Studies in the Theory of Descent, Volume I

Variety I.– Light green (Fig. 7, Pl. III.); dorsal line, blackish-green, strongly marked; subdorsal line broad, pure white, edged above with dark green; spiracular line, chrome-yellow; horn, black, with yellow tip and blue sides. Spiracles, blackish-brown, with narrow yellow border; legs, and extremities of prolegs, vermilion-red.

Variety II.– Blackish-brown (Fig. 6, Pl. III.); head and prothorax, yellowish-brown; markings the same as above.

Variety III.– Blackish-green or greenish-black (Figs. 10 and 11, Pl. III.); subdorsal line with blackish-green border above, gradually passing into a light green ground-colour; spiracular line, chrome-yellow; head and prothorax, greenish-yellow.

Variety IV.– Light green (Figs. 4 and 12, Pl. III.); dorsal line quite feeble; subdorsal broad, only faintly edged with dark green; subspiracular line, faint yellowish; head and prothorax, green.

Variety V.– Brownish-violet (Fig. 8, Pl. III.); the black dorsal line on a reddish ground either narrow or broad.

From these five varieties we see that the different types do not stand immediately next to one another; they are, in fact, connected by numerous transitional forms, the ground-colour varying greatly, being dark or light, yellowish or bluish. (Compare Figs. 4, 5, 7, and 12.) The markings remain the same in all, but may be of very different intensities. The dorsal line is often only very feebly indicated, and the subdorsal line is frequently but faintly edged; the latter is also sometimes deep black above and bordered rather darkly beneath, the sides then being of a dark green, often with blackish dots on the yellow spiracular line (Fig. 5, Pl. III.), this likewise being frequently edged with black. Only the horn and legs are alike in all forms. The green ground-colour passes into blackish-green, greenish or brownish-black, and again, from reddish-brown to lilac (Fig. 3), this last being the rarest colour.

The designation “polymorphism” may here appear very inapplicable, since we have no sharply distinct forms, but five very variable ground-colours connected by numerous intermediate modes of coloration. Should, however, the term “variability” be suggested, I am in possession of an observation which tends to show that the different colours have to a certain extent become fixed. I found a brown caterpillar, the five front segments of which were light green on the left side, and the fifth segment brown and green mixed (Fig. 9, Pl. III.). Such parti-coloration can evidently only appear where we have contending characters which cannot become combined; just as in the case of hermaphrodite bees, where one half of a segment is male and the other half female, the two characters never becoming fused so as to produce a truly intermediate form.107 From this observation, I conclude that some of the chief varieties of Stellatarum have already become so far removed from one another that they must be regarded as intermediate fixed forms, the colours of which no longer become fused together when they occur in one individual, but are developed in adjacent regions. Other facts agree with this conclusion. Thus, among the 140 adult larvæ which I bred from the batch of eggs above mentioned, the transition forms were much in the minority. There were forty-nine green and sixty-three brown caterpillars, whilst only twenty-eight were more or less transitional.

On these grounds I designate the phenomenon as “polymorphism,” although it may not yet have reached, as such, its sharpest limits. This would be brought about by the elimination of the intermediate forms.108

Immediately before pupation, all the caterpillars, both green and brown, acquire a lilac coloration. The fifth stage lasts seven days, and the whole larval development twenty-three days, the period from the deposition of the eggs to the appearance of the moth being only thirty-one days.

I have treated of the polymorphism of Stellatarum in detail, not only because it has hitherto remained unknown, and an analysis of such cases has been completely ignored,109 but more particularly because, it appears to me, that important conclusions can be drawn therefrom. Moreover, such an extreme multiplicity of forms is interesting, since, so far as I know, polymorphism to this extent has not been observed in any insect.

The theoretical bearing of this polymorphism will be treated of subsequently. It is not in any way connected with a more advanced development of the markings, since M. Stellatarum shows in this respect a very low state of development. This species displays only two stages: – (1), complete absence of all markings; and (2), a simple subdorsal, with dorsal and spiracular lines. We must therefore admit that the phyletic development of the markings has for a long time remained at a standstill, or, what expresses the same thing, that the marking which the adult larva now possesses is extremely old.

In order to complete my observations on M. Stellatarum, I now add some remarks on the pupa, the colour variations of which it appeared of importance to investigate, owing to the extraordinary variability of the caterpillar. The pupa varies but very slightly; the ochreous yellow ground-colour sometimes passes into reddish, and sometimes into greenish; the rather complicated blackish-brown marking of streaky lines is very constant, especially on the wing portions, being at most only more or less strongly pronounced. The minute colour variations of the pupa therefore have no connection with the colour of the caterpillar, both green and brown larvæ furnishing sometimes reddish-yellow and sometimes greenish-yellow pupæ.

The comparison of M. Stellatarum with the other known species of the genus, brings scarcely any addition to our knowledge of the phyletic development. Thus, the two European species of which the caterpillars are known, viz. M. Fuciformis and Bombyliformis,110 show essentially the same markings as Stellatarum, the chief element being a well-developed subdorsal line. The Indian M. Gilia, Herrich-Schäf., possesses also this line,111 and, together with the East Indian M. Corythus, Walk.,112 has oblique stripes in addition; the stripes do not, however, cross this line, but commence underneath it, and probably originated at a later period than the subdorsal line. Should this be the case, we must regard M. Corythus as representing a later phyletic stage. According to Duponchel’s figures, in both M. Fuciformis and Bombyliformis small oblique stripes (red) occur near the spiracles, but these have nothing to do with the oblique stripes of M. Gilia just mentioned, as they run in a contrary direction. Of the two European species, I have only seen the living caterpillar of Fuciformis, and this possessed no oblique stripes.

To these five species I am now enabled to add a sixth, viz. Macroglossa Croatica,113 a species inhabiting Asia Minor and Eastern Europe, of which a specimen and notice were kindly forwarded to me by Dr. Staudinger. The adult caterpillar much resembles that of M. Stellatarum in form and marking, but the subdorsal line appears much less distinctly defined, and the dorsal and spiracular lines seem to be entirely absent. The colour is generally green, but varies to red, and the subdorsal is more distinct and sharper in the young than in the adult larva. The markings of this species do not therefore in any way surpass those of Stellatarum, but are, on the contrary, much simpler.114

THE GENUS PTEROGON, BOISD. 115

Although I am acquainted with only a small portion of the developmental history of a single species of this genus, I will here proceed to record this fragment, since, taken in connection with two other species, it appears to me sufficient to determine, at least broadly, the direction of development which this genus has taken.

Pterogon Œnotheræ, Fabr

The adult larva, as made known by many, and for the most part good figures, has very complicated markings, which do not seem derivable from any of the elements of marking in the Sphingidæ hitherto considered. I was therefore much surprised at finding a young caterpillar of this species, only twelve millimeters in length, of a light green colour, without any trace of the subsequent latticed marking, and with a broad white subdorsal line extending along all the twelve segments. (Pl. VII., Fig. 63). Judging from the size and subsequent development, this caterpillar was probably in the third stage.

The same colouring and marking remained during the following (fourth) stage; but in the position occupied by the caudal horn in other Sphingidæ, there could now be observed the rudiment of a future ocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The subdorsal line disappears suddenly in the fifth stage, when the larva becomes dark green (rarely) or blackish-brown; the latticed marking and the small oblique stripes are also acquired, together with the beautifully developed eye-spots, consisting of a yellow mirror with black nucleus and ground-area (Pl. VII., Fig. 65).

The North American Pterogon Gauræ and P. Abboti116 also show markings precisely similar to those of this European species in the adult state; but in the two former the markings are of special interest as indicating the manner in which the primary Sphinx-marking has become transformed into that of the apparently totally different adult P. Œnotheræ. P. Gauræ is green, with a complicated latticed marking, which closer observation shows to arise from the dorsal line being resolved into small black dots, whilst the subdorsal line is broken up into black, white-bordered triangles. This caterpillar therefore gives fresh support to the remarkable phenomenon that the animals as well as the plants of North America are phyletically older than the European fauna and flora, a view which also appeared similarly confirmed by Deilephila Lineata, the representative form of D. Livornica. In entire accordance with this is the fact that the larva of P. Gauræ is without the eye-spot on the eleventh segment, and instead thereof still shows the original although small caudal horn. The perfect insect also resembles our P. Œnotheræ in colour and marking, but not in the form of the wings.

That the caterpillars of the genus Pterogon originally possessed the caudal horn we learn from P. Gorgoniades, Hübn.,117 a species now inhabiting south-east Russia, and for a knowledge of which I am indebted to Dr. Staudinger’s collection. There are in this about eight blown specimens, from 3.7 to 3.9 centimeters in length, which show a marking, sometimes on a red and sometimes on a green ground, which unites this species with the young form of P. Œnotheræ, viz., a broad white subdorsal line, extending from the small caudal horn to the head. In addition to this, however, the caterpillar possesses an extraordinarily broad white red-bordered infra-spiracular line, a fine white dorsal stripe, and a similar line between the subdorsal and spiracular, i. e. a supra-spiracular line.

The caterpillars in Staudinger’s collection, notwithstanding their small size, all belong to the last stage, as the moth itself does not measure more than 2.6 centimeters in expanse, and is therefore among the smallest of the known Sphingidæ. This species has therefore in the adult condition a marking very similar to that of Œnotheræ when young – it bears to Œnotheræ the same relationship that Deilephila Hippophaës does to D. Euphorbiæ, only in the present case the interval between the two species is greater. Gorgoniades is obviously a phyletically older species, as we perceive from the marking and from the possession of a horn. We certainly do not yet know whether Œnotheræ possesses a horn in its earliest stages, although in all probability it does so; in any case the ancestor of Œnotheræ had a horn, since the closely allied P. Gauræ now possesses one.

We thus see that also in the genus Pterogon the marking of the caterpillars commences with a longitudinal line formed from the subdorsal; an infra-spiracular or also a supra-spiracular line (Gorgoniades) being added. A latticed marking is developed from the linear marking by the breaking up of the latter into spots or small patches, which finally (in Œnotheræ) become completely independent, their connection with the linear marking being no longer directly perceptible.

THE GENUS SPHINX, LINN

Of this genus (in the narrow sense employed by Gray) I have only been able, in spite of all trouble, to obtain fertile eggs of one species. The females cannot be induced to lay in confinement, and eggs can only be obtained by chance.

I long searched in vain the literature of this subject for some account of the young stages of these caterpillars, and at length found, in a note to Rösel’s work, an observation of Kleemann’s on the young forms of Sphinx Ligustri, which, although far from complete, throws light on certain points.

From a female of S. Ligustri Kleemann obtained 400 fertile eggs. The caterpillars on emerging are “at first entirely light yellowish-green, but become greener after feeding on the fresh leaves;” the horn is also at first light green, and then becomes “darker.” The young larvæ spin webs, by which they fasten themselves to the leaves of their food-plant (this, so far as I know, has not been observed in any species of Sphingidæ). They moult four times, the border round the head and the purple stripes appearing after the third moult, these stripes “having previously been entirely white.” The ecdyses follow at intervals of about six days, increasing to about ten days after the fourth moult.118

From this short account we gather that in the third stage the marking consists of seven oblique white stripes, which acquire coloured edges in the fourth stage, a fact which I have myself frequently observed. On the most important point Kleemann’s observations unfortunately give no information – the presence or absence of a subdorsal line in the youngest stages. That he does not mention this character, can in no way be considered as a proof of its actual absence. I am rather inclined to believe that it is present in the first, and perhaps also in the second stage. There occur, however, species of the genus Sphinx (sensû strictiori) which possess a subdorsal line when young, as I think may be certainly inferred from the fact that the remains of such a line are present in the adult larva of S. Convolvuli.

This conclusion becomes still more certain on comparing the markings with those of a nearly allied genus; without such comparison the separation of the genus Macrosila, Boisd., from Sphinx is scarcely justifiable. If to these two genera we add Dolba, Walk., and Acherontia, Ochs., we must be principally struck with the great similarity in the markings, which often reaches to such an extent that the differences between two species consist entirely in small shades of colour, while the divergence of the moths is far greater.

Of the genera mentioned, I am acquainted altogether with fourteen species of caterpillars: —Macrosila Hasdrubal, Rustica,119 and Cingulata;119 Sphinx Convolvuli, Ligustri, Carolina,119 Quinquemaculata,119 Drupiferarum,119 Kalmiæ,119 and Gordius;119 Dolba Hylæus;119 Acherontia Atropos, Styx,120 and Satanas.120 With one exception all these caterpillars possess oblique stripes of the nature of those of the Smerinthus larvæ, and most of them are without any trace of a subdorsal line; one species – the North American M. Cingulata– has a completely developed subdorsal; and the typical European species, S. Convolvuli, has a rudimentary subdorsal line. The ground-colour in most of these species is of the same green as that of the leaves of their food-plants; some are brown, i. e. earth-coloured, and in these the markings do not appear so prominently; others again possess very striking colours (A. Atropos), the oblique stripes in these cases being very vivid. Only M. Hasdrubal121 separates itself completely from this system of classification, since this species is deep black with narrow yellow rings, the horn and last segment being red.

The large and most striking caterpillar of M. Hasdrubal is the same which Wallace has made use of for his theory of the brilliant colours of caterpillars. The explanation of the origin of this widely divergent mode of marking could only be furnished by the ontogeny, in which one or another of the older phyletic stages will certainly have been preserved.

Strictly speaking the same should be said of the other species – nevertheless their comparison with the so similarly marked Smerinthinæ, together with the circumstance that in certain species a subdorsal line can be traced, makes it appear correct to suppose that here also the subdorsal was the primary marking, this line being subsequently entirely replaced by the oblique stripes. The Sphinginæ would therefore be a younger group than the Smerinthinæ, a conclusion which is borne out by the fact that in the former the oblique stripes have reached a higher development, being always of two, and sometimes even of three colours (S. Drupiferarum, white, red, black), whilst in the species of Smerinthus they only occasionally possess uniformly coloured borders.

THE GENUS ANCERYX, BOISD

Although this genus is not admitted into most of the European catalogues – the solitary European species representing it being referred to the genus Sphinx, Linn.122– its separation from Sphinx appears to me to be justified, not because of the striking differences presented by the moths, but because the caterpillars, judging from the little we know of them, likewise show a similar degree of difference.

I have frequently succeeded in obtaining fertile eggs of Anceryx Pinastri and I will now give the developmental history of this caterpillar, which has already been figured with great accuracy in Ratzeburg’s excellent work on forest insects. Rösel was acquainted with the fact that the “pine moth” laid its eggs singly on the needles of the pine in June and July, and he described them as “yellowish, shining, oval, and of the size of a millet seed.”

On emerging, the caterpillars are six millimeters in length, of a light yellow colour, the head shining black with a yellow clypeus. The caudal horn, which is forked at the tip, is also at first yellowish, but soon becomes black. No particular marking is as yet present, but a reddish stripe extends along the region of the dorsal vessel, and the course of the spiracles is also marked by an orange-red line. (Fig. 53, A & B, Pl. VI.)

As soon as the young larvæ are filled with food they acquire a greenish streak. The first moult occurs after four days, and immediately after this there is still an absence of distinct markings, with the exception of a greenish-white spiracular line. In the course of some hours, however, the original light green ground-colour becomes darker, and at the same time a sharp, greenish-white subdorsal line appears, together with a parallel line extending above the spiracles, which, in Pterogon Gorgoniades, has already been designated as the “supra-spiracular.” The dorsal line is absent: the head is light green, with two narrow blackish-brown lines surrounding the clypeus; the horn and thoracic legs are black; claspers, reddish green; length, twelve to thirteen millimeters. (Fig. 54.)

Third Stage

After another period of four days the second moult occurs, neither colour nor marking being thereby affected. Only the horn, now no longer forked, becomes brownish with a black tip. The young caterpillars are now, as before, admirably adapted to the pine needles, on which they feed by day, and from which they can only be distinguished with difficulty.

Fourth Stage

The third moult also brings no essential change. The ground-colour and marking remain the same, only the spiracles, which were formerly dull yellowish, are now of a vivid brick-red. The horn becomes yellowish-red at the base.

Fifth Stage

The marking is only completely changed in the fifth and last stage. A broad reddish-brown dorsal line replaces the subdorsal, more or less completely. The supra-spiracular line also becomes broken up into numerous short lengths, whilst the green ground-colour in some specimens becomes more or less replaced by a brownish shade extending from the back to the sides. Horn, black; the upper part of the first segment with a corneous plate, similar to that of the Deilephila larvæ.

This stage is very variable, as shown by the figures in various works. The variations arise on the one hand from the struggle between the green ground-colour and the reddish-brown extending from above, and, on the other hand, from a more or less complete disappearance of the associated longitudinal lines. The latter are sometimes completely retained, this being the case in a caterpillar figured by Hübner (Sphinges, III., Legitimæ C, b), where both the subdorsal and supra-spiracular lines are continuous from segment 11 to segment 1, an instance which may perhaps be regarded as a reversion to the primary form.

The entire change of the marking from the fourth to the fifth stage depends upon the fact that the young larvæ resemble the needles of the pine, whilst the adults are adapted to the branches. I shall return to this later.

The ontogeny of A. Pinastri makes us acquainted with three different forms of marking: (1) simple coloration without marking; (2) a marking composed of three pairs of parallel longitudinal lines; (3) a complicated marking, arising from the breaking up of the last and the addition of a darker dorsal line.

Of the fourteen species placed by Gray in the genus Anceryx, I find, in addition to the one described, notices of only two caterpillars: —

A. Coniferarum,123 a North American species, lives on Pinus Palustris, and was figured by Abbot and Smith. Colour and marking very similar to A. Pinastri.

A. Ello, Linn.,124 according to the authority of Mérian, is described by Clemens125 as dark brown, “with a white dorsal line, and irregular white spots on the sides.” It lives on a “species of Psidium or Guava.”

Most of the species of Anceryx appear to live on Coniferæ, to which they show a general and decided adaptation. In the absence of decisive information, I partly infer this from the names, as Anceryx Juniperi (Africa). It has long been known that in our A. Pinastri the mixture of brown and fir-green, interspersed with conspicuous irregular light yellowish and white spots, causes the adult larva to present a very perfect adaptation to its environment. Of this caterpillar Rösel states: – “After eating it remains motionless, and is then difficult to see, because it is of the same colour as its food, since its brown dorsal line has almost the colour of the pine twigs; and who is not familiar with the fact that beneath the green needles there is also much yellow to be found?”

This adaptation to the needles and twigs obviously explains why this caterpillar in the adult condition is so far removed from those of the genus Sphinx, while the moths are so nearly related that they were only separated as a distinct genus when we became acquainted with a large number of species.

II. Conclusions from Phylogeny

The considerations previously set forth are entirely based on Fritz Müller’s and Haeckel’s view, that the development of the individual presents the ancestral history in nuce, the ontogeny being a condensed recapitulation of the phylogeny.

Although this law is generally true – all recent investigations on development having given it fresh confirmation – it must not be forgotten that this “recapitulation” is not only considerably abbreviated, but may also be “falsified,” so that a searching examination into each particular case is very desirable.

The question thus arises, in the first place, as to whether the markings of caterpillars, so distinct at the different stages of growth, are actually to be regarded as residual markings inherited from the parent-form; or whether their differences do not depend upon the fact that the caterpillar, in the course of growth, is exposed to different external conditions of life, to which it has adapted itself by assuming a different guise.