Studies in the Theory of Descent, Volume II. Weismann August
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.14
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.”15 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, 181916) 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.17
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.
This
[For Mr. A. G. Butler’s observations on the genus
[The following characters are given in Stainton’s “Manual of British Butterflies and Moths,” vol. i. p. 114: – “Larva of very variable form: at one extreme we find the singular
Encyl. Meth. ix. p. 310.
[The genus