Butterfly resemblances—A living leaf—How spiders trap
butterflies—Butterfly doubles—Suggested explanation—More
evidence wanted—Warning coloration—A theory on trust—A
straightforward test—Advice to naturalists—A strange
omission.
SOME of the most remarkable instances of protective resemblances amongst
insects are exhibited by butterflies, one, perhaps, being the most
perfect existing under nature; however, I only say perhaps. This is the
world-renowned leaf butterfly of Sumatra, and elsewhere in the Malay
Archipelago. Of the great purple emperor family, it is purple on the
upper surface, and gleams like a meteor as it shoots about in the rich,
sun-bathed atmosphere of the tropics, its conspicuousness being enhanced
by a sort of miniature, sharp-pointed swallow-tail, in which the hinder
pair of wings end, and a broad, orange bar, like a sash or scarf of
honour, running right across the anterior wings. It flies boldly and
strongly, and when it descends upon a bush or shrub it is as though a
little purple torch had shot through the foliage; but all at once, even
though you see it come down just in front of you, it has vanished
utterly—the torch has gone out. You may look and look, but unless you
know the trick, and have seen the settling, and never taken your eyes
off the exact spot, you will never find the butterfly, or see anything
more of it until, all at once, it gleams in the air again. For the under
part of the leaf butterfly’s lovely purple wings is like the leaf
indeed—“the sere, the yellow leaf”—with a midrib running down the centre
veinings on either side, a curled tip at the top, a stalk at the bottom,
and everything proper to leaves, but not as a rule to butterflies. All
four wings join in this effect, for being thrown up in the usual way
when the insect settles, the leaf-like shape is thus brought about,
one-half of the under surface being seen on each side in clear profile,
whilst the purple now lies hid within, like the pictures on a folded
screen. As for the body of the butterfly, that is hidden inside the
wings too; the legs are all but invisible, and the two little pointed
swallow-tails, just touching the plant’s stem with their mutual tip,
make the stalk of the leaf. Even on the wall of a room or a curtain it
would seem as though a dead leaf were sticking there; how much more
when, as is always the case, the butterfly flies into some bush or
thicket crowded with dry, brown leaves, and settles all amongst them. It
is not that you don’t see it there that makes you miss it, but that you
see it and scores of brown leaves all about it, every one of which looks
just the same as itself.
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[Illustration: PROTECTIVE MIMICRY]
The picture at the top shows birds pursuing butterflies, while
in the one below the same birds have lost their prey, as the
butterflies have alighted and show only the underside of
their wings, which are practically indistinguishable from
the neighbouring leaves.
To make the matter plainer, in case this is not a very accurate
description, here is the account of an eye-witness: “This species,” says
Dr. Wallace, “was not uncommon in dry woods and thickets, and I often
endeavoured to capture it without success, for after flying a short
distance it would enter a bush among dry or dead leaves, and however
carefully I crept up to the spot, I could never discover it till it
would suddenly dart out again, and then disappear in a similar place. At
length I was fortunate enough to see the exact spot where the butterfly
settled, and though I lost sight of it for some time, I at length
discovered that it was close before my eyes, but that in its position of
repose it so closely resembled a dead leaf attached to a twig as almost
certainly to deceive the eye, even when gazing full upon it.” Then
follows a minute explanation of the imposture. “The end of the upper
wings terminates in a fine point, just as the leaves of many tropical
shrubs and trees are pointed, while the lower wings are somewhat more
obtuse, and are lengthened out into a short, thick tail. Between these
two points there runs a dark, curved line exactly representing the
midrib of a leaf, and from this radiate on each side a few oblique
marks, which well imitate the lateral veins. The tint of the under
surface varies much, but it is always some ashy brown or reddish colour,
which matches with those of dead leaves. The habit of the species is
always to rest on a twig and among dead or dry leaves, and in this
position, with the wings closely pressed together, their outline is
exactly that of a moderately-sized leaf, slightly curved or shrivelled.
The tail of the hind wings forms a perfect stalk, and touches the stick,
while the insect is supported by the middle pair of legs, which are not
noticed among the twigs and fibres that surround it. The head and
antennæ are drawn back between the wings, so as to be quite concealed,
and there is a little notch hollowed out at the very base of the wings,
which allows the head to be retracted sufficiently. All these varied
details combine to produce a disguise that is so complete and marvellous
as to astonish everyone who observes it, and the habits of the insect
are such as to utilise all these peculiarities, and render them
available in such a manner as to remove all doubt of the purpose of this
singular case of mimicry, which is undoubtedly a protection to the
insect. Its strong and swift flight is sufficient to save it from its
enemies when on the wing, but if it were equally conspicuous when at
rest it could not long escape extinction, owing to the attacks of the
insectivorous birds and reptiles that abound in the tropical
forests.”[104]
Dr. Wallace then speaks of another closely allied species which is
common in India, on the under surface of whose wings there are
sometimes, to the boot of all that has been described, in the way of
disguise, “patches and spots formed of small black dots, so closely
resembling the way in which minute fungi grow on leaves that it is
almost impossible, at first, not to believe that fungi have grown on the
butterflies themselves.”[104] The minuteness of a resemblance like this
is really very surprising, for it seems as though the butterfly-hunting
bird or insect—some powerful wasp may represent the latter—was capable
of minutely examining the object in question, and saying to itself, as
it were, “I don’t think that can be a leaf, because there are no black
spots upon it,” or _vice versâ_. In reality, however, it is no doubt the
general effect, to which every detail contributes, that tells. What such
resemblances do seem to me to show—and this, I think, is a new idea—is
the accuracy and precision of some insects’ sight. How insects see
things has long been a question, and many, I suppose, think it quite
uncertain whether a leaf, for instance, throws the same picture on their
retina that it does on ours. But if, to deceive them, the copy must be
such that it also deceives us, is it not clear that it does? Otherwise
the effect of the original could probably be reproduced by a less
accurate copy. How little, after all, does the finest painting really
resemble nature! The effect alone does so, not the means by which it is
arrived at. Surely, then, if an insect, looking at a leaf or any other
object, received but a general impression of colour, with an outline
more or less blurred, or ill-defined, these copies of nature by
nature—made to deceive—would bear witness to the fact. A study of
protective resemblances is perhaps the best way of forming an idea as to
how creatures, other than ourselves, see the world. It is even possible
that such resemblances exist, which we, because we see things
differently, are totally incapable of detecting.
I do not know if any other striking case of resemblance to an inanimate
object (if plant life can be included under this term) is offered by the
butterfly world, though there are several more of the same kind, but I
cannot remember one just now. No doubt there are many which have not yet
been discovered. We have, however, various instances of concealment even
here in England, as, for instance, the peacock butterfly; but these, as
well as special resemblances, are, for the most part, more marked in
moths. The lappet moth, indeed, though it does not quite get the shape,
looks very like a dead brown leaf, whilst in the buff-tip moth we are
supposed to have a special resemblance to a piece of rotting wood,
clothed with moss or lichen, and broken at each end. Personally, I have
never received the impression of such a definite object, but only a
general one of rot and decay. Even here, however, I do not believe I
could ever be taken in, for the yellow head and tips of the wings, which
are supposed to offer a perfect resemblance to the two broken ends of
the piece of wood, are to me the tell-tale parts, and instantly cry out,
“Moth!” In fact, soft as is the colouring of the buff-tip, it still
seems to me a salient object, and I do not think very much of that
bird’s eyesight who fails to detect it under anything like favourable
circumstances.
Another moth that flies by day, and is not uncommon in the United
States, bears, when sitting on a leaf, a much stronger resemblance to a
bird-dropping, but in this not uncommon form of imitation moths, and all
other insects, are outdone by spiders, who use it aggressively against
them, and particularly, it would seem, against butterflies, as the
following instances will show. Mr. Forbes once, whilst travelling in
Java, saw a butterfly settled upon a bird-dropping. He watched it for
some time, and then, wondering at its long stay, approached cautiously,
and, slowly extending his hand, actually caught it by its wings, between
his finger and thumb—no mean feat, as it seemed, yet there was nothing
to boast of. As he lifted the butterfly only the wings came away, the
rest of it staying with the supposed bird-dropping, which was now seen
to be a spider, who, having caught the butterfly by means of this
shameful imposture, was quietly occupied in eating it. The disguise in
this case was of the most wonderful perfection. “Such excreta,” says Mr.
Forbes, who discovered this one, “consist of a central and denser
portion of a pure white, chalk-like colour, streaked here and there with
black, and surrounded by a thin border of the dried-up, more fluid
part.”[105] The appearance of each of these constituent parts was
successfully counterfeited by the spider in question, who, in its own
person, represented the more solid material, and spun the rest with its
web.
As I know from early experience, when a naturalist makes a prize, all at
once, of some interesting specimen, for some time afterwards he expects,
or, rather, feels as if he would see some other on every leaf or twig;
but time went by and no more of these “vain, delusive” spiders presented
themselves. At length, years afterwards, the same naturalist found
himself by the banks of the Moesi river, in Sumatra (which sounds much
more interesting than the Thames, for instance), and this was his second
experience. “I was,” he says, “rather dreamily looking on the shrubs
before me, when I became conscious of my eyes resting on a
bird-excreta-marked leaf. How strange, I thought, it is that I have
never got another specimen of that curious spider I found in Java, which
simulated a patch just like this! I plucked the leaf by the petiole
while so cogitating and looked at it half-listlessly for some moments,
mentally remarking how closely that other spider had copied nature,
when, to my delighted surprise, I discovered that I had actually secured
a second specimen, but the imitation was so exquisite that I really did
not perceive how matters stood for several moments. The spider never
moved while I was plucking or twirling the leaf, and it was only when I
placed the tip of my little finger on it that I observed that it was a
spider, when it, without any displacement of itself, flashed its falces
into my flesh.”[105] (He means it bit him.)
Not all butterflies are entrapped by the kind of simulacrum here
noticed. Nature can adapt herself to every taste, and in South Africa
there are spiders who make themselves attractive by appearing to be
flowers. Of some of these and their _modus operandi_ Mr. Rowland Trimen,
who was curator of the Cape Town Museum, gives the following interesting
account: “Many species of spiders,” he says, “are well adapted to
succeed by being coloured in resemblance to the flowers in or on which
they await the arrival of their victims. One that inhabits Cape Town is
of the exact rose-red of the flowers of the oleander, and, to more
effectually conceal it, the palpi, top of the cephalothorax, and four
lateral stripes on the abdomen, are white, according remarkably with the
irregular white markings so frequent on the petals of _Nerium_.”[106]
These, indeed, must be beautiful spiders, and one would like to hear a
little more of them, but Mr. Trimen goes at once from red to yellow. “I
was led,” he continues, “to notice a yellow spider of the same group in
consequence of seeing that two of a number of butterflies on the flowers
of _Senecio pulugera_ did not, on my approach, fly off with their
companions. Each of these unfortunates turned out to be in the clutches
of a spider, and when I released them I observed their captors very
narrowly, and I found that the latter’s close resemblance to the
_Senecio_ flowers was not one of colour alone, but due also to attitude.
This spider, holding on to the flower stalk by the two hinder pairs of
legs, extended the two long front pairs upward and laterally. In this
position it was scarcely possible to believe that it was not a flower
seen in profile, the rounded abdomen representing the central mass of
florets and the extended legs the ray florets, while to complete the
illusion the femora of the front pair of legs, adpressed to the thorax,
have each a longitudinal red stripe, which represents the ferruginous
stripe on the sepals of the flower.”[106]
Later on, Mr. Trimen was so fortunate as actually to see a butterfly
caught by another flowery impostor:—“The butterfly,” he tells us, “was
engaged in honey-sucking on a white flower-head of _Lantana_, and
explored each individual flower with its proboscis. While I was watching
it, the butterfly touched and partly walked over what looked like a
slightly folded or crumpled flower about the middle of the cluster. This
turned out to be a spider, which instantly seized the butterfly,
throwing forward its front legs, somewhat after the fashion of a mantis.
In this spider the effect of the little depressions on the limb of the
corolla was given by some depressed lines on the back of its smooth
white abdomen.”[106]
Other spiders resemble snail-shells, others ants, and one, at least, is
like a small scorpion, but we will return to the butterflies. As I have
said, except for that wonderful copy of a leaf, already described, I
cannot think of any very extraordinary resemblances amongst them,
belonging to that class, but there are others which form a little class
of their own. In the last chapter we have seen bees imitating bees, and
in this we will make the acquaintance of certain butterflies which, as
it were, pretend to be of a species which they do not really belong to.
Thus in Brazil, by the great River Amazons, a number of large showy
butterflies are found which belong to the family of the _Heliconea_, and
wherever these fly they are accompanied by various other butterflies,
belonging to quite different families, which are nevertheless so
extremely like them that even Mr. Bates, who, for eleven years, ran up
and down the Amazons with a butterfly-net in his hand, could never be
quite sure which kind it was that he was going to catch. Often, when he
thought he had got a _Heliconea_ he was perfectly thunderstruck to find
it was really a _Papilio_, _Pieris_, _Euterpe_, _Leptalis_,
_Protogonius_, _Ithoneis_, _Dioptis_, _Pericopis_, _Hyelosia_, or
something of that sort; or again, when it was one of these he was after,
and at last he thought he had it in the net, he would be petrified, on
looking more closely, to find that what he had really caught was a
_Heliconea_.
But now, as all these butterflies were alike or nearly alike, how could
Mr. Bates tell—or how had anybody been able to tell before him—that they
were really not all the same species—that a _Heliconea_ was not a
_Papilio_, or that a _Papilio_, _Pieris_, _Euterpe_, etc., were not all
of them _Heliconeas_? This, at first sight, seems a difficult question
to answer, but really it is not, because, in all these families of
butterflies, the various species composing them bear a kind of
generalised resemblance to one another: there is a family likeness, in
fact, and this is not only the case in regard to their outward
appearance—the shape and colour of their wings, etc.—but it applies, in
a still greater degree, to their structure and internal economy. Thus,
however strongly a _Pieris_, or one of those others, might resemble a
_Heliconea_, the trained eye of an entomologist could easily see that it
really was a member of another family, and since, in resembling the
_Heliconea_, it departed from the general type of pattern and colouring
exhibited by the family to which it belonged, whilst this one species of
_Heliconea_ it resembled was like the others, it might be inferred that
the latter was the imitated and not the imitating form. Again—and this
is still more decisive evidence in the cases where it applies—the
resemblance is often confined to one sex of the copying species, viz.
the female, so that whilst she is hardly to be distinguished from the
model on which she has founded herself, the male retains the appearance,
together with all the other characteristics, of the race to which both
he and she belong.
But now came a further question, the most puzzling or, at any rate, the
most important one of all, viz. Why should the one butterfly imitate, or
rather resemble the other, in such an extraordinary degree—a degree
seeming to preclude the possibility of mere chance having brought it
about? This question Mr. Bates is supposed to have been the first to
answer, though I cannot help thinking, myself, that he has only extended
an explanation, which, in some cases, was so obvious that no one had
thought of pointing it out, to these other cases where it was not nearly
so easy to see. For what can be plainer, as Mr. Bates himself remarks,
than that a moth, for instance, by closely resembling a hornet, would
escape the attacks of birds that might otherwise have devoured it? I
cannot think but that so patent an explanation had been in the minds of
many long before 1862, and though no one previous to that date may have
applied the principle of natural selection to such cases, it must be
remembered that natural selection had been established by Darwin some
ten or twelve years before.
Bates, however, besides making an ingenious application of the above
principle to a special case, gave a real reason for something which was
not at all obvious, viz. why one butterfly should be a gainer by closely
resembling another; and this no one had hitherto been able to do. His
surmise, which has since in many instances been confirmed, is as
follows. Having first pointed out that the _Heliconea_ butterflies are a
numerous, flourishing race, whilst those species that imitate them are
poor in numbers, he says, “What advantages the _Heliconidæ_ possess to
make them so flourishing a group, and, consequently, the object of so
much mimetic resemblance, it is not easy to discover. There is nothing
apparent in their structure or habits which could render them safe from
persecution by the numerous insectivorous animals which are constantly
on the watch in the same parts of the forest which they inhabit. It is
probable they are unpalatable to insect enemies. Some of them have
glands near the end of the abdomen which they protrude when roughly
handled; it is well known that similar organs in other families secrete
fetid liquids or gases and that these serve as a protection to the
species. They have all a peculiar smell. I never saw the flocks of
slow-flying _Heliconidæ_ in the woods persecuted by birds or
dragon-flies, to which they would have been easy prey; nor when at rest
on leaves did they appear to be molested by lizards, or the predaceous
flies which were very often seen pouncing on butterflies of other
families. If they owe their flourishing existence to this cause it would
be intelligible why species whose scanty number of individuals reveals a
less protected condition, should be disguised in their dress, and thus
share their immunity. Is it not probable, seeing the excessive abundance
of the one species and the fewness of individuals of the other, that the
_Heliconea_ is free from the persecution to which the _Leptalis_ is
subjected?”[107]
No sooner was this suggestion made than naturalists all over the world
began to test it, or rather to say that it ought to be tested. Some
experiments have been made, but they have not been very numerous, and it
can hardly be said that they entirely support Bates’s view. Sometimes
they do and sometimes they do not, so as there is no reason to suppose
that every butterfly is relished by every kind of insect-eating
creature, this is not conclusive, till the same tests are employed in
regard to butterflies that are not imitated in this way; for if the
latter have not been imitated on that account, it need not be on that
account that others have been imitated. Thus Belt says, as the result of
his observations, “The _Heliconidæ_ are distasteful to most animals; I
have seen even spiders drop them out of their webs again; and small
monkeys, which are extremely fond of insects, will not eat them, as I
have proved over and over again.”[108] He also “observed a pair of birds
that were bringing butterflies and dragon-flies to their young, and
although the _Heliconidæ_ swarmed in the neighbourhood, and are of weak
flight so as to be easily caught, the birds never brought one to their
nest.”[109] This seems very good evidence of the truth of Bates’s
theory, but then, as against it, we learn from the same observer that
“another spider that frequented flowers seemed to be fond of these very
same butterflies,” and as to the spiders which were seen to drop them
out of their webs, they may resolve themselves into one, since farther
on Belt says, “A large species of spider also used to drop them out of
its web when I put them into it.”[109] Then we are told that “there is,
however, a yellow and black-banded wasp that catches them to store his
nest with”; and which, having done so, “would quietly bite off its
wings, roll it up into a ball, and fly off with it.”[110] Professor
Poulton calls these cases “interesting exceptions,” and easily accounts
for them. But might not further observation keep adding to the number of
exceptions, until at last, they become so numerous that all one could
say would be this: “There is a great choice of insects in tropical
America, and some creatures may prefer one kind and some another, to
whatever species they belong.” In India, again, where there is another
family of butterflies having doubles, or understudies, only one species
was refused by all the mantids which a French naturalist gave them to.
Others were eaten by all of them.
Has any man tried eating one of these butterflies? That was what
Professor Wheeler did to test another supposed case of the same sort.
Here the insect was a large and very conspicuously coloured day-flying
moth. This moth has not an understudy, as far as is known, but it was
supposed, then, to be an example of what is called “warning coloration,”
that is to say, its bright colours were believed to be flourished in the
face of any and every animal it might meet with, in order to warn them
that it was not good to eat. Otherwise some bird, or lizard, or other
creature might kill it before it had time to find out that it wasn’t.
For instance, had it been just a brown moth—there are so many of these
and most of them good to eat—how was it to be distinguished from others?
But such a get-up as _that_—black and white wings and a black and orange
body—once seen it was not to be forgotten. It was like the red flag at a
rifle range, warning one off, and this is the theory of warning
coloration. So Professor Wheeler, as he rode through the deserts of
Wyoming, with the moths all about him, resolved to test this theory
which had lived for a long time, and still goes on living a good deal on
trust. “He dismounted from his horse and proceeded to masticate the body
of one of the moths. To his astonishment the little flavour that it
contained was mild and pleasant, one may almost say nut-like.”[111]
Perhaps it may be thought that, on the “_de gustibus_” principle, what
is pleasant to a human being might be disagreeable to a bird or a
lizard; but Professor Wheeler tried another experiment. “Another
day-flying moth, common in our eastern States, has deep black wings,
each adorned with a pair of large yellow spots, and there is a dash of
orange on its legs. It certainly cannot be a mimetic species (if it
were, of course, one would not expect it to be nasty) as there is no
other day-flying moth which could serve as its model. Several of these
moths were given to some lizards that had previously been well fed on
house-flies and could not, therefore, be very hungry. The moths were
seized at once, and devoured, with evident signs of relish.”[111]
As a result of these experiments Professor Wheeler concludes that
“naturalists should be more careful in imputing nauseous or disagreeable
qualities to some conspicuously coloured animals,” and he suggests that
“if every field entomologist could only bring himself to repeat the
writer’s experiment on one of many cases of ‘flaunted nauseousness’ and
place his taste impressions on record, we should in the course of time
have a really valuable body of evidence, for we can hardly assume that
beasts, birds, and reptiles can find things ‘nauseous’ which are quite
tasteless, or even pleasant, to the human palate.”[111] “_Il n’y a pas
de réplique à cela_,” and how it is that so simple a plan did not occur
to Mr. Bates during all the eleven years he was on the Amazon it is not
very easy to imagine. On the whole, perhaps, it may be said that the
reason why certain butterflies are imitated by other butterflies has not
been so satisfactorily settled as the fact that they are so imitated.
But, on the other hand, there is some—perhaps much—evidence of the truth
of Bates’s theory, and, moreover, that theory is in itself so plausible
that it seems to require a good deal of evidence to overthrow it.
It is not only in South America that butterflies dress up like one
another. Instances similar to those here given occur also in Africa and
the Malay Archipelago, as well as in other parts of the world. There is
even one doubtful case in England, both the copy and original being
moths. Moths, especially the day-flying ones, are represented in these
phenomena as well as butterflies, which are sometimes imitated by them.
-----
Footnote 104:
Wallace, The Malay Archipelago, pp. 100-2.
Footnote 105:
Forbes, A Naturalist’s Wanderings in the Eastern Archipelago.
Footnote 106:
Trimen, Protective Resemblance and Mimicry in Animals.
Footnote 107:
“Contributions to an Insect Fauna of the Amazon Valley,” in
_Transactions of the Linnean Society_, vol. 23, p. 495.
Footnote 108:
Belt, The Naturalist in Nicaragua, p. 109.
Footnote 109:
Ibid., pp. 316, 317.
Footnote 110:
Ibid., p. 109.
Footnote 111:
“Impostors among Animals,” in the _Century Magazine_, July, 1901.
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