excluded when we turn our attention to more complicated cases of
colour-adaptation, which are not confined to the simple, general
coloration, but are helped by markings and colour-patterns, that is, by
schemes of colour.
Thus numerous caterpillars exhibit definite lines and spots on their
ground-colouring, which, in one way or another, aid in protecting them
from their enemies.
The green grass-eating caterpillar of many of our _Satyridæ_ has two or
more darker or lighter lines running down the sides of its body, which
make it much less conspicuous among the grasses on which it feeds than
if it were a uniform green mass (Fig. 2). Not infrequently the colour
and form present a remarkably close resemblance to the inflorescences
or fruit-ears of the grasses. Caterpillars marked thus are never found
on the leaves of trees, where they would immediately catch the eye. It
is true that longitudinal striping often occurs on caterpillars which
live on other plants besides grass, but as these other plants grow
among the grasses the protective efficacy is just the same. This is
the case with the Pieridæ (Garden Whites).
All the caterpillars of our Sphingidæ, on the other hand, which live
on bushes and trees, have on the sides of the segments light oblique
stripes, seven in number, which are disposed to the longitudinal axis
of the body at the same angle as the lateral veins of a leaf of their
food-plant have to the mid-rib. It cannot of course be said that the
caterpillar thereby gains the appearance of a leaf, indeed, if one sees
it apart from its food-plant it does not look in the least like a leaf,
but among the leaves of a bush or tree this marking secures it in a
high degree from discovery. Thus the caterpillar of the eyed hawk-moth
(_Smerinthus ocellatus_), when it is sitting among the crowded foliage
of a willow, is often very difficult to find, because its large green
body does not appear as a single green spot, but is divided by the
oblique lateral stripes into sections like the half of a willow leaf,
so that even a searching glance is led astray, there being nothing to
focus attention on the animal as distinguished from its surroundings
(Fig. 3). As a boy I often had the interesting experience of
overlooking a caterpillar which was sitting just before me, until after
a time I chanced to hit upon the exact spot in the field of vision.
[Illustration: FIG. 2. Longitudinally striped caterpillar of a Satyrid.
After Rösel.]
[Illustration: FIG. 3. Full-grown caterpillar of the Eyed Hawk-moth,
_Smerinthus ocellatus_. _sb_, the subdorsal stripe.]
In the majority of these caterpillars with oblique stripes, the
likeness to the half of a leaf is heightened by the fact that the light
oblique row is accompanied by a broader coloured band, suggesting the
shade of the leaf's mid-rib. The caterpillar of _Sphinx ligustri_
has a lilac band, and that of _Sphinx atropos_ a blue one. In both
cases it is difficult to believe that such striking colours can
secure the animals from discovery, yet among the blending shadows
of the leaf-complex of their food-plant they greatly increase their
resemblance to a leaf-surface. Of the death's-head caterpillar
(_Sphinx atropos_) this sounds almost incredible, for this form is
chiefly a bright golden yellow, and the narrow white oblique stripes
have sky-blue borders becoming darker towards the under side; but it
must not be forgotten that the potato is not the true food-plant of
the species, for it lives, in its true home in Africa, and also in the
south of Spain, on wild solanaceous plants, which, we are informed
by Noll, have precisely these colours--golden-yellow and blue in the
blossom, the fruit, and in part also in the leaves and stem. There the
caterpillars sit the whole day long on the plants, while with us they
have formed the habit of feeding only in the twilight and at night, and
concealing themselves in the earth by day, a habit that is found in
other caterpillars also, and which we must again ascribe to a process
of natural selection.
[Illustration: FIG. 4. Full-grown caterpillar of the Elephant Hawk-moth
(_Chærocampa elpenor_) in its "terrifying attitude."]
Some caterpillars exhibit other, more complex markings, which do not
protect them by rendering them difficult to detect, but by terrifying
the enemy who has discovered them, and warning him away. Such
terrifying or aggressive colours are to be found, for instance, in the
caterpillars of the Sphingid genus _Chærocampa_ in the form of large
eye-like spots, which occur in pairs close together on the fourth
and fifth segments of the animal. Children and those unfamiliar with
animals take these for true eyes; and as the caterpillar, when it is
threatened by an enemy, draws in the head and anterior segments, so
that the fourth one is greatly distended, the eye-spots seem to stand
on a thick head (Fig. 4), and it cannot be wondered at that the smaller
birds, lizards, and other enemies are so terrified that they refrain
from attacking. Even hens hesitate to seize such a caterpillar in its
defiant attitude, and I once looked on for a long time in a hen-coop
while one hen after another rushed to pick up a caterpillar I had
placed there, but, when close to it, hastily drew back the head already
prepared to strike. Even a gallant cock was a long time in making
up his mind to attack the terrible beast, and drew back repeatedly
before he at length ventured to strike a deadly blow with his bill.
After the first stroke the caterpillar, of course, was lost. Thus even
this disguise is only a _relative_ protection, effective only against
smaller enemies. But that these are really frightened away, I had once
an opportunity of observing, when I put a caterpillar of the common
elephant hawk-moth (_Chærocampa elpenor_) in the feeding-trough of a
hencoop, and a sparrow flew down to feed from the trough. It descended
at first with its back to the caterpillar and fed cheerily. But when by
chance it turned round, and spied the caterpillar, it scurried hastily
away.
Among Lepidoptera, too, eye-spots often occur on the wings, and to
some extent, at least, they have in this case also the significance of
warning marks. Take, for instance, the large blue and black eye-spots
on the posterior wings of the eyed hawk-moth (_Smerinthus ocellatus_).
When the insect is sitting quietly the two spots are not visible, as
they are covered by the anterior wings, but as soon as the creature is
alarmed it spreads all four wings, and now both eyes stand boldly out
on the red posterior wings and alarm the assailant, as they give the
impression of the head of a much larger animal (see Fig. 5). There are
also eye-like spots which have not this significance and effect, as,
for instance, the 'eye-spots' on the train-feathers of the peacock and
the Argus pheasant, or the little eye-like spots on the under surface
of many diurnal butterflies. In the first case, it is a matter of
decoration; in the second, perhaps of the mimicry of dewdrops, which
increases still further the resemblance to a withered leaf; but there
are undoubtedly many cases in which the eye-spots serve as means of
frightening off enemies, and these cases are especially common among
butterflies.
[Illustration: FIG. 5. The Eyed Hawk-moth in its 'terrifying attitude.']
Such warning marks are in no way contradictory to the sympathetic
colouring of the rest of the body, and indeed we usually find them
in combination with it. In some cases the eye-spot, though very
conspicuous, is covered, as in the eyed hawk-moth, when at rest, by
the sympathetically coloured parts--in this instance the anterior
wings. In other cases eye-spots of considerable size lie clearly
exposed, but exhibit the same sympathetic colours as the whole of the
rest of the wing-surface. In this case they do not interfere with
the protective influence of general colouring, because they are only
visible from a very short distance. This is the case in the large
_Caligo_ species of South America, which only fly for a short time in
the early morning and in the evening, remaining concealed throughout
the day in dark shadowy places, where the mingled colouring of brown,
grey, yellow, and black on the under surfaces of the wings prevents
their being recognized from a distance as butterflies at all. But even
the best sympathetic colouring is not an absolute protection, and when
the insect is discovered by an enemy near at hand, the terrifying mark,
a large deep-black spot on the posterior wing, comes into effect, and
scares the assailant away.
[Illustration: FIG. 6. Under surface of the wings of _Caligo_.]
In such cases the sympathetic colouring was probably the first to
arise, and the eye-spot was developed later by a new process of
selection, brought about by the necessity of protecting the species
more effectively than by mere inconspicuousness alone. In many cases
it can be proved that the power of scaring off an enemy did not begin
with the formation of the eye-spot, but with the development of a new
instinct. When the caterpillar of _Chærocampa elpenor_ is attacked
it immediately assumes the defiant attitude described above, but the
same striking attitude is assumed by the caterpillars of the allied
American genus _Darapsa_, as I learn from an old illustration by Abbot
and Smith, although this form possesses no eye-spots (Fig. 7). Thus,
then, metaphorically speaking, the caterpillar at first attempted
to scare off its enemy by a terrifying attitude alone, and it was
only subsequently, in the course of the phyletic evolution, that the
eye-spots were added, in the elephant hawk-moths and other species,
to heighten the terrifying effect. But that the eye-spot did not make
its appearance suddenly is proved by several American species of
_Smerinthus_, in which they are much less perfectly developed than in
the European species. In these Sphingidæ, too, the defiant attitude
was evolved earlier than the eye-spots, as we may see from our poplar
hawk-moth (_Smerinthus populi_), which, when alarmed, spreads out all
four wings in the same peculiar manner which in the eyed hawk-moth
(_Smerinthus ocellatus_) displays the eye-spots; it strikes about with
its wings as if to scare off the enemy, an effect which will certainly
be more surely achieved if, at the same time, a pair of eyes becomes
suddenly visible.
Sympathetically coloured caterpillars are, however, by no means the
only ones; there are some with such striking, glaring colours that,
far from rendering their possessors inconspicuous, they make them
visible from a long way off; but this apparent contradiction of the
theory of the colour-adaptation of animals that require protection
has been explained by the acuteness of Alfred Russel Wallace. We know
that among insects, and also among caterpillars, there are many which
have a repulsive taste. In any case, certain caterpillars are rejected
by many birds and lizards. Such species are, therefore, relatively
safe from being devoured. If they were protectively coloured, or if,
moreover, they resembled caterpillars with an agreeable taste, they
would gain little advantage from their unpalatability; for the birds
would at first take them for eatable, and would only discover their
repulsiveness on attempting to eat them. But a caterpillar which has
received a single stroke from a bird's bill is doomed to death. It must
therefore be of the greatest advantage for unpalatable caterpillars,
and unpalatable animals generally, to be in their colouring as
conspicuously distinguishable as possible from the edible species.
Hence, then, the glaring colours, which we can now refer without any
further difficulty to the process of natural selection, for every
individual of an ill-tasting species that is more conspicuously
coloured than its fellows must have an advantage over them, and must
have a better chance of surviving, because it will be less easily
mistaken for a member of an edible species.
[Illustration: FIG. 7. Caterpillar of a North American _Darapsa_ in its
"terrifying attitude" (after Abbot and Smith).]
I should like to discuss one other phenomenon, which is well calculated
to give us a deeper insight into the transformation processes of
organisms--I refer to the remarkable dimorphism of colour which occurs
in many of the species of caterpillar just described.
The caterpillar of the convolvulus hawk-moth (_Sphinx convolvuli_) is
in its full-grown stage green, like the wild convolvulus on which it
lives, or brown like the ground on which its food-plant grows. It thus
shows a double adaptation, each of which is capable of protecting it
to a certain extent, and we might think to the _same_ extent. But that
is not so, the brown colouring is a more effective protection than the
green, as we may learn from two facts. In the first place, the four
young stages of the caterpillar are green, and it only becomes brown in
the last stage, though sometimes even then it remains green. This shows
that the brown is a relatively modern adaptation, and it could not have
arisen had it not been better than the original green. In the second
place, the green-coloured caterpillars of the convolvulus hawk-moth
are nowadays much less numerous than the brown ones, and this implies
that the latter survive oftener in the struggle for existence. We have
here an interesting case of an easily recognizable process of selection
still going on between the old green and the newer brown variety.
It is hardly necessary to ask why the brown colour should in this case
be a better protection than the green, for it is obvious that such a
large green body as that of the full-grown convolvulus-caterpillar
would be but badly concealed among the little leaves of the convolvulus
plant in spite of its green colour; while the brown caterpillar, on
the brown soil, with its pebbles, hollows, and irregular shadows, is
excellently protected, especially if it passes the day concealed in the
ground, as is actually the case.
Our view is materially strengthened by the fact that the same
phenomenon of double colouring occurs in several allied species of
Sphingidæ, but in a manner which shows us that we have to do with a
similar process of transformation, only at a more advanced stage. The
caterpillar of _Chærocampa elpenor_ (Fig. 4) shows the same state of
things as that of the convolvulus hawk-moth; it is brown or green,
and the green form is the less common. But in the two other European
species of _Chærocampa_ the full-grown caterpillar is always brown,
and indeed it becomes brown in the fourth stage, instead of, like
_Chærocampa elpenor_, only in the fifth and last. Another indigenous
sphingid species, _Deilephila vespertilio_, only remains green during
the first two stages, and assumes in the third stage the grey-brown
colour which it afterwards retains. The dark colour has obviously
prevailed among the full-grown caterpillars for a considerable
length of time, for it is in this, the largest and most conspicuous
stage, that the change of colour must have been most necessary, and
consequently the process of selection must have begun in it, and only
after the more protective brown became general would it have extended
to the next stage below, if it were of use there too, and, later on,
to still earlier stages in the life-history.
One might be inclined to ascribe this shunting back of a new character
from the later to the earlier stages of development to purely internal
forces, which brought it about of necessity, and quite independently
of whether the extension of the character was useful or injurious.
We shall come back to this later, and try to find out how far this
is the case, but in the meantime we may regard at least so much as
established, that this shunting back does not take place everywhere and
without limits, but that natural selection calls a halt as soon as its
effect would be injurious.
[Illustration: FIG. 8. Caterpillar of the Buckthorn Hawk-moth,
_Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_, ring-spots.]
There could be no continuance of insect-metamorphosis if every
character of the final stage had to be shunted back to the one next
below, for then, for instance, the characters of the butterfly must,
in the course of the phyletic evolution, be carried back to the pupa
and larva. But even in the larval stage alone it can be seen that
this carrying back is kept within well-defined limits. Thus, for
instance, in the dimorphic caterpillars of the Sphingidæ the brown of
the full-grown stage never comes so far down as the earliest stages,
for the little caterpillars are all green, like the leaves and stems
on which they sit. On the other hand, there are species in which the
green persists, as apparently the most advantageous colour. Thus in
the buckthorn hawk-moth (_Deilephila hippophaës_) (Fig. 8), which
lives in the warm valleys of the Alps, and especially in Valais, the
caterpillars are grey-green in all stages, and are exactly of the
shade of the lower surface of the buckthorn leaves; they possess no
oblique lines, for these would not make them more like the leaves, as
the full-grown caterpillars are much bigger than an individual leaf
of buckthorn, on which, moreover, the lateral veins are not very
conspicuous. Nevertheless the caterpillar enjoys very fair security, as
it does not feed through the day, but only in twilight and at night;
it passes the daytime concealed in the dry leaves and earth about the
base of the bush. Its resemblance to the leaves is very great, and is
increased by the fact that it bears on the last segment a comparatively
large orange-coloured spot (_r_), exactly the colour of the buckthorn
berry, which ripens just at the time that the caterpillar attains its
full growth.
But butterflies are as much persecuted, and have as much need of
protection, as caterpillars, and among them, too, we find many
instances of protective colouring, which are the more interesting in
that they occur, as a rule, only on such parts of the body as remain
visible when the insect is at rest, which is exactly what we should
expect if the coloration has been wrought out in the course of natural
selection. But it is well known that the resting position of diurnal
Lepidoptera is quite different from that of the nocturnal forms, and is
not even the same among all families, and in accordance with this we
find the sympathetic colouring occurs on quite different areas in the
different families.
The reason why the butterflies only require to be protected by their
colour in the sleeping or resting position is that no colour whatever
could make a flying butterfly invisible to its enemies, because the
background against which its body shows is continually changing during
its flight, and, moreover, the movement alone is enough to betray it,
even if it is of a dull colour.
Thus, in general, only those parts of a butterfly's wing that are
invisible at rest could safely bear bright or conspicuous colour, while
the visible portions had to acquire sympathetic coloration through
natural selection.
As the diurnal butterflies, when at rest, turn their wings upward and
bring them together, it is only the under side which is sympathetically
coloured, and that only as far as it is visible, that is, the whole of
the posterior wing, and as much of the anterior one as is not covered
by it. Many diurnal butterflies, when at rest, fold the anterior wing
so far back that only its tip remains visible, and in such cases only
this tip is protectively coloured, while in other forms, which have not
this habit, almost the whole surface of the wing is sympathetically
coloured.
A very simple protective colouring is exhibited by our 'lemon
butterfly' (_Rhodocera rhamni_), in which the under surface is a
whitish yellow, which protects the insect well when it settles on
the dry leaves on the ground in the light woods which it is fond of
frequenting.
Our gayest diurnal butterflies, the species of _Vanessa_, all have the
under surface of a dusky colour, sometimes passing into a blackish
brown, as in the peacock-butterfly, _Vanessa_ (_v. io_), sometimes
more into greyish brown, or brown-yellow, or reddish brown. They are
never simple colours, but always consist of mixtures of different
colour-tones--indeed, there is often a complex mingling of many
colours, as grey, brown, black, white, green, blue, yellow, and red,
made up of dots, strokes, spots, and rings, into a wonderful and
very constant pattern, which, taken as a whole, has the effect of
being uniform, and harmonizes with the soil, or with the highway, on
which the species loves to settle, with much greater accuracy than a
monochrome grey or brown would do. When the 'painted lady' (_Vanessa
cardui_) settles on the ground it is hardly distinguishable from
it, and this species in particular has a preference for settling
on the ground. Other species of _Vanessa_, such as the peacock and
the Camberwell beauty (_Vanessa antiopa_), are underneath of a dark
blackish grey, or even black; when resting they press themselves into
the darkest corners and crevices, and are thus most effectively secured
from discovery.
Many diurnal Lepidoptera, on the other hand, especially the
wood-butterflies of the family Satyridæ, have the habit of resting
on the trunks of trees, as _Satyrus proserpina_ does on the great
beech-trunks of the forest clearings. These large butterflies, coloured
conspicuously on the upper surface in deep velvety black and white, are
marked on the under surface exactly to match the whitish bark of the
great beech, covered over with white, grey, blackish-brown, and yellow
spots, and the butterfly whose flight one has just been carefully
following disappears as it suddenly alights on such a tree-trunk. As I
have already stated, the protective colour only extends over as much
of the insect as is seen when it is at rest. As the anterior wings are
folded far back between the posterior ones, the protective colouring is
limited to the whole surface of the posterior wing, and the tip of the
anterior one, as far as that is visible in the resting attitude; the
protectively coloured area is somewhat sharply bounded, and it is often
of very different extent in quite nearly allied species, according
to whether the species folds the anterior wing far back or not. Thus
in our common small tortoiseshell-butterfly (_Vanessa urticæ_) the
protective area is considerably wider than in the large tortoiseshell
(_Vanessa polychloros_), much as the two resemble each other in other
details.
This harmony between the wing tips and the posterior wings is nowhere
wanting, where the under side is protectively coloured at all, but in
many cases the protective colouring spreads over almost the whole of
the anterior wings, and these are then not folded far back when at
rest, as will be seen later in the so-called leaf-butterflies.
There is one genus of diurnal butterflies which seems to contradict
the law that all the surface that is visible in the resting position
exhibits the protective coloration--the South American wood-butterflies
of the genus _Ageronia_. They have on the upper surface a very
complicated bark-like pattern of mingled grey on grey, and this
confirms the usual rule, for we know that these butterflies--a striking
exception among all the other diurnal forms--settle with outspread
wings on the trunk of a tree in exactly the same attitude as many of
the nocturnal Lepidoptera of the family of the Loopers or Geometridæ,
in which the upper surface is also deceptively like the bark of the
tree on which they rest.
[Illustration: FIG. 9. _Hebomoja glaucippe_, from India; under surface.
_A_, in flight. _B_, in resting attitude.]