Having thus established the independence in the variability of
the individual stages of metamorphosis, I will now turn to the
consideration of the question as to how far a parallelism is displayed
in the phyletic development of these stages. Is there a complete
congruence of form-relationship between larvæ on the one hand and
imagines on the other? does the classification founded on the
morphology of the imagines agree with that based on the morphology of
the larvæ or not?
If, according to Claus,[176] we divide the order Lepidoptera into six
great groups of families, it is at once seen that these groups, which
were originally founded exclusively on imaginal characters, cannot by
any means be so clearly and sharply defined by the larval characters.
This is certainly the case with the _Geometræ_, of which the larvæ
possess only ten legs, and on this account progress with that peculiar
“looping” movement which strikes even the uninitiated. This group,
which is very small, is however the only one which can be founded on
the morphology of the larvæ; it comprises only two nearly related
families (_Phytometridæ_ and _Dendrometridæ_), and it is not yet
decided whether these should not be united into one group comprising
the family characters of the whole of the “loopers.”
Neither the group of Micro-lepidoptera, nor those of the _Noctuina_,
_Bombycina_, _Sphingina_, and _Rhopalocera_, can be based
systematically on larval characters. Several of these groups are indeed
but indistinctly defined, and even the imagines present no common
characteristics by which the groups can be sharply distinguished.
This is well shown by the _Rhopalocera_ or butterflies. These insects,
in their large and generally brilliantly coloured wings, which are
usually held erect when at rest, and in their clubbed antennæ, possess
characters which are nowhere else found associated together, and
which thus serve to constitute them a sharply defined group.[177]
The caterpillars, however, show a quite different state of affairs.
Although the larval structure is so characteristic in the individual
families of butterflies, these “larval-families” cannot be united
into a larger group by any common characters, and the “_Rhopalocera_”
would never have been established if only the larvæ had been known.
It is true that they all have sixteen legs, that they never possess a
Sphinx-like horn, and that they are seldom hairy, as is the case with
many _Bombycidæ_,[178] but these common _negative_ characters occur
also in quite distinct groups.
In the butterflies, therefore, a perfect congruence of
form-relationship does not exist, inasmuch as the imagines constitute
one large group of higher order whilst the larvæ can only be formed
into families. If it be admitted that the common characters of
butterflies depend on their derivation from a common ancestor, the
imagines must have retained certain common characters which enable them
to be recognized as allies, whilst the larvæ have preserved no such
characters from the period at which the families diverged.
Without going at present into the causes of these phenomena I will pass
on to the consideration of further facts, and will now proceed to
investigate both the form-relationships within the families. Here there
can be no doubt that in an overwhelmingly large majority of cases the
phyletic development has proceeded with very close parallelism in both
stages; larval and imaginal families agree almost completely.
Thus, under the group _Rhopalocera_ there is a series of families
which equally well permit of their being founded on the structure of
the larva or on that of the imago, and in which the larvæ and imagines
therefore deviate from one another to the same extent. This is the
case, for instance, with the families of the _Pieridæ_, _Papilionidæ_,
_Danaidæ_, and _Lycænidæ_.
But there are also families of which the limits would be very different
if the larvæ were made the basis of the classification instead of the
butterflies as heretofore. To this category belongs the sub-family
_Nymphalinæ_. Here also a very characteristic form of caterpillar
indeed prevails, but it does not occur in all the genera, being
replaced in some by a quite different form of larva.
In the latest catalogue of Diurnal Lepidoptera, that of Kirby (1871),
112 genera are comprised under this family. Of these most of the larvæ
possess one or several rows of spines on most or on all the segments, a
character which, as thus disposed, is not met with in any other family.
This character is noticeable in genera 1 to 90, if, from those
genera of which the larvæ are known, we may draw a conclusion with
reference to their allies. I am acquainted with larvæ of genus 2,
_Agraulis_, Boisd. (_Dione_, Hübn.); of genus 3, _Cethosia_, Fabr.; 10,
_Atella_, Doubl.; 12, _Argynnis_, Fabr.; 13, _Melitæa_,[179] Fabr.;
19, _Araschnia_, Hübn.; 22, _Vanessa_, Fabr.; 23, _Pyrameis_, Hübn.;
24, _Junonia_, Hübn.; 31, _Ergolis_, Boisd.; 65, _Hypolimnas_, Hübn.
(_Diadema_, Boisd.); 77, _Limenitis_, Fabr.; 81, _Neptis_, Fabr.; 82,
_Athyma_, Westw.; and finally with those of genus 90, _Euthalia_,
Hübn.--which, according to Horsfield’s figures, possess only two rows
of spines, these being remarkably long and curved, and fringing both
sides. It may be safely assumed that the intermediate genera would
agree in possessing this important character of the Nymphalideous
larvæ, viz., spines.
After the genus 90 there are 22 more genera, and these are spineless,
at least in the case of the two chief genera, 93, _Apatura_, and
104, _Nymphalis_. Of the remainder I know neither figures nor
descriptions.[180] In the two genera named the larvæ are provided with
two or more spine-like tentacles on the head, and the last segment
ends in a fork-like process directed backwards. The body is otherwise
smooth, and differs also in form from that of the larvæ of the other
_Nymphalinæ_, being thickest in the middle, and tapering anteriorly and
posteriorly; neither is the form cylindrical, but somewhat flattened
and slug-shaped. If therefore we were to arrange these butterflies by
the larvæ instead of by the imagines, these two genera and their allies
would form a distinct family, and could not remain associated with the
90 other Nymphalideous genera.
We have here a case of _incongruence_; the imagines of the genera 1-90
and 91-112 are more closely allied than their larvæ.
From still another side there arises a similar disagreement. The larvæ
of the genera _Apatura_ and _Nymphalis_ agree very closely in their
bodily form and in their forked caudal appendage with the caterpillars
of another sub-family of butterflies, the _Satyrinæ_, whilst their
imagines differ chiefly from those of the latter sub-family in the
absence of an enlargement of certain veins of the fore-wings, an
essential character of the _Satyrinæ_.
This double disagreement has also been noticed by those systematists
who have taken the form of the caterpillar into consideration.
Thus, Morris[181] attempted to incorporate the genera _Apatura_ and
_Nymphalis_ into the family _Libytheidæ_, placing the latter as
transitional from the _Nymphalidæ_ to the _Satyridæ_. But although the
imagines of the genera _Apatura_, _Nymphalis_, and _Libythea_ may be
most closely related--as I believe they actually are--the larvæ are
widely different, being at least as different as are those of _Apatura_
and _Nymphalis_ from the remaining _Nymphalinæ_.
Now if we could safely raise _Apatura_ and _Nymphalis_ into a distinct
family--an arrangement which in the estimation of Staudinger[182] is
correct--and if this were interpolated between the _Satyridæ_ and
_Nymphalidæ_, such an arrangement could only be based on the larval
structure, and that of the imagines would thus remain unconsidered,
since no other common characters can be found for these two genera than
those which they possess in common with the other Nymphalideous genera.
The emperor-butterflies (_Apatura_), by the ocelli of their fore-wings
certainly put us somewhat in mind of the _Satyrinæ_, in which such
spots are always present; but this character does not occur in the
genus _Nymphalis_, and is likewise absent in most of the other genera
of this group. The genus _Apatura_ shows in addition a most striking
similarity in the markings of the wings to the purely Nymphalideous
genus _Limenitis_, and it is therefore placed, by those systematists
who leave this genus in the same family, in the closest proximity to
_Limenitis_. This resemblance cannot depend upon mimicry, since not
only one or another but _all_ the species of the two genera possess a
similar marking; and further, because similarity of marking alone does
not constitute mimicry, but a resemblance in colour must also be added.
The genus _Limenitis_ actually contains a case of imitation, but in
quite another direction; this will be treated of subsequently.
It cannot therefore be well denied that in this case the larvæ show
different relationships to the imagines.
If the “natural” system is the expression of the genetic relationship
of living forms, the question arises in this and in similar cases
as to whether the more credence is to be attached to the larvæ
or to the imagines--or, in more scientific phraseology, which
of the two inherited classes of characters have been the most
distinctly and completely preserved, and which of these, through its
form-relationship, admits of the most distinct recognition of the
blood-relationship, or, inversely, which has diverged the most widely
from the ancestral form? The decision in single instances cannot but be
difficult, and appears indeed at first sight impossible; nevertheless
this will be arrived at in most cases as soon as the ontogeny of the
larvæ, and therewith a portion of the phylogeny of this stage, can be
accurately ascertained.
As in the _Rhopalocera_ most of the families show a complete congruence
in the form-relationship of the caterpillars and perfect insects, so
a similar congruence is also found in the majority of the families
belonging to other groups. Thus, the two allied families of the group
_Sphingina_ can also be very well characterized by their larvæ;[183]
both the _Sphingidæ_ and the _Sesiidæ_ possess throughout a
characteristic form of larva.
Of the group _Bombycina_ the family of the _Saturniidæ_ possess
thick cylindrical caterpillars, of which the segments are beset with
a certain number of knob-like warts. It is true that two genera of
this family (_Endromis_ and _Aglia_) are without these characteristic
warts, but the imagines of these genera also show extensive and common
differences from those of the other genera. A distinct family has in
fact already been based on these genera (_Endromidæ_, Boisd.). Thus the
congruence is not thereby disturbed.
So also the families _Liparidæ_, _Euprepiidæ_, and _Lithosiidæ_ appear
sharply defined in both forms; and similar families occur likewise
under the _Noctuina_, although in this group the erection of families
presents great difficulties owing to the near relationship of the
genera, and is always to some extent arbitrary. It is important,
however, that it is precisely the transitional families which present
intermediate forms both as larvæ and as imagines.
Such an instance is offered by the _Acronyctidæ_, a family belonging
to the group _Noctuina_. The imagines here show in certain points an
approximation to the group _Bombycina_; and their larvæ, which are
thickly covered with hairs, likewise possess the characteristics of
many of the caterpillars of this group.[184]
A second illustration is furnished by the family _Ophiusidæ_, which is
still placed by all systematists under the _Noctuina_, its affinity to
the _Geometrina_, however, being represented by its being located at
the end of the _Noctuina_. The broad wings and narrow bodies of these
moths remind us in fact of the appearance of the “geometers;” and the
larvæ, like the imagines, show a striking resemblance to those of the
_Geometrina_ in the absence of the anterior abdominal legs. For this
reason Hübner in his work on caterpillars has termed the species of
this family “_Semi-Geometræ_.”
All these cases show a complete congruence in the two kinds of
form-relationship; but exceptions are not wanting. Thus, the family
_Bombycidæ_ would certainly never have been formed if the larval
structure only had been taken into consideration, since, whilst the
genera _Gastropacha_, _Clisiocampa_, _Lasiocampa_, _Odonestis_, and
their allies, are thickly covered with short silky hairs disposed in
a very characteristic manner, the caterpillars of the genus _Bombyx_,
to which the common silkworm, _B. Mori_, belongs, are quite naked and
similar to many Sphinx-caterpillars (_Chærocampa_). Are the imagines of
the genera united under this family, at any rate morphologically, as
unequally related as their larvæ? Whether it is correct to combine them
into one family is a question that does not belong here; we are now
only concerned with the fact that the two stages are related in form in
very different degrees.
An especially striking case of incongruence is offered by the family
_Notodontidæ_, under which Boisduval, depending only on imaginal
characters, united genera of which the larvæ differed to a very great
extent. In O. Wilde’s work on caterpillars this family is on this
account quite correctly characterized as follows:--“Larvæ of various
forms, naked or with thin hairs, sixteen or fourteen legs.”[185]
In fact in the whole order Lepidoptera there can scarcely be found
associated together such diverse larvæ as are here placed in one
imago-family; on one side the short cylindrical caterpillars of
the genus _Cnethocampa_, Steph. (_C. Processionea_, _Pithyocampa_,
&c.), which are covered with fine, brittle, hooked hairs, and are
very similar to the larvæ of _Gastropacha_ with which they were
formerly united; and on the other side there are the naked, humped,
and flat-headed larvæ of the genus _Harpyia_, Ochs., with their two
long forked appendages replacing the hindmost pair of legs, and the
grotesquely formed caterpillars of the genera _Stauropus_, Germ.,
_Hybocampa_, Linn., and _Notodonta_, Ochs.
The morphological congruence between larvæ and imagines declares itself
most sharply in genera, where it is the rule almost without exception.
In this case we can indeed be sure that a genus or sub-genus founded
on the imagines only will, in accordance with correct principles,
present a corresponding difference in the larvæ. Had the latter been
known first we should have been led to construct the same genera as
those which are now established on the structure of the imagines, and
these, through other circumstances, would have stood in the same degree
of morphological relationship as the genera founded on the imagines.
There is therefore a congruence in a double sense; in the first place
the differences between the larvæ and imagines of any two genera are
equally great, and, in the next place, the common characters possessed
by these two stages combined cause them to form precisely the same
groups defined with equal sharpness; the genera coincide completely.
So also the butterflies of the sub-family _Nymphalinæ_ can well be
separated into genera by the characters of the larvæ, and these, as
far as I am able to judge, would agree with the genera founded on the
imagines.
The genus _Melitæa_, for example, can be characterized by the
possession of 7-9 fleshy tubercles bearing hairy spines; the genus
_Argynnis_ may be distinguished by always having six hairy unbranched
spines on each segment, and the genus _Cethosia_ by two similar spines
on each segment; the genus _Vanessa_ shows sometimes as many as
seven branched spines; and the genus _Limenitis_ never more than two
branched blunt spines on each segment, and so forth. If we go further
into details it will be seen that the most closely related imagines,
as might indeed have been expected, likewise possess the most nearly
allied larvæ, whilst very small differences between the imagines are
also generally represented by corresponding differences in the larvæ.
Thus, for instance, the genus _Vanessa_ of Fabricius has been divided
into several genera by later authors. Of these sub-genera, _Grapta_,
Doubl. (containing the European _C.-album_, the American _Fabricii_,
_Interrogationis_, _Faunus_, _Comma_, &c.), is distinguished by the
fact that the larvæ not only possess branched spines on all the
segments with the exception of the prothorax, but these spines are
also present on the head; in the genus _Vanessa_ (_sensû strictiori_),
Doubl., the head and prothorax are spineless (_e.g._ _V. Urticæ_); in
the tropical genus _Junonia_, Hübn., which was also formerly (Godart,
1819[186]) united with _Vanessa_, the larvæ bear branched spines on all
the segments, the head and prothorax included.
It is possible to go still further and to separate two species of
_Vanessa_ as two new genera, although they have hitherto been preserved
from this fate even by the systematists most given to “splitting.”
This decision is certainly justifiable, simply because these species
at present stand quite alone, and the practical necessity of forming a
distinct genus does not make itself felt, and this practical necessity
moreover frequently comes into conflict with scientific claims: science
erects a new genus based on the amount of morphological difference, it
being quite immaterial whether one or many species make up this genus;
such an excessive subdivision is, however, a hindrance to practical
requirements, as the cumbrous array of names thereby becomes still
further augmented.
The two species which I might separate from _Vanessa_ on the ground of
their greater divergence, are the very common and widely distributed
_V. Io_ and _Antiopa_, the Peacock Butterfly and the Camberwell Beauty.
In the very remarkable pattern of their wings, both show most marked
characteristics; _Io_ possesses a large ocellus on each wing, and
_Antiopa_ has a broad light yellow border which is not found in any
other species of _Vanessa_. There can be no doubt but that each of
these would have been long ago raised into a genus if similarly marked
species of _Vanessa_ occurred in other parts of the world, as is the
case with the other species of the genus. Thus, it is well known that
there is a whole series of species resembling our _V. Cardui_, and
another series resembling our _V. C.-album_, the two series possessing
the same respective types of marking; indeed on these grounds the
sub-genera _Pyrameis_ and _Grapta_ have been erected.[187]
I should not have considered it worth while to have made these remarks
if it had not been for the fact that the caterpillars of _V. Io_ and
_V. Antiopa_ differ in small particulars from one another and from the
other species of the genus. These differences relate to the number and
position of the spines, as can be seen from the following table:--