which is sympathetically coloured, if protective coloration has been
developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ, the
anterior wings are grey banded with darker zigzag lines, and mottled
with many shades of black, grey, yellow, red, and even violet. As the
anterior wings cover the body and the posterior wings like a roof,
they make the resting insect very inconspicuous when it has settled
on wooden fences, trunks of trees, or even old timber. When bright
colours--red, yellow, or blue--occur in these moths it is always on
the posterior wings, which are covered when at rest. This can best be
observed in the species of the genus _Catocala_.
Let us now, however, interrupt our survey of the facts for a moment,
and let us inquire whether all the cases of protective colouring in
Lepidoptera we have considered can be referred to natural selection, or
whether it is not conceivable that other causes may have evoked them.
[Illustration: FIG. 10. _Xylina vetusta_, after Rösel. _A_, in flight.
_B_, at rest.]
The first thing to be said is that the Lamarckian principle of the
inherited effects of use and disuse cannot here be taken into account,
as the colours of the surface of the body do not exercise any active
function at all; their effect is due simply to their presence, and
it is for them quite indifferent whether and how often they have
opportunity to protect their bearers from enemies, or whether no
enemies ever chance to appear. It has frequently been suggested, too,
that these colorations are associated with the differences in the
strength of the illumination to which the different parts and surfaces
are exposed. But this again is untenable, as is proved even by the
dimorphism frequently occurring in caterpillars, for the green and
the brown individuals are exposed to precisely the same light; and
still more clearly by the sympathetic colouring, which is so exactly
defined and yet so different on the under surface of the diurnal
butterflies. Yet there are isolated cases in which it seems as if
the direct influence of the light had brought about certain striking
differences in the colouring of the parts of an insect, and I shall
describe perhaps the prettiest of these cases, to which Brunner von
Wattenwyl directed attention. It concerns one of the Orthoptera of
Australia, a Phasmid, _Tropidoderus childreni_, Grey, which has a
general colouring of leaf-green, but with singular deviations from it
on certain areas of the body. In this insect the anterior wings which
form the wing covers or elytra (Fig. 11, _V_) are so short that they
scarcely cover the half of the long abdomen. Their place is taken by
the anterior margin of the posterior wing (_H. horn_), which is hard
and horny like the elytra, and in the resting position protects the
whole abdomen. All these covering parts are grass-green, except at the
places where they overlap; on these areas they have a faded look, and
are yellowish instead of green. Brunner says of this: 'The phenomenon
gives the impression that the more brilliant colour is a character due
to daylight. If several sheets of white paper of unequal dimensions be
placed one above the other, ... and exposed to the sun, after a short
time silhouettes of the smaller sheets will appear on the larger ones,
either in a lighter or in a darker colour. Probably this "fading"
of the covered parts in the Phasmid also belongs to this "category
of photographs."' This seems convincing, but analogous phenomena in
other insects prevent our regarding the pretty comparison with the
photograph as a sufficient explanation. If it were a question of a
diurnal butterfly, such an assumption would have to be rejected on this
ground alone, that the wing colouring is developed in the pupa, and
appears perfect and unalterable as soon as the perfect insect emerges.
But in the pupa the position of the wings is exactly the reverse
of that seen in the resting attitude of a butterfly, that is, the
protectively coloured under side of the wing is not turned towards the
light but away from it. Moreover, in the pupa the anterior wings cover
the posterior ones completely, no matter what the wing position may be
later in the perfect insect. Furthermore, the thick and often darkly
coloured sheath of the pupa prevents the light having any effect, and
not a few species pass their pupal stage in such dark places--for
instance, under stones, as in the case of many 'Blues'--that the light
can hardly reach them. And if the light did exercise an influence, how
could it produce such diverse coloration as the protective colours of
diurnal butterflies, on the one side dark, even to blackness, on the
other side, yellow, reddish, and even white and pure green; and how
should the same rays of light call forth complicated colour patterns
on one and the same surface, for instance, the white, sprinkled with
green, of the Aurora butterfly (_Anthocharis cardaminis_)? Finally, we
have only to remember that numerous nocturnal Lepidoptera pass through
their pupa stage underground, although they exhibit brilliant as well
as protective colours in the most appropriate distribution, to reject
once for all the hypothesis that the influence of light plays any
decisive rôle in determining the distribution of the colours on the
wings of Lepidoptera.
But it is otherwise with _Tropidoderus_. In this case the wings grow
gradually during the slow growth of the animal, which takes place in
full light, and the wings of the young insect probably lie one above
the other, in exactly the same position, and cover the same places as
in the full-grown form; we might, therefore, from the facts of the
case, admit the possibility that the yellow of the covered portions is
due to the exclusion of light.
[Illustration: FIG. 11. _Tropidoderus childreni_, after Brunner von
Wattenwyl, in flying pose. _V_ anterior wing. _H. häut_, membranous
part of posterior wing. _H. horn_, horny portion.]
But as soon as the conditions that obtain among Lepidoptera are
also taken into consideration we recognize the insufficiency of the
interpretation suggested, for among butterflies we have precisely the
same phenomenon--sharp limitation of the protective colouring to the
parts visible in the resting position, a fact which, in the case of
the said butterflies, admits of no other interpretation than that of
natural selection. Let us therefore see if we cannot, in the case of
_Tropidoderus_, arrive at some better understanding of the phenomenon
than that implied in the theory of direct light-influence. Obviously,
the yellow parts of the animal do not require to be green, since they
are not visible in the sitting position, and the locust in flight could
not by any device be made invisible. It therefore only remains to be
explained why the yellow parts are not colourless, and why they are
not also green. We cannot at present answer with any confidence; it is
possible that the colouring matter which causes the green only becomes
green under the influence of direct sunlight, and otherwise remains
yellow; it is possible, too, that, as in Lepidoptera (see Fig. 9), the
full protective colour is only developed by natural selection in the
places which are visible in the sitting position, and that the covered
parts take on any indifferent colour, which might be readily afforded
by the metabolism of the insect. But this much is certain, that the
covered parts would be green, if that were advantageous to the survival
of the species, just as the under surface of the wings of some diurnal
butterflies is green. Had it been required, the green colour would have
resulted in the course of natural selection, just as it has resulted
in the most different parts of the most diverse insects, even in those
whose development takes place entirely removed from the influence of
light. Therein lies the difference between our interpretation and that
of Brunner von Wattenwyl: without natural selection no explanation of
this case is possible.
[Illustration: FIG. 12. _Notodonta camelina_, after Rösel. _A_, in
flight. _B_, at rest.]
Hitherto I have spoken only of the diurnal butterflies in which the
anterior wings show an extension of the protective colouring which
marks the whole surface of the posterior wings, and it was always the
tips of the anterior wings that were thus coloured. But among the
nocturnal Lepidoptera there are corresponding cases, in which a little
tip of the posterior wing forms the continuation of the protective
surface of the anterior wing. Some species of _Notodonta_ and allied
genera show in the posterior corner of the otherwise whitish posterior
wings a little grey spot, and a hair tuft which in colour, and--when it
is big enough--in marking, exactly resembles the protectively coloured
anterior wings (Fig. 12). The 'why' is at once clear, when one looks at
the insect in the resting position, for only this little corner of the
wing projects beyond the covering anterior wing. This has been regarded
as telling against natural selection, for such a little spot could not
possibly, by its colour, turn the scale as to the life or death of the
individual, and so could not be selected. But one might say the same of
the tip of the anterior wing in the diurnal forms, although there the
protective surface is larger, often much larger. But who is to decide
how large an exposed, unprotected spot must be in order to attract
the attention of an enemy on the look out for food? Or who can prove
that the best and most familiar protective colouring really protects
its possessors? What if, after all, it is all a game, a joke, which
the Creator is playing with us poor mortals? Did not a trustworthy
observer recently watch carefully, and see how a pair of sparrows daily
cleared a wooden fence on which moths of the genus _Catocala_ and other
species of nocturnal Lepidoptera, excellently furnished with protective
colours, were wont to settle by day? They did their work thoroughly,
and hardly overlooked a single individual. But who has a right to see
anything more in this than--what surely goes without saying--that the
best protective colouring is not an absolute protection, and never
preserves all from destruction, but always only some, and it may be
very few.
How else could there be such a high ratio of elimination, and such a
constancy in the number of individuals of a species on any unchanging
area? These sparrows had simply made full use of an experience,
probably acquired by chance to begin with, and their vision had become
sharpened for this particular species on the almost similarly coloured
wooden fence, just as that of the expert butterfly collector does. It
certainly does not follow from this that the protective colouring was
useless, nor can we regard the harmony between the protruding tip of
the anterior or posterior wing and the large protectively coloured
surface of the covering wing as of no importance. On the contrary, if
the tips were white or conspicuously coloured like the rest of the
posterior wing, they would assuredly attract the sharp eye of hungry
enemies to the spot, and so betray the victim. Instead of this, the
spot in question is not only dark, but, in the case of _Notodonta_, is
furnished with a tuft of hairs, which, in the insect's resting position
(Fig. 12, _B_), lies on the back, and looks like a dark, somewhat
curved projecting tooth, in front of which there stands another, quite
similar, which arises from the anterior wing, and behind there are
other seven, rather smaller, dark teeth of the same kind, springing
from the outer edge of the anterior wing. Taken altogether, they mimic
the dentated edge of a withered leaf, and thus, in spite of their
diverse origins, form a unified picture, and one with a considerable
protective value. How is it possible to doubt that each of these
hair-tufts has arisen under the influence of natural selection, and
that its absence or imperfect development might result in the discovery
and elimination of the insect concerned?
These cases seem to me particularly beautiful proofs of the productive
efficiency of selection. The wing is protected just as far as it
protrudes from beneath the other--not a millimetre further! How should
it be otherwise, when the colouring of the parts just beside these is
indifferent for the species, so that any variations in these parts in
the direction of protective colouring never survive to be transmitted
and accumulated?
It is precisely this restriction to what is absolutely necessary that
is the surest sign, here and elsewhere, that the character in question
has been brought about by natural selection. And if this is the only
possible, and at the same time quite sufficient explanation of the
remarkably well-defined colour deliminations in all Lepidoptera,
there can be no reason why we should try to drag in any other factor
to explain the case of _Tropidoderus_, the less so as here again
selection alone can account for the green of the exposed surfaces;
and furthermore, the modification, common in other Phasmidæ, of the
most anterior green stripe of the posterior wing into a firm cover
protecting the soft abdomen, also points to natural selection; the
cover-wings proper have here become too short, and so the edge of
the posterior wing has been modified into a hard rib, which protects
the soft body of the insect (Fig. 11, _H. horn_). No differences in
illumination, and no _direct_ effect of any external influence whatever
could have brought that about.
How much more I might adduce in this connexion! The manifold diversity
of colour and form adaptation is so great among insects, to which
protection from their enemies is so necessary, and especially among
butterflies, that I should never come to an end if I were to try to
give even an approximate idea of it. Let us, therefore, turn now from
such cases to a higher--the highest--grade of adaptation, that in which
there is not only a mimicry of special and complex coloration, but in
which the whole animal has become like some external object, and is
thereby secured from discovery.
We must first consider the case of our lappet moth (_Gastropacha
quercifolia_), which in its copper-red colour and in the remarkable
shape and dentated edges of its wings, and finally in the quite
extraordinary clucking-hen-like attitude of the wings when at rest,
greatly resembles some dry oak-leaves lying one above the other.
Not unlike this is a 'shark' moth found in this country, _Xylina
obsoleta_, which, as the name indicates, looks when at rest like a
broken bit of half-rotten wood (Fig. 10, p. 77). It 'feigns death,' as
we commonly say, that is, it draws the legs and antennæ close to the
body, and does not move; indeed, one may lift it up and throw it on the
ground without its betraying by a single twitch that it lives. Only
after it has been left undisturbed for some time does it show signs of
life again, and makes off hastily, to find a better hiding-place. The
colouring of this moth is so curiously mingled--brown, whitish, black,
and yellow--and traced with acute-angled lines and curves, that one
cannot distinguish it at sight from a bit of rotten wood. I experienced
that myself once when, passing a hedge, I thought I saw a _Xylina_
sitting on the ground, and picked it up to examine it. I thought it was
a bit of wood, and, disappointed, I threw it down again on the grass,
but then I felt uncertain, and picked it up once more--to find that it
was a moth after all[1]!
[1] Rösel says in this connexion: 'The marvellous form of this Papilio
preserves it from injuries, for, when he hangs freely on a trunk of a
tree, he would be taken ten times sooner for a piece of bark than for
a living creature. By day, too, he is so little sensitive, that if he
be thrown down from his resting-place he falls to the ground as if
lifeless, and remains lying motionless. One may also throw him into
the air, or turn him about, and he will rarely give a sign of life.
I have impaled many of them on needles, without seeing any sign of
sensitiveness on their part. This is the more remarkable that these
birds (sic), after they have submitted to all the torment and misery
one can inflict on them, without showing any sign of feeling, will,
whenever they are left in peace and have no further disturbances
to fear, quickly creep off to a dark corner and attempt to conceal
themselves from future attacks.'--_Insektenbelustigungen_, Nürnberg,
1746, vol. i. p. 52.
This case of _Xylina_ is hardly less remarkable, and its likeness to
the mimicked object is scarcely less wonderful than that of the often
discussed mimicry of a leaf, with stalk, midrib, and lateral veins, by
many of the forest butterflies of South America and India.
[Illustration: FIG. 13. _Kallima paralecta_, from India, right under
side of the butterfly at rest. _K_, head. _Lt_, maxillary palps. _B_,
limbs. _V_, anterior wing. _H_, posterior wing. _St_, 'tail' of the
latter, corresponding to the stalk of the leaf. _gl_^1 and _gl_^2,
transparent spots. _Aufl_, eye-spots. _Sch_, mould-spots.]
The best known of these is the Indian _Kallima paralecta_, which, when
it settles, is deceptively like a dead leaf, or rather like a dry or
a half-withered one, on which brown alternates with red, and on which
there are one or two translucent spots, without scales, presumably
representing dewdrops. The upper surface of this butterfly is simply
marked, but gorgeously coloured--blue-black with a reddish yellow,
or bluish white band--and quite constant. The under surface, on the
other hand, although it always resembles a dead leaf, shows very
varied ground colours, being sometimes greyish, sometimes yellowish,
or reddish yellow, or even greenish. Often it shows the lateral
veining of the leaf quite as distinctly as in Fig. 13, but often quite
indistinctly, and the black, mouldy spots (_Sch_) of our figure may
be more strongly marked, or they may be absent. It would seem as if
the mimicry of different kinds of leaves was here aimed at--so to
speak--just as in the case of the varied and numerous species of the
South American genus _Anæa_, which usually live in the woods, and are
all more or less leaf-like, but each species is like a different leaf,
or like a leaf in a different condition, dry, moist, or decomposing.
It is simply astounding to see this diversity of leaf mimicry, and
the extraordinary faithfulness with which the impression of the leaf
is reproduced. But it is by no means always the venation which causes
the resemblance, for this is often inconspicuous; the high degree
of deceptiveness is due to the silvery-clear yellow, dark yellow,
red-brown to dark black-brown ground-colouring, which is never quite
uniform, and over which there usually spreads a whitish ripple,
combined with the remarkable imitation of the sheen of many leaves. The
upper side of this butterfly is almost always conspicuously decorated
with violet, dark blue or red, but always without any relation to the
under surface. Not in all, but in many of the species of this genus,
we find the round, translucent mirrors on the wing already mentioned
in the case of _Kallima_, and in some species quite remarkable means
are made use of to make the resemblance to a leaf thoroughly deceptive.
Thus _Anæa polyxo_, when sitting, looks like a leaf out of the edge
of which a caterpillar has eaten a little piece; in reality there is
nothing missing from the wing, but on the front margin of the anterior
wing a semicircular spot of a bright, soft, yellow colour stands out so
sharply from the rest of the chestnut-brown wing surface, that it has
the effect of a hole in the leaf.
[Illustration: FIG. 14. _Cœnophlebia archidona_, from Bolivia, in its
resting attitude. _mr_, midrib of the apparent leaf. _st_, the apparent
stalk.]
A modern opponent of the selection theory (Eimer) has suggested
that the marking of the lateral veins, and other resemblances to a
leaf in _Kallima_, represent nothing more than the pattern which
was present in any case, inherited from ancestors, and which in the
course of time arranged itself in a particular manner according to
internal developmental laws. Not selection--that is, adaptation to
surroundings--but the internal developmental impulse has brought about
the resemblance to the leaf. It is astonishing how a preconceived
idea can blind a man and weaken his judgment! It goes without saying
that the adaptations do not start from a _tabula rasa_, but from
what is already present; of course, natural selection makes use of
the markings inherited from ancestors; it takes what already exists,
and alters or extends it as suits best. Thus it is easy to prove
that the clear mirrors (Fig. 13, _gl_^1 and _gl_^2) on the wings of
_Kallima_ have arisen from a modification of the nuclei of eye-spots,
just as the dark mould-spots which often occur, frequently develop
in association with the inherited eye-spots; not always however, for
many such accumulations of black scales occur in spots on which there
has never been an eye-spot. Thus, too, the 'midribs' of the butterfly
have in part arisen from a gradual displacing, extending, and altering
of the direction of inherited stripes as, for instance, is clearly
recognizable in the posterior wing of Fig. 13, but sometimes they are
new formations. But the veining of a leaf is never found on the wing
of any butterfly of a species which has not the habit of resting among
leaves, or which has not had it at one time, and it never corresponds
to the natural marking of any genus which does not live in forests.
This impression of leaf-venation has obviously arisen from quite
different patterns of markings, and it has been reached now by one
way, now by another. We can see this from the fact that, in different
butterflies, it lies in quite different positions on the wing. In
the _Kallima_ species the stalk of the leaf lies in the tail of the
posterior wing, the tip of the midrib lies near the tip of the wing; in
_Cœnophlebia archidona_ it is exactly reversed, the tip of the anterior
wing (Fig. 14) is prolonged, and forms the stalk, while a broad,
dark, stripe, the midrib (_mr_), runs from there across the middle of
both wings, and seems to give off two or three lateral ribs running
outwards. If it be asked whether this butterfly always sits down so
artistically that the 'upward turning leaf-stalk is in juxtaposition
to a twig,' we may answer that a bird flying fast is not likely to
look to see whether every leaf in the profusion of foliage in the
primitive forests is properly fastened to its stalk or not, any more
than we should do in the case of a painted bush, on which many a leaf
has the appearance of floating in the air, just as in nature, or in its
faithful copy, the photograph.
[Illustration: FIG. 15. _Cærois chorinæus_, from the lower Amazon, in
its resting attitude. _V_, anterior wing. _H_, posterior wing. _mr_,
midrib of the apparent leaf. _sr_, lateral veins. _st_, hint of a
leaf-stalk.]
Quite different from the leaf-marking either of _Cœnophlebia_ or
_Kallima_ is that of one of the Satyrides of the lower Amazon valley,
_Cærois chorinæus_ (Fig. 15). If one spreads this butterfly out in the
usual way it does not look in the least like a leaf, and one only sees
a number of curiously placed disconnected stripes on the under surface
of the wing. But if the wings be folded together to correspond with
the sitting position of the butterfly, there appears the figure of a
leaf, of which, however, only half is present, and whose midrib (_mr_)
runs obliquely forward from the inner angle of the posterior wing.
Here, again, it is not difficult to guess that this straight stripe has
arisen, by displacement and straightening, from a curved line inherited
from some remote ancestor, and it is these precise changes which are
the work of the adaptive processes of natural selection. The same
applies to the lateral ribs (_sr_), which are here four in number.
But even the division of the wing surface by a single dark line, such
as that which crosses the middle of the posterior wing of _Hebomoja_
(Fig. 9), an Indian butterfly, heightens not inconsiderably the
resemblance of the resting butterfly to a leaf, a resemblance which
has already been shown in the form and colour. Indeed, even the sharp
division of the wing surface into a darker inner and a lighter outer
portion, which occurs in many species of _Anæa_, gives a very vivid
impression of a leaf crossed by a midrib.