Part 3

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:--

SPECIES OF THE GENUS VANESSA, FABR.

+-------------------+-----------------------------------------------+ | | Number of Spines on the head and segments | | | of the larva. | | +-----+-----+-----+-----+-----+-----+-----+-----+ | |Head.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.| | | | I. | II. | III.| IV. | V. |VI.- | XII.| | | | | | | | | XI. | | | +-----+-----+-----+-----+-----+-----+-----+-----+ |V. Io | 0 | 0 | 2 | 2 | 4 | 6 | 6 | 4 | |V. Antiopa | 0 | 0 | 4 | 4 | 6 | 6 | 7 | 4 | |V. Urticæ | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. Polychloros | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. Ichnusa | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. Atalanta | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. C.-album | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. Interrogationis | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | |V. Levana | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 | +-------------------+-----+-----+-----+-----+-----+-----+-----+-----+

This character of the number of spines will not be considered as too unimportant when we observe how perfectly constant it remains in the nearly allied species. This is the case in the three consecutive forms, _Urticæ_, _Polychloros_, and _Ichnusa_. Now when we see that two species which differ in their imaginal characters present correspondingly small differences in their larvæ, this exact systematic congruence indicates a completely parallel phyletic development.

Exceptions are, however, to be met with here. Thus, Hübner has united one group of the species of _Vanessa_ into the genus _Pyrameis_ just mentioned, on account of certain characteristic distinctions of the butterflies. I do not know, however, how this genus admits of being grounded on the structure of the larvæ; the latter, as appears from the above table, agree exactly in the number and position of the spines with the caterpillars of _Vanessa_ (_sensû strictiori_), nor can any common form of marking be detected which would enable them to be separated from _Vanessa_.

Still more striking is the incongruence in the genus _Araschnia_, Hübn. (_A. Prorsa-Levana_), which, like the genus _Pyrameis_, is entirely based on imaginal characters. This is distinguished from all the other sub-genera of the old genus _Vanessa_ by a small difference in the venation of the wings (the discoidal cell of the hind-wings is open instead of closed). Now it is well-known that in butterflies the wing-venation, as most correctly shown by Herrich-Schäffer, is the safest criterion of “relationship.” It thus happens that this genus, typified by the common _Levana_, is in Kirby’s Catalogue separated from _Vanessa_ by two genera, and according to Herrich-Schäffer[188] by forty genera! Nevertheless, the larvæ agree so exactly in their spinal formula with _Grapta_ that we should have no hesitation in regarding them as a species of this sub-genus. It appears to me very probable that in this case the form-relationship of the caterpillar gives more correct information as to the blood-relationship of the species than that of the imago--in any case the larvæ show a different form-relationship to the imagines.

Just as in the case of butterflies there are many genera of _Sphingidæ_ which can be based on the structure of the larvæ, and which agree with those founded on the imagines.

Thus, the genus _Macroglossa_ is characterized by a straight anal horn, a spherical head, and by a marking composed of longitudinal stripes, these characters not occurring elsewhere in this combination. The nearly allied genus _Pterogon_, on the other hand, cannot be based on the larvæ only, since not only is the marking of the adult larva very distinct in the different species, but the anal horn is present in two species, whilst in a third (_P. Œnotheræ_) it is replaced by a knob-like eye-spot. The genus _Sphinx_ (_sensû strictiori_) is distinguished by the simple, curved caudal horn, the smooth, egg-shaped head and smooth skin, and by a marking mainly composed of seven oblique stripes. The genus _Deilephila_ is distinguished from the preceding by a dorsal plate, situated on the prothorax and interrupting the marking, as well as by the pattern, which here consists of a subdorsal line with ring-spots more or less numerous and developed; the skin also is rough, “shagreened,” although it must be admitted that there are exceptions (_Vespertilio_). The genus _Chærocampa_ admits also of being based on the form-relationship of its caterpillars, although this is certainly only possible by disregarding the marking and taking alone into consideration the peculiar pig-like form of the larvæ. The genus _Acherontia_, so nearly related to _Sphinx_, possesses in the doubly curved caudal horn a character common to the genus (three species known[189]). Finally may be mentioned the genus _Smerinthus_, of which the larvæ, by their anteriorly tapering form, their shagreened skin and almost triangular head with the apex upwards, their simply curved anal horn, and by their seven oblique stripes on each side, constitute a genus as sharply defined as that formed by the moths.

Although in all the systematic divisions hitherto treated of there are cases where the form-relationship of the larva does not completely coincide with that of the imago, such incongruences are of far more frequent occurrence in the smallest systematic group, viz. species.

The larvæ of two species have very frequently a much nearer form-relationship than their imagines. Thus, the caterpillars of _Smerinthus Ocellatus_ and _S. Populi_ are closely allied in structure, marking, and colouring, whilst the moths in these two last characters and in the form of the wings are widely separated.[190] Judging from the larvæ we should expect to obtain two very similar moths, but in fact both _Populi_ and _Ocellatus_ have many near allies, and these closely related species sometimes possess larvæ which differ more widely than those of more distantly related species of imagines.

Thus, in Amur-land and North America there occur species of _Smerinthus_ which closely resemble our _Ocellatus_ in colour, marking, and form of wing, and which possess the characteristic large blue ocellus on the hind-wings. _S. Excæcatus_ is quite correctly regarded as the representative American form of our _Ocellatus_, but its caterpillar, instead of being leaf-green, is of a chrome-yellow, and possesses dark green instead of white oblique stripes, and has moreover a number of red spots, and a red band on the head--in brief, in the very characters (colour and certain of the markings) in which the imagines completely agree it is widely different from _Ocellatus_. It appears also to be covered with short bristles, judging from Abbot and Smith’s figure.[191]

Just in the same way that the species having the nearest conceivable form-relationship to _Ocellatus_ possesses a relatively strongly diverging larva, so does the nearest form-relation of _Populi_ (imago) offer a parallel case. This species, which is also North American, lives on _Juglans Alba_. The imago of _Smerinthus Juglandis_ differs considerably from _S. Populi_ in the form of the wings, but it resembles the European species so closely in marking and colouring that no doubt can exist as to the near relationship of the two forms. The caterpillar of _S. Juglandis_,[192] however, differs to a great extent from that of _Populi_ in colour--it is not possible to confound these two larvæ; but those of _Populi_ and _Ocellatus_ are not only easily mistaken for one another, but are distinguished with difficulty even by experts.