Studies in the Theory of Descent, Volume I

Of the North African D. Mauritanica, which likewise belongs to the Galii group, I have not been able to obtain specimens or figures of the younger stages. The adult caterpillar is very similar to that of Euphorbiæ, but differs in the absence of the second row of ring-spots. For this reason it must be regarded as a retarded form at an older stage of phyletic development.

I now proceed to the Livornica group.

Deilephila Livornica, Esper

This, the only European species here to be considered, possesses almost the same markings as Galii in its fourth stage, i. e., a subdorsal line with interpolated ring-spots. The species is known to be rare, and I have not been able to obtain living specimens, but I have examined several blown larvæ, all of which agree in having the ring-spots sharply distinct from the whitish subdorsal line, so that the latter is thereby interrupted. Figures of the adult larva are given in the works of Hübner, Boisduval, and Duponchel. In most specimens the ground-colour is brown, although Boisduval[93 - Fig. 62, Pl. VII., is copied from Boisduval.] also figures a light green specimen; from which it may be inferred, from analogy with Galii and Vespertilio, that the first stages are green. In Dr. Staudinger’s collection there is a young larva, probably in the fourth stage, the ground-colour of which is light ash-grey. The dorsal and subdorsal lines are white, the latter showing in the positions where the ring-spots subsequently appear, small white “mirrors” with red nuclei, exactly corresponding to the stage of Vespertilio represented in Fig. 49 A, Pl. VI. The “mirrors” are nothing more than dilatations of the subdorsal line, which is not therefore interrupted by them. The black “ground-area” does not surround the “mirrors” completely, but borders them only above and below, and is much more strongly developed above, extending in this direction to the dorsal line.

The fourth group comprises the two species D. Lineata, Fabr., and D. Zygophylli, Ochs., the former being the North American representative of our D. Livornica, but differing in remaining permanently at the fourth stage of this last species. I am acquainted with D. Lineata only through the figure of the adult larva given by Abbot and Smith, which figure, judging from the position and form of the spots, I am compelled to believe is not quite correct, notwithstanding the excellence of the other illustrations. The ground-colour of the caterpillar is green; the subdorsal yellow, bordered with black, slightly curved, arched lines, which nowhere interrupt its continuity. This North American species appears therefore to be an older form than our Livornica.

Deilephila Zygophylli, Ochsenheimer

This species, which is the next allied form to D. Lineata, is an inhabitant of Southern Russia. I have seen four specimens of the caterpillar in Dr. Staudinger’s collection, three of which are certainly in the last ontogenetic stage. The ground-colour appears ash-grey, ash-brown, or blackish with whitish granulations. A broad white subdorsal line extends to the base of the black caudal horn, this line in one specimen appearing at first sight not to possess a trace of spot rudiments (Fig. 50). On closer investigation, however, there could be observed, in the same position where the ring-spots stand in the other species of Deilephila, small black crescents above and below the subdorsal line. In other specimens the white subdorsal line had also become expanded in these positions into distinct spots; indeed, in one individual light white mirror-spots, bordered above and below by black crescents, stood on the subdorsal line (Fig. 50 A).

It is thus in this distinguishing character that the caterpillar is extremely variable, and we may suppose either that this species is now in a state of transition to a higher stage of phyletic development, or else that the ring-spots were formerly more strongly developed, and are now degenerating. The developmental history of the larva could alone decide which of these two views is correct. There would be no difficulty in procuring materials for this purpose if one of the numerous and zealous Russian naturalists would take up the subject.

Deilephila Hippophaës, Esper

This is the only representative of the fifth and oldest group known to me. The moth resembles D. Euphorbiæ to the extent of being sometimes confounded with it, a circumstance which is made the more remarkable by the fact that the caterpillars are so completely different.

The adult larva of this local moth has been made known by the figures, more or less exact, in the works of Hübner, Boisduval, and Duponchel. Wilde also gives a description of it, although from a foreign source. I will not here delay myself by criticizing the different descriptions and figures; they are partly correct, partly inexact, and sometimes altogether erroneous; they were of no avail for the question which here primarily concerns us, and new observation had to be undertaken.

I have been able to compare altogether about forty caterpillars, thirty-five of which were living. All these specimens possessed nearly the same greyish-green ground-colour, and most of them had exactly the simple marking as represented, for instance, in Hübner’s figure, i. e., a rather broad greenish-white subdorsal line, somewhat faded at the edges, and without a trace of spots on any of the segments with the exception of the eleventh, on which there was a yellowish, black-bordered mirror-spot, with a broad, diffused, vivid orange-red nucleus. Specimens also occur, and by no means uncommonly, in which no other markings are to be seen than those mentioned; there were nine among twenty-eight examples compared from this point of view.

In many other individuals of this species small red spots appear on the subdorsal line, exactly in the positions where the ring-spots are situated in the other species of the genus (Fig. 60), so that these spots are thus repetitions of the single ring-spot – a fact which must appear of the greatest interest in connection with the development of the markings throughout the whole genus. But this is not all, for again in other specimens, these red spots stand on a large yellow “mirror,” and in one individual (Fig. 59), they had become developed into well-formed ring-spots through the addition of a black border. We have thus presented to us in one and the same stage of a species, the complete development of ring-spots from a subdorsal line.

These facts acquire a still greater interest, as showing how new elements of marking are produced. The spots on the subdorsal line decrease from the posterior to the anterior segments, so that they must undoubtedly be regarded as a repetition or transference of the ring-spot previously developed on the eleventh segment. I will now proceed to furnish proofs in support of this statement.

I have never met with any specimens having ring-spots on all the segments – in the most prominent instances these spots were present on segments 10–5. This was the case in three out of the twenty-eight caterpillars minutely examined. On all these segments, however, the ring-spots were not equally developed, but increased in perfection from the posterior towards the anterior segments. In the larva represented in Fig. 59 for example, there is a completely developed ring-spot on segment 10, which, although possessing but a feeble black “ground-area,” is still distinctly bordered; on segment 9 this border is less sharp, and not so dark, and it is still less sharp and much lighter on segments 8 and 7, whilst it has completely disappeared from segment 6, the yellow “mirror” having at the same time lost in size. On segment 5, only two small contiguous reddish spots, the first rudiments of the nucleus,[94 - The fading of the red anteriorly has not been represented in the figure.] can be recognized on close inspection.

Specimens in which the spots extend from the eleventh to the seventh segment are of more frequent occurrence, five having been found among the twenty-eight. In these the spots diminish anteriorly in size, perfection, and intensity of colour. Still more frequently (in eleven specimens) are the ring-spots or their rudiments restricted to the tenth and ninth segments, the spot on the latter being without exception less developed than that on the former segment.

An anteriorly progressing formation of ring-spots thus undoubtedly occurs, the spots generally diminishing in perfection very suddenly towards the front segments; and specimens, such as that represented in Fig. 60, Pl. VII., in which traces of ring-spots are to be seen on all the segments from the tenth to the fifth, are of rare occurrence.

From what elements of marking are these secondary ring-spots resulting from transference developed? They do not, as in the case of the primary eye-spots of the Chærocampinæ, originate in the separation of one portion of the subdorsal line, and the subsequent formation of this detached spot into a “mirror;” but they arise from the formation of a nucleus, first one and then two of the shagreen-dots on the subdorsal line acquiring a yellowish or reddish colour (Fig. 61, Pl. VII., segments 6 and 7). The ground on which these two spots are situated then becomes yellow (Fig. 61, Pl. VII., segment 8), and a more or less distinct black border, having the form of two small crescents, is afterwards formed. At a later period these two crescents and also the two primary nuclei coalesce, producing a ring-spot which, as in Fig. 61, Pl. VII., segment 9, can be distinctly resolved into two portions.

It certainly cannot be denied that these facts may also be theoretically interpreted in a reverse sense. We might interpret the phenomena in this case, as also in that of D. Zygophylli, as a gradual disappearance from the front towards the hind segments of ring-spots formerly present, a view which could only be refuted by the ontogeny of the species. I have not been fortunate enough to procure eggs of D. Hippophaës, so that the younger stages are unknown to me. Among my caterpillars, however, there were two in the fourth stage of development, but these did not show ring-spots on all the segments, as we should expect on the above view; on the contrary, no trace of such spots could be seen on any of the segments with the exception of the eleventh, on which there was a ring-spot less perfectly developed than in the last stage.

In this fourth stage the larva of D. Hippophaës is of a lighter green (Fig. 58), the subdorsal yellowish with sharp boundaries, and the infra-spiracular line pure white, as in the next stage. The shagreening is present, but none of the shagreen-dots are red or reddish, and no trace of a ring-spot can be detected on the subdorsal line with the exception of that on the eleventh segment. In this last position this line is somewhat widened, and a long, diffused, rose-red spot can there be recognized upon it (Fig. 58 A). The black “ground-area” present in the fifth stage is as yet absent, and the spot is not so sharply separated anteriorly from the subdorsal line as it becomes later.

From these observations we might venture to expect that in the third stage of Hippophaës, the subdorsal line would also be free from this spot on the eleventh segment, and it is possible that in the second stage this line is itself absent.

The Genus Deilephila: Summary of Facts and Conclusions

Regarding only the adult larvæ of the species of Deilephila, these represent in their five groups, five stages in the phyletic development of the genus; but if we also take into consideration the developmental history, two more stages must be added, viz., that in which the caterpillar possesses no particular marking, as was found to be the case in the first stage of the development of D. Euphorbiæ and D. Dahlii; and a second stage with a subdorsal line, but without any ring-spot formations. Seven stages of phyletic development must therefore be distinguished.

Stage 1.– No species with entire absence of marking in the adult form now occurs.

Stage 2.– A subdorsal, accompanied by a spiracular line, extends from the caudal horn to the first segment. This also no longer forms the final stage of the ontogeny, but is, however, undoubtedly retained in the second stage of several species (D. Vespertilio, Livornica, Lineata, and perhaps also Galii).

Stage 3.– The subdorsal line bears a ring-spot on the penultimate segment; the other markings as in the last stage. D. Hippophaës only belongs to this stage, a small number of specimens, however, showing a transition to the following stage by the transference of ring-spots from the posterior to the anterior segments.

Stage 4.– Open ring-spots appear on the subdorsal line on all the segments from the eleventh to the first. D. Zygophylli and the North American D. Lineata belong here.

Stage 5.– Closed ring-spots are situated on the subdorsal line. Of the known species, only D. Livornica concludes its development at this phyletic stage.

Stage 6.– A single row of ring-spots replaces the subdorsal line. D. Galii, Vespertilio, and Mauritanica represent this stage at the conclusion of their ontogeny.[95 - [The caterpillar of Deilephila Euphorbiarum, figured by Burmeister (Lép. Rép. Arg., Pl. XVI, Fig. 1) belongs to this stage. R.M.]]

Stage 7.– A double row of ring-spots. Only D. Dahlii, Euphorbiæ, and Nicæa attain to this highest stage of Deilephila marking, the two first species in the fourth stage, and Nicæa in the third stage of its ontogeny.

Although our knowledge of the history of the development of the individual species is still so fragmentary, we may conclude with certainty that the development of the markings has been uniform throughout – that it has proceeded in the same manner in all species. All the species appear to be making for the same goal, and the question thus arises whether there may not be an innate force urging their phyletic development. The rigorous examination of this conception must be reserved for a later section. Here, as we are only occupied essentially in establishing facts, it must be remarked that retrogression has never been observed. The young larval forms of a species never show the markings of a later phyletic stage than the older larval forms; the development takes the same course in all species, only making a greater advance in the same direction in some than in others.

Thus, Nicæa and Euphorbiæ have advanced to the seventh phyletic stage, Zygophylli and Hippophaës only to the third, and some specimens of Zygophylli to the fourth. But at whatever phyletic stage the ontogeny of a species may terminate, the young larval stages always display the older phyletic stages. Thus, Galii in its last ontogenetic stage reaches the sixth phyletic stage; in its penultimate stage it reaches the fifth phyletic stage; and in its third stage; the fourth phyletic stage is represented, so that little imagination is required to anticipate that in the second stage the third or second phyletic stage would be pictured.

If we tabulate the development of the various species, indicating the ontogenetic stages by Arabic numerals, and the stages of the phylogeny which are reached in each stage of the ontogeny by Roman numerals, we obtain a useful synopsis of the series of developments, and, at the same time, it shows how many gaps still remain to be filled up in order to complete our knowledge even of this small group of species.

Table of Development of the Species of Deilephila

From this very incomplete table we perceive that, in certain instances, the stages can be represented as a continuous series of phyletic steps, as in the case of D. Galii; that in others certain steps may be omitted, as with D. Euphorbiæ, in which grade I. of stage 1 is immediately followed by grade V. in stage 2. In reality the gap caused by this omission is still greater than would appear, as grade V. is only indicated, and not actually reached, the subdorsal not being present as a sharply-defined line, but only as a faint stripe. The suppression of phyletic steps increases with the advancement in phyletic development. The higher the step to which a species finally attains, the greater is the tendency of the initial stages to be compressed, or omitted altogether.

From what has thus far been seen with respect to the development of D. Hippophaës, there may be drawn what to me appears to be a very important conclusion, viz. that the ring-spots of Deilephila first originated on the segment bearing the caudal horn, and were then gradually transferred as secondary spots to the preceding segments. Complete certainty would be given to this conclusion by a knowledge of the young forms of other phyletically retarded species, especially those of the American D. Lineata, and perhaps also those of Zygophylli and Livornica. The few observations on the development of D. Galii already recorded give support to this view, since the absence of ring-spots on the three front segments in the young caterpillar (one instance), or their less perfect formation on these segments (second instance), indicates a forward transference of the spots.

If the foregoing view be accepted, there follows from it a fundamental difference between the development of the genera Chærocampa and Deilephila. In the former the formation of the eye-spots proceeds from a subdorsal line, but they first appear on two of the front segments, and are then transferred to the posterior segments. In Deilephila, on the other hand, a single ring-spot is formed on the penultimate segment bearing the caudal horn, and this is repeated on the anterior segments by secondary transference. With respect to the origination of the ring-spot also, there is a distinction between this genus and Chærocampa, inasmuch as the first step towards the eye-formation in the latter consists in the separation of a curved portion of the subdorsal line, whilst in Deilephila the nuclear spot first seems to originate and the separation of the mirror-spot from the subdorsal line appears to occur secondarily. It is difficult here to draw further conclusions, since the first appearance of the primary ring-spot has not yet been observed, and no more certain inference respecting the history of the formation of the primary ring-spots can be drawn from the manner in which the secondary ring-spots are formed. Because in Hippophaës the formation of the secondary ring-spots begins with the red coloration of one or two shagreen-dots, it does not follow that the primary spot on the eleventh segment also originated in this manner; and this is not without importance when we are concerned with the causes which underlie the formation of ring-spots. In Chærocampa also, the formation of the primary eye-spots appears to differ from that of the secondary – in the latter the black “ground-area” first appearing, and in the former the “mirror-spot.” The secondary eye-spots certainly remain rudimentary in this last genus, so that the evidence in support of this conclusion is thus much weakened; but it must be admitted that we are here on ground still too uncertain to admit of wider conclusions being based thereon.

As a final result of the investigation, we may advance the opinion that the existing species of the genus Deilephila have reached five different phyletic stages, and that their very different external appearance is explained by their different phyletic ages; the appearance from these caterpillars of moths so extremely similar, can otherwise be scarcely understood.

It may appear almost unnecessary to bring forward additional proofs in support of this interpretation of the facts, but in a field where the data are so scanty, no argument which can be drawn from them should be considered as superfluous. The variations which occasionally occur in the larvæ, however, to a certain extent furnish a proof of the correctness of the theoretical interpretation offered.

When, in the ontogeny of these species, we actually see before us a series of stages of phyletic development, we must admit that ordinary reversion may occur, causing an adult caterpillar to show the characters of the young. Forms reverting to an earlier phyletic stage must, on the whole, occur but seldom, as this stage is removed further back in the ontogeny. Thus, indications of the subdorsal line must occur but rarely in the adult larvæ of Euphorbiæ, and still less frequently in Nicæa, whilst they must be expected to be of more common occurrence in Vespertilio, and also, as has already been seen, in Dahlii. In this last species, as also in Vespertilio, the completely-developed subdorsal line is still present in the third stage, whilst it is possessed by Euphorbiæ only in the second stage, and then in a rudimentary condition.

The state of affairs may in fact be thus described: Among several hundred adult larvæ of Dahlii found in Sardinia by Dr. Staudinger, there were some which did not actually possess a distinct subdorsal line, but in place thereof, and as its last indication, a feeble light stripe. One of Dr. Staudinger’s caterpillars showed also a distinct line between the closed eye-spots. In the last stage of Vespertilio this line appears still more frequently, whilst in Euphorbiæ it is extremely rare, and when present it only appears as a faint indication. This is the case with one of the specimens figured in Hübner’s work as an “aberration,” and also with one in Dr. Staudinger’s collection. Of Nicæa I have at most seen only eight specimens, none of which showed any trace of the long-vanished subdorsal line.

It must be expected that any ontogenetic stage would most readily revert to the preceding phyletic stage, so that characters present in the preceding stage are consequently those which would most commonly arise by reversion. This postulate of the theory also finds confirmation in the facts. Caterpillars which, when full grown, belong to the seventh phyletic stage, e. g.D. Euphorbiæ, not unfrequently show variations corresponding to the sixth stage, i. e. only one instead of two rows of ring-spots – the upper and first-appearing series. On the other hand, forms reverting to the fifth phyletic stage (ring-spots with connecting subdorsal line) occur but very rarely. I have never met with such cases in adult living caterpillars of D. Euphorbiæ, although in one instance such a larva was found in the fourth ontogenetic stage; but the strikingly dark, brownish subdorsal line which connected the otherwise perfectly developed ring-spots, completely disappeared in the fifth stage of the ontogeny. Those larvæ which, in the adult state, belong to the sixth phyletic stage, not unfrequently show the characters of the fifth stage more or less developed, as, for example, D. Vespertilio.[96 - [In concluding this account of the Chærocampinæ I may call attention to the following species, which have since been figured by Burmeister: —Pachylia Ficus, Linn. (loc. cit. Pl. XIV., Figs. 1 and 2); during the three first stages the larva is uniformly green, with a yellow subdorsal line, and below this ten oblique yellow stripes slanting away from the head; after the third moult the colour completely changes, the whole area of the body being divided into two distinct portions by the subdorsal line, above which the colour is red, and underneath of a pale green; the oblique stripes have almost disappeared; no occelli nor annuli are present. Pachylia Syces, Hübn. (loc. cit. Fig. 3); very similar to the last species in its young stages (figured also by Mérian, Surin. pl. 33). Philampelus Vitis, Linn. (loc. cit. Figs. 4 and 5); two stages represented; between first and second moults green, with oblique paler stripes slanting in same direction as in Pachylia, and each one containing a red streak surrounding the spiracle. When adult, the ground-colour is yellow above and green beneath, the whole surface being mottled with deep black and red transverse markings; the oblique stripes whitish, bordered with black at their lower extremities (figured also by Mérian, pls. 9 and 39). Philampelus Anchemolus, Cram. (loc. cit. Pl. XV., Fig. 1; Mérian, pl. 47); green when young, with seven oblique red stripes; when adult, uniformly brown, with seven pale yellow lateral markings, the first four of which are spots, and the remainder broad oblique stripes slanting forwards. Philampelus Labruscæ, (see note 82 (#cn_85), p. 195 (#Page_195)). R.M.]]

THE GENUS SMERINTHUS, LATREILLE

The caterpillars of this genus are very similar in appearance, and all possess extremely simple markings. The occurrence of numerous stages of development of these markings is thus excluded, and the study of the ontogeny therefore promised to furnish less information concerning the phyletic development of the genus than in the case of the preceding genera. This investigation has nevertheless also yielded interesting results, and the facts here recorded will be found of value in likewise throwing light on the causes which have produced the markings of caterpillars.

I shall commence, as in former cases, with the developmental history. I have easily been able to obtain fertile eggs of all the species of Smerinthus known to me. Impregnated females laid large numbers of eggs in confinement, and also bred females of the commoner species can readily be made to copulate, when pinned, and exposed in a suitable place in the open air. A male soon appears under these circumstances, and copulation is effected as readily as though the insect were not fastened in the way indicated.

Smerinthus Tiliæ, Linn.[97 - [Mimas Tiliæ of Butler’s revision. The author states that this genus is “easily distinguished from Laothoë by the form of the wings, the outer margin of secondaries deeply excavated below the apex, and the secondaries narrow and not denticulated.” Here again we have a clashing of the results arrived at by a study of the ontogeny of the larvæ, on the one hand, and the founding of genera on the characters of the imagines only, on the other. Of the three species discussed by Dr. Weismann, Mr. Butler, following other authors, refers Tiliæ to the genus Mimas, Populi to Laothoë, and Ocellatus to Smerinthus. It is to be hoped that when our knowledge of the developmental history of larvæ is more complete in all groups, a reconciliation between the results of the biological investigator and the pure systematist will be brought about, so that a genus may not, as at present, have such very different values when regarded from these two points of view. R.M.]]

The light green eggs are nearly spherical, and after fourteen days (beginning of July) the young larvæ emerge. These are also of a light green colour, and are conspicuous for the great length of the caudal horn, which is nearly half as long as the body. This horn is likewise of a light green at first, but becomes dark violet in the course of an hour. No trace of any markings can be detected at this stage.

As soon as the caterpillars are hatched they commence to nibble the empty egg shells; then they run about with great activity, and after several hours take up their position on the largest vein on the under side of the lime leaves, where they remain for a long period. In this situation they have the same form and colour as the leaf-vein, and are very difficult to discover, which would not be the case if they reposed obliquely or transversely to the vein. In about 4–5 days the caterpillars undergo their first moult, and enter upon the second stage. On each side of the segments 11–4, there now appear seven oblique whitish stripes on a somewhat darker green ground; these slope in the direction of the caudal horn. Owing to the transparency of the skin, a dark green dorsal line appears in the position of the underlying dorsal vessel, the green contents of the alimentary canal being distinctly visible through the absence of adipose matter in the tissues. The larvæ possess also a fine whitish subdorsal line, which extends from the horn to the head. The horn at this stage becomes black with a yellowish red base.

In the third stage, which occurs after six or seven days, the oblique stripes appear darker, and the subdorsal line disappears.

Fourth Stage