The general prevalence of protective colouring among caterpillars has
already been so frequently treated of that it is not here my intention
to recall particular instances. In order to judge of the effect of
marking, however, it will be well to bear in mind that these insects,
being generally defenceless and thus requiring protection, have
acquired the most diverse means of rendering themselves in some measure
secure from their foes.
The sharp spines which occur on the caterpillars of many butterflies
(_Vanessa_, _Melitæa_, _Argynnis_), and the hairs on those of
many moths, serve for protective purposes. Among other means of
protection--although in a different sense--we have in all the species
of the great family of the _Papilionidæ_ the strikingly coloured
(yellowish red) odour-emitting tentacles concealed near the head,
and suddenly protruded for terrifying foes; and likewise the forked
horn at the tail of the caterpillars of the genus of moths _Harpyia_,
the tentacles of which can be suddenly protruded in a similar
manner. Adaptive colours and forms combined with certain habits[129]
are, however, much more common than defensive weapons. Thus, the
caterpillars of the _Noctuæ_ belonging to the genus _Catocala_ and its
allies, feed only at night on the green leaves of various forest-trees;
by day they rest in crevices of the bark on the tree trunk, which they
resemble so perfectly in the colour of their peculiar glossy dull grey
or brownish skin beset with small humps, that only sharp eyes can
detect them, even when we are familiar with their habits.[130]
The striking resemblance of many moths to splinters of wood is well
known, and to this is added a habit which helps their disguise, viz.,
that of remaining stiff and motionless on the approach of danger,
just like a splinter projecting from the branch.[131] Among the moths
coming under this category may be mentioned _Cucullia Verbasci_, and
particularly those of the genus _Xylina_, which, when at rest, closely
resemble a broken splinter of wood in the colour and marking of their
fore wings, and when touched, have a habit of drawing in their legs and
falling without opening their wings as though dead.
That simple adaptive colouring prevails widely among caterpillars
is shown by the large number of green species.[132] It may be fairly
said that all caterpillars which possess no other means of protection
or defence are adaptively coloured. These facts are now well known;
so also is the explanation of the varied and striking colours of many
caterpillars given by Wallace.[133] There is, however, novelty in the
proof contained in the foregoing descriptions of larval development,
as to the manner in which the di- and polymorphism of caterpillars
can be explained from the external phenomena which they present,
these phenomena being well adapted for showing the great importance
of protective colouring to the larvæ. We have here presented a
double adaptation, although not quite of the nature of that which I
formerly admitted on hypothetical grounds.[134] In the first place,
from the developmental history there results the conclusion that all
Sphinx-larvæ which, in the adult state, are di- or polymorphic, are
unicolorous when young. Thus, the caterpillars of _Chærocampa Elpenor_
all remain green till the fourth stage, when they mostly become
light or dark brown, and only very seldom retain their green colour.
_Chærocampa Porcellus_ behaves in a precisely similar manner; as also
does _Pterogon Œnotheræ_, which inhabits the same localities, and is
found on the same food-plant, but is not very closely related to the
_Chærocampa_. In this species also (_P. Œnotheræ_) the brown is more
common than the green form in the adult state, both varieties showing a
complicated marking. The young larvæ possess only a light green colour,
and a pure white subdorsal line as the only marking; they are so well
adapted to the leaves of their food-plants, _Epilobium Hirsutum_,
and _E. Rosmarinifolium_, that they can only be detected with great
difficulty. After the third moult they become brown, and can be easily
seen when at rest on their food-plant.
Now in all known caterpillars brown colours are adaptive, sometimes
causing a resemblance to the soil, and at others to dead leaves or
branches. As soon, therefore, as the caterpillars have attained a
considerable size, they remain concealed by day.[135] The truth of this
observation not only appears from various entomological notes, but I
have frequently convinced myself of its accuracy. I well remember from
the earliest times that _C. Elpenor_, especially when the larva is
adult, always rests by day among the dead branches and leaves of its
shrub-like food-plant, _Epilobium Hirsutum_; and even when this species
lives on the low-growing _Epilobium Parviflorum_, it conceals itself
by day on the ground, among the tangled leaves and branches. I have
observed that _Sphinx Convolvuli_ has a precisely similar habit, for
which reason it is difficult to obtain, even in localities where it
occurs very commonly.
In the neighbourhood of Basle I once found at mid-day a brown
caterpillar of _Pterogon Œnotheræ_ on an isolated dead branch
of _Epilobium Rosmarinifolium_, and I was informed by Herr
Riggenbach-Stähelin--a collector of great experience who accompanied
me--that these caterpillars always rest (by day) on withered plants as
soon as they become brown, but before this change they are only to be
found on green plants.
Thus, it cannot well be doubted that the change of colour is associated
with a change in the habits of life, and the question arises as to
which has been the primary change.
If the view here entertained, that the later brown coloration is
adaptive, be correct, the species must have first acquired the habit of
concealing itself by day on the ground and among dead herbage, before
the original green colour could have been changed into brown by natural
selection. This must represent the actual facts of the case.
Nearly allied species which at an advanced age are not dimorphic, but
are darkly coloured in all individuals, are especially calculated
to throw some light on this point. For instance, the caterpillar of
_Deilephila Vespertilio_, which comes under this denomination, is light
green when young, and rests both by day and night on the leaves of the
plant on which it feeds. As soon as it acquires its dark colour--after
the third moult--it changes its habits, concealing itself by day on the
ground and feeding only by night. For this reason collectors prefer
seeking for it in the evening, or with a lantern by night.
The most instructive case, however, is that of _Deilephila Hippophaës_,
in which no change of colour is associated with age, the caterpillar,
throughout its whole life, remaining of a greyish green, which exactly
matches the colour of the leaves of its food-plant, _Hippophae
Rhamnoides_. Nevertheless this species also possesses the habit of
feeding only at night as soon as it has attained to a considerable
size, hiding itself by day at the root of its food-plant. Collectors
expressly state that this larva can scarcely be found by day, and
recommend that it should be sought for at night with a lantern.
From the foregoing facts and considerations it may fairly be concluded,
that the habit of hiding by day, possessed by these and other allied
caterpillars, was acquired when they resembled the leaves in colour,
and that the adaptation to the colour of the soil, or dead foliage and
withered branches, ensued as a secondary consequence.
But why have these caterpillars acquired such a habit, since they
appear to be perfectly protected by their resemblance in colour to
the green leaves? The answer to this question is easily given when we
consider in which species this habit generally occurs.
Does the habit prevail only among the species of the one genus
_Deilephila_, and in all the species of this genus? This is by no
means the case, since, on the one hand, many species of _Deilephila_,
such as _D. Euphorbiæ_, _Galii_, _Nicæa_, and _Dahlii_, do not possess
the habit, and, on the other hand, it occurs in species of other
genera, such as _Macroglossa Stellatarum_, _Sphinx Convolvuli_, and
_Acherontia Atropos_.
The habit in question must therefore be the result of certain external
conditions of life common to all those species which rest by day. The
mode of life common to them all is that they do not live on trees with
large leaves or with thick foliage, but on low plants or small-leaved
shrubs, such as the Sea Buckthorn.[136] I believe I do not err when
I attribute the habit possessed by the adult larvæ, of concealing
themselves by day, to the fact that the green colour is protective only
so long as they are small--or, more precisely speaking, as long as
their size does not considerably exceed that of a leaf or twig of their
food-plant. When they become considerably larger, they must become
conspicuous in spite of their adaptive colour, so that it would then be
advantageous for them to conceal themselves by day, and to feed only
by night. This habit they have acquired, and still observe, even when
the secondary adaptation to the colour of the soil, &c., has not been
brought about. We learn this from _D. Hippophaës_, which remains green
throughout its whole larval existence; and no less from the green forms
of the adult larvæ of _Sphinx Convolvuli_, _Chærocampa Elpenor_, and
_Porcellus_, all of which conceal themselves by day in the same manner
as their brown allies.
It may be objected that there are Sphinx-larvæ--instances of which I
have myself adduced--which live on low small-leaved plants, and which
nevertheless do not hide themselves by day. This is the case with the
spurge-feeding _D. Euphorbiæ_, so common in many parts of Germany. This
caterpillar must, however, be classed with those which, on account
of their distastefulness, or for other reasons to be subsequently
considered, are rejected by birds and other larger foes, and which, as
Wallace has shown, derive advantage from being coloured as vividly as
possible. I shall return to this subject later, when treating of the
biological value of special markings.
On the other hand, it is readily conceivable that, from the conditions
of life of caterpillars living on trees or shrubs with dense foliage,
the habit of resting by day and descending from the tree for
concealment would not have been acquired. Such larvæ are sufficiently
protected by their green colour among the large and numerous leaves;
and I shall have occasion to show subsequently that their markings
increase this protective resemblance.
The di- or polymorphism of the larvæ of the _Sphingidæ_ does not
therefore depend upon a _contemporaneous_ double adaptation, but
upon the replacement of an old protective colour by a new and better
one, and therefore upon a _successive_ double adaptation. The adult
caterpillars of _C. Elpenor_ are not sometimes brown and sometimes
green because some individuals have become adapted to leaves and others
to the soil, but because the anciently inherited green has not yet
been completely replaced by the newly acquired brown coloration, some
individuals still retaining the old green colour.
When, in another place,[137] I formerly stated “that a species can
become adapted in this or that manner to given conditions of life, and
that by no means can only one best adapted form be allowed for each
species,” this statement is theoretically correct speaking generally,
but not in its application to the present class of cases. A comparison
with one another of those caterpillars which repose by day, distinctly
shows that they all possess a tendency to abandon the green and assume
a dull colour, but that this process of replacement has advanced
further in some species than in others. It will not be without interest
to follow this operation in some detailed cases, since we may thus
obtain an insight into the processes by which polymorphism has arisen,
as well as into the connection between this phenomenon and simple
variability.
In _D. Hippophaës_ the process has either not yet commenced, or is
as yet in its first rudiments. If we may trust the statements of
authors, together with the ordinary green form there occurs, rarely,
a silver-grey variety, which may be regarded as the beginning of a
process of colour substitution. Among thirty-five living specimens of
this scarce species which I was able to procure, the grey form did not
occur, neither have I found it in collections.
In _Macroglossa Stellatarum_ we see the transforming process in full
operation. A large number of individuals (about thirty-five per cent.)
are still green; the number of dark-coloured individuals reaches
forty-six per cent., these, therefore, preponderating; whilst between
the two extremes there are about nineteen per cent. of transition
forms, showing all possible shades between light green and dark
blackish-brown or brownish-violet, and even, in solitary individuals,
pure violet (See Figs. 3-12, Pl. III.). The relatively small number
of the intermediate forms, taken in connection with the fact that all
the 140 specimens employed in my investigation were obtained from one
female, leads to the conclusion that these forms owe their existence to
cross-breeding. It would be superfluous to attempt to prove this last
conclusion with reference to the before-mentioned case, in which a
caterpillar was streaked with brown and green (Fig. 9, Pl. III.).
The process of transformation, as already mentioned, advances in such
a manner that the intermediate forms diminish relatively to the dark
individuals. This is found to be the case with _Sphinx Convolvuli_,
and almost to the same extent with _Chærocampa Elpenor_, in both of
which species the green caterpillars are the rarest.[138] Forms truly
intermediate in colour between green and brown no longer occur, but
apparently only different shades of light and dark brown, passing into
brownish-black.
The process has again made a further advance in _Chærocampa Porcellus_
and _Celerio_ as well as in _Pterogon Œnotheræ_. In all these species
the green form occurs,[139] but so rarely that very few collectors have
seen it. The brown form has therefore in these cases nearly become the
predominant type, and the solitary green specimens which occasionally
occur, may be regarded as reversions to an older phyletic stage.
_Deilephila Livornica_ appears to have reached a similar stage, but the
caterpillar of this species has been so imperfectly observed, that it
is difficult to determine, even approximately, the relative proportion
of the brown to the green individuals. I have only seen one of the
latter in Dr. Staudinger’s collection (Compare Fig. 62, Pl. VII.).
In _Deilephila Vespertilio_, _Euphorbiæ_, _Dahlii_, _Mauritanica_,
_Nicæa_, and _Galii_, the green form has completely disappeared. The
blackish olive-green colour shown by many caterpillars of the two last
species, can be considered as a faint retention of the light green
colour which they formerly possessed, and which they both show at the
present time in their young stages.
Beginning with the appearance of single darker individuals, we pass
on in the first place to a greater variability of colouring, and
from this, by the greater diminution of the intermediate forms,
to polymorphism; the complete extermination of these forms ending
in dimorphism. The whole process of transformation has been thus
effected:--As the new colouring always prevailed over the old, the
latter was at length completely displaced, and the caterpillars, which
were at first simply variable, became polymorphic and then dimorphic,
finally returning to monomorphism.
We thus see the process of transformation still going on, and no
doubt can arise as to its inciting causes. When a character can with
certainty be ascribed to adaptation, we can explain its origin in no
other way than by the action of natural selection. If, as I believe,
it can and has been shown, not only that caterpillars in general
possess adaptive colours, but that these colours can change during the
lifetime of one and the same species, in correspondence with external
conditions, we must certainly gain a very high conception of the power
which natural selection exerts on this group of living forms.[140]