Studies in the Theory of Descent, Volume I

The second group, consisting also of three species, shows a great resemblance to Euphorbiæ, but has only one row of ring-spots. It contains D. Vespertilio, D. Galii, and the Algerian D. Mauritanica.

For the third group I only know one representative, D. Livornica, Esp., which possesses a single row of ring-spots connected by a subdorsal line.

Another group is composed of D. Zygophylli, which occurs on the shores of the Caspian Sea, and the North American D. Lineata; these species possessing a strongly marked subdorsal line, associated with more or less distinct ring-spots, which I shall designate as “open rings,” because their black border does not intersect the subdorsal line, but has the form of an arch above and below it.

In the last group, represented by D. Hippophaës, which occurs at the foot of the Alps (Wallis), and southward as far as Andalusia, there is only a broad subdorsal line, generally without any trace of a row of spots.

The important differences of marking displayed by these five groups are not in any way accidental, but they represent different stages of phyletic development; or, in other words, the five groups are of different ages, the first (Euphorbiæ, &c.) being the youngest, and the last (Hippophaës) the oldest of the genus.

According to their phyletic age, the groups follow each other in inverse order, the first being Hippophaës, the second that of Zygophylli, the third that of Livornica, the fourth that of Galii, and the fifth and youngest that of Euphorbiæ. Only in this last am I acquainted with the complete development of one species, for which reason I commence with this group, thus proceeding from the youngest to the oldest forms, instead of taking the more natural course from the simplest and oldest to the youngest and most complicated.

Deilephila Euphorbiæ, Linn

Some captured females were at once placed in an enclosure about the size of a small sitting-room. It was evident that they did not feel quite at home under these conditions, frequently beating their heads and wings against the tarlatan, but some of them nevertheless laid eggs at the base of the leaves of Euphorbia Cyparissias. The eggs much resemble those of Chærocampa Elpenor, being spheroidal in form, but rather smaller, and of a somewhat darker green. They were laid in small clusters composed sometimes of as many as seven, the single eggs being placed near together, but never touching, and seldom at the point of the leaf, but generally near the end of a twig, where young shoots are in close proximity. During the embryonic development the eggs become coloured, first yellow and partly blackish, and finally completely black.

First Stage

The young caterpillars (Fig. 37, Pl. V.) immediately after hatching measure four millimeters in length; they are at first rather light, but in the course of half-an-hour they are seen by the naked eye to become of a deep velvety black; later, on increasing in size, they again become paler, appearing of a greenish-black, and subsequently blackish-green. On further increasing in size (Fig. 38), they are blackish-green, with the horn, head, legs, and a crescent-shaped chitinous plate on the back of the prothorax black. There are also on the last segment a double and two single black chitinous plates. Of the later marking of the caterpillar there is scarcely anything present. The spiracles appear as white spots, and on each segment there are a number (mostly ten) of small warts, each of which emits a single bristle.

When the young larvæ have attained a length of seven millimeters they are olive-green, and do not contrast so brilliantly with the green of the Euphorbia leaves as before; neither do they as yet possess any markings.

Second Stage

The first ecdysis occurs after five days, and with this there appears quite suddenly a very complicated pattern. The ground-colour is now a light yellowish-green (Fig. 39), and on each of the twelve segments, near the front border, there is a pure white round spot in the middle of a large black transverse spot. I shall designate these, in accordance with the nomenclature employed for Chærocampa, as the white “mirrors” on black “ground-areas,” both together constituting “ring-spots,” as distinguished from “eye-spots” proper, in which a “nucleus,” the pupil of the eye, is also added. In many, but not in all specimens, very distinct traces of a subdorsal line can be seen as a light whitish stripe connecting the white spots. The horn, the thoracic and prolegs, and some spots on the head, are black.

The caterpillars remain unaltered till after four days, when, having a length of 17 millimeters, the second moult takes place, bringing with it changes quite as great as those which occurred with the first.

Third Stage

The caterpillar now assumes the shagreened appearance which it possesses in the adult state. Small white warts are arranged in rows from the dorsal to the spiracular line, and again underneath this line on the abdominal legs. These dots are not only of value as a character for differentiating the genera Deilephila and Chærocampa, but they also play a part in the peculiar spot-marking which will be shown later on. The ground-colour of the caterpillar is now light green (Fig. 40), replaced by black on certain parts. From the black “ground-area” of the ring-spots, two black triangles extend towards the posterior borders of the segments, but usually without reaching them.

The ring-spots are not essentially changed, although it may be observed that in most specimens the shagreen-dots under each ring-spot are somewhat larger, and stand closer together than in other places. In the following stage they become fused into a second white “mirror,” so that two ring-spots stand one above the other, their black ground-areas meeting. The formation of the second ring-spot sometimes takes place in the present stage (Fig. 42).

The subdorsal line has now completely vanished, whilst the spiracular line[89 - To be accurate this should be designated the infra-spiracular line; but this term cannot be well applied except in cases where there is also a supra-spiracular line, as, for instance, in Anceryx (Hyloicus) Pinastri.] appears as a broad stripe above the legs. The horn is yellow with a black point, and the black spots on the head have increased in size.

Fourth Stage

The third moult, which again occurs after four days, is not accompanied by such important changes. The green ground-colour has now completely disappeared, and is replaced by a dull black. The caterpillars are now, as also in the previous stage, extremely variable. Thus, for example, a triangular patch of the green ground-colour may be retained on the posterior edge of the segments (Fig. 41), those specimens which possess this character generally having their markings retarded in development, as shown by the absence of the second “mirror” of the ring-spots.

In Fig. 41 the shagreen-dots from which this second “mirror” is subsequently formed, are distinctly larger than the others, and on the eleventh segment two of them have already coalesced.

Fifth Stage

After another period of four days, the fourth moult takes place. The marking remains the same, but the colours become more vivid; the brick-red of the head, horn, dorsal line and legs, changing into a fiery red. The spiracular line, formerly green alternating with yellow, generally becomes resolved into a row of reddish-yellow spots. Ten days later the caterpillar (8.5 centimeters in length), ceases to feed, and prepares for pupation.

In this last stage also there is great variability of colour, but although each particular character is subject to fluctuation, the individuals of the same brood show but little variation among themselves.[90 - Upon this fact obviously depends the statement of that extremely accurate observer Rösel, that the caterpillar of Euphorbiæ is but very slightly variable (“Insektenbelustigungen,” Bd. iii. p. 36). I formerly held the same opinion, till I convinced myself that this species is very constant in some localities, but very variable in others. It appears that local influences make the caterpillar variable.] Thus, the dorsal line is sometimes black, and sometimes red, or again, this colour interrupted with black, so that only small red spots mark its course. The head may be entirely red, or this colour mixed with black. On the under side of the caterpillar, red generally predominates, but in some specimens this is replaced by black. The ground-colour is also variable, being generally a shining brownish-black, but sometimes dull coaly black. The shagreen-dots are sometimes white and sometimes yellow, and the “mirrors” of the ring-spots are also often yellowish.

The most interesting variation, however, appears to me to be the following: – In many specimens from Kaiserstuhl (Breisgau), the red was unusually vivid, and was not limited to the ordinary places, but occupied also the triangles on the posterior edges of the segments (Fig. 44), which are green in the third and fourth stages (Fig. 42). This variety has also been figured by Hübner. In one individual (Fig. 43), the under ring-spots were wanting, whilst the upper ones possessed a beautiful red nucleus fading away anteriorly, and showing the first step in the formation of a complete eye-spot.

I cannot positively assert that a fifth moult occurs in the last ten days, although I am very doubtful whether this is the case. It is certain, however, that some time before pupation, and whilst the larva is still feeding, the striking colours fade out, and become replaced chiefly by black.

The ontogeny of this species is obviously but a very incomplete representation of its phyletic development. This is at once apparent from the large gap between the first and second stages. It is not possible that a row of ring-spots can have arisen suddenly; in all probability they have been developed from a subdorsal line, which in Euphorbiæ is now only indicated in the second stage by a faint line. This conjecture is raised to a certainty when we call in the aid of the remaining species of Deilephila.

Deilephila Nicæa, De Prunner

I only know this species from blown larvæ in Staudinger’s collection, and Duponchel’s figure, of which Fig. 51, Pl. VI. is a copy. The adult insect possesses two perfectly separated rows of ring-spots. Duponchel figures also two younger stages, of which the youngest is probably the third stage. The larva is 18 millimeters in length, of a leaf-green colour, and shows no trace of a subdorsal line, but possesses the two rows of ring-spots, which only differ from those of the succeeding stages in the green colour of the “mirror.”

Deilephila Dahlii, Treitschke

I am familiar with numerous specimens in various stages, collected in Sardinia by Dr. Staudinger, and preserved by inflation.

The first stage is blackish, and shows no kind of marking; thus agreeing with the corresponding stage of Euphorbiæ. The second stage is unfortunately not represented in Staudinger’s collection.

The third stage shows a row of ring-spots, which are, however, connected by a very distinct and sharply defined subdorsal line. In the fourth stage a second row of (under) ring-spots is added, whilst the subdorsal line generally at the same time disappears.

The caterpillar remains unchanged during the fifth stage, when it shows a great resemblance in marking to Euphorbiæ; neither does it appear to differ essentially from this species in colour, so far as can be judged from preserved specimens and single figures (in Duponchel and Hübner). I have, moreover, seen several larvæ in the last stage, and the subdorsal could be distinctly recognized as a broad light stripe.

Of the four groups, the second (that of Galii), appears to me to be of but very little importance, as I shall now proceed to show from the development of D. Vespertilio.

Deilephila Vespertilio, Fabricius

Hitherto I have unfortunately been unable to obtain fertile eggs of this species, so that I can say nothing about the first stage. The latter would have been of interest, not only because of the marking, but also because of the presence of a residual caudal horn.

I am likewise only acquainted with the end of the second stage, having found, at the end of June 1873, a single caterpillar on Epilobium Rosmarinifolium, just previous to its second ecdysis. In the case of such young caterpillars, however, the new characters which appear in the succeeding stage are generally perceptible through the transparent chitinous skin at the end of the preceding stage, so that the markings of the insect are thus caused to change. The caterpillar found was about 16 millimeters long, and of a beautiful smooth and shining grass-green (Fig. 13). A broad white subdorsal line extended from the first to the penultimate segment, from which the horn was completely absent. On close inspection the first traces of the ring-spots could be detected near the anterior edge of each segment as feeble, round, yellow, ill-defined spots, situated on the subdorsal line itself (Fig. 13). On the first segment only there is no spot, and here no ring-spot is afterwards formed. Besides these markings, there was only to be seen a yellowish-white spiracular line.

This solitary specimen unfortunately buried itself before the moult for which it had prepared itself had occurred; but this ecdysis is associated with a very important transformation. This statement is founded on a blown specimen in Staudinger’s collection; it is only 18 millimeters in length, but already shows the later grey colouring in place of the beautiful green. In this, the third stage, the broad white subdorsal line bears on each segment a red spot enclosed between black crescents above and below (Fig. 49 A). In the fourth stage, during which I have seen many living caterpillars, the subdorsal line is still distinctly present in some individuals (Fig. 14), but the spots (“mirrors”) are now completely surrounded by a narrow black ring (“ground-area”), which sharply separates them from the subdorsal line (Fig. 49 B). In the fifth stage this ring becomes a somewhat irregularly formed black “ground-area,” whilst the subdorsal line completely vanishes (Figs. 51 and 49 C). The mirrors are white, but generally have a reddish nucleus, which obviously corresponds to the primary yellow spots from which the whole development of the ring-spots originates. This character is, however, sometimes absent; and many other variations also occur in the earlier stages, all of which can be easily explained as cases of arrested, or retarded development. Thus, the subdorsal line often disappears earlier, and is only present in the fourth stage as a feeble light stripe.

Deilephila Galii, Fabricius

The markings of this species appear to be developed in a precisely similar manner to those of D. Vespertilio. The adult larva, as in the last species, shows no trace of a subdorsal line. A row of large black spots, each having an irregular round, yellowish-white nucleus, is situated on an olive-green, blackish-brown, brown, or dirty yellow ground. I have, unfortunately, also in this case been unable to procure fertile eggs. There is, however, one figure of a caterpillar, 2.5 centimeters long, by Hübner, which is of a light green colour, and has five longitudinal lines; one dorsal, two subdorsal, and a spiracular line. The subdorsal is white, and bears in the place of the ring-spots small red dots, whilst the line itself is bordered with black where the red spots are situated. Hübner has probably figured the third stage, so that we may venture to conclude that in the second stage there is a subdorsal line either quite free from spots, or only showing such feeble rudiments as are to be seen in the second stage of Vespertilio.

I found two specimens in the fourth stage in the Upper Engadine. One of these (Fig. 45) was already of a dark, blackish-green ground-colour[91 - The green is considerably too light in Fig. 45.] with a broad, greenish-white subdorsal line sharply defined throughout its entire length, and containing ring-spots of a sulphur-yellow with an orange-red nucleus; the black “ground-area” did not encroach upon the subdorsal line, but was confined to two faint crescents situated above and below the “mirror.” Only the two foremost “mirrors” (on the second and third segments) were without nuclei.

The remaining peculiarities of coloration are shown in the figure. I may here only point out the shagreening present on the sides and a portion of the under surface.

The specimen figured was 3.3 centimeters long; a second example measured 2.8 centimeters in length, and was essentially similar, but showed that a considerable amount of variability must prevail at this stage of development. It was pitchy black, with a very indistinct subdorsal line and a few ring-spots, the “mirrors” of which were also sulphur-yellow, with the orange-red nucleus. The shagreening was quite as strong as in the first specimen, the dots being yellow instead of white. It is specially to be observed, because of its important theoretical bearing, that in this larva the ring-spots were absent on the three front segments, and on the fourth only, a faint indication of one could be perceived. In the caterpillar figured the ring-spots increase also in distinctness from the tail to the head.

Fifth Stage

The two specimens just mentioned, after moulting, acquired the well-known markings of the adult caterpillar already briefly described above. The fifth is the last stage.

The larva is known to occur in several variations, Rösel having figured it in three forms; light green, olive-green, and dirty yellow. It has not been since considered worth the trouble to attend to the subject of caterpillar coloration. Thus, Wilde,[92 - “Die Pflanzen und Raupen Deutschlands.” Berlin, 1860, p. 83.] in his well-known work, takes no notice of Rösel’s observation, but simply describes the caterpillar of Galii as “blackish olive-green.”

Having had an opportunity of observing twenty-five adult specimens of this somewhat scarce species at one time, I am able to state that it is not in this instance di- or polymorphism, but a case presenting a great degree of variability, with which we have to deal. There are not several sharply-defined types of coloration; but the extremes are connected by numerous intermediate forms. The extreme forms, however, certainly preponderate.

I have never met with Rösel’s light green form; neither was there a dark green specimen among the twenty-five mentioned, and I only know this variety from single individuals, found at a former period. Among the twenty-five caterpillars; all gradations of colour occurred, from pitchy black to light clay-yellow, and even to an almost whitish-yellow; some were brownish-black, others of a beautiful chestnut-brown, and others yellowish brown, dark clay-yellow, or brownish-red. Out of twenty-one specimens of which the ground-colours were noted, there were nine black, nine clay-yellow, and three brown; each of the three groups again showing various minor modifications of colour. The other colours also varied somewhat. Thus, the “mirrors” were sometimes white, sometimes strong yellow, and occasionally they also contained a reddish nucleus.

The variations in the shagreening were especially interesting, inasmuch as these appeared to have a striking connection with the general colouring of the caterpillar. Black specimens seldom show such sparse shagreening as that represented in Pl. V., Fig. 46, but are generally thickly scattered with large shagreen-dots right up to the dorsal line (Fig. 47, Pl. VI.), then strikingly resembling the adult larva of D. Euphorbiæ. The light ochreous-yellow individuals, on the other hand, were sometimes entirely without shagreening (Fig. 48, Pl. VI.), being smooth, and much resembling the light ochreous-yellow or yellowish-red caterpillar of D. Nicæa (Fig. 51, Pl. VI.). I have never seen a caterpillar of Galii which showed traces of the subdorsal line in the last stage, nor have I ever met with one which possessed a second row of “mirror” spots; so that retrogression or a sudden advance in development does not appear to occur.