Natural selection—Protective resemblances—A locust’s
stratagem—Mock leaf-cutting ants—Flowery dissemblers—A Malay
explanation—Snake-suggesting caterpillars—A prudent
lizard—Inconclusive experiments—A bogus ant—Flies that live
with bees—A caterpillar that dresses up—A portrait-modelling
caterpillar.
EVERYBODY knows nowadays how all the different species of animals and
plants, living and extinct, have come into existence. It was quite
simple. All they had to do was to keep on varying. Some of them varied
in a way that was good for them, some in a way that was bad. The latter
died, but the others increased and multiplied, and as the process was
always going on, and it is impossible to vary long without becoming
changed, it happened that creatures which had started with a certain
appearance got in time to have quite another one, so that they would not
have been recognised by the people who used to know them, if these same
people had kept alive. However, as the process was so slow that it took
millions of years, and is still going on, awkward things of this sort
never happened, and so, as nobody had ever seen one species of animal
change into another before their eyes, they found it difficult to
believe that they ever had done so; for the ordinary person says
“seeing’s believing,” though he believes in all sorts of things that
neither he nor anyone else ever has seen, or is ever likely to. Still,
for all that, he thinks his own eyesight must be better than anyone
else’s.
This process of eternal change, with the changes for the better
surviving, and those for the worse dying out, is what is called natural
selection, and if we understand it—as there are few now who do not—we
can understand this, that if any kind of creature is so strong and
formidable that it would be an advantage for weaker creatures to be
mistaken for it, then it is not at all unlikely that some of these
weaker creatures will get more and more to resemble it, until at last
they are so mistaken. For instance, our common wasps, who are armed with
a formidable sting, and are very skilful in using it, are not attacked
by any other insect, excepting hornets, which are not common. Any fly or
moth, therefore, that resembles a wasp will be generally left alone, and
the more so the more it resembles it. Accordingly we do find flies and
moths that look very like wasps, and live safely in consequence. Still
more would it be of advantage to look like a hornet, and there is a moth
so like one that it is called the Hornet-clear-wing.
On this same principle of being mistaken for something that is safe from
attack or annoyance, all sorts of animals, and in a special degree
insects, have come to look like various objects around them, and amidst
which they live, such as stems of grass, pieces of moss or stick,
leaves, flowers, and so on—some of the resemblances being more special
and extraordinary. These are things which, though the eye may see, it
does not as a rule dwell upon, because there are so many others round
about. Who, for instance, would look at any particular blade of grass?
So a bird that would pounce down upon an insect that it saw moving
amongst the leaves of a tree, if there was no doubt that it was an
insect, would not take any pains to examine what only looked like one
leaf amongst many.
Thus, throughout nature we have these curious resemblances of certain
creatures to certain other creatures, or to the plants or inanimate
objects around them, but it is principally amongst insects that the
phenomenon is met with, probably because they increase and multiply so
quickly that there has been more time both for the laws of inheritance
and for the great controlling one of natural selection to have come into
play. Whatever is the reason, there is no doubt about the fact, which
will be best illustrated by one or two salient instances.
Ants, though they fall a prey to various animals larger than
themselves—such as birds or ant-eaters—yet in their relations with other
insects occupy a position of comparative safety, on account of their
weapons and pugnacity, and, still more, of their numbers. The driver
ants of equatorial Africa, and their South American representatives, the
_ecitons_, are indeed, when they set out on their foraging expeditions,
the terror, not only of insects, but of all animal life. “Wherever they
move,” says Bates, referring to the latter, “the whole animal world is
set in commotion, and every creature tries to get out of their way.”[91]
This, however, as they climb trees, and send out encircling columns
which enclose a considerable extent of ground, is difficult, or rather,
impossible, for all such as cannot fly some distance without alighting;
for if an ant or two once seize upon them all is over. An insect,
therefore, that cannot evade the onset of such an enemy is lucky if it
has some such means of ensuring its safety as has been above referred
to. One at least thus specially favoured inhabits Nicaragua. “I was much
surprised,” says Mr. Belt, “with the behaviour of a green leaf-like
locust. This insect stood immovably amongst a host of ants, many of
which ran over its legs, without ever discovering there was food within
their reach. So fixed was its instinctive knowledge that its safety
depended on its immovability, that it allowed me to pick it up and
replace it amongst the ants without making a single effort to escape. It
might easily have escaped from the ants by using its wings, but it would
only have fallen into as great a danger, for the numerous birds that
accompany the army of ants are ever on the outlook for any insect that
may fly up, and the heavy flying locusts, grasshoppers, and cockroaches
have no chance of escape.”[92] This locust resembled a green leaf which,
as we have seen, was a very protective resemblance indeed. It might,
however, had it been a smaller insect, have resembled one of the ants
themselves, and in that case could have run about with them, pretending
or appearing to forage, also with perfect impunity. Whether the _Eciton_
has such a double I know not, but various ants have. With some it is a
spider that assumes their form. With others, as we have seen to a
partial extent, a caterpillar, but the _Sauba_, or leaf-cutting
ant—which is also the mushroom-growing one—is understudied, leaf and
all, by an insect which, though the order to which it belongs has been
determined, has not yet apparently received a name. “An example,” says
Professor Poulton, “of protective mimicry, which I believe to be more
wonderful in its detail and complexity than any which has been hitherto
described, was observed and interpreted by my friend Mr. W. L. Sclater,
in 1886, during his investigations in British Guiana. Mr. Sclater and
his native servant had been collecting insects by shaking the branches
of a tree over a sheet. The servant, although described as a very acute
observer, saw an insect on the sheet which he mistook for one of the
abundant Cooshie ants (perhaps the native name), carrying its little
jagged segment of leaf over its back. Mr. Sclater looked more closely,
and saw that it was an entirely different insect belonging to the order
_Homoptera_. Its length was about that of an ant carrying its leaf. The
leaf was represented by the thin flattened body of the insect which in
its dorsal part is so compressed laterally that it is no thicker than a
leaf” (or as we would say, which along the back is no thicker than a
leaf), “and terminates in a sharp, jagged edge. The head and legs were
brown, and suggested the appearance of that part of an ant which is
uncovered by the piece of leaf. The jagged dorsal line, when seen in
profile, evidently corresponds to the roughly gnawed edge of the
fragment of leaf, for Mr. Sclater tells me that the contour of the
latter is generally shaped by the mandibles of the ant rather than due
to the natural margin.”[93]
The above-mentioned insect is a dweller in trees, and one might have
supposed that a general resemblance to the leaves among which it moves
would have been a sufficient protection for it. This probably was the
beginning of the deception, which became more complex as time went on.
In the leaf-like back of the insect we see probably the original
disguise, but as the eyes of birds became more acute they began to
pierce through it, more especially when the creature walked. Round about
the would-be leaf, however, the leaf-cutting ants—distasteful to the
birds that so affected it—were constantly moving and walking. If only it
could get to resemble one of these it might be as active as it pleased,
and especially if its motions, as well as its appearance, became ant- or
ant and leaf-like. And this, indeed, was what gradually began to take
place. Variation was always going on, and natural selection was always
at hand to mould and shape its results. The two insects were, to begin
with, of much the same size, and the general leaf-like appearance of the
one was a good basis on which the more particular resemblance to the cut
piece of leaf, carried by the other, might be founded. A few deeper
washes of brown, some not very profound modifications of contour, and an
ant-suggesting legs and body began to appear beneath it. Meanwhile,
however, hundreds of thousands—nay, millions—of bad or mediocre copies
were swept away, the species became rarer and rarer—trembled, perhaps,
on the verge of extinction; but just when it might have appeared to the
birds, who were no longer able to obtain a once much-enjoyed morsel,
that it really was extinct, it was saved; nature’s object had been
gained. A certain number of individuals were left, were close at hand
even; even now, at that very moment, one might be crawling on the same
twig where a despondent bird sat, only it was not to be distinguished
from a leaf-cutting ant. Such are the ways of nature, such the slaughter
that attends her victories.
In Borneo, and the Malay Archipelago generally, there is a pretty pink
flower known as the “Straits Rhododendron.” Once a gentleman was looking
at one of these flowers and admiring it, when all at once it turned
round and stared him in the face. It was not a flower, but a mantis; its
flattened legs—pink like them—made the petals; its abdomen, turned up
over the back and held thus motionless, resembled an opening bud. “When
I held the branch on which the insect had established itself in my hand
I could not tell exactly where animal tissue commenced and where flower
ended, so perfectly was the one assimilated to the other, both in colour
and surface-texture.”[94] When once established on a flower this mantis
would remain there quite motionless, if undisturbed, until it had
occasion to leave it; and of course, in nature, had any insect settled
on or near it, it would have instantly been seized. The ways of the
mantis are well known. “Under a most sanctimonious aspect,” says Fabre,
speaking of the little green one of Provence, “it hides the morals of a
cannibal”;[95] and, indeed, the female, which is larger and stronger
than the male, will often turn upon the latter and devour it in the very
midst of a love-passage. This it does, as in all other cases, by
suddenly launching forward one or both of its fore-arms—which have been
previously held in an attitude of prayer—and enclosing the body of the
victim between their first and second segments, each of which is toothed
along the edge like a saw. The double row of teeth meet in the body,
which, held aloft, and writhing on either side of the trap, is devoured
piecemeal by the mantis, who, with its sharp jaws, tears little
mouthfuls out of it as long as it, or its appetite, lasts. This process,
made more interesting by the way in which it was brought about, was
witnessed in the case of the above-mentioned species. Small flies
frequently settled upon it as it sat motionless, flower-like amidst
flowers. “These it made no attempt either to drive off or to capture;
its motions seeming rather to attract than repel them. After a short
time a larger _Dipteron_, as big as a common house-fly, alighted on the
inflorescence within reach of the predatory limbs. Then the mantis
became active immediately; the fly was seized, torn in pieces, and
devoured.”[96] Such are the real propensities of the seeming flower, and
such, too, it may be observed, are those of some actual flowers—to wit,
insectivorous ones.
To the Malays, however, whose minds are not yet open to the doctrines of
protective or aggressive resemblance, or to evolution generally, this
mantis _is_ a flower, they “know not seems.” The blossoms of “the
_sendudok_” have become alive, and perhaps some analogies suggested by
their own life-experience temper their surprise at such an apparent
change of disposition. They say, too, that few men ever see more than
one flower-mantis in the whole course of their lives, so rare a creature
is it. In this, no doubt, they are right; yet it would be possible,
perhaps, even for a Malay to see several without knowing anything about
it. Native eyes are almost always sharper and better than those of the
Europeans who come amongst them; but, on the other hand, no native goes
about like a modern entomologist, with his eyes specially open in one
direction and the possibilities of protective resemblance in his mind.
The same naturalist, during the same expedition, was singularly
delighted to secure a larva, whose resemblance to a snake was “so
startlingly accurate that I was, for a moment, completely deceived.”[96]
A description follows which, as it is of that kind which deals longly
and learnedly in details without producing any particular general
effect, may be left out. It would seem, however, that this caterpillar,
like many others, has the power of withdrawing its actual head into a
fold or two of its skin, which is here so marked that it performs the
office of a mask, obscuring and taking the place of the real head thus
obliterated. The mask is furnished with two spots, which at once become
the creature’s eyes, and both in colour, shape, and general appearance
bear a remarkable resemblance to those of a snake; whilst a wrinkled
fold, running back on either side from what appears to be the snout,
suggests the mouth, and the flattened head with its characteristic
arrangement of broad, flattened scales is also indicated by certain
markings and colours on the required part of the caterpillar’s body. An
apparent head like this, thrown suddenly up as though threatening to
dart forward with a hiss and distended jaws, might alarm anyone, and
such a mock demonstration is evidently required to give full effect to
the disguise. Thus we are told that “when the larva was moving about
with the anterior segments well expanded the resemblance to a snake was
not so startling; but directly it was touched the terrifying attitude
was assumed, the anterior segments being drawn in and the front of the
body turned towards the aggressor. When, at the same time, the hinder
part of the body was hidden by leaves the deception became complete, and
if effective enough to deceive, even temporarily, a human being, it must
surely be equally effective in deterring less highly organised and timid
foes.”
For the “timid” certainly, but for the “less highly organised” the
conclusion does not seem so plain. No sight is better than a bird’s,
and it is practice that makes perfect in any particular direction.
Still, unless we suppose the disguise to be accidental merely—and this
no one with a knowledge of the whole subject can do—the object of it
seems clearly apparent, and we may, therefore, assume that, on the
whole, it is successful—to the extent, at any rate, of keeping the
species in existence. In such matters, however, there is nothing like
practical experiments, if one has the chance of making them, as the
finder of the caterpillar in question must have had, since he says,
“Unfortunately I was unable to test the efficacy of the disguise, for
fear of losing the larva, which I was anxious to rear for the purpose
of identification.”[96] To me this appears a false judgment. Such a
test would have been much more interesting, surely, especially if
resulting in the way anticipated, than a dry pinned specimen and a
Latin name.
Another large snake-resembling caterpillar was found by Bates in the
forests of Brazil, and the likeness was sufficiently striking to alarm
several people to whom he showed it. But it is not necessary to go so
far afield, for here in England, according to Professor Poulton, we have
an excellent example of this kind of protective resemblance. This is no
other than the caterpillar of the elephant hawk-moth, which by
withdrawing its head into its body—just as does the Bornean
species—produces a similar false face, with a pair, or, indeed, two
pairs of fierce-looking eyes.[97] This caterpillar feeds on the great
willow herb, and when at rest keeps amongst the dead brown leaves at the
base of the stem. “As soon,” says Professor Poulton, “as the leaves are
rustled by an approaching enemy, the caterpillar swiftly draws its head
and the three first body-rings into the two next rings, bearing the
eye-like marks. These two rings are thus swollen, and look like the head
of the animal, upon which four enormous, terrible-looking eyes are
prominent. The effect is greatly heightened by the suddenness of the
transformation, which endows an innocent-looking and inconspicuous
animal with a terrifying and serpent-like appearance.”[98]
With this caterpillar, since naturalists know what to call it, and there
is no chance of its handing down any of their names in Latin to
posterity, it has been possible to make experiments, and on the whole
perhaps they have been in favour of the protective resemblance theory.
The most interesting one—that I have read, that is to say—was made by
Professor Poulton with a full-sized green lizard, and is thus described
by him: “The lizard was evidently suspicious, and yet afraid to attack
the caterpillar, which maintained the terrifying attitude in the most
complete manner throughout. The lizard kept boldly advancing, and then
retreating in fright; but at each advance it approached rather nearer to
the caterpillar. After this had taken place many times and nothing had
happened, the lizard grew bolder and ventured to gently bite what
appeared to be the head of the caterpillar; it then swiftly retired, but
finding that there was no retaliation it again advanced and gave it a
rather harder bite. After a few bites had been given in this cautious
manner, the lizard appeared satisfied that the whole thing was a fraud,
and devoured the caterpillar in the ordinary manner.”[98] Professor
Poulton has no doubt as to the lizard having been alarmed at first by
the appearance of the caterpillar, and adds that he has never seen one
act in the same way on any other occasion; other large hawk-moth
caterpillars being eaten at once with entire _sang-froid_. It may be
observed, however, that if every lizard were to act in the way recorded,
under natural conditions, the advantage to the caterpillar would be nil,
since though a species may survive through not being eaten, it certainly
will not through being eaten with hesitation. And why should a lizard be
more timid in the open air than in a box or a fern-case? Unless we
assume, therefore, that this particular one was bolder than most others
would be, the result of the experiment was not for, but against, the
theory it was designed to test; and since we have no business to assume,
the only thing to do is to get more caterpillars, and give them to more
lizards. Small birds, however—and this in a country like England is more
to the point—seem really to fear these pseudo-snakes to the extent of
flying away from them.[99] But would an ordinary large caterpillar of
the _Sphingidæ_—say, of a privet hawk or death’s-head moth—frighten them
in the same way? If so, then again we are nowhere.
Perhaps a still more extraordinary instance of protective resemblance
than any of the foregoing is that of a caterpillar which pretends to be
an ant—one provided with an efficient sting, and of an irritable
disposition. Here, as in the snake cases, it is by one portion of the
body only that the fraud is perpetrated, but this, instead of being the
front, is the hind part, in which, perhaps, it offers a unique example
of the sort. The colour of the caterpillar is exactly that of the ant,
and whilst its extremity represents the latter’s head, two black spots
which are there situated bear an equally close resemblance to the eyes.
The jaws are represented by the last pair of false legs or claspers,
which are of disproportionate size, and can upon occasion be stretched
widely apart, whilst a number of thin, tentacle-like processes, attached
in pairs to the segments of the body, have all the appearance of an
ant’s legs and antennæ. Armed with these properties, which, however, in
a state of quiescence are not very recognisable, the caterpillar waits,
as one may say, to have its feelings ruffled, when, by flinging the
hinder part of its body into the air, each separate appurtenance begins
at once to act the part assigned it, and the whole becomes a startling
make-up. The head, with eyes, is jerked from side to side, the jaws
gape, the legs move, the antennæ quiver, and an angry, threatening ant
starts, as by magic, into being. “When,” says Mr. Annandale, “the
caterpillar is seen in an end-on position, or when the anterior
two-thirds of the body are hidden, the resemblance is positively
startling,” so that “it is difficult to imagine how a lizard or a frog
with a previous experience of the ant could fail to be deterred.”[100]
In the light of the above cases, that of the cuckoo-bees does not seem
so very wonderful, since both the species are bees, and all or most of
the members of any group or family of animals as a rule bear some
resemblance to one another, since they descend from a common and not
very remote ancestor. Many flies, however, have almost as close a
resemblance to various bees and wasps, whilst one of the latter is even
the model for a species of cricket, which would otherwise fall a victim
to it and others of its family. There is a beetle, too, so like a wasp,
not only in its appearance, but in the way in which it runs about and
moves its antennæ, that anyone almost would be taken in. Whether, under
this disguise, it enters wasps’ nests and preys upon the larvæ, as the
bee-like _Volucella_ flies enter the nests of the humble-bees they
imitate, I do not know, nor, I think, does anyone, but this might very
well be the case. These flies, however, now I come to think of it, do
not really injure the bees. It used to be the idea that they did, but
lately it has been discovered that they are only scavengers, feeding on
all the waste products of their hosts, and even on their dead bodies
should such opportunities arise.[101] The humble-bees, on their part,
seem to appreciate these services, though we are not entitled to say
that they admit the flies into their nests on this account, since they
probably do so owing to their likeness to themselves.
Of the walking-stick insects, which are hardly to be distinguished, even
with close attention, from the grass or twigs on which they cling,
everyone has heard or read, and the caterpillars, common enough in
England, which remain motionless, projecting like a twig from its stem,
and looking just the same as one, are almost as good instances of
unconscious deception. But neither these caterpillars, nor any of the
other insects that have been mentioned, do anything, except through the
attitudes they assume, to produce their wonderful disguises. They have
nothing to do with the cutting out of the material. They do not dress up
for the part themselves. That, however, is what some caterpillars do.
There is one, for instance, in Borneo, that has a number of spines
arranged in pairs down its back, and on each of these spines it fixes
several little buds of the plant on which it is feeding, such buds, and
not the leaves of the plant, being the actual food it eats. Consequently
the caterpillar, which is quite a small one, looks like a spray of tiny
buds itself, and can hardly, by possibility, be noticed amidst its
flowery chaplet. The buds are not impaled on the spines, as might be
supposed, but are attached to them with silk, which the caterpillar
weaves for the purpose, and the whole process of the thing has been
observed by the gentleman who gives the account, and who is no less
competent a person than the curator of the Sarawak Museum. This is what
he says: “A bud would be shorn off with the mandibles, then held in the
two front pairs of legs, and covered all over with silk issuing from the
mouth of the caterpillar. The caterpillar then twisted the front part of
its body round, and attached with silk the bud to one of the spinous
processes, and another bud would then be attached to this, and so on
until a sufficiently long string—generally three or four buds—was made,
when operations on another spine would be commenced. The caterpillar fed
on these buds, scooping out the interior, and when not hurried, using
the empty shells in preference to whole buds for its covering. When
irritated it curled up, and remained thus for fifteen or twenty minutes.
At other times it would sway about, looking like a branchlet blown by
the breeze.”[102]
In time this caterpillar made “a silk cocoon covered with buds,” but it
never turned into a butterfly, for ants attacked it, and its life was
nipped in the bud. It appears to be a very rare caterpillar, and nobody
knows what butterfly it belongs to, or what is its full Latin name.
Since it is a _Geometer_, however, why not _Geometer ignota_ under a
sketch (as given in _Nature_, June 25th, 1903), in the cabinet—which
would, in all cases, be the better plan?
I really do not know whether this or another caterpillar of South
America be the more extraordinary, for if the one makes itself like
something, the other makes something like itself. _Anæa_ (_sp?_)—I give
the name as I find it—is a little green caterpillar having a very funny
nondescript sort of shape—as much like a little piece of gnawed-out
leaf, left hanging to the midrib, as anything else. Such an object,
however, is not one of the common ones of nature, and if it stood alone
might be unrecognised or misinterpreted. The caterpillar, therefore,
feeding along the midrib of the leaf, gnaws out a number of such little
pieces, more or less like itself, and leaves them sticking upright along
it, attached by a point or two. All the rest of the leaf at that part of
the midrib, it apparently eats, or bites away, so that there remains
only the slender, bare stalk, with several bits of leaf upon it, one of
which is the caterpillar. To say which bit is he is now very difficult,
and it looks as if none of them were. This caterpillar is, of course,
green, like the leaf he feeds on, but he is not the same colour all
over. He is light above and dark below, and this exactly suits—I have it
on authority—the chiaroscuro of the situation, so that, both in light
and shadow, he looks for all the world like a little elongated bit of
green leaf attached to the midrib by a couple of stalks.[103] One would
say, “Some caterpillars must have been eating that leaf”; but one would
never think the caterpillar that had been eating it was still there.
-----
Footnote 91:
Bates, The Naturalist on the River Amazon (second edition, 1864), p.
420.
Footnote 92:
Belt, The Naturalist in Nicaragua, pp. 19, 20.
Footnote 93:
Proc. Zool. Society for 1891, pp. 462, 463.
Footnote 94:
Proc. Zool. Society for 1900, pp. 837-69.
Footnote 95:
Fabre, Insect Life, p. 165.
Footnote 96:
Proc. Zool. Society for 1900, pp. 837-69.
Footnote 97:
There are two British species of Elephant Hawk-Moth, the large and
small. The caterpillar of the former has four false eyes, that of the
latter only two.
Footnote 98:
Poulton, The Colour of Animals, pp. 258-61.
Footnote 99:
Leland O. Howard, The Insect Book.
Footnote 100:
Proc. Zool. Society for 1900, pp. 837-69.
Footnote 101:
Leland O. Howard, The Insect Book.
Footnote 102:
Nature, June 25th, 1903.
Footnote 103:
Poulton, The Colour of Animals, pp. 258-61.
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