escape from their enemies and obtain their food in an infinite number of
ways; and that their varied habits and instincts are in every case
adapted to the conditions of their existence. The porcupine and the
hedgehog have a defensive armour that saves them from the attacks of
most animals. The tortoise is not injured by the conspicuous colours of
his shell, because that shell is in most cases an effectual protection
to him. The skunks of North America find safety in their power of
emitting an unbearably offensive odour; the beaver in its aquatic habits
and solidly constructed abode. In some cases the chief danger to an
animal occurs at one particular period of its existence, and if that is
guarded against its numbers can easily be maintained. This is the case
with many birds, the eggs and young of which are especially obnoxious to
danger, and we find accordingly a variety of curious contrivances to
protect them. We have nests carefully concealed, hung from the slender
extremities of grass or boughs over water, or placed in the hollow of a
tree with a very small opening. When these precautions are successful,
so many more individuals will be reared than can possibly find food
during the least favourable seasons, that there will always be a number
of weakly and inexperienced young birds who will fall a prey to the
enemies of the race, and thus render necessary for the stronger and
healthier individuals no other safeguard than their strength and
activity. The instincts most favourable to the production and rearing of
offspring will in these cases be most important, and the survival of the
fittest will act so as to keep up and advance those instincts, while
other causes which tend to modify colour and marking may continue their
action almost unchecked.
It is perhaps in insects that we may best study the varied means by
which animals are defended or concealed. One of the uses of the
phosphorescence with which many insects are furnished, is probably to
frighten away their enemies; for Kirby and Spence state that a ground
beetle (Carabus) has been observed running round and round a luminous
centipede as if afraid to attack it. An immense number of insects have
stings, and some stingless ants of the genus Polyrachis are armed with
strong and sharp spines on the back, which must render them unpalatable
to many of the smaller insectivorous birds. Many beetles of the family
Curculionidae have the wing cases and other external parts so excessively
hard, that they cannot be pinned without first drilling a hole to
receive the pin, and it is probable that all such find a protection in
this excessive hardness. Great numbers of insects hide themselves among
the petals of flowers, or in the cracks of bark and timber; and finally,
extensive groups and even whole orders have a more or less powerful and
disgusting smell and taste, which they either possess permanently, or
can emit at pleasure. The attitudes of some insects may also protect
them, as the habit of turning up the tail by the harmless rove-beetles
(Staphylindidae) no doubt leads other animals besides children to the
belief that they can sting. The curious attitude assumed by sphinx
caterpillars is probably a safeguard, as well as the blood-red tentacles
which can suddenly be thrown out from the neck, by the caterpillars of
all the true swallow-tailed butterflies.
It is among the groups that possess some of these varied kinds of
protection in a high degree, that we find the greatest amount of
conspicuous colour, or at least the most complete absence of protective
imitation. The stinging Hymenoptera, wasps, bees, and hornets, are, as a
rule, very showy and brilliant insects, and there is not a single
instance recorded in which any one of them is coloured so as to resemble
a vegetable or inanimate substance. The Chrysididae, or golden wasps,
which do not sting, possess as a substitute the power of rolling
themselves up into a ball, which is almost as hard and polished as if
really made of metal,--and they are all adorned with the most gorgeous
colours. The whole order Hemiptera (comprising the bugs) emit a powerful
odour, and they present a very large proportion of gay-coloured and
conspicuous insects. The lady-birds (Coccinellidae) and their allies the
Eumorphidae, are often brightly spotted, as if to attract attention; but
they can both emit fluids of a very disagreeable nature, they are
certainly rejected by some birds, and are probably never eaten by any.
The great family of ground beetles (Carabidae) almost all possess a
disagreeable and some a very pungent smell, and a few, called bombardier
beetles, have the peculiar faculty of emitting a jet of very volatile
liquid, which appears like a puff of smoke, and is accompanied by a
distinct crepitating explosion. It is probably because these insects are
mostly nocturnal and predacious that they do not present more vivid
hues. They are chiefly remarkable for brilliant metallic tints or dull
red patches when they are not wholly black, and are therefore very
conspicuous by day, when insect-eaters are kept off by their bad odour
and taste, but are sufficiently invisible at night when it is of
importance that their prey should not become aware of their proximity.
It seems probable that in some cases that which would appear at first to
be a source of danger to its possessor may really be a means of
protection. Many showy and weak-flying butterflies have a very broad
expanse of wing, as in the brilliant blue Morphos of Brazilian forests,
and the large Eastern Papilios; yet these groups are tolerably
plentiful. Now, specimens of these butterflies are often captured with
pierced and broken wings, as if they had been seized by birds from whom
they had escaped; but if the wings had been much smaller in proportion
to the body, it seems probable that the insect would be more frequently
struck or pierced in a vital part, and thus the increased expanse of the
wings may have been indirectly beneficial.
In other cases the capacity of increase in a species is so great that
however many of the perfect insect may be destroyed, there is always
ample means for the continuance of the race. Many of the flesh flies,
gnats, ants, palm-tree weevils and locusts are in this category. The
whole family of Cetoniadae or rose chafers, so full of gaily-coloured
species, are probably saved from attack by a combination of characters.
They fly very rapidly with a zigzag or waving course; they hide
themselves the moment they alight, either in the corolla of flowers, or
in rotten wood, or in cracks and hollows of trees, and they are
generally encased in a very hard and polished coat of mail which may
render them unsatisfactory food to such birds as would be able to
capture them. The causes which lead to the development of colour have
been here able to act unchecked, and we see the result in a large
variety of the most gorgeously-coloured insects.
Here, then, with our very imperfect knowledge of the life-history of
animals, we are able to see that there are widely varied modes by which
they may obtain protection from their enemies or concealment from their
prey. Some of those seem to be so complete and effectual as to answer
all the wants of the race, and lead to the maintenance of the largest
possible population. When this is the case, we can well understand that
no further protection derived from a modification of colour can be of
the slightest use, and the most brilliant hues may be developed without
any prejudicial effect upon the species. On some of the laws that
determine the development of colour something may be said presently. It
is now merely necessary to show that concealment by obscure or imitative
tints is only one out of very many ways by which animals maintain their
existence; and having done this we are prepared to consider the
phenomena of what has been termed "mimicry." It is to be particularly
observed, however, that the word is not here used in the sense of
voluntary imitation, but to imply a particular kind of resemblance--a
resemblance not in internal structure but in external appearance--a
resemblance in those parts only that catch the eye--a resemblance that
deceives. As this kind of resemblance has the same effect as voluntary
imitation or mimicry, and as we have no word that expresses the required
meaning, "mimicry" was adopted by Mr. Bates (who was the first to
explain the facts), and has led to some misunderstanding; but there need
be none, if it is remembered that both "mimicry" and "imitation" are
used in a metaphorical sense, as implying that close external likeness
which causes things unlike in structure to be mistaken for each other.
_Mimicry._
It has been long known to entomologists that certain insects bear a
strange external resemblance to others belonging to distinct genera,
families, or even orders, and with which they have no real affinity
whatever. The fact, however, appears to have been generally considered
as dependent upon some unknown law of "analogy"--some "system of
nature," or "general plan," which had guided the Creator in designing
the myriads of insect forms, and which we could never hope to
understand. In only one case does it appear that the resemblance was
thought to be useful, and to have been designed as a means to a definite
and intelligible purpose. The flies of the genus Volucella enter the
nests of bees to deposit their eggs, so that their larvae may feed upon
the larvae of the bees, and these flies are each wonderfully like the bee
on which it is parasitic. Kirby and Spence believed that this
resemblance or "mimicry" was for the express purpose of protecting the
flies from the attacks of the bees, and the connection is so evident
that it was hardly possible to avoid this conclusion. The resemblance,
however, of moths to butterflies or to bees, of beetles to wasps, and of
locusts to beetles, has been many times noticed by eminent writers; but
scarcely ever till within the last few years does it appear to have been
considered that these resemblances had any special purpose, or were of
any direct benefit to the insects themselves. In this respect they were
looked upon as accidental, as instances of the "curious analogies" in
nature which must be wondered at but which could not be explained.
Recently, however, these instances have been greatly multiplied; the
nature of the resemblances has been more carefully studied, and it has
been found that they are often carried out into such details as almost
to imply a purpose of deceiving the observer. The phenomena, moreover,
have been shown to follow certain definite laws, which again all
indicate their dependence on the more general law of the "survival of
the fittest," or "the preservation of favoured races in the struggle for
life." It will, perhaps, be as well here to state what these laws or
general conclusions are, and then to give some account of the facts
which support them.
The first law is, that in an overwhelming majority of cases of mimicry,
the animals (or the groups) which resemble each other inhabit the same
country, the same district, and in most cases are to be found together
on the very same spot.
The second law is, that these resemblances are not indiscriminate, but
are limited to certain groups, which in every case are abundant in
species and individuals, and can often be ascertained to have some
special protection.
The third law is, that the species which resemble or "mimic" these
dominant groups, are comparatively less abundant in individuals, and are
often very rare.
These laws will be found to hold good, in all the cases of true mimicry
among various classes of animals to which we have now to call the
attention of our readers.
_Mimicry among Lepidoptera._
As it is among butterflies that instances of mimicry are most numerous
and most striking, an account of some of the more prominent examples in
this group will first be given. There is in South America an extensive
family of these insects, the Heliconidae, which are in many respects very
remarkable. They are so abundant and characteristic in all the woody
portions of the American tropics, that in almost every locality they
will be seen more frequently than any other butterflies. They are
distinguished by very elongate wings, body, and antennae, and are
exceedingly beautiful and varied in their colours; spots and patches of
yellow, red, or pure white upon a black, blue, or brown ground, being
most general. They frequent the forests chiefly, and all fly slowly and
weakly; yet although they are so conspicuous, and could certainly be
caught by insectivorous birds more easily than almost any other
insects, their great abundance all over the wide region they inhabit
shows that they are not so persecuted. It is to be especially remarked
also, that they possess no adaptive colouring to protect them during
repose, for the under side of their wings presents the same, or at least
an equally conspicuous colouring as the upper side; and they may be
observed after sunset suspended at the end of twigs and leaves where
they have taken up their station for the night, fully exposed to the
attacks of enemies if they have any. These beautiful insects possess,
however, a strong pungent semi-aromatic or medicinal odour, which seems
to pervade all the juices of their system. When the entomologist
squeezes the breast of one of them between his fingers to kill it, a
yellow liquid exudes which stains the skin, and the smell of which can
only be got rid of by time and repeated washings. Here we have probably
the cause of their immunity from attack, since there is a great deal of
evidence to show that certain insects are so disgusting to birds that
they will under no circumstances touch them. Mr. Stainton has observed
that a brood of young turkeys greedily devoured all the worthless moths
he had amassed in a night's "sugaring," yet one after another seized and
rejected a single white moth which happened to be among them. Young
pheasants and partridges which eat many kinds of caterpillars seem to
have an absolute dread of that of the common currant moth, which they
will never touch, and tomtits as well as other small birds appear never
to eat the same species. In the case of the Heliconidae, however, we
have some direct evidence to the same effect. In the Brazilian forests
there are great numbers of insectivorous birds--as jacamars, trogons,
and puffbirds--which catch insects on the wing, and that they destroy
many butterflies is indicated by the fact that the wings of these
insects are often found on the ground where their bodies have been
devoured. But among these there are no wings of Heliconidae, while those
of the large showy Nymphalidae, which have a much swifter flight, are
often met with. Again, a gentleman who had recently returned from Brazil
stated at a meeting of the Entomological Society that he once observed a
pair of puffbirds catching butterflies, which they brought to their nest
to feed their young; yet during half an hour they never brought one of
the Heliconidae, which were flying lazily about in great numbers, and
which they could have captured more easily than any others. It was this
circumstance that led Mr. Belt to observe them so long, as he could not
understand why the most common insects should be altogether passed by.
Mr. Bates also tells us that he never saw them molested by lizards or
predacious flies, which often pounce on other butterflies.
If, therefore, we accept it as highly probable (if not proved) that the
Heliconidae are very greatly protected from attack by their peculiar
odour and taste, we find it much more easy to understand their chief
characteristics--their great abundance, their slow flight, their gaudy
colours, and the entire absence of protective tints on their under
surfaces. This property places them somewhat in the position of those
curious wingless birds of oceanic islands, the dodo, the apteryx, and
the moas, which are with great reason supposed to have lost the power of
flight on account of the absence of carnivorous quadrupeds. Our
butterflies have been protected in a different way, but quite as
effectually; and the result has been that as there has been nothing to
escape from, there has been no weeding out of slow flyers, and as there
has been nothing to hide from, there has been no extermination of the
bright-coloured varieties, and no preservation of such as tended to
assimilate with surrounding objects.
Now let us consider how this kind of protection must act. Tropical
insectivorous birds very frequently sit on dead branches of a lofty
tree, or on those which overhang forest paths, gazing intently around,
and darting off at intervals to seize an insect at a considerable
distance, which they generally return to their station to devour. If a
bird began by capturing the slow-flying, conspicuous Heliconidae, and
found them always so disagreeable that it could not eat them, it would
after a very few trials leave off catching them at all; and their whole
appearance, form, colouring, and mode of flight is so peculiar, that
there can be little doubt birds would soon learn to distinguish them at
a long distance, and never waste any time in pursuit of them. Under
these circumstances, it is evident that any other butterfly of a group
which birds were accustomed to devour, would be almost equally well
protected by closely resembling a Heliconia externally, as if it
acquired also the disagreeable odour; always supposing that there were
only a few of them among a great number of the Heliconias. If the birds
could not distinguish the two kinds externally, and there were on the
average only one eatable among fifty uneatable, they would soon give up
seeking for the eatable ones, even if they knew them to exist. If, on
the other hand, any particular butterfly of an eatable group acquired
the disagreeable taste of the Heliconias while it retained the
characteristic form and colouring of its own group, this would be really
of no use to it whatever; for the birds would go on catching it among
its eatable allies (compared with which it would rarely occur), it would
be wounded and disabled, even if rejected, and its increase would thus
be as effectually checked as if it were devoured. It is important,
therefore, to understand that if any one genus of an extensive family of
eatable butterflies were in danger of extermination from insect-eating
birds, and if two kinds of variation were going on among them, some
individuals possessing a slightly disagreeable taste, others a slight
resemblance to the Heliconidae, this latter quality would be much more
valuable than the former. The change in flavour would not at all prevent
the variety from being captured as before, and it would almost certainly
be thoroughly disabled before being rejected. The approach in colour and
form to the Heliconidae, however, would be at the very first a positive,
though perhaps a slight advantage; for although at short distances this
variety would be easily distinguished and devoured, yet at a longer
distance it might be mistaken for one of the uneatable group, and so be
passed by and gain another day's life, which might in many cases be
sufficient for it to lay a quantity of eggs and leave a numerous
progeny, many of which would inherit the peculiarity which had been the
safeguard of their parent.
Now, this hypothetical case is exactly realized in South America. Among
the white butterflies forming the family Pieridae (many of which do not
greatly differ in appearance from our own cabbage butterflies) is a
genus of rather small size (Leptalis), some species of which are white
like their allies, while the larger number exactly resemble the
Heliconidae in the form and colouring of the wings. It must always be
remembered that these two families are as absolutely distinguished from
each other by structural characters as are the carnivora and the
ruminants among quadrupeds, and that an entomologist can always
distinguish the one from the other by the structure of the feet, just as
certainly as a zoologist can tell a bear from a buffalo by the skull or
by a tooth. Yet the resemblance of a species of the one family to
another species in the other family was often so great, that both Mr.
Bates and myself were many times deceived at the time of capture, and
did not discover the distinctness of the two insects till a closer
examination detected their essential differences. During his residence
of eleven years in the Amazon valley, Mr. Bates found a number of
species or varieties of Leptalis, each of which was a more or less exact
copy of one of the Heliconidae of the district it inhabited; and the
results of his observations are embodied in a paper published in the
Linnean Transactions, in which he first explained the phenomena of
"mimicry" as the result of natural selection, and showed its identity in
cause and purpose with protective resemblance to vegetable or inorganic
forms.
The imitation of the Heliconidae by the Leptalides is carried out to a
wonderful degree in form as well as in colouring. The wings have become
elongated to the same extent, and the antennae and abdomen have both
become lengthened, to correspond with the unusual condition in which
they exist in the former family. In colouration there are several types
in the different genera of Heliconidae. The genus Mechanitis is generally
of a rich semi-transparent brown, banded with black and yellow; Methona
is of large size, the wings transparent like horn, and with black
transverse bands; while the delicate Ithomias are all more or less
transparent, with black veins and borders, and often with marginal and
transverse bands of orange red. These different forms are all copied by
the various species of Leptalis, every band and spot and tint of colour,
and the various degrees of transparency, being exactly reproduced. As if
to derive all the benefit possible from this protective mimicry, the
habits have become so modified that the Leptalides generally frequent
the very same spots as their models, and have the same mode of flight;
and as they are always very scarce (Mr. Bates estimating their numbers
at about one to a thousand of the group they resemble), there is hardly
a possibility of their being found out by their enemies. It is also
very remarkable that in almost every case the particular Ithomias and
other species of Heliconidae which they resemble, are noted as being very
common species, swarming in individuals, and found over a wide range of
country. This indicates antiquity and permanence in the species, and is
exactly the condition most essential both to aid in the development of
the resemblance, and to increase its utility.