, Fig. 5), not only in
the beautiful pure white and deep black of the wing surface, but also
in the distribution of these colours to form a pattern.
We have thus in Africa four different females of _Papilio merope_,
each of which mimics a protected species of Danaid. They are not
always locally separate, so that each is exclusively restricted to a
particular region, for their areas of distribution often overlap, and,
at the Cape for instance, one male form and three different forms of
female have been reared from one set of eggs. In addition, we have the
fact that between the two local forms of _Danais echeria_ transition
forms occur, and that the mimetic females of _Papilio merope_ show
the same transition forms locally, and we must admit that all these
facts harmonize most beautifully with the selection interpretation,
but defy any other. And that the last doubt may be dispelled, nature
has preserved _the primitive female form_ on the continent of
Africa--namely, in Abyssinia, where, along with the mimetic females,
there are others which are tailed like the males (Pl. I, Fig. 1), and
are like them in form and colour, a few minor differences excepted.
Thus we have in _Papilio merope_ a species which, in the course of
its distribution through Africa, has scarcely varied at all in the
male sex, but in the female has almost everywhere lost the outward
appearance of a _Papilio_, and has assumed that of a Danaid, which is
protected by being unpalatable, and not even everywhere the appearance
of the same species, but in each place that of the prevailing one, and
sometimes of several in one region. These females thus show at the
present day a polymorphism which consists of four chief mimetic forms,
to which has to be added the primitive form--that resembling the male.
This has survived in Abyssinia alone, and even there it is not the only
one, but occurs along with some of the mimetic forms.
To the question why only the females are mimetic in this and other
cases, Darwin and Wallace have answered that the females are more in
need of protection. In the first place, the males among butterflies
are considerably in the majority, and, secondly, the females must
live longer in order to be able to lay their eggs. Moreover, the
females, which are loaded with numerous eggs, are heavier in flight,
and during the whole period of egg-laying--that is, for a considerable
time--they are exposed to the attacks of numerous enemies. Whether
one of the abundant males is devoured sooner or later is immaterial
to the persistence of the species, since one male is sufficient to
fertilize several females. The death of a single female, on the other
hand, implies a loss of several hundred descendants to the species.
It is, therefore, intelligible that, in species already somewhat
rare, the female must first of all be protected; that is to say, that
all variations tending in the direction of her protection would give
rise to a process of selection resulting in an augmentation of the
protective characters.
But there are also butterflies in which both sexes mimic a protected
model. Thus many imitators of the unpalatable Acræides (Pl. II, Fig.
21) resemble the model in both sexes, and of the South American Whites
which mimic the Heliconiidæ there are some which have the appearance
of the Heliconiidæ even in the male sex (Pl. II, Fig. 18, 19), while
others look like ordinary Whites (for instance, _Archonias potamea_).
But in many of these species, which are mimetic in the female sex, we
find also in the male some indications of the mimetic colouring, but
in the first instance only on the under surface. Thus the females of
_Perhybris pyrrha_ (Pl. II, Fig. 17) resemble in their black, yellow,
and orange-red colour-pattern the immune American Danaid, _Lycorea
halia_ (Pl. II, Fig. 12), but their mates are, on the upper surface,
like our common Whites, though they already show on the under surface
the orange-red transverse stripes of the _Lycorea_ (Pl. II, Fig. 16).
In other mimetic species of Whites a similar beginning is even more
faintly hinted at, and in others, again, the upper surface of the male
is also provided with protective colours, and only a single white spot
on the posterior, or sometimes even on the anterior wing as well, shows
the original white of the Pieridæ (Fig. 18).