caterpillar removed from a jumping bean. The figures are a little larger
than life-size, as is shown by the line drawn near the caterpillar
giving its actual length. The shape of the “beans,” as segments of a
tripartite sphere, is seen. One shows a round hole, with a lid-like
piece marked _a_, removed from the hole. This hole did not exist when
the bean first came into my possession in November 1908. At that time
the caterpillar within was active, and the bean or fruit-segment often
jumped. In April the caterpillar cut this round hole from within,
leaving the circular lid in place, and became a chrysalis. The lid was
pushed out, as shown in the drawing, by the moth when it escaped from
the chrysalis in July. (Drawn from nature for this work.)]
A curious product of the relationship of an insect and a plant is the
so-called “jumping bean,” which is brought to this country from Mexico,
and may be purchased in some of the London shops which deal in
“miscellaneous” articles. They have been known for some years, but are
becoming now a regular article of commerce. As one buys them (Fig. 52)
they are segments of a globular fruit which has divided into three,
comparable to the familiar segments of an orange, but less numerous.
They are about one-third of an inch long, light, quite dry, and
apparently hollow, without any visible opening. Two sides of the little
capsule are flat, and the third side is bulged and rounded, so that the
capsule easily rocks when resting on that side. When these dry fruits or
segments of a fruit are brought into a warm room or placed near a fire
so as to make them as warm as the hand, they commence to rock and move
with curious little jerks. They jump as much as one-eighth of an inch
from the ground, and advance as much as a quarter of an inch at a time,
though by rolling they may progress a good deal more. They will often
move seven or eight times in the same direction so as to make a progress
of a couple of inches on a flat surface, and I have found that if a cool
surface or protection from warmth is within reach they will in the
course of time arrive at that cool area and come to rest. When the plate
on which they are placed becomes cool or the temperature of the room
falls to what we should call “chilly,” they cease to move, but can be
roused again by renewed warmth.
How and why do these “beans,” or, rather, fruit-segments (for they are
not beans), move in this determined purposeful manner? The whole
proceeding has a mysterious and uncanny aspect. They have no legs, no
spring; they are simple little smooth capsules, and yet they jump and
seemingly “walk” about. The explanation is that there is a grub inside
each so-called “bean.” Cut one of the beans or capsules open, and you
find that it is a very thin-walled and hollow case, but coiled on itself
in the cavity you open, and about half filling it, is a yellowish white
grub (Figs. 52 and 53). It is not a “maggot,” but a “caterpillar,” that
is to say, it is not legless, but has eight pairs of legs—namely, three
pairs of short walking legs in front, four pairs of sucker-like legs,
and a hinder pair of larger size called “claspers.” It has a hard brown
plate on its head, and possesses hard jaws. It refuses to leave the
opened capsule, and crawls back again if forcibly removed, and in the
course of a few hours spins a silken cover to replace the piece of
“shell” you have cut away. Mr. Rollo has lately succeeded in getting the
caterpillar to patch up its injured residence with a thin piece of
glass, such as is used by microscopists, which he put in place of a side
of the capsule removed by a knife. He was thus able subsequently to
watch through the glass the movements of the little creature when it
causes the mended capsule or “bean” to jump. It rears itself from the
lower surface of the capsule, and gives a series of sharp blows to the
roof, projecting its body with each blow, and thus overbalances the
capsule, or, if the flat side is lying downwards, jerks it along much as
one may sit with one’s feet on the rail of a chair and cause it to jerk
along the floor by the swinging movements of the body. The caterpillar
does not die at once when removed from the capsule; it has been kept
alive in a glass tube for a month.
[Illustration: FIG. 53.—The caterpillar of the moth, _Carpocapsa
saltitans_, removed from the jumping bean: magnified three diameters.
Observe the jaws (with which the circular plate is cut in the bean
before the grub becomes a chrysalis), eyes, three pairs of pointed legs,
four sucker legs placed in the middle region, and followed by three
segments without legs, and a terminal segment with a pair of suckers.
(Drawn from nature for this work.)]
So far so good. The next questions are: What Mexican plant is it that
forms the capsule or tripartite fruit in which the caterpillar is found?
How did the caterpillar get there? What kind of an insect does it turn
into, and when? I will answer the last question first. The caterpillar
turns into a chrysalis in the early part of the year, having first cut a
perfectly circular ring in the shell of the capsule. The circular plate
thus within the ring is not disturbed, and cannot be observed without
very close inspection. The making of this perfectly circular cut without
removing the piece marked out must be effected by a rotation of the
caterpillar’s head and jaws as a centre-bit—an astonishing performance.
But when the moth emerges from the chrysalis, a gentle push is enough to
cause the little circular plate to fall out, and the moth creeps through
the hole to the outer world. The moth, which comes out of the
chrysalis-coat, is a very pretty little creature (see Fig. 54),
measuring two-thirds of an inch across the opened wings, which are
marked with dark and reddish-brown-coloured bands. It is a close ally of
the British codling moth, the caterpillar of which eats its way into the
core of apples, and is familiar to all growers and eaters of that fruit.
The codling moth and the Mexican “jumper” belong to a group of small
moths called _Tortricinæ_, and they are named respectively _Carpocapsa
saltitans_ (the one whose grub or caterpillar inhabits the “jumping
bean”) and _Carpocapsa pomonana_, the codling moth. There are other
British species of _Carpocapsa_, the grubs of which eat into the acorn,
the walnut, the chestnut, and the beechnut—a distinct kind or species
for each. None of these grubs cause the nuts they attack to “jump.”
[Illustration: FIG. 54.—The moth, _Carpocapsa saltitans_, which escapes
from the jumping bean or segment of the fruit of the Mexican spurge,
_Sebastiana palmeri_, in which its caterpillar and chrysalis have passed
their lives. The crossed lines indicate the natural size of the moth.
(Drawn from nature for this work.)]
The “jumping bean” of Mexico is a segment of the triply divided fruit of
a large spurge, which is called _Sebastiana palmeri_. The spurges are
known in England as little green-leaved annuals, with yellow-green
flowers and a milky juice. Botanists call them the _Euphorbiaceæ_, and
in that “natural order” are included the boxwood tree and some tropical
trees of great value and importance. None other than the Brazilian
indiarubber tree, Hevea, of which we hear so much nowadays, its rubber
to the value of £14,000,000 being exported every year from Brazil, is
one of them. So also is the Chinese candle-tree, which furnishes a
tallow-like fat, made into candles in China. Others are the croton oil
and the castor oil shrubs, natives of India, and the manihot or tapioca
plant. The fruits of _Sebastiana_ (the jumping bean) are very much like
those of the croton; and as there are crotons (though not the one of the
purgative oil) in abundance in Mexico, it has taken some time to make
sure that the “jumping bean” is not the fruit of a croton, but that of
the allied plant _Sebastiana_. It appears that there is no commercial
value for this plant, and that those capsules which happen to contain a
grub and move are collected from the ground by the native Mexican boys
and sold as curiosities.
The moth (_Carpocapsa saltitans_) lays its eggs on the Sebastian shrub,
and the young grub, on hatching, eats its way into the young fruit when
the latter is still quite soft and the seed unformed, and so leaves no
hole to mark its entrance. As the fruit swells the grub eats out the
seed and surrounding pulp of the segment of the fruit into which it
entered early in life. By the time the fruits are dry and fall to the
ground the caterpillar is fully grown. Of course, it is only a very few
of the capsules which are thus invaded by a grub.
The question very naturally arises, “Why should the caterpillar put
itself to the great muscular effort of making the little capsule in
which it is contained jump and move over the ground?” It seems probable
that these movements are made in order to bring the capsule from an
exposed position when it falls on to the ground—where it might be
crushed or eaten by some animal—into a position of shelter, either into
a hole, or under some stone or fallen wood. The warmth of the sun in an
exposed position excites the caterpillar to activity, which ceases when
it has reached the shade offered by some protecting cranny. In the same
way I have applied artificial heat and, alternatively, shelter from
heat, so as to cause the movements or the resting of the jumping bean in
a London sitting-room.
These things and others of absorbing interest may be seen in the truly
wonderful museum of Kew Gardens, where perhaps the visitor will be
disposed to spend more time in cold weather than in the summer. The park
at Kew Gardens, with its splendid forest and lakes, and its Italian
tower, is one of the beautiful things of England, and it has a special
quality even in this season of mist and veiled sunshine. I found there
recently, under the trees, as I did fifty years ago, a rare and
strange-looking fungus, the _Phallus impudicus_ of botanists,—a furtive
denizen of the glades which in late spring are purple with wild
hyacinths. The same spot in June presents within a few minutes’ journey
from the smoke and smell and noise of Piccadilly a perfect sample of
what is, perhaps, the most beautiful sight in Nature—bright sunlight
breaking through the young green leaves of a forest on to green herbage.
And close by are the azaleas!
XXXIII
PROTECTIVE COLOURING IN ANIMALS
Every one is familiar with some of the instances in which the natural
colour of an animal helps to hide it from view. Green caterpillars, for
instance, are less visible when among the green leaves which they eat
than they would be were they brown, blue, red, yellow, or black. The
little green tree-frog is difficult to see when he is clinging to a
leaf, because his colour is the same as that of the leaf.
Sandy-brown-coloured animals, birds, reptiles, and beasts of prey, are
found on the sands of the desert; white birds, foxes, hares, and bears
on the Arctic snow. The similarity of the colouring of these animals to
that of the ground on which they live results in their escaping the
observation of man’s eye, and we are entitled to believe that they
escape for the same reason the observation of other animals. They are
thus in many cases protected from the attacks of enemies searching for
them as prey, or in other cases they may themselves be enabled the more
easily in consequence of their concealing colour to creep upon other
animals and seize them as food. Some of the simpler cases of this
resemblance between an animal and its surroundings are easy to observe,
and the value of the resemblance as protection, or as a means of secret
attack, is plain enough.
But there are far more numerous cases in which the significance of
colour as concealment, is not so immediately obvious. There are the
curious stick insects, with long bodies and delicate long legs,
sometimes with bud-like knobs on the body which look like bits of the
branches of trees, not merely on account of their colour, but on account
of their shape. Shape or modelling has a great deal to do with the
effective concealment of an animal. Then, too, there is the curious fact
that some insects (and also some birds) when at rest on the stems of
trees, are practically invisible, but if they spread their wings are
conspicuous. The beech-leaf butterfly of Assam and Africa is of a purple
colour, marked with a great orange-coloured bar on each fore-wing when
the wings are open, and it is obvious enough. But when the wings are
closed and the insect is at rest, the undersides only are seen, and are
coloured so as to represent the veining and fungus marks of a dry brown
leaf, so that not even a human observer, let alone a bird or a lizard,
can distinguish at two-feet distance the butterfly from dried leaves
placed near it.
A well-known little moth, with pale green mottled wings, is the only
case in which I have myself watched the protection afforded by colour at
work. It was on a summer’s evening, when I saw this little moth
zigzagging up and down with the most extraordinarily irregular flight,
and a bird pursuing it. Twice the bird swooped and just missed his prey
owing to a sudden turn and drop on the part of the moth. And then to my
great delight the moth flopped against the stem of a tree on which was
growing a greenish-grey lichen. The bird swooped again close to the
tree, but failed to see the insect, and quitted the chase. It took me an
appreciable time to detect the little moth resting against the lichen,
and closely matching it in colour. There are endless examples known of
such “protective resemblances,” some of them (such as that of the
buff-tip moth, which, with its wings closed, looks like a broken birch
twig) being most unexpected and fascinating. In the forests of
Madagascar, the whitish-grey tree lichens are imitated by thread-like
growths on beetles, tree-bugs, locusts, and even lizards, with a
wonderful concealing effect, and some other flat membrane-like insects
are so much like the greenish and yellowish bark of trees, that we
actually lost a specimen for some time in the case labelled “Mimicry,”
in which a series of these things was arranged by me for the edification
of visitors to the Natural History Museum. It was found, after a day or
two, to have been present all the time with other specimens on a piece
of bark, from which it was indistinguishable.
Some eight years ago a distinguished American painter, Mr. Abbott
Thayer, was able to add very importantly to our knowledge of the ways in
which colour serves to conceal animals when in their natural
surroundings. Mr. Thayer was able to do this owing to the fact that he
was a devoted student of woodland life. This, however, alone was not
enough. Mr. Thayer had the special ability to deal with this subject
which comes from the trained eye of an artist. He had, above all, the
knowledge of “tone values” and of the illusive and delusive effects of
false shading and of colour-spots and bars, and of complementary colours
and “irradiation”—which only a painter who deals every day in the most
practical way with these matters can attain to. Mr. Thayer showed eight
years ago—and demonstrated conclusively by means of models, one of
which he presented to the Natural History Museum at my request—that in
very many cases it is of no use for an animal to be of the same colour
as its surroundings, since if the animal (a bird, or a quadruped, or a
fish) is of plump and rounded shape and is observed under the open
canopy of heaven, a deep shadow will exist on its lower surface and make
it as obvious as a shaded charcoal drawing on a piece of light-brown
paper. But if the back of the animal is of a dark tint and its belly
white or whitish, then the effect of light and shade is (Mr. Thayer
showed) completely counteracted and the animal becomes totally invisible
in its natural surroundings.
Mr. Thayer’s model demonstrating this consists of two life-size wooden
models of ducks seated on a stick—one to the left, the other to the
right. The stick, with the two models on it, is fixed horizontally in a
box, which is open above (that is, has no lid) and is also open in
front. The box is, in fact, a little stage, lit from above by the light
of the sky, and its three remaining sides are sufficiently high to form
a complete background to the model ducks, whose perch runs across the
“scene” at some 7 in. or 8 in. from the floor of the box. The box itself
is lined with a pale purplish-brown flannel, and each bird is tightly
covered with the same material. When so prepared the box is placed on a
table under a skylight (where it is to stay), the table being high
enough to bring the ducks just below the line of sight. Of course, deep
shadows are formed by the top-light on the under side of the beak, head,
and body of the models, and in spite of their colour being itself
identical with that of the walls of the box, they are as obvious as it
is possible for anything to be. Now Mr. Thayer takes his paints and very
carefully darkens the back of one of the ducks and whitens its belly and
the under side of its head and beak. The light and dark regions merge
into one another along the side of the bird by skilful gradation. When
this shading and whitening is finished (and, of course, the perfection
of the result depends on the continuance of the right amount of
sunlight, which is not a thing one can always ensure in a London
museum) the duck-model so treated is absolutely invisible at a distance
of 10 ft. or 15 ft.—and even when one is nearer escapes notice—looking
like a haze or vague shadow of a bird even to an observer who knows
nevertheless that it is there and is really as solid and large as the
untreated model by its side. If now some one stretches out his hand so
as to cut off the top-light falling on the painted model, it immediately
becomes as solid to the eye as the untreated one, and when the hand is
withdrawn it melts away again like Banquo’s ghost. The models made by
Mr. Thayer were, so long as I was director, exhibited in the small room
between the fish gallery and the central hall of the Natural History
Museum, and, if they have not yet been removed, are well worth a visit.
Mr. Thayer’s models work perfectly, and astonish every one who sees
them. The great point of interest about them, however, is, that the bird
with dark back and light belly is really in the condition which is quite
common in a number of birds, especially ducks and wading birds, where it
must act as a means of concealing the bird—just as it does in the
painted model. Of course, there are vast numbers of birds not so shaded,
but it is possible to explain the darker and lighter colouring, in
various arrangements seen in birds, as helping to produce concealment or
disappearance from view, when the habits and natural surroundings of the
bird are known. So, too, with many hairy quadrupeds (mammals, or
“animals,” or “beasts,” as they are often called). The white hair under
the tail and about the rump, helps a running animal to escape the vision
of its pursuer—blending, as Mr. Thayer shows that it does—with the
white colour of the sky-line. In the case of fish—especially
fresh-water fish—the dark back and light belly are very common, and
although they do not help to conceal the fish when seen from above,
swimming over a light-coloured river-bed, yet when looked at by other
fishes or by otters in the water, the effect of the light from above on
this disposition of dark and light tints on the fish’s body must be the
same as that demonstrated by Mr. Thayer’s “disappearing duck,” and must
often render the fish absolutely invisible, even at close quarters.
Mr. Thayer has pursued this subject during the past seven years, and
last autumn he gave some interesting demonstrations in the Zoological
Gardens in London. He showed a model of a white egret, which was but
little noticeable when standing up clear against a bright, white-clouded
sky. The long plumes on the wings, developed in the breeding season,
were shown (by putting them on and taking them off) to assist in causing
invisibility, since they made the side of the body flat and concealed
the shadow on its rounded underside. A similar bird-model marked with
strong black on the neck and legs—the rest being white—refused (so to
speak) to shape itself as a bird at all, and looked at a distance of
twenty yards like a bit of rock or stump of wood with a twig and dead
leaf attached. The effect of different tones of brown cardboard cut into
the form of a butterfly, when seen on different backgrounds, was shown;
but the most interesting experiment was made with a black-green piece of
cloth cut to the shape of a butterfly and fastened on to a sheet of
dead-black cloth in the open air, in the presence of white cloud light
of moderate brilliancy. At five yards one could see the outline of the
dark-green butterfly-shaped piece; at fifteen yards one could just
distinguish the edge separating the dark-green piece from the black
cloth. Now Mr. Thayer stuck in the middle of the dark-green
butterfly-wing a small circle of pure white (about one-third of an inch
across). The effect was entirely to obliterate the previously visible
edge; one could no longer see the dark-green area at all—one only saw
a white spot on a continuous dark ground, the dark-green and the black
were merged into one. That is no doubt due to the powerful stimulation
of the sensitive “retina” of the eye by the white light of the spot; the
feeble stimulation by the dark-green and black, though these remain
physically as distinct from one another as before, ceases to affect the
brain, which is, as it were, entirely occupied with the strong white
spot. This, according to Mr. Thayer, is the value to butterflies and
other animals of a violently contrasted white spot or band on a dark
general colouring. The fringe of white dots and connected white flakes
nearer the centre of the wing—common on the wings of butterflies—has,
similarly, the result of rendering the wing-outline imperceptible and
the butterfly invisible. Many such relations of colour spots and bands,
as well as of dark and light markings, have been elucidated by Mr.
Thayer, and will be illustrated by coloured drawings in the book which
he is preparing on the subject.
While it is the fact that Mr. Thayer has thrown new light on the
colour-protection and invisibility of animals, it must be remembered
that there are other explanations of certain cases of brilliant
colouring in animals besides that which he has so well illustrated.
“Warning” colours, recognition marks, and sexually attractive colouring
all certainly and demonstrably exist in well-known and well-studied
kinds of animals. It is very possible that some of these colour-markings
have been produced by a slight change in what were previously
“concealing” patterns or colour-markings. The tendency of the human
observer is to regard any colour, spot, or pattern on a bird, fish,
beast, or insect as a “mark” or distinguishing “sign.” We examine these
things at close quarters, and do not, unless we reflect a good deal on
the matter and experiment with the object, realise that what is a mark
of distinction or recognition when seen at a few inches’ distance may be
an illusive and obscuring colour-scheme when seen at a distance of some
feet, and in natural and habitual surroundings. It is not unlikely that
we shall arrive at definite knowledge of the psychological “sight
interpretations” of animals by a further study of this subject. It is in
the highest degree probable that the retinal picture produced in an
animal’s eye by certain spots of colour, shade, and light exhibited by
another animal, are not interpreted by the receptive animal in the same
way as they would be by a scrutinising, inquiring, reasoning man, even
one who is what we call a “savage.” Moreover, though many English
naturalists have travelled and seen “life and light” in the sunny
regions of the earth, there are few students of the colour-markings of
animals in our museums, especially in great cities, who have adequate
experience of what colour-markings really can effect in the way of
concealment and illusion when light and surrounding objects are as they
are, in the tropics or sub-tropical regions. It is a fashion nowadays in
the best-provided museums of natural history to exhibit stuffed beasts,
birds, and insects in what are called “their natural surroundings.” The
fatal objection to such exhibitions is that were the beasts, birds, and
insects placed in their most usual “natural surroundings,” they would be
invisible!