PHYLETIC DEVELOPMENT OF THE MARKINGS OF THE SPHINGIDÆ: SUMMARY AND
CONCLUSION.
If, from the form possessed by many of the caterpillars of the
_Sphingidæ_ on their emergence from the egg, we may venture to draw
a conclusion concerning the oldest phyletic stage, these larvæ were
originally completely destitute of marking. The characteristic caudal
horn must be older than the existing markings, since it is present in
the younger stages (except in cases where it is altogether wanting),
and is generally even larger than at a later age.
There is, however, further evidence that there were once Sphinx-larvæ
without any markings. Such a species now exists. I do not mean the
boring caterpillars of the _Sesiidæ_,[160] which live in the dark,
and are therefore colourless, but I refer to a large larva (over six
centimeters long) preserved in spirit in the Berlin Museum,[161] which,
from its form, belongs to the _Smerinthus_ group. It possesses a caudal
horn, and on the whole upper surface is covered with short and sparsely
scattered bristles, such as occur in the _Sesiidæ_. The colour of this
unknown insect appears to have been light green, although it now shows
only a yellowish shade. Every trace of marking is absent, and it thus
corresponds exactly with the youngest stages of the majority of the
existing Sphinx-larvæ--even to the short bristles sparsely scattered
over the whole upper surface of its body. We have therefore, so to
speak, a living fossil before us, and it would be of great interest to
ascertain its history.
All the data furnished by the developmental history go to show that
of the three kinds of markings which occur in the _Sphingidæ_, viz.,
longitudinal and oblique stripes and spots, the first is the oldest.
Among the species which are ornamented with oblique stripes or spots
there are many which are longitudinally striped in their young stages,
but the reverse case never occurs--young larvæ never show spots or
oblique stripes when the adult is only striped longitudinally.
The first and oldest marking of the caterpillars of the _Sphingidæ_ was
therefore the longitudinal striping, or, more precisely speaking, the
subdorsal, to which dorsal and spiracular lines may have been added.
That this second stage of phyletic development has also been preserved
in existing species has already been sufficiently shown; the greater
portion of one group, the _Macroglossinæ_, has indeed remained at this
stage of development.
From the biological value which must be attributed to this kind of
marking, its origination by natural selection presents no difficulty.
The first rudiments of striping must have been useful, since they must
have broken up the large surface of the body of the caterpillar into
several portions, and would thus have rendered it less conspicuous to
its enemies.
Thus it is not difficult to perceive how a whole group of genera could
have made shift with this low grade of marking up to the present time.
Colour and marking are not the only means of offence and defence
possessed by these insects; and it is just such simply-marked larvæ as
those of the _Macroglossinæ_ which have the protective habit of feeding
only at night, and of concealing themselves by day. Moreover, under
certain conditions of life the longitudinal stripes may be a better
means of protection, even for a Sphinx-larva, than any other marking;
and all those species in which this pattern is retained at the present
time live either among grasses or on _Coniferæ_.
It cannot be properly said that the second form of marking--the
oblique stripes--has been developed out of the first. If these had
arisen by the transformation of the longitudinal stripes, the two
forms could not exist side by side. This is the case, however, both
in certain species in the adult state (_Calymnia Panopus_[162]), as
well as in others during their young stages (most beautifully seen
in _Smerinthus Populi_, Fig. 56). Various facts tend to show that
the oblique stripes appeared in the phyletic development _later_
than the longitudinal lines. In the first place they appear later
than the latter in the ontogeny of certain species. This is the case
with _Chærocampa Elpenor_ and _Porcellus_, in which, however, they
certainly do not reach a high state of development. Then again, the
longitudinal lines disappear completely in the course of the ontogeny,
whilst the oblique stripes alone maintain their ground. Thus, the
subdorsal line vanishes at a very early stage, with the exception of
a small residue,[163] in all native species of _Smerinthus_. I have
already attempted to show that new characters are only acquired _in the
last stage_, and that if still newer ones are then added, the former
disappear from the last stage, and are transferred back to a younger
one. _Characters vanish therefore from a stage in the same order as
they were acquired._
Finally, among the genera with longitudinal stripes (_e.g.
Macroglossa_) we know certain species which, when at an advanced
age, possess oblique stripes (_M. Fuciformis_), although these slant
in a direction opposite to those of most of the other larvæ of the
_Sphingidæ_. These are, however, always species which differ from their
allies in their mode of life, not feeding on grasses or low plants, but
on large-leaved shrubs. If we were able to ascertain the ontogeny of
these species, we should find that the oblique stripes appeared late in
life, as has already been shown in the case of _Pterogon Œnotheræ_.
If it be asked why the longitudinal lines were first formed, and then
the oblique stripes, it may be replied that the physical constitution
of these caterpillars would be more easily able to give rise to simple
longitudinal lines than to complicated oblique stripes crossing their
segments.[164] It may perhaps also be suggested that the oldest
_Sphingidæ_ lived entirely on low plants among grasses, and in the
course of time gradually took to shrubs and trees. At the present time
the majority of the Sphinx-larvæ still live on low plants, and but few
on trees, such caterpillars generally belonging to certain special
genera.
The character of oblique stripes becomes perfected by the addition
of coloured edges, the latter, as is self-evident, having been added
subsesequently.
The third chief constituent of the Sphinx-markings, _i.e._ the
spots--whether perfect ocelli or only ring-spots--in two of the special
genera here considered, arise on the subdorsal, where they are either
deposited (_Deilephila_), or built up from a fragment of this line
(_Chærocampa_). That these markings can, however, also originate
independently of the subdorsal, is shown by the ocellus of _Pterogon
Œnotheræ_, situated on the segment bearing the caudal horn. In this
case, however, the ontogeny teaches us that the spot also succeeds the
subdorsal, so that we can state generally that all these spot-markings
are of later origin than the longitudinal striping.
The question as to the relative ages of the oblique stripes and the
spot-marking does not admit of a general answer. In some cases (_C.
Elpenor_ and _Porcellus_) the oblique stripes disappear when the ocelli
reach complete development, and we may therefore venture to conclude
that in these cases the former appeared earlier in the phylogeny.
But it is very probable that oblique stripes arose independently at
different periods, just as longitudinal lines occur irregularly in
quite distinct families. It would be a great error if we were to
ascribe the possession of oblique stripes solely to descent from a
common ancestor. The oblique markings found on certain species of
_Macroglossa_ (_M. Corythus_ from India) have not been inherited from
a remote period, but have been independently acquired by this or by
some recent ancestral species. They have nothing to do _genetically_
with the oblique stripes which occur in some species of _Chærocampa_
(_e.g._ in _C. Nessus_, from India), or with those of the species of
_Smerinthus_ and _Sphinx_. They depend simply on analogous adaptation
(Seidlitz[165]), _i.e._ on adaptation to an analogous environment.
The case is similar with the spot-markings. I have already shown that
under certain conditions ring-spots may assume the exact appearance of
eye-spots by the formation of a nucleus in the “mirror,” such as occurs
occasionally in _Deilephila Euphorbiæ_ (Fig. 43), more frequently
in _D. Galii_, and as a rule in _D. Vespertilio_. Nevertheless,
these markings arise in quite another manner to the eye-spots of the
_Chærocampinæ_, with which they consequently have no genetic relation;
the two genera became separated at a time when they neither possessed
spot-markings. Further, in _Pterogon Œnotheræ_ we find a third kind
of spot-marking, which is most closely allied to the ocelli of the
_Chærocampa_-larvæ, but is situated in quite another position, and must
have originated in another manner, and consequently quite independently
of these eye-spots.
It can also be readily understood why the first and second elements of
the markings of the _Sphingidæ_ should be mutually exclusive, and not
the second and third or the first and third.
A light longitudinal line cutting the oblique stripes, considerably
diminishes that resemblance to a leaf towards which the latter have a
tendency, and it is therefore only found in cases where an adaptive
marking can be of no effect on account of the small size of the
caterpillar, _i.e._ in quite young stages. (See, for instance, Fig.
56, the first stage of _S. Populi_.) At a later period of life the
old marking must give way to the new, and we accordingly find that
the subdorsal line vanishes from all the segments on which oblique
stripes are situated, and is only retained on the anterior segments
where the latter are wanting. In some few cases both elements of
marking certainly occur together, such as in _Calymnia Panopus_ and
_Macroglossa Corythus_; but the oblique stripes are, under these
circumstances, shorter, and do not extend above the subdorsal line, and
in _Darapsa Chœrilus_ even become fused into the latter.[166]
In certain cases there may also be a special leaf structure imitated
by the longitudinal lines, but on the whole the latter diminish the
effect of the oblique stripes; and we accordingly find that not only
has the subdorsal disappeared from those segments with oblique stripes,
but that most larvæ with this last character are also without the
otherwise broad spiracular and dorsal lines. This is the case with all
the species of _Smerinthus_[167] known to me, as well as with all the
species of the genera _Sphinx_, _Dolba_, and _Acherontia_.
Oblique stripes and spot-markings are not, however, necessarily
mutually exclusive in their action, and we also find these in certain
cases united in the same larva, although certainly never in an equal
state of perfection. Thus, _Chærocampa Nessus_[168] possesses strongly
marked oblique stripes, but feebly developed ocelli; and, on the other
hand, _Chærocampa Elpenor_ shows strongly developed eye-spots, but the
earlier oblique stripes are at most only present as faint traces. This
is easily explained by the mode of life. These caterpillars--at least
such of them as are perfectly known--do not live on plants with large,
strongly-ribbed leaves, and are even in the majority of individuals
adapted to the colour of the soil; the oblique stripes have therefore
in these cases only the significance of rudimentary formations.
That the first and third forms of markings also are not always mutually
prejudicial in their action is shown by the case of _Chærocampa
Tersa_, in which the eye-spots certainly appear to possess some
other significance than as a means of causing terror. In most of the
_Chærocampa_-larvæ the subdorsal line disappears in the course of the
phylogeny, and it can be understood that the illusive appearance of the
eye-spots would be more perfect if they did not stand upon a white
line.
If we consider the small number of facts with which I have here been
able to deal, the result of these investigations will not be deemed
unsatisfactory. It has been possible to show that each of the three
chief elements of the markings of the _Sphingidæ_ have a biological
significance, and their origin by means of natural selection has thus
been made to appear probable. It has further been possible to show
that the first rudiments of these markings must also have been of
use; and it thus appears to me that their origin by means of natural
selection has been proved to demonstration. Moreover, it has not been
difficult to understand the displacement of the primary elements of the
markings by secondary characters added at a later period, as likewise
an essential effect of natural selection. Finally, it has been possible
to explain also the subordinate or accessory elements of the markings,
partly by the action of natural selection, and partly as the result of
markings formerly present acting by correlation.
From the origin and gradual evolution of the markings of the
_Sphingidæ_ we may accordingly sketch the following picture:--
The oldest Sphinx-larvæ were without markings; they were probably
protected only by adaptive colouring, and a large caudal horn, and by
being armed with short bristles.
Their successors, through natural selection, became longitudinally
striped; they acquired a subdorsal line extending from the horn to the
head, as well as a spiracular, and sometimes also a dorsal, line. The
caterpillars thus marked must have been best hidden on those plants
in which an arrangement of parallel linear parts predominated; and we
may venture to suppose that at this period most of the larvæ of the
_Sphingidæ_ lived on or among such plants (grasses).
At a later period oblique stripes were added to the longitudinal
lines, the former (almost always) slanting across the seven hindmost
segments from the back towards the feet in the direction of the
caudal horn. Whether these stripes all arose simultaneously, or, as
is more probable, whether only one at first appeared, which was then
transferred to the other segments by correlation assisted by natural
selection, cannot, at least from the facts available, at present be
determined.
On the whole, as the oblique stripes became lengthened towards the
back, the longitudinal lines disappeared, since they injured the
deceptive effect of the stripes. In many species also there were formed
dark or variegated coloured edges to the oblique stripes, in imitation
of the shadow lines cast by the leaf-ribs.
Whilst one group of _Sphingidæ_ (_Sphinx_, _Smerinthus_) were thus
striving to make their external appearance approximate more and more to
that of a ribbed leaf, others of the longitudinally striped species
became developed in another manner.
Some of the latter lived indeed on bush-like leaved plants, but no
oblique stripes were developed, because these would have been useless
among the dense, narrow, and feebly-ribbed leaves of the food-plants.
These caterpillars, from the earlier markings, simply retained the
longitudinal lines, which, combined with a very close resemblance to
the colour of the leaves, afforded them a high degree of protection
against discovery. This protection would also have been enhanced if
other parts of the food-plant, such as the berries (_Hippophaës_), were
imitated in colour and position in such a manner that the large body
of the caterpillar contrasted still less with its environment. In this
way the first ring-spot probably arose in some species on only one--the
penultimate segment.
As soon as this first pair of ring-spots had become an established
character of the species, they had a tendency to become repeated
on the other segments, advancing from the hind segments towards
the front ones. Under certain conditions this repetition of the
ring-spots might have been of great disadvantage to the species, and
would therefore have been as far as possible prevented by natural
selection (_Hippophaës_); in other cases, however, no disadvantage
would have resulted--the caterpillar, well adapted to the colour of its
food-plant, would not have been made more conspicuous by the small
ring-spots, which might thus have become repeated on all the segments
(_Zygophylli_). In cases like the two latter, striking colours must
have been eliminated when inherited from an immediate ancestor; but on
this point nothing can as yet be said with certainty.
In other cases the repetition of the ring-spots with strongly
contrasted colours was neither prejudicial nor indifferent, but could
be turned to the further advantage of the species. If a caterpillar fed
on plants containing acrid juices (_Euphorbiaceæ_) which, by permeating
its alimentary system, rendered it repulsive to other animals, the
ring-spots commencing to appear (by repetition) would furnish an
easy means for natural selection to adorn the species with brilliant
colours, which would protect it from attack by acting as signals of
distastefulness.
But if the dark spots stood on a light ground (_Nicæa_), they would
present the appearance of eyes, and cause their possessors to appear
alarming to smaller foes.
From the developmental histories and biological data at present before
us, it cannot with certainty be said which of these two functions of
the ring-spots was first acquired in the phylogeny, but we may perhaps
suppose that their significance as a means of causing alarm was arrived
at finally.
It may also be easily conceived that as the ring-spots became more and
more complicated, they would occasionally have played other parts,
being fashioned once again in these stages into imitations of portions
of plants, such as a row of berries or flower-buds. For this, however,
there is as yet no positive evidence.
As the ring-spots became detached from the subdorsal line out of which
they had arisen, the latter disappeared more and more completely from
the last ontogenetic stage, and receded towards the younger stages of
life of the caterpillar--it became _historical_. This disappearance
of the subdorsal may also be explained by the fact that the original
longitudinal stripe imitating the linear arrangement of leaves would
become meaningless, even if it did not always diminish the effect of
the ring-spots. But characters which have become worthless are known
in the course of time to become rudimentary, and finally to disappear
altogether. I do not believe that disuse alone causes such characters
to vanish, although in the case of active organs it may have a large
share in this suppression. With markings it cannot, however, be a
question of use or disuse--nevertheless they gradually disappear as
soon as they become meaningless. I consider this to be the effect of
the arrest of the controlling action of natural selection upon these
characters (suspension of the so-called “conservative adaptation,”
Seidlitz). Any variations may become of value if the character
concerned is met with in the necessary state of fluctuation. That this
process of extinction does not proceed rapidly, but rather with extreme
slowness, is seen in the ontogeny of several species of _Deilephila_,
which retain the now meaningless subdorsal line through a whole series
of stages of life.
In another group of Sphinx-larvæ with longitudinal stripes, an eye-spot
became developed independently of the subdorsal line, in the position
of the caudal horn, which has here vanished with the exception of a
small knob-like swelling. This character--which we now see perfected in
_Pterogon Œnotheræ_--undoubtedly serves as a means of causing terror;
but whether the incipient stages possessed the same significance,
cannot be decided in the isolated case offered by the one species of
the genus _Pterogon_ possessing this marking.
In a third group of longitudinally striped caterpillars, the younger
genus _Chærocampa_, eye-spots were developed directly from portions of
the subdorsal line, at first only on the fourth and fifth segments. It
can be here positively asserted that this character served as a means
of alarm from its very commencement. It is certainly for this reason
that we see the subdorsal line in the immediate neighbourhood of the
spots disappear at an early stage, whilst it is retained on the other
segments for a longer period. A portion of the younger (tropical)
species of this group then developed similar, or nearly similar, ocelli
on the remaining segments by correlation; and it may now have occurred
that in solitary cases the eye-spots acquired another significance
(_C. Tersa_?), becoming of use as a disguise by resembling berries or
flower-buds. It is also conceivable that the eye-spots may in other
cases have been converted into a warning sign of distastefulness.
In all those larvæ which possessed purely terrifying markings,
however, not only was the original protective colouring preserved,
but in most of them this colour gradually became replaced by a
better one (adaptation of the adult larva to the soil). The oblique
stripes imitating the leaf-ribs also are by no means lost, but are
almost always present, although but feebly developed, and often only
temporarily.
The pattern formed by the oblique stripes may also be retained, even
with perfect adaptation to the soil, and may be converted to a new use
by losing its sharpness, and, instead of imitating definite parts of
plants, may become transformed into an irregular and confused marking,
and thus best serve to represent the complicated lights and shadows,
stripes, spots, &c., cast on the ground under low-growing plants from
between the stems and dead leaves.
Just as in the case of ocellated species where caterpillars without
eye-spots may retain and newly utilize their older markings, so larvæ
having oblique stripes with the most diversely coloured edges may show
the same markings in allied (younger?) species, both in a rudimentary
and in a transformed condition. These markings may thus contribute to
the formation of a latticed or reticulated pattern. Even the oldest
marking, the subdorsal line, may still play a part, since its remnants
cause certain portions of the complicated pattern to appear more
strongly marked (_S. Convolvuli_). Finally, when an adaptation to a
changing environment intersected by lights and shadows is required,
new markings may be here added as in other cases, viz., dark streaks
extending over the light surface of the whole caterpillar.
In concluding this essay, I may remark that, with respect to the
wide and generally important question which gave rise to these
investigations, a clearer and simpler result has been obtained than
could have been expected, considering the complexity of the characters
requiring to be traced to their causes, as well as our still highly
imperfect knowledge of ontogenetic and biological facts.
For a long time I believed that it was not possible to trace all the
forms of marking and their combinations to those causes which are
known to produce transformation; I expected that there would be an
inexplicable residue.
But this is not the case. Although it cannot yet be stated at first
sight with certainty in every single instance how far any particular
element of marking may have a biological value in the species
possessing it, nevertheless it has been established that each of
the elements of marking occurring in the larvæ of the _Sphingidæ_
originally possessed a decided biological significance, which was
produced by natural selection.
In the case of the three chief elements of the markings of the
_Sphingidæ_, it can be further shown that not only the initial stages
but also their ultimate perfection--the highest stages of their
development, are of decided advantage to their possessors, and have
a distinct biological value, so that the gradual development and
improvement of these characters can be traced to the action of natural
selection.
But although natural selection is the factor which has called into
existence and perfected the three chief forms and certain of the
subsidiary markings, in the repetition of the local character on the
other segments, as well as in the formation of new elements of marking
at the points of intersection of older characters now rudimentary, we
can recognize a second factor which must be entirely innate in the
organism, and which governs the uniformity of the bodily structure
in such a manner that no part can become changed without exerting a
certain action on the other parts--an innate law of growth (Darwin’s
“correlation”).
Only once during the whole course of the investigations was it for an
instant doubtful whether a phyletic vital force did not make itself
apparent, viz., in the red spots accompanying the oblique stripes
in several _Smerinthus_-larvæ. Closer analysis, however, enabled us
to perceive most distinctly the wide gulf that separates “analogous
variation” from the mystic phyletic vital force. Nothing further
remains therefore for the action of this force in respect to the
marking and colouring of the _Sphingidæ_, since several even of the
subordinate markings can be traced to their causes, only the “dorsal
spots” of our two native species of _Chærocampa_ having been referred
to correlation without decided proof. From the temporary inability
to explain satisfactorily such an insignificant detail, no one will,
however, infer the existence of such a cumbrous power as a phyletic
vital force.
The final result to which these investigations have led us is
therefore the following:--The action of a phyletic vital force cannot
be recognized in the marking and colouring of the _Sphingidæ_; the
origination and perfection of these characters depend entirely on the
known factors of natural selection and correlation.
=II.=