The following questions now present themselves: Have the markings
of caterpillars any biological value, or are they in a measure only
sports of nature? Can they be considered as partially or entirely the
result of natural selection, or has this agency had no share in their
production?
The problem here offers itself more distinctly than in any other group
of living forms, because it presents an alternative without a third
possibility. In other words, if it is not possible to show that larval
markings have a distinct biological significance, there remains only
for their explanation the assumption of a phyletic force, since the
direct action of the environment is insufficient to account for such
regularity of development throughout a series of forms. The explanation
by sexual selection is excluded _ab initio_, since we are here
concerned with larvæ, and not with reproductive forms.[141]
The biological significance of marking--if such significance it
possess--will be most easily investigated by examining whether species
with similar markings have any conditions of life in common which would
permit of any possible inference as to the significance of the markings.
Among the _Sphingidæ_ we find four chief forms of marking; (1) complete
absence of all marking; (2) longitudinal stripes; either a simple
subdorsal or this together with a spiracular and dorsal line; (3)
oblique stripes; (4) eye-spots and ring-spots, single, paired, or in
complete rows.
Now if we consider in which species these four kinds of marking are of
general occurrence, not only in the small group of the _Sphingidæ_
but in the whole order Lepidoptera, we shall arrive at the following
results:--
1. _Complete absence of marking_, so common in the larvæ of
other insects, such as the Coleoptera, is but seldom found among
Lepidopterous caterpillars.
To this category belong all the species of _Sesiidæ_ (the genera
_Sesia_, _Trochilia_, _Sciapteron_, _Bembecia_, &c.), the larvæ of
which, without exception, are of a whitish or yellowish colour, and
live partly in the wood of trees and shrubs and partly in the shoots
of herbaceous plants. Subterranean larvæ also, living at the roots
of plants, such as _Hepialus Humuli_ at the roots of hop, and _H.
Lupulinus_ at those of _Triticum Repens_, possess neither colour nor
marking. These, like the foregoing, are yellowish-white, evidently
because they are deprived of the influence of light.[142] The larvæ of
certain small moths, such as _Tortrix Arbutana_ and _Pomonana_, which
live in fruit, and many case-bearing _Tineina_, are likewise without
marking and devoid of bright colour, being generally whitish. Many
of the small caterpillars which feed exteriorly are also--so far as my
experience extends--without definite markings, these being among the
most minute, such as the greenish leaf-mining species of _Nepticula_.
It is among the larger species that we first meet with longitudinal
and oblique stripes. Eye-spots do not occur in any of these larvæ, a
circumstance of the greatest importance for the biological significance
of this character, as will be shown subsequently. The small size of
the caterpillars cannot be the sole cause of the absence of such
eye-spots, since in young _Smerinthus_ caterpillars one centimeter
long, the oblique stripes are beautifully developed, and the larvæ of
many of the smaller moths considerably exceed this size. The surface of
these caterpillars therefore, _i.e._, the field on which markings are
displayed, is not absolutely too small for the development of such a
character.
Besides the larvæ of the Micro-lepidoptera and of those species living
in the dark, there is also a complete absence of marking in the young
stages of many caterpillars. Thus, all the _Sphingidæ_ of which I have
been able to observe the development, show no markings immediately
after emergence from the egg; in many they appear very soon, even
before the first moult, and, in other species, after this period.
2. _The second category of markings, longitudinal stripes_, is very
widely distributed among the most diverse families. This character
is found among the larvæ of butterflies, _Sphingidæ_, _Noctuæ_,
Micro-lepidoptera, &c., but in all these groups it is absent in many
species. This last fact is opposed to the view that this character
is purely morphological, and leads to the supposition that it may
have a biological value, being of service for the preservation of the
individual, and therefore of the species.
I find that such marking is of service, stripes extending
longitudinally along the upper surface of the caterpillar generally
making the latter less conspicuous. This, of course, does not hold
good under all circumstances, since there are many species with very
striking colours which possess longitudinal stripes. Let us consider,
however, a case of adaptive colouring, such as a green caterpillar,
which, on this account only, is difficult to see, since it accords
with the colour of the plant on which it lives. If it is a small
caterpillar, _i.e._, if its length and thickness do not considerably
exceed that of the parts of its food-plant, it can scarcely be better
concealed--stripes would hardly confer any special advantage unless the
parts of the plant were also striped. But the case is quite different
if the caterpillar is considerably larger than the parts of the plant
(leaves, stalks, &c.). The most perfect adaptive colouring would not
now prevent it from standing out conspicuously as a larger body, among
the surrounding parts of the plants. It must be distinctly advantageous
therefore to such a caterpillar to be striped, since these markings
to a certain extent divide the large body into several longitudinal
portions--they no longer permit it to be seen as a whole, and thus act
more effectively than mere assimilative colouring in causing it to
escape detection. This protection would be the more efficacious if the
stripes resembled the parts of the plant in colour and size, such, for
instance, as the lines of light and shadow produced by stalks or by
long and sharp-edged leaves.
If this view be correct, we should expect longitudinal stripes to be
absent in the smallest caterpillars, and to be present more especially
in those species which live on plants with their parts similarly
disposed, _i.e._, on plants with numerous thin, closely-growing stalks
and grass-like leaves, or on plants with needle-shaped leaves.
It has already been mentioned that the smallest species are devoid of
longitudinal striping. The larvæ of the Micro-lepidoptera show no such
marking, even when they do not live in the dark, but feed either on the
surface or in superficial galleries of the leaves (_Nepticula_, &c.),
in which they must be exposed to almost as much light as when living
on the surface. The fact that the subdorsal line sometimes appears in
very young Sphinx-larvæ is explained, as has already been shown, by the
gradual backward transference of adaptational characters acquired in
the last stage of development.
It can easily be demonstrated that longitudinally striped caterpillars
mostly live on plants, of which the general appearance gives the
impression of a striped arrangement. We have only to consider in
connection with their mode of life, any large group of adaptively
coloured species marked in this manner. Thus, among the butterflies,
nearly all the _Satyrinæ_ possess larvæ conspicuously striped--a fact
which is readily explicable, because all these caterpillars live on
grasses. This is the case with the genera _Melanargia_, _Erebia_,
_Satyrus_, _Pararge_, _Epinephele_, and _Cænonympha_, no species of
which, so far as the larvæ are known, is without longitudinal stripes,
and all of which feed on grasses. It is interesting that here also,
as in certain _Sphingidæ_, some species are brown, _i.e._, adapted to
the soil, whilst the majority are green, and are therefore adapted to
living grass. Just as in the case of the _Sphingidæ_ also, the brown
species conceal themselves by day on the earth, whilst some of the
green species have likewise acquired this habit. I have already shown
how this habit originates from the increasing size of the growing
larva, which would otherwise become too conspicuous, in spite of
adaptive colour and marking. A beautiful confirmation of this view is
found in the circumstance that only the largest species of _Satyrus_,
such as _S. Proserpinus_, _Hermione_, _Phædrus_, &c., possess brown
caterpillars. I should not be surprised if a more exact investigation
of these species, which have hitherto been but seldom observed,
revealed in some cases a dimorphism similar to that of the _Sphingidæ_;
and I believe that I may venture to predict that the young stages
of all these brown larvæ--at present quite unknown--are, as in the
last-named group, green.
Besides the _Satyrinæ_, most of the larvæ of the _Pierinæ_ and
_Hesperidæ_ possess longitudinal stripes, which are generally less
strongly pronounced than in the former subfamily. Some of the _Pierinæ_
live on _Cruciferæ_, of which the narrow leaves and thin leaf- and
flower-stalks present nothing but a linear arrangement; other species
of this group, however, feed on _Leguminosæ_ (_Lathyrus_, _Lotus_,
_Coronilla_, _Vicia_), and some few on broad-leaved bushes (_Rhamnus_).
This last fact may appear to be opposed to the theory; but light
lateral stripes, such for example, as those possessed by _Gonepteryx
Rhamni_, can never be disadvantageous, and may be of use, even on large
leaves, so that if we consider them as an inherited character, there
is no reason for natural selection to eliminate them. In the case of
caterpillars living on vetch, clover, and other _Leguminosæ_, it must
not be forgotten that, although their food-plants do not present any
longitudinal arrangement of parts, they always grow among grasses, the
species feeding on such plants always resting between grass stems, and
very frequently on the grass itself, so that they can have no better
protective marking than longitudinal stripes. The striping of the
_Hesperidæ_ larvæ, which partly feed on grasses but mostly on species
of _Leguminosæ_, can be explained in a similar manner.
It is not here my intention to go through all the groups of Lepidoptera
in this manner. The instances adduced are quite sufficient to prove
that longitudinal stripes occur wherever we should expect to find them,
and that they really possess the biological significance which I have
ascribed to them. That these markings are occasionally converted into
an adaptive imitation of certain special parts of a plant, is shown
by the larvæ of many moths, such for example as _Chesias Spartiata_,
which lives on broom (_Spartium Scoparium_), its longitudinal stripes
deceptively resembling the sharp edges of the stems of this plant.[143]
3. _Oblique striping._ Can the lilac and white oblique stripes on the
sides of a large green caterpillar, such as those of _Sphinx Ligustri_;
or the red and white, or white, black, and red stripes of _Smerinthus
Tiliæ_ and _Sphinx Drupiferarum_ respectively, be of any possible use?
Have we not here just one of those cases which clearly prove that such
a character is purely morphological, and worthless for the preservation
of the individual? Does not Nature occasionally sport with purposeless
forms and colours; or, as it has often been poetically expressed, does
she not here give play to the wealth of her phantasy?
At first sight this indeed appears to be the case. We might
almost doubt the adaptive importance of the green ground-colour
on finding coloured stripes added thereto, and thus--as one might
suppose--abolishing the beneficial action of this ground-colour, by
making the insect strikingly conspicuous. But this view would be
decidedly incorrect, since oblique stripes are of just the same
importance as longitudinal stripes. The former serve to render the
caterpillar difficult of detection, by making it resemble, as far as
possible, a leaf; they are imitations of the leaf-veins.
Nobody who is in the habit of searching for caterpillars will
doubt that, in cases where the oblique stripes are simply white or
greenish-white, it is extremely difficult to see the insect on its
food-plant, _e.g._ _S. Ocellatus_ on _Salix_; not only because it
possesses the colour of the leaves, but no less because its large body
does not present an unbroken green surface, which would bring it into
strong contrast with the leaves, and thus arrest the attention. In the
case of the species named, the coloured area of the body is divided by
oblique parallel stripes, just in the same manner as a willow leaf.
In such instances of course we have not presented to us any special
imitation of a leaf with all its details--there is not a perfect
resemblance of the insect to a leaf, but only an arrangement of lines
and interspaces which does not greatly differ from the division of a
leaf by its ribs.
That this view is correct is shown by the occurrence of this form
of marking. It is on the whole rare, being found, besides in
many _Sphingidæ_, in isolated cases in various families, but is
always confined to those larvæ which live on ribbed leaves, and
never occurring in species which feed on grasses or on trees with
needle-shaped leaves. This has already been shown with respect to
the _Sphingidæ_, in which the oblique stripes are only completely
developed in the subfamilies _Smerinthinæ_ and _Sphinginæ_. The species
of _Smerinthus_ all live on trees such as willows, poplars, lime,
oak, &c., and all possess oblique stripes. The genus _Anceryx_ also
belongs to the _Sphinginæ_, and these caterpillars, as far as known,
live on trees with needle-shaped leaves. The moths of this last genus
are very closely allied to the species of _Sphinx_, not only in form
and colour, but also in many details of marking. The larvæ are however
different, this distinction arising entirely from their adaptation
to needle-shaped leaves, the _Sphinx_ caterpillars being adapted
to ordinary foliage. The species of _Anceryx_, as has been already
shown, are brown mixed with green, and never possess even a trace of
the oblique stripes, but have a latticed marking, consisting of many
interrupted lines, which very effectively serves to conceal them among
the needles and brown bark of the _Coniferæ_.
Of the _Sphinginæ_ living on plants with ordinary foliage, not a single
species is without oblique stripes. I am acquainted with ten species of
caterpillars and their respective food-plants, viz. _Sphinx Carolina_,
_Convolvuli_, _Quinquemaculata_, _Prini_, _Drupiferarum_, _Ligustri_;
_Macrosila Rustica_ and _Cingulata_; _Dolba Hylæus_ and _Acherontia
Atropos_.
Besides among the _Sphinginæ_, oblique stripes occur in the larvæ of
certain butterflies, viz. _Apatura Iris_, _Ilia_, and _Clytie_, all of
which live on forest trees (aspen and willows), and are excellently
adapted to the leaves by their green colour. In addition to these, I am
acquainted with the larvæ of some few moths, viz. of _Aglia Tau_ and
_Endromis Versicolora_, both of which also live on forest trees.
Oblique stripes also occasionally occur in the smaller caterpillars
of _Noctuæ_, _Geometræ_, and even in those of certain _Pyrales_, in
all of which they are shorter and differently arranged. In these cases
also, my theory of adaptation holds good, but it would take us too far
if I attempted to go more closely into them. I will here only mention
the extraordinary adaptation shown by the caterpillar of _Eriopus
Pteridis_. This little Noctuid lives on _Pteris Aquilina_; it possesses
the same green colour as this fern, and has double oblique white
stripes crossing at a sharp angle on each segment, these resembling the
lines of _sori_ of the fern-frond so closely, that the insect is very
difficult to perceive.
After all these illustrations it can no longer remain doubtful that
the oblique stripes of the _Sphingidæ_ are adaptive. But how are
the coloured edges bordering these stripes in so many species to be
explained?
I must confess that I long doubted the possibility of being able to
ascribe any biological value to this character, which appeared to me
only conspicuous, and not protective. Cases may actually occur in which
the brightly coloured edges of the oblique stripes make the caterpillar
conspicuous--just in the same manner as any marking may bring about a
conspicuous appearance by presenting a striking contrast of colour.
I am acquainted with no such instance, however. As a rule, in all
well-adapted caterpillars, considering their colour in its totality,
this is certainly not the case. The coloured edges, on the contrary,
enhance the deceptive appearance by representing the oblique shadows
cast by the ribs on the under-side of the leaf; all these caterpillars
rest underneath the leaves, and never on the upper surface.
This explanation may, perhaps, at first sight appear far-fetched,
but if the experiment be made of observing a caterpillar of _Sphinx
Ligustri_ on its food-plant, not immediately before one’s eyes in a
room, but at a distance as under natural conditions, it will be found
that the violet edges do not stand out brightly, but show a colour
very similar to that of the shadows playing about the leaves. The
coloured edges, in fact, produce a more effective breaking up of the
large green surface of the caterpillar’s body, than whitish stripes
alone. Of course if the insect was placed on a bare twig in the sun, it
would be easily visible at a distance; the larva never rests in such
a position, however, but always in the deep shadow of the leaves, in
which situation the coloured edges produce their peculiar effect. It
may be objected that the oblique white stripes, standing simply on
a dark green ground-colour, would produce the same effect, and that
my explanation therefore leaves the bright colouring of these edges
still unaccounted for. I certainly cannot say why in _Sphinx Ligustri_
these edges are lilac, and in _S. Drupiferarum_, _S. Prini_, and
_Dolba Hylæus_ red, nor why they are black and green in _Macrosila
Rustica_, and blue in _Acherontia Atropos_. If we knew exactly on what
plants these caterpillars fed originally, we might perhaps indulge in
comparing with an artistic eye the shadows playing about their leaves,
seeing in one case more red, and in another more blue or violet. The
coloured stripes of the _Sphingidæ_ must be regarded as the single
strokes of a great master on the countenance of a human portrait.
Looked into closely, we see red, blue, or even green spots and strokes;
but all these colours, conspicuous when close, disappear on retreating,
a general effect of colour being then produced, which cannot be
precisely described by words.
Quite in accordance with this explanation, we see caterpillars with
the brightest coloured stripes concealing themselves in the earth by
day, and betaking themselves to their food-plants only in the dusk of
the evening or dawn of morning and even during the night; _i.e._ in a
light so faint that feeble colours would produce scarcely any effect.
The bright blue of _Acherontia Atropos_, for example, would give the
impression of oblique shadows without any distinctive colour.
It is precisely the case of this last caterpillar, which formerly
appeared to me to present insurmountable difficulties to the
explanation of the coloured stripes by adaptation, and I believed that
this insect would have to be classed with those species which are
brightly coloured because they are distasteful, and are avoided by
birds. But although we have no experiments on this point, I must reject
this view. Unfortunately, we know scarcely anything of the ontogeny
of this caterpillar; but we know at least that the young larvæ (stage
four) are greener than the more purely yellow ones of the fifth stage
(which, however, are also frequently green), and we know further that
some adults are of a dark brownish-grey, without any striking colours.
From analogy with the dimorphism of the species of _Chærocampa_ and
_Sphinx_, fully considered previously, it must therefore be concluded
that in this case also, a new process of adaptation has commenced--that
the caterpillar is becoming adapted to the soil in and on which it
conceals itself by day.[144] An insect which acquires undoubted
protective colours cannot, however, be classed with those which possess
an immunity from hostile attacks.
That the coloured edges are correctly explained as imitations of the
oblique shadows of the leaf-ribs, may also be proved from another point
of view. Let us assume, for the sake of argument, that these coloured
stripes are not adaptive, and that they have not been produced by
natural selection, but by a hypothetical phyletic force. We should
then expect to see them appear at some period in the course of the
phyletic development--perhaps at first only in solitary individuals,
then in several, and finally in all; but we certainly could not expect
that at first single, irregular, coloured spots should arise in the
neighbourhood of the oblique white stripes--that these spots should
then multiply, and fusing together, should adhere to the white stripes,
so as to form an irregular spot-like edge, which finally becomes formed
into a straight, uniformly broad stripe. The phyletic development of
the coloured edges takes place, however, in such a manner, the species
of _Smerinthus_, as has already been established, showing this with
particular distinctness. In _S. Tiliæ_ the course of development can
be followed till the somewhat irregular red border is formed. In the
species of _Sphinx_ this border has become completely linear. It is
very possible that the ontogeny of _S. Ligustri_ or _Drupiferarum_
would reveal the whole process, although it may also be possible that
owing to the contraction of the development, much of the phylogeny is
already lost.
I have now arrived at the consideration of the last kind of marking
which occurs in the _Sphingidæ_, viz.:--
4. _Eye-spots and Ring-spots._--These markings, besides among the
_Sphingidæ_, are found only in a very few caterpillars, such as certain
tropical _Papilionidæ_ and _Noctuæ_. I know nothing of the conditions
of life and habits of these species, however, and without such
knowledge it is impossible to arrive at a complete explanation.
With Darwin, I take an eye-spot to be “a spot within a ring of another
colour, like the pupil within the iris,” but to this central spot
“concentric zones” maybe added. In the _Chærocampa_ larvæ and in
_Pterogon Œnotheræ_, in which complete ocelli occur, there are always
three zones--a central spot, the pupil, or, as I have called it, the
“nucleus;” then a light zone, the “mirror;” and, surrounding this
again, a dark zone (generally black), the “ground-area.”
As ring-spots I will consider those ocelli which are without the
nucleus (pupil), and which are not therefore, strictly speaking,
deceptive imitations of an eye, but present a conspicuous light spot
surrounded by a dark zone.
Between these two kinds of markings there is, however, no sharp
boundary, and morphologically they can scarcely be separated. Species
with ring-spots sometimes have nuclei, and ocellated larvæ in some
cases possess only a pale spot instead of a dark pupil. I deal here
with the two kinds separately, because it happens that they appear
in two distinct genera, in each of which they have their special
developmental history. Ring-spots originate in a different position,
and in another manner than eye-spots; but it must not, on this
account, be assumed without further inquiry, that they are called
into existence by the same causes; they must rather be investigated
separately, from their origin.
Eye-spots are possessed by the genera _Chærocampa_ and _Pterogon_;
ring-spots by the genus _Deilephila_. In accordance with the data
furnished by the above-given developmental histories, the origination
of these markings in the two genera may be thus represented:--
In the genera named, eye-spots and ring-spots are formed by the
transformation of single portions of the subdorsal line.
In _Chærocampa_ the primary ocelli originate on the fourth and fifth
segments by the detachment of a curved portion of the subdorsal, this
fragment becoming the “mirror,” and acquiring a dark encircling zone
(“ground-area”). The nucleus (pupil) is added subsequently.
In _Deilephila_ we learn from the development of _D. Hippophaës_, that
the primary annulus arises on the segment bearing the caudal horn (the
eleventh) by the deposition of a red spot on the white subdorsal line,
which is somewhat enlarged in this region. The formation of a dark
“ground-area” subsequently occurs, and with this, at first the partial,
and then the complete, detachment of the mirror-spot from the subdorsal
line takes place.
In both genera the spots arise at first locally on one or two segments,
from which they are transferred to the others as a secondary
character. In _Chærocampa_ this transference is chiefly backwards, in
_Deilephila_ invariably forwards.
We have now to inquire whether complete eye-spots--such as those of the
_Chærocampa_ larvæ--have any significance at all, and whether they are
of biological importance. It is clear at starting, that these spots do
not belong to that class of markings which make their possessors more
difficult of detection; they have rather the opposite effect.
We might thus be disposed to class ocellated caterpillars with those
“brightly coloured” species which, like the _Heliconinæ_ and _Danainæ_
among butterflies, possess a disgusting taste, and which to a certain
extent bear the signal of their distastefulness in their brilliant
colours. But even if I had not found by experiment that our native
_Chærocampa_ larvæ were devoured by birds and lizards, and that they
are not therefore distasteful to these insect persecutors, from the
circumstance that these caterpillars are all protectively coloured,
it could have been inferred that they do not belong to this category.
It has been found that all adaptively coloured caterpillars are
eaten, and one and the same species cannot possibly be at the same
time inconspicuously (adaptively) and conspicuously coloured; the one
condition excludes the other.
What other significance can eye-spots possess than that of making the
insects conspicuous? Had we to deal with sexually mature forms, we
should, in the first place, think of the action of sexual selection,
and should regard these spots as objects of taste, like the ocelli
on the feathers of the peacock and argus-pheasant. But we are here
concerned with larvæ, and sexual selection is thus excluded.
The eye-spots must therefore possess some other significance, or else
they are of no importance at all to the life of the insect, and are
purely “morphological characters;” in which case, supposing this could
be proved, they would owe their existence exclusively to forces innate
in the organism itself--a view which very closely approaches the
admission of a phyletic vital force.
I am of opinion, however, that eye-spots certainly possess a biological
value as a means of terrifying--they belong to that numerous class of
characters which occur in the most diverse groups of animals, and which
serve the purpose of making their possessors appear as alarming as
possible.
The caterpillars of the _Sphingidæ_ are known to behave themselves in
different manners when attacked. Some species, such, for instance,
as _Sphinx Ligustri_ and _Smerinthus Ocellatus_, on the approach of
danger assume the so-called Sphinx attitude; if they are then actually
seized, they dash themselves madly to right and left, by this means not
only attempting to get free, but also to terrify their persecutor.
This habit frequently succeeds with men, and more especially with
women and children; perhaps more easily in these cases than with their
experienced foes, birds.
The ocellated _Chærocampa_ larvæ behave differently. They remain quiet
on being attacked, and do not put on a Sphinx-like attitude, but only
withdraw the head and three small front segments into the large fourth
segment, which thus becomes much swollen, and is on this account taken
for the head of the insect by the inexperienced.[145] Now the large
eye-spots are situated on the fourth segment, and it does not require
much imagination to see in such a caterpillar an alarming monster with
fiery eyes, especially if we consider the size which it must appear to
an enemy such as a lizard or small bird. Fig. 28 represents the larva
of _C. Porcellus_ in an attitude of defence, although but imperfectly,
since the front segments can be still more withdrawn.
These facts and considerations do not, however, amount to scientific
demonstration, and I therefore made a series of experiments, in order
to determine whether these caterpillars did actually frighten small
birds. The first experiment proved but little satisfactory. A jay,
which had been domesticated for years, to which I threw a caterpillar
of _Chærocampa Elpenor_, did not give the insect any time for
manœuvring, but killed it immediately by a strong blow with its bill.
This bird had been tame for years, and was in the habit of pecking at
everything thrown to him. Perhaps a wild jay (_Garrulus Glandarius_)
would have treated the insect differently, but it is hardly possible
that such a large and courageous bird would have much respect for
our native caterpillars. I now turned to wild birds. A large brown
_Elpenor_ larva was placed in the food-trough of an open fowl-house
from which the fowls had been removed. A flock of sparrows and
chaffinches (_Fringilla Domestica_ and _Cœlebs_) soon flew down from
the neighbouring trees, and alighted near the trough to pick up stray
food in their usual manner. One bird soon flew on to the edge of the
trough, and was just about to hop into it when it caught sight of the
caterpillar, and stood jerking its head from side to side, but did not
venture to enter. Another bird soon came, and behaved in a precisely
similar manner; then a third, and a fourth; others settled on the perch
over the trough, and a flock of ten or twelve were finally perched
around. They all stretched their heads and looked into the trough, but
none flew into it.
I now made the reverse experiment, by removing the caterpillar and
allowing the birds again to assemble, when they hopped briskly into the
trough.
I often repeated this experiment, and always with the same result. Once
it could be plainly seen that it was really fear and not mere curiosity
that the birds showed towards the caterpillar. The latter was outside
the trough amongst scattered grains of food, so that from one side it
was concealed by the trough. A sparrow flew down obliquely from above,
so that at first it could not see the caterpillar, close to which it
alighted. The instant it caught sight of the insect, however, it turned
in evident fright and flew away.
Of course these experiments do not prove that the larger insectivorous
birds are also afraid of these caterpillars. Although I have not
been able to experiment with such birds, I can certainly prove that
even fowls have a strong dislike to these insects. I frequently
placed a large _Elpenor_ larva in the poultry yard, where it was soon
discovered, and a fowl would run hastily towards it, but would draw
back its head just when about to give a blow with the bill, as soon
as it saw the caterpillar closely. The bird would now run round the
larva irresolutely in a circle--the insect in the meantime assuming
its terrifying attitude--and stretching out its head would make ten
or twenty attempts to deal a blow with its bill, drawing back again
each time. All the cocks and hens acted in a similar manner, and it
was often five or ten minutes before one particularly courageous bird
would give the first peck, which would soon be followed by a second
and third, till the caterpillar, appearing palatable, would finally be
swallowed.
These experiments were always made in the presence of several persons,
in order to guard myself against too subjective an interpretation of
the phenomena; but they all invariably considered the conduct of the
birds to be as I have here represented it.[146]
If it be admitted that the ocelli of caterpillars are thus means of
exciting terror, the difficulty of their occurring in protectively
coloured species at once vanishes. They do not diminish the advantage
of the adaptive colouring, because they do not make the caterpillars
conspicuous, or at least any more easily visible at a distance,
excepting when the insects have assumed their attitude of alarm. But
these markings are of use when, in spite of protective colouring, the
larva is attacked by an enemy. The eye-spots accordingly serve the
caterpillar as a second means of defence, which is resorted to when the
protective colouring has failed.
By this it must not be understood that the ocelli of the _Chærocampa_
larvæ invariably possess only this, and no other significance for the
life of the insect. Every pattern can be conceived to render its
possessor in the highest degree conspicuous by strongly contrasted
and brilliant colouring, so that it might be anticipated that perfect
eye-spots in certain unpalatable species would lose their original
meaning, and instead of serving for terrifying become mere signals of
distastefulness. This is perhaps the case with _Chærocampa Tersa_ (Fig.
35), the numerous eye-spots of which make the insect easily visible.
Without experimenting on this point, however, no certain conclusion
can be ventured upon, and it may be equally possible that in this case
the variegated ocelli with bright red nuclei resemble the blossoms of
the food-plant (_Spermacoce Hyssopifolia_).[147] I here mention this
possibility only in order to show how an inherited form of marking,
even when as well-defined and complicated as in the present case, may,
under certain circumstances, be turned in quite another direction
by natural selection, for the benefit of its possessor. Just in the
same manner one and the same organ, such, for instance, as the limb
of a crustacean, may, in the course of phyletic development, perform
very different functions--first serving for locomotion, then for
respiration, then for reproduction or oviposition, and finally for the
acquisition of food.
I now proceed to the consideration of the biological value of
incomplete eye-spots, or, as I have termed them, ring-spots. Are these
also means of terrifying, or are they only signals of distastefulness?
I must at the outset acknowledge that on this point I am able to offer
but a very undecided explanation. The decision is only to be arrived
at by experiments conducted with each separate species upon which
one desires to pronounce judgment. It is not here legitimate to draw
analogical inferences, and to apply one case to all, since it is not
only possible, but very probable, that the biological significance
of ring-spots changes in different species. Nothing but a large
series of experiments could completely establish this. Unfortunately
I have hitherto failed in obtaining materials for this purpose. I
would have deferred the publication of this essay for a year, could
I have foreseen with certainty that such materials would have been
forthcoming in sufficient quantity during the following summer; but
this unfortunately depends very much upon chance, and I believed that
a preliminary conclusion would be preferable to uncertainty. Perhaps
some entomologist to whom materials are more easily accessible, may, by
continuing these experiments, accomplish this object.
The experiments hitherto made by other observers, are not sufficient
for deciding the question under consideration. Weir,[148] as is
well known, showed that certain brightly coloured and conspicuous
larvæ were refused by insectivorous birds; and Butler[149] proved the
same for lizards and frogs. These experiments are unfortunately so
briefly described, that in no case is the species experimented with
mentioned by name, so that we do not know whether there were any Sphinx
caterpillars among them.[150] I have likewise experimented in this
direction with lizards, in order to convince myself of the truth of
the statement that (1) there are caterpillars which are not eaten on
account of their taste, and (2) that such larvæ possess bright colours.
I obtained positive, and on the whole, very decided results. Thus,
the common orange and blue striped caterpillars of _Bombyx Neustria_
enjoyed complete immunity from the attacks of lizards, whilst those of
the nearly allied _Eriogaster Lanestris_ and _L. Pini_ were devoured,
although not exactly relished. That the hairiness is not the cause
of their being unpalatable, is shown by the fact that _L. Pini_ is
much more hairy than _B. Neustria_. The very conspicuous yellow and
black ringed caterpillar of _Euchelia Jacobææ_ gave also most decided
results. I frequently placed this insect in a cage with _Lacerta
Viridis_, but they would never even notice them, and I often saw the
caterpillars crawl over the body, or even the head of the lizards,
without being snapped at. On every occasion the larvæ remained for
several days with the lizards without one being ever missed. The
reptiles behaved in a precisely similar manner with respect to the moth
of _E. Jacobææ_, not one of which was ever touched by them. The yellow
and black longitudinally striped caterpillars of _Pygæra Bucephala_
were also avoided, and so were the brightly coloured larvæ of the
large cabbage white (_Pieris Brassicæ_), which when crushed give a
disagreeable odour. This last property clearly shows why lizards reject
this species as distasteful. Both caterpillar and butterfly possess
a blood of a strong yellow colour and oily consistency, in which,
however, I could not detect such a decided smell as is emitted by that
of the _Heliconinæ_ and _Danainæ_.[151]
I next made the experiment of placing before a lizard a caterpillar
as much as possible like that of _E. Jacobææ_. Half grown larvæ of
_Bombyx Rubi_ likewise possess golden yellow (but narrower) transverse
rings on a dark ground, and they are much more hairy than those of
_E. Jacobææ_. The lizard first applied its tongue to this caterpillar
and then withdrew it, so that I believed it would also be avoided;
nevertheless it was subsequently eaten. The caterpillars of _Saturnia
Carpini_ were similarly devoured in spite of their bristly hairs,
and likewise cuspidate larvæ (_Dicranura Vinula_), notwithstanding
their extraordinary appearance and their forked caudal horn.[152]
These lizards were by no means epicures, but consumed large numbers
of earth-worms, slugs, and great caterpillars, and once a specimen of
the large and powerfully biting Orthopteron, _Decticus Verucivorus_.
Creatures which possessed a strongly repugnant odour were, however,
always rejected, this being the case with the strongly smelling
beetle, _Chrysomela Populi_, as also with the stinking centipede,
_Iulus Terrestris_, whilst the inodorous _Lithobius Forficatus_ was
greedily eaten. I will call particular attention to these last facts,
because they favour the supposition that with rejected caterpillars a
disgusting odour--although perhaps not always perceptible by us--is the
cause of their being unpalatable.
Striking colours are of course only signals of distastefulness, and the
experiment with _Bombyx Rubi_ shows that the lizards were from the
first prejudiced against such larvæ, the prejudice only being overcome
on actually trying the specimen offered. A subsequent observation
which I made after arriving at this conclusion, is most noteworthy.
After the lizard had learnt by experience that there might be not only
distasteful caterpillars (_E. Jacobææ_), but also palatable ones banded
with black and yellow (_B. Rubi_), it sometimes tasted the _Jacobææ_
larvæ, as if to convince itself that the insect was actually as it
appeared to be, viz., unpalatable!
A striking appearance combined with a very perceptible and penetrating
odour is occasionally to be met with, as in the caterpillar of the
common Swallow-tail, _Papilio Machaon_. I have never seen a lizard
make the slightest attempt to attack this species. I once placed two
large specimens of this caterpillar in the lizard vivarium, where they
remained for five days, and finally pupated unharmed on the side of the
case.
I have recorded these experiments, although they do not thus far
relate to Sphinx-caterpillars, with the markings of which we are
here primarily concerned, because it appeared to me in the first
place necessary to establish by my own experiments that signals of
distastefulness did occur in caterpillars.
I now come to my unfortunately very meagre experience with _Deilephila_
larvæ, with only two species of which have I been able to experiment,
viz., _D. Galii_ and _Euphorbiæ_.
The first of these was constantly rejected. Two large caterpillars, one
of the black and the other of the yellow variety, were left for twelve
hours in the lizard vivarium, without being either examined or touched.
It thus appears that _D. Galii_ is a distasteful morsel to lizards;
and the habits of the caterpillar are quite in accordance with this,
since it does not conceal itself, but rests fully exposed by day on a
stem, so that it can scarcely escape being detected. It is almost as
conspicuous as _D. Euphorbiæ_.
I was much surprised to find, however, that this last species was not
rejected by lizards. On placing a large caterpillar, six to seven
centimeters long, in the vivarium, the lizard immediately commenced
to watch it, and as soon as it began to crawl about, seized it by the
head, and, after shaking it violently, commenced to swallow it. In
spite of its vigorous twisting and turning, the insect gradually began
to disappear, amidst repeated shakings; and in less than five minutes
was completely swallowed.[153] With regard to lizards, therefore, the
prominent ring-spots of this larva are not effective as a means of
alarm, nor are they considered as a sign of distastefulness.
Unfortunately I have not hitherto been able to make any experiments
with birds. It would be rash to conclude from the experience with
lizards that ring-spots were of no biological value. There is scarcely
any one means of protection which can render its possessor secure
against _all_ its foes. The venom of the most poisonous snakes does
not protect them from the attack of the secretary bird (_Serpentarius
Secretarius_) and serpent eagle (_Spilornis Cheela_); and the adder,
as is well known, is devoured by hedgehogs without hesitation. It
must therefore be admitted that many species which are protected by
distastefulness, may possess certain foes against which this quality
is of no avail. Thus, it cannot be said that brightly coloured
caterpillars, which are not eaten by birds and lizards, are also spared
by ichneumons. It is readily conceivable therefore, that the larva of
_D. Euphorbiæ_ may not be unpalatable to lizards, because they swallow
it whole; whilst it is perhaps distasteful to birds, because they must
hack and tear in order to swallow it.
From these considerations it still appears most probable to me that
_D. Euphorbiæ_, and the nearly allied _D. Dahlii_ and _Mauritanica_,
bear conspicuous ring-spots as signs of their being unpalatable to the
majority of their foes. The fact that these species feed on poisonous
_Euphorbiaceæ_, combined with their habit of exposing themselves openly
by day, so as to be easily seen at a distance, may perhaps give
support to this view. As these insects are not protectively coloured,
this habit would long ago have led to their extermination; instead
of this, however, we find that in all situations favourable to their
conditions of life they are among the commonest of the _Sphingidæ_.
Thus, _D. Euphorbiæ_ occurs in large numbers both in South and North
Germany (Berlin); and Dr. Staudinger informs me that in Sardinia the
larvæ of _D. Dahlii_ were brought to him by baskets full.
But if the conspicuous ring-spots (combined of course with the other
bright colours) may be regarded as signals of distastefulness in many
species of _Deilephila_, this by no means excludes the possibility that
in some species these markings play another part, and are effective as
a means of alarm. It even appears conceivable to me that in one and the
same caterpillar they may play both parts against different foes, and
it would certainly be of interest to confirm or refute this supposition
by experiment.
In the light yellow variety of the caterpillar of _D. Galii_ the
ring-spots may serve as means of alarm, and still more so in that of
_D. Nicæa_, the resemblance of which to a snake has struck earlier
observers.[154]
In those species of _Deilephila_ which conceal themselves by day, the
ring-spots cannot be considered as signals of distastefulness, and they
must therefore have some other meaning. As examples of this class may
be mentioned _D. Vespertilio_, which is protectively coloured both in
the young and in the adult stages; and likewise _D. Hippophaës_, in
which this habit of concealment is associated with adaptive colouring.
In the case of the first-named species, it appears possible that the
numerous large ring-spots may serve to alarm small foes, but the
truth of this supposition could only be decided by experiment. In _D.
Hippophaës_, on the other hand, such an interpretation must be at once
rejected, since most individuals possess but a single ring-spot, which
shows no resemblance whatever to an eye.
I long sought in vain for the meaning of this ring-spot, the
discovery of which would in this particular case be of the greatest
value, because we have here obviously the commencement of the whole
development of ring-spots before us--the initial stage from which the
marking of all the other species of _Deilephila_ has proceeded.
I believe that I have now found the correct answer to this riddle, but
unfortunately at a period of the year when I am unable to prove it
experimentally. I consider that the ring-spots are crude imitations of
the berries of the food-plant. The latter are orange-red, and exactly
of the same colour as the spots; the agreement in colour between the
latter and the berries is quite as close as that between the leaves and
the general colouring of the caterpillar. I know of no species which
more closely resembles the colour of the leaves of its food-plant,
the dark upper side and light under side corresponding in the leaves
and caterpillars. The colour of the _Hippophae_ is not an ordinary
green, but a grey-green, which shade also occurs, although certainly
but rarely, in the larvæ. I may expressly state that I have repeatedly
shown to people as many as six to eight of the large caterpillars on
one buckthorn branch, without their being able at once to detect them.
It is not therefore mere supposition, but a fact, that this species
is protected by its general colouring. At first the orange-red spots
appear rather to diminish this protection--at least when the insects
are placed on young shoots bearing no berries. But since at the same
time when the berries become red (end of July and the beginning of
August) the caterpillars are in their last stage of development (_i.e._
possess red spots), it appears extremely probable that these spots are
vague representations of the berries. For the same reason that these
caterpillars have acquired the habit of feeding only at dusk and during
the morning twilight, or at night, and of concealing themselves by
day, it must be advantageous for them to have the surface of their
large bodies not only divided by white stripes, but also interrupted
in yet another manner. How could this be better effected than by two
spots which, in colour and position, represent the grouping of the red
berries on the branches? When feeding, the insect always rests with
the hind segments on a branch, the front segments only being more or
less raised and held parallel to the leaves; the red spots thus always
appear on the stem, where the berries are likewise situated. It might
indeed be almost supposed that the small progress which the formation
of secondary ring-spots on the other segments has made up to the
present time, is explicable by the fact that such berry-like spots on
other portions of the caterpillar would be rather injurious than useful.
It may, however, be asked how an imitation of red berries, which are
eaten by birds just as much as other berries, can be advantageous to a
caterpillar, since by this means it would rather attract the attention
of its enemies?
Two answers can be given to this. In the first place, the berries are
so numerous on every plant that there is but a very small chance of the
smaller and less conspicuous berry-spots catching the eye of a bird
before the true berries; and, secondly, the latter, although beginning
to turn red when the caterpillars are feeding, do not completely
ripen till the autumn, when the leaves are shed, and the yellowish-red
clusters of berries can be seen at a distance. The caterpillar,
however, pupates long before this time.
I have considered this case in such detail because it appears to me
of special importance. It is the only instance which teaches us that
the rows of ring-spots of the _Deilephila_ larvæ proceed from one
original pair--the only instance which permits of the whole course of
development being traced to its origin. Were it possible to arrive at
the causes of the formation of these spots, their original or primary
significance would thereby be made clear.
I will now briefly summarise the results of the investigation of the
biological value of the _Deilephila_ ring-spots.
In the known species of the genus now existing these spots have
different meanings.
In some species (certainly in _Galii_, and probably in _Euphorbiæ_
and _Mauritanica_) the conspicuous ring-spots serve as signals of
distastefulness for certain enemies (not for all).
In a second group of species they serve as a means of alarm, like the
eye-spots of the _Chærocampa_ larvæ (_Nicæa_? light form of _Galii_?).
Finally, in a third group, of which I can at present only cite
_Hippophaës_, they act as an adaptive resemblance to a portion of a
plant, and enhance the efficacy of the protective colouring.
5. _Subordinate Markings._--If, from the foregoing
considerations, it appears that the three chief elements of the
Sphinx-markings--longitudinal and oblique stripes, and spot
formations--are not purely morphological characters, but have a very
decided significance with respect to their possessors, there should be
no difficulty in referring the whole of the markings of the _Sphingidæ_
to the action of natural selection, supposing that these three kinds of
marking were the only ones which actually occurred.
In various species, however, there appear other patterns, which I have
comprised under the term “subordinate markings,” some of which I will
select, for the purpose of showing the reasons which permit of their
being thus designated.
I ascribe to this category, for example, that fine network of dark
longitudinal streaks which often extends over the whole upper side
of the caterpillar, and which is termed the “reticulation.” This
character is found chiefly in the adult larvæ of _Chærocampa_, being
most strongly pronounced in the brown varieties: it occurs also in
_Deilephila Vespertilio_, _Pterogon Œnotheræ_, and _Sphinx Convolvuli_.
As far as I know, it is only associated with adaptive colours, and
indeed occurs only in those caterpillars which rest periodically at
the base of their food-plants among the dead leaves and branches. I do
not consider this reticulation to be a distinct imitation, but only as
one of the various means of breaking up the large uniform surface of
the caterpillar so as to make it present inequalities, and thus render
it less conspicuous. There can be no doubt as to the dependence of this
character upon natural selection.
There is, however, a second group of markings, which must be referred
to another origin. To this group, for instance, belong those light dots
in _Chærocampa Porcellus_ and _Elpenor_ which have been termed “dorsal
spots.” I know of no other explanation for these than that they are the
necessary results of other new formations, and depend on correlation
(Darwin), or, as I may express it, they are the result of the action of
the law governing the organization of these species.
As long as we are confined to the mere supposition that the character
in question may be the outward expression of an innate law of growth,
it is permissible to attempt to show that a quite similar formation in
another species depends upon such a law.
Many of the dark specimens of _Sphinx Convolvuli_ show whitish dots
on segments six to eleven, one being situated on the front edge of
each of these segments, at the height of the completely vanished
subdorsal line (Fig. 52). These spots vary much in size, lightness,
and sharpness of definition. Now it might be difficult to attribute
any biological significance to this character, but its origin becomes
clear on examining light specimens in which the oblique white stripes
are distinct on the sides and the subdorsal line is retained at least
on the five or six anterior segments. It can then be seen that the
spots are located at the points of intersection of the subdorsal and
the oblique stripes (Fig. 16, Pl. III.), and they can accordingly be
explained by the tendency to the deposition of light pigment being
twice as great in these positions as in other portions of the two
systems of light lines. Light spots are thus formed when the lines
which cross at these points are partially or completely extinct
throughout their remaining course.
A marking is therefore produced in this case by a purely innate law of
growth--by the superposition of two ancient characters now rudimentary.
Many other unimportant details of marking must be regarded as having
been produced in a similar manner, although it may not be possible to
prove this with respect to every minute spot and stripe. The majority
of “subordinate markings” depend on the commingling of inherited, but
now meaningless, characters with newly acquired ones.
It would be quite erroneous to attribute to natural selection only
those characters which can be demonstrated to still possess a
biological value in the species possessing them. They may be equally
due to heredity. Thus, it is quite possible that the faint and
inconspicuous ring-spots of _Deilephila Vespertilio_ are now valueless
to the life of the species--they may be derived from an ancestral form,
and have not been eliminated by natural selection simply because they
are harmless. I only mention this as a hypothetical case.
In the case of markings of the second class, _i.e._ oblique stripes,
a transference to later phyletic stages can be demonstrated, although
the stripes thereby lose their original biological value. Thus, the
_Chærocampa_ larvæ, when they were green throughout their whole life
and adapted to the leaves, appear to have all possessed light oblique
stripes in imitation of the leaf-ribs. All the species of the older
type of colouring and marking, such as _Chærocampa Syriaca_ (Fig. 29)
and _Darapsa Chœrilus_ (Fig. 34), and also the light green young forms
of _C. Elpenor_ (Fig. 20), and _Porcellus_ (Figs. 25 and 26), show
these oblique stripes. In these last species the foliage imitation
is abandoned at a later stage, and a dark brown, or blackish-brown,
ground-colour acquired. Nevertheless the oblique stripes do not
disappear, but show themselves--in the fourth stage especially, and
sometimes in the fifth--as distinct dirty yellow stripes, although not
so sharply defined as in the earlier stages. These persistent stripes,
in accordance with their small biological value, are very variable,
since they are only useful in so far as they help to break up the
large surface presented by the caterpillar, and are of no value as
imitations of surrounding objects.
The oblique stripes of _Sphinx Convolvuli_ offer a precisely similar
case; and it may be safely predicted that the young forms of this
species would possess sharply defined light oblique stripes, since
more or less distinct remnants of these markings occur in all the
adult larvæ, and especially in the green form. The entire pattern of
this caterpillar depends essentially on the commingling of characters
persisting from an earlier period, _i.e._ of residues of the subdorsal
and oblique stripes, both these markings being extraordinarily
variable. The black reticulation was added to the ground-colour as
a new means of adaptation, this character appearing only in the
phyletically younger brown form, and being entirely absent, or only
faintly indicated, in the older green variety.