flounder and the skate, are exactly the colour of the gravel or sand on
which they habitually rest. Among the marine flower gardens of an
Eastern coral reef the fishes present every variety of gorgeous colour,
while the river fish even of the tropics rarely if ever have gay or
conspicuous markings. A very curious case of this kind of adaptation
occurs in the sea-horse (Hippocampus) of Australia, some of which bear
long foliaceous appendages resembling seaweed, and are of a brilliant
red colour; and they are known to live among seaweed of the same hue, so
that when at rest they must be quite invisible. There are now in the
aquarium of the Zoological Society some slender green pipe-fish which
fasten themselves to any object at the bottom by their prehensile tails,
and float about with the current, looking exactly like some cylindrical
algae.
It is, however, in the insect world that this principle of the
adaptation of animals to their environment is most fully and strikingly
developed. In order to understand how general this is, it is necessary
to enter somewhat into details, as we shall thereby be better able to
appreciate the significance of the still more remarkable phenomena we
shall presently have to discuss. It seems to be in proportion to their
sluggish motions or the absence of other means of defence, that insects
possess the protective colouring. In the tropics there are thousands of
species of insects which rest during the day clinging to the bark of
dead or fallen trees; and the greater portion of these are delicately
mottled with gray and brown tints, which though symmetrically disposed
and infinitely varied, yet blend so completely with the usual colours of
the bark that at two or three feet distance they are quite
undistinguishable. In some cases a species is known to frequent only one
species of tree. This is the case with the common South American
long-horned beetle (Onychocerus scorpio) which, Mr. Bates informed me,
is found only on a rough-barked tree, called Tapiriba, on the Amazon. It
is very abundant, but so exactly does it resemble the bark in colour and
rugosity, and so closely does it cling to the branches, that until it
moves it is absolutely invisible! An allied species (O. concentricus) is
found only at Para, on a distinct species of tree, the bark of which it
resembles with equal accuracy. Both these insects are abundant, and we
may fairly conclude that the protection they derive from this strange
concealment is at least one of the causes that enable the race to
flourish.
Many of the species of Cicindela, or tiger beetle, will illustrate this
mode of protection. Our common Cicindela campestris frequents grassy
banks and is of a beautiful green colour, while C. maritima, which is
found only on sandy sea-shores, is of a pale bronzy yellow, so as to be
almost invisible. A great number of the species found by myself in the
Malay islands are similarly protected. The beautiful Cicindela gloriosa,
of a very deep velvety green colour, was only taken upon wet mossy
stones in the bed of a mountain stream, where it was with the greatest
difficulty detected. A large brown species (C. heros) was found chiefly
on dead leaves in forest paths; and one which was never seen except on
the wet mud of salt marshes was of a glossy olive so exactly the colour
of the mud as only to be distinguished when the sun shone, by its
shadow! Where the sandy beach was coralline and nearly white, I found a
very pale Cicindela; wherever it was volcanic and black, a dark species
of the same genus was sure to be met with.
There are in the East small beetles of the family Buprestidae which
generally rest on the midrib of a leaf, and the naturalist often
hesitates before picking them off, so closely do they resemble pieces of
bird's dung. Kirby and Spence mention the small beetle Onthophilus
sulcatus as being like the seed of an umbelliferous plant; and another
small weevil, which is much persecuted by predatory beetles of the genus
Harpalus, is of the exact colour of loamy soil, and was found to be
particularly abundant in loam pits. Mr. Bates mentions a small beetle
(Chlamys pilula) which was undistinguishable by the eye from the dung of
caterpillars, while some of the Cassidae, from their hemispherical forms
and pearly gold-colour, resemble glittering dew-drops upon the leaves.
A number of our small brown and speckled weevils at the approach of any
object roll off the leaf they are sitting on, at the same time drawing
in their legs and antennae, which fit so perfectly into cavities for
their reception that the insect becomes a mere oval brownish lump, which
it is hopeless to look for among the similarly coloured little stones
and earth pellets among which it lies motionless.
The distribution of colour in butterflies and moths respectively is very
instructive from this point of view. The former have all their brilliant
colouring on the upper surface of all four wings, while the under
surface is almost always soberly coloured, and often very dark and
obscure. The moths on the contrary have generally their chief colour on
the hind wings only, the upper wings being of dull, sombre, and often
imitative tints, and these generally conceal the hind wings when the
insects are in repose. This arrangement of the colours is therefore
eminently protective, because the butterfly always rests with his wings
raised so as to conceal the dangerous brilliancy of his upper surface.
It is probable that if we watched their habits sufficiently we should
find the under surface of the wings of butterflies very frequently
imitative and protective. Mr. T. W. Wood has pointed out that the little
orange-tip butterfly often rests in the evening on the green and white
flower heads of an umbelliferous plant, and that when observed in this
position the beautiful green and white mottling of the under surface
completely assimilates with the flower heads and renders the creature
very difficult to be seen. It is probable that the rich dark colouring
of the under side of our peacock, tortoiseshell, and red-admiral
butterflies answers a similar purpose.
Two curious South American butterflies that always settle on the trunks
of trees (Gynecia dirce and Callizona acesta) have the under surface
curiously striped and mottled, and when viewed obliquely must closely
assimilate with the appearance of the furrowed bark of many kinds of
trees. But the most wonderful and undoubted case of protective
resemblance in a butterfly which I have ever seen, is that of the
common Indian Kallima inachis, and its Malayan ally, Kallima paralekta.
The upper surface of these insects is very striking and showy, as they
are of a large size, and are adorned with a broad band of rich orange on
a deep bluish ground. The under side is very variable in colour, so that
out of fifty specimens no two can be found exactly alike, but every one
of them will be of some shade of ash or brown or ochre, such as are
found among dead, dry or decaying leaves. The apex of the upper wings is
produced into an acute point, a very common form in the leaves of
tropical shrubs and trees, and the lower wings are also produced into a
short, narrow tail. Between these two points runs a dark curved line
exactly representing the midrib of a leaf, and from this radiate on each
side a few oblique lines, which serve to indicate the lateral veins of a
leaf. These marks are more clearly seen on the outer portion of the base
of the wings, and on the inner side towards the middle and apex, and it
is very curious to observe how the usual marginal and transverse striae
of the group are here modified and strengthened so as to become adapted
for an imitation of the venation of a leaf. We come now to a still more
extraordinary part of the imitation, for we find representations of
leaves in every stage of decay, variously blotched and mildewed and
pierced with powdery black dots gathered into patches and spots, so
closely resembling the various kinds of minute fungi that grow on dead
leaves that is it impossible to avoid thinking at first sight that the
butterflies themselves have been attacked by real fungi.
But this resemblance, close as it is, would be little use if the habits
of the insect did not accord with it. If the butterfly sat upon leaves
or upon flowers, or opened its wings so as to expose the upper surface,
or exposed and moved its head and antennae as many other butterflies do,
its disguise would be of little avail. We might be sure, however, from
the analogy of many other cases, that the habits of the insect are such
as still further to aid its deceptive garb; but we are not obliged to
make any such supposition, since I myself had the good fortune to
observe scores of Kallima paralekta, in Sumatra, and to capture many of
them, and can vouch for the accuracy of the following details: These
butterflies frequent dry forests and fly very swiftly. They were never
seen to settle on a flower or a green leaf, but were many times lost
sight of in a bush or tree of dead leaves. On such occasions they were
generally searched for in vain, for while gazing intently at the very
spot where one had disappeared, it would often suddenly dart out and
again vanish twenty or fifty yards further on. On one or two occasions
the insect was detected reposing, and it could then be seen how
completely it assimilates itself to the surrounding leaves. It sits on
a nearly upright twig, the wings fitting closely back to back,
concealing the antennae and head, which are drawn up between their bases.
The little tails of the hind wings touch the branch and form a perfect
stalk to the leaf, which is supported in its place by the claws of the
middle pair of feet, which are slender and inconspicuous. The irregular
outline of the wings gives exactly the perspective effect of a
shrivelled leaf. We thus have size, colour, form, markings, and habits,
all combining together to produce a disguise which may be said to be
absolutely perfect; and the protection which it affords is sufficiently
indicated by the abundance of the individuals that possess it....
We will now endeavour to show how these wonderful resemblances have most
probably been brought about. Returning to the higher animals, let us
consider the remarkable fact of the rarity of white colouring in the
mammalia or birds of the temperate or tropical zones in a state of
nature. There is not a single white land-bird or quadruped in Europe,
except the few arctic or alpine species to which white is a protective
colour. Yet in many of these creatures there seems to be no inherent
tendency to avoid white, for directly they are domesticated white
varieties arise, and appear to thrive as well as others. We have white
mice and rats, white cats, horses, dogs, and cattle, white poultry,
pigeons, turkeys, and ducks, and white rabbits. Some of these animals
have been domesticated for a long period, others only for a few
centuries; but in almost every case in which an animal has been
thoroughly domesticated, parti-coloured and white varieties are produced
and become permanent.
It is also well known that animals in a state of nature produce white
varieties occasionally. Blackbirds, starlings, and crows are
occasionally seen white, as well as elephants, deer, tigers, hares,
moles, and many other animals; but in no case is a permanent white race
produced. Now there are no statistics to show that the normal-coloured
parents produce white offspring oftener under domestication than in a
state of nature, and we have no right to make such an assumption if the
facts can be accounted for without it. But if the colours of animals do
really, in the various instances already adduced, serve for their
concealment and preservation, then white or any other conspicuous colour
must be hurtful, and must in most cases shorten an animal's life. A
white rabbit would be more surely the prey of hawk or buzzard, and the
white mole, or field mouse, could not long escape from the vigilant owl.
So, also, any deviation from those tints best adapted to conceal a
carnivorous animal would render the pursuit of its prey much more
difficult, would place it at a disadvantage among its fellows and in a
time of scarcity would probably cause it to starve to death. On the
other hand, if an animal spreads from a temperate into an arctic
district, the conditions are changed. During a large portion of the
year, and just when the struggle for existence is most severe, white is
the prevailing tint of nature, and dark colours will be the most
conspicuous. The white varieties will now have an advantage; they will
escape from their enemies or will secure food, while their brown
companions will be devoured or will starve; and "as like produces like"
is the established rule in nature, the white race will become
permanently established, and dark varieties, when they occasionally
appear, will soon die out from their want of adaptation to their
environment. In each case the fittest will survive, and a race will be
eventually produced adapted to the conditions in which it lives.
We have here an illustration of the simple and effectual means by which
animals are brought into harmony with the rest of nature. That slight
amount of variability in every species, which we often look upon as
something accidental or abnormal, or so insignificant as to be hardly
worthy of notice, is yet the foundation of all those wonderful and
harmonious resemblances which play such an important part in the economy
of nature. Variation is generally very small in amount, but it is all
that is required, because the change in the external conditions to which
an animal is subject is generally very slow and intermittent. When
these changes have taken place too rapidly, the result has often been
the extinction of species; but the general rule is, that climatal and
geological changes go on slowly, and the slight but continual variations
in the colour, form and structure of all animals, has furnished
individuals adapted to these changes, and who have become the
progenitors of modified races. Rapid multiplication, incessant slight
variation, and survival of the fittest--these are the laws which ever
keep the organic world in harmony with the inorganic and with itself.
These are the laws which we believe have produced all the cases of
protective resemblance already adduced, as well as those still more
curious examples we have yet to bring before our readers.
It must always be borne in mind that the more wonderful examples, in
which there is not only a general but a special resemblance as in the
walking leaf, the mossy phasma, and the leaf-winged butterfly--represent
those few instances in which the process of modification has been going
on during an immense series of generations. They all occur in the
tropics, where the conditions of existence are the most favourable, and
where climatic changes have for long periods been hardly perceptible. In
most of them favourable variations both of colour, form, structure, and
instinct or habit, must have occurred to produce the perfect adaptation
we now behold. All these are known to vary, and favourable variations
when not accompanied by others that are unfavourable, would certainly
survive. At one time a little step might be made in this direction, at
another time in that--a change of conditions might sometimes render
useless that which it had taken ages to produce--great and sudden
physical modifications might often produce the extinction of a race just
as it was approaching perfection, and a hundred checks of which we can
know nothing may have retarded the progress towards perfect adaptation;
so that we can hardly wonder at there being so few cases in which a
completely successful result has been attained as shown by the abundance
and wide diffusion of the creatures so protected.
[Here are given many detailed examples of insects which gainfully mimic
one another.]
We will now adduce a few cases in which beetles imitate other insects,
and insects of other orders imitate beetles.
Charis melipona, a South American Longicorn of the family Necydalidae,
has been so named from its resemblance to a small bee of the genus
Melipona. It is one of the most remarkable cases of mimicry, since the
beetle has the thorax and body densely hairy like the bee, and the legs
are tufted in a manner most unusual in the order Coleoptera. Another
Longicorn, Odontocera odyneroides, has the abdomen banded with yellow,
and constricted at the base, and is altogether so exactly like a small
common wasp of the genus Odynerus, that Mr. Bates informs us he was
afraid to take it out of his net with his fingers for fear of being
stung. Had Mr. Bates's taste for insects been less omnivorous than it
was, the beetle's disguise might have saved it from his pin, as it had
no doubt often done from the beak of hungry birds. A larger insect,
Sphecomorpha chalybea, is exactly like one of the large metallic blue
wasps, and like them has the abdomen connected with the thorax by a
pedicle, rendering the deception most complete and striking. Many
Eastern species of Longicorns of the genus Oberea, when on the wing
exactly resemble Tenthredinidae, and many of the small species of
Hesthesis run about on timber, and cannot be distinguished from ants.
There is one genus of South American Longicorns that appears to mimic
the shielded bugs of the genus Scutellera. The Gymnocerous capucinus is
one of these, and is very like Pachyotris fabricii, one of the
Scutelleridae. The beautiful Gymnocerous dulcissimus is also very like
the same group of insects, though there is no known species that exactly
corresponds to it; but this is not to be wondered at, as the tropical
Hemiptera have been comparatively so little cared for by collectors.
The most remarkable case of an insect of another order mimicking a
beetle is that of the Condylodera tricondyloides, one of the cricket
family from the Philippine Islands, which is so exactly like a
Tricondyla (one of the tiger beetles), that such an experienced
entomologist as Professor Westwood placed it among them in his cabinet,
and retained it there a long time before he discovered his mistake! Both
insects run along the trunks of trees, and whereas Tricondylas are very
plentiful, the insect that mimics it is, as in all other cases, very
rare. Mr. Bates also informs us that he found at Santarem on the Amazon,
a species of locust which mimicked one of the tiger beetles of the genus
Odontocheila, and was found on the same trees which they frequented.
There are a considerable number of Diptera, or two-winged flies, that
closely resemble wasps and bees, and no doubt derive much benefit from
the wholesome dread which those insects excite. The Midas dives, and
other species of large Brazilian flies, have dark wings and metallic
blue elongate bodies, resembling the large stinging Sphegidae of the same
country; and a very large fly of the genus Asilus has black-banded wings
and the abdomen tipped with rich orange, so as exactly to resemble the
fine bee Euglossa dimidiata, and both are found in the same parts of
South America. We have also in our own country species of Bombylius
which are almost exactly like bees. In these cases the end gained by the
mimicry is no doubt freedom from attack, but it has sometimes an
altogether different purpose. There are a number of parasitic flies
whose larvae feed upon the larvae of bees, such as the British genus
Volucella and many of the tropical Bombylii, and most of these are
exactly like the particular species of bee they prey upon, so that they
can enter their nests unsuspected to deposit their eggs. There are also
bees that mimic bees. The cuckoo bees of the genus Nomada are parasitic
on the Andrenidae, and they resemble either wasps or species of Andrena;
and the parasitic humble-bees of the genus Apathus almost exactly
resemble the species of humble-bees in whose nests they are reared. Mr.
Bates informs us that he found numbers of these "cuckoo" bees and flies
on the Amazon, which all wore the livery of working bees peculiar to the
same country.
There is a genus of small spiders in the tropics which feed on ants, and
they are exactly like ants themselves, which no doubt gives them more
opportunity of seizing their prey; and Mr. Bates found on the Amazon a
species of Mantis which exactly resembled the white ants which it fed
upon, as well as several species of crickets (Saphura), which resembled
in a wonderful manner different sand-wasps of large size, which are
constantly on the search for crickets with which to provision their
nests.
Perhaps the most wonderful case of all is the large caterpillar
mentioned by Mr. Bates, which startled him by its close resemblance to a
small snake. The first three segments behind the head were dilatable at
the will of the insect, and had on each side a large black pupillated
spot, which resembled the eye of the reptile. Moreover, it resembled a
poisonous viper, not a harmless species of snake, as was proved by the
imitation of keeled scales on the crown produced by the recumbent feet,
as the caterpillar threw itself backward!
The attitudes of many of the tropical spiders are most extraordinary and
deceptive, but little attention has been paid to them. They often mimic
other insects, and some, Mr. Bates assures us, are exactly like flower
buds, and take their station in the axils of leaves, where they remain
motionless waiting for their prey.
I have now completed a brief, and necessarily very imperfect, survey of
the various ways in which the external form and colouring of animals is
adapted to be useful to them, either by concealing them from their
enemies or from the creatures they prey upon. It has, I hope, been shown
that the subject is one of much interest, both as regard a true
comprehension of the place each animal fills in the economy of nature,
and the means by which it is enabled to maintain that place; and also as
teaching us how important a part is played by the minutest details in
the structure of animals, and how complicated and delicate is the
equilibrium of the organic world.
My exposition of the subject having been necessarily somewhat lengthy
and full of details, it will be as well to recapitulate its main
points.