Although by no means in favour of the excessive subdivision of genera, I am of opinion that Ochsenheimer’s genusDeilephilahas been correctly separated by Duponchel into the two generaChærocampaandDeilephila,sensû strictiori. Such a division may appear but little necessary if we examine the perfect insects only; but the developmental history of the caterpillars shows that there is a wide division between the two groups of species, these groups however being branches of one stem.
Some captured females laid single eggs sparsely on grass, wood, and especially on the tarlatan with which the breeding-cage was covered. The eggs are nearly spherical, but somewhat compressed, of a grass-green colour, a little lighter, and somewhat larger (1.2 millim.) than those ofDeilephila Euphorbiæ. During the development of theembryo the eggs first became yellowish-green, and finally yellowish.
First Stage.
The young caterpillars are four millimeters in length, and immediately after hatching are not green, but of a yellowish-white opalescent colour, the large and somewhat curved caudal horn being black. The caterpillars were so transparent that under a low magnifying power the nervous, tracheal, and alimentary systems could be beautifully seen. As soon as the larvæ began to feed (onEpilobium parviflorum) they became green in consequence of the food appearing through the skin, but the latter also gradually acquired a dark green colour (Pl. IV., Fig. 17). All the specimens (some twenty in number) were exactly alike, and showedno trace of marking.
Second Stage.
The first ecdysis occurred after 5–6 days, the length of the caterpillars being from nine to ten millimeters. After this first moult they appeared of a shining green, the horn, which was black during the first stage, becoming a little red at the base, while a fine white subdorsal line extended from the horn to the head (Fig. 18). The head and legs were green; the divisions between the segments appeared as fine light rings, and the entire upper surface of the segments was also crossedby fine transverse rings, as was also the case in the first stage.
At the beginning of the present stage no trace of the eye-spots could be detected; but a few days after the first moult it was observed that the white subdorsal line was no longer straight on the fourth and fifth segments, but had become curved upwards into two small crescents. The latter soon stood out more strongly, owing to the filling up of their concavities with darker green. These are the first rudiments of the eye-spots (Figs. 19 and 30). A very fine white line now connected the spiracles (infra-spiracular line), and could be traced from the last segment to the head. This line takes no further part in the subsequent development of the markings, but disappears in the following stage. The blood-red colour of the base of the black caudal horn is retained till the fifth stage, and then also disappears.
Before the second moult, which occurs after another period of 5–6 days, the caterpillars, which were about 1.3 centimeters in length, had assumed their characteristic tapering, slug-like form. I did not notice that the larvæ at this stage possessed the power of withdrawing the three foremost segments into the two succeeding ones, as is so frequently to be observed in the adults; neither were these two segments so strikingly enlarged as they are at an earlier period.
Third Stage.
After the second ecdysis the marking and colouring only undergo change with respect to the eye-spots. The concavities of the crescent-shaped portions of the subdorsal line becomeblack,70the remainder of this line at the same time losing much of its whiteness, and thus becoming less distinct, whilst the crescents assume the appearance of small eye-spots (Fig. 20). During this stage the curved, crescent-formed portions become prepared for complete separation from the remainder of the subdorsal line; and just before the third moult the eye-spots become sharply defined both in front and behind, whilst the black ground-colour curves upwards, and the white spots gradually become lenticular and commence to enlarge (Fig. 21).
Fourth Stage.
The third moult takes place after another interval of 5–6 days, the eye-spots then becoming very prominent. The white nucleus of the front spot is kidney-shaped, and that of the hind spot egg-shaped; whilst the black ground-colour extends as a slender border upwards along the sides of the spots, but does not completely surround them till towards the end of the present stage (Fig. 21). Thecentral portion of the white spots at the same time becomes of a peculiar violet-brown colour inclining to yellow above, the peripheral region alone remaining pure white.
Of the subdorsal line only traces are now to be recognized, and these are retained, with almost unchanged intensity, sometimes into the last stage, remaining with the greatest persistence on the three front and on the penultimate segments, whilst on those containing the eye-spots,i.e., the fourth and fifth, not a trace remains. At the present stage the peculiar mingling of colours becomes apparent over the whole of the upper surface; the green is no longer uniform, but a mixture of short and gently sinuous, dark green striations on a lighter ground now appear. On the sides of the caterpillar these stripes, which are at first indistinct, but become more strongly pronounced in the next stage, are arranged obliquely on the spiracles, with the lower portions directed forwards.
Fifth Stage.
The fourth moult occurs 7–8 days after the third, the caterpillar being 4–5 centimeters in length. Whilst all the specimens hitherto observed were with one exception light green, they now mostly changed their colour and became dark brown. In one case only did the brown colour appear in the previous (fourth) stage. The striationspreviously mentioned appear as dull and interrupted dirty yellow streaks, the same dirty yellow colour showing itself continuously on the sides of the four front segments. Of the subdorsal line only a distinct trace is now to be seen on the eleventh and on the three front segments, whilst on the third segment the formation of another eye-spot commences to be plainly perceptible by a local deposition of black (Fig. 23). This third spot does not, however, become completely developed, either in this or in the last stage, but the subdorsal line remains continuous on the three front segments. Among other changes at this stage, there occurs a considerable shortening of the caudal horn, which at the same time loses its beautiful black and red colours and becomes brownish.
The two large eye-spots have now nearly attained complete development. The kidney-shaped white spot has become entirely surrounded by black; and on the brown, red, and yellow tints present in this spot during the last stage, a nearly black spot has been developed—the pupil of the eye (Fig. 33). In order to establish a definite terminology for the different portions of the eye-spot, I shall designate the pupil as the “nucleus,” the light ground on which the pupil stands as the “mirror,” and the black ground which surrounds the mirror as the “ground-area.”
In this fifth stage the larva attains a lengthof six centimeters, after which the fifth moult takes place, the caterpillar becoming ready for pupation in the sixth stage. No striking changes of colouring or marking occur after the present stage, but only certain unimportant alterations, which are, however, of the greatest theoretical interest.
Sixth Stage.
In this stage the eye-like appearance of the spots on the front segments becomes still more distinct than in the fifth stage; at the same time these spots repeat themselves on all the other segments from the fifth to the eleventh, although certainly without pupils, and appearing only as diffused, deep black spots, of the morphological significance of which, however, there cannot be the least doubt. They are situated in precisely the same positions on the 5–11 segments as those on the third and fourth—near the front, and above and below the subdorsal line. A feeble indication of the latter can often be recognized (Fig. 23).
In all dark brown specimens the repeated spots can only be detected in a favourable light, and after acquiring an intimate knowledge of the caterpillar; but in light brown and green specimens they appear very sharply defined.
There is one other new character which I have never observed at an earlier period than the sixthstage, viz. the small dots which appear in pairs near the posterior edge of segments 5–11. These dots cannot have been developed from the subdorsal line, as they are situated higher than the latter. Their colour varies according to the ground-colour of the caterpillar, but it is always lighter, being light green in green specimens, dull yellow in those that are light brown, and grey in the blackish-brown caterpillars. These “dorsal spots,” as I shall term them, are chiefly of interest because they are present inChærocampa Porcellus, in which species they appear one stage earlier than inC. Elpenor.
Females captured on the wing, laid in the breeding-cage single eggs of a light green colour, spheroidal in form, and very similar to those ofC. Elpenor.
First Stage.
The caterpillars on first hatching measure 3.5 millimeters in length, and are of a uniform light green colour, with a fine white transverse line on the posterior edge of each segment, precisely similar to that which appears in the second stage ofC. Elpenor. They resemble the latter species still further in showing a fine white subdorsal line, which can easily be recognized by the naked eye (Fig. 24). Although the adult larva isdistinguished from all the other known species ofChærocampaby the absence of a caudal horn, a distinct but very small one is nevertheless present at this first stage, and is indeed retained throughout the entire course of development, but does not increase further in size, and thus gradually becomes so small in proportion to the size of the caterpillar that it may be entirely overlooked.
The first moult takes place after 4–5 days.
Second Stage.
The blue-green coloration remains unchanged; but a somewhat darker green dorsal line becomes apparent down the middle of the back (the dorsal vessel?), and the subdorsal line now becomes very broad and pure white, being much more conspicuous than in any stage ofC. Elpenor(Fig. 25). The tapering of the three front segments occurs at this stage, and oblique, dark green striations on a lighter ground stand out distinctly on the spiracles. As withC. Elpenor, the first traces of the future eye-spots appear during the second stage; not in the present case as a curvature of the subdorsal line, but as a spot-like widening of the latter, of a brighter white than the somewhat greenish colour of the remainder of the line.
Third Stage.
After the second moult, the formation of the dark “ground-area” of the eye-spots commences bythe appearance of a little brown on the under edge of the foremost of the white spots, this coloration gradually increasing in extent and in depth. At the same time both spots become more sharply distinguishable from the subdorsal line, which becomes constantly greener (Fig. 27). The brown colour soon grows round the white of the front eye-spot, which becomes so far perfected; whilst the completion of the hind spot is effected slowly afterwards. The formation of the eye-spots does not therefore proceed any more rapidly in this species than inC. Elpenor.
At the end of the present stage the length of the caterpillar is about four centimeters; the ground colour is still sea-green; the subdorsal line is much diminished, completely fading away at its lower edge, but remaining sharply defined above, against the green ground-colour (Fig. 26).
Fourth Stage.
After the third moult all the caterpillars (5) became brown, this change occurring therefore one stage earlier than is generally the case withC. Elpenor. In single instances the brown colour appeared in the third stage. The subdorsal line had disappeared from all the segments but the three first and the last. The eye-spots now rapidly attained complete development; they contained a black pupil, and gave the insect a truly repulsive appearance when, on being threatened by danger,it drew in the front segments, and expanded the fourth (Fig. 28). The eye-spots of the fifth segment are much less developed than inC. Elpenor; they remain small, and are not readily detected. On the other hand, there now appear on all the segments with the exception of the last, just as in the sixth stage ofC. Elpenor, distinct rudiments of eye-spots, which present the appearance of irregular, roundish, black spots on the front borders of the segments, at the height of the former subdorsal line. In this latter region the black pigment is disposed as a longitudinal streak, and to this a median line is added, the whole forming a marking which perhaps makes the caterpillar appear still more alarming to its foes. This marking is, however, only to be distinctly recognized on the three first segments. The “dorsal spots” mentioned in the case ofC. Elpenorthen appear very distinctly on segments 5–11.
The caterpillars continued to feed for eleven days after the third moult, at the end of which period the fourth moult took place, but without the occurrence of any change of marking. The larvæ then buried themselves, the complete development having taken 28–29 days.
The development of thePorcelluscaterpillar was twice followed; in 1869 in twelve, and in 1874 in five specimens. In no case did I obtain caterpillars which remained green throughout the entire course of development, although this colouris stated in the books to occur occasionally in these larvæ; neither have I been able to find any figure of an adult green specimen, so that it must in the meantime be admitted that such specimens, if they occur at all, are exceptionalinstances.71The theoretical bearing of this admission will appear later on.
The first stage ofElpenorshows that the most remote ancestor of the genus possessed no kind of marking, but was uniformly green. At a later period, the white longitudinal stripe which I have designated the “subdorsal line” made its appearance, and at a still later period this line vanished, with the exception of a few more or less distinct remnants, whilst, at the same time, from certainportions of it, the eye-spots of the fourth and fifth segments became developed. After the perfecting of the eye-spots, weak repetitions of the latter appeared as black spots on all the segments except the last.
InPorcellusthe caterpillar emerges from the egg with the subdorsal line, the first stage ofElpenorbeing omitted. From this fact we may venture to conclude thatPorcellusis the younger species, or, what comes to the same thing, that it has further advanced in development. The whole subsequent history ofPorcellusagrees with this view, its course of development being essentially but a repetition of the phenomena displayed byElpenor, and differing only in one point, viz. that all new characters make their appearance one stage earlier than in the latter species. This is the case with the transformation of the green into a brown ground-colour; with the repetition of the eye-spots on the remaining segments in the form of suffused black spots; and with the appearance of the light “dorsal spots.” Only the eye-spots themselves appear, and the snout-like tapering of the front segments occurs in the same stage as inElpenor,i.e.the second.
From these data alone, we may venture to infer the occurrence of four chief stages in the phyletic development of the genus. The first stage was simply green, without any marking; the second showed a subdorsal line; the third, eye-spots onthe third and fourth segments; and the fourth stage showed a repetition of the eye-spots, although but rudimentary, on all the remaining segments with the exception of the twelfth.
Now if we compare the other known species ofChærocampalarvæ with the above, we shall arrive at the interesting conclusion that all these species can be arranged in three groups, which correspond exactly with the three last phyletic stages as just deduced from the ontogeny ofC. ElpenorandPorcellus.
Of the genusChærocampa,72over fifty species have beendescribed,73of which the larvæ of onlyfifteen are known in the form which they possess at the last ontogenetic stage.
Group1.—I can furnish but little information with respect to this group. The first species with which I became acquainted wasChærocampa Syriaca,74of which I saw two blown caterpillars in Staudinger’s collection, and which I have figured inPl. IV., Fig. 29. The larva is green, and has the short oblique stripes over the legs common to so many species ofChærocampa, the only marking besides these being a simple white subdorsal line, without any trace of eye-spots. This species exactly corresponds therefore with the second ontogenetic stage ofC. ElpenorandPorcellus. The account of the species, both in the larval and perfect state, is unfortunately so imperfect, that we cannot with certainty infer the age of the two caterpillars from their size. If the moth were of the same size asElpenor, then the caterpillar figured, having a length of 5.3 centimeters, would not be in the last but in the penultimate stage, and it remains doubtful whether it may not acquire eye-spots in the last stage.
That species exist, however, which in their last stage correspond to the second stage ofElpenor, is shown by two of the forms belonging to Walker’s genusDarapsa, which was founded onthe characters of the imagines only. Ten species of this genus are given in Gray’s catalogue, the adult larva of two of these being known through the excellent figures of Abbot andSmith.75These two caterpillars possess the characteristic tapering form in a very marked degree; one is figured in the attitude so often assumed by our species ofChærocampaon the approach of danger, the three front segments being withdrawn into the fourth. (Fig. 34,Pl. IV., is copied from this Plate). There are no eye-spots either inD. MyronorD. Chœrilus,76but only a broad white subdorsal line; underneath which, and to a certain extent proceeding from it, there are oblique white stripes, precisely similar to those which meet the subdorsal line in the third stage ofC. Porcellus.77
Group2.—This group contains numerous species which, like our nativeC. ElpenorandPorcellus, show eye-spots on the fourth and fifth segments, whilst these markings are absent, or at most only present in traces, on the remainder. To this section there belong, besides the two species mentioned, five others, viz. in Europe,C. CelerioandAlecto(not certainly known?);78in India,C. Nessus, Drury, andLucasii,Boisduval;79and an unnamed species from Port Natal.
In the species belonging to this group the subdorsal line may be more or less retained. Thus,C. Celerio, according to Hübner’s figure, has a broad yellow line extending from the horn to the sixth segment, whilst it is completely absent on the three front segments. In the unnamed species from PortNatal80the subdorsal line extends to the front edge of the fifth segment, and on the fourth segment only is there a perfect eye-spot, whilst on the succeeding segments traces of such markings can be recognized as dark spots similarto those inElpenorandPorcellus. The transition to the third group is through another unnamed species fromMozambique,81in which rather large eye-spots have become developed on the fourth and fifth segments and these are followed by a subdorsal line, which only appears distinctly at certain places. On this broken subdorsal line, and not completely separated from it, there are small, roundish eye-spots, situated near the front edge of each segment; these being, therefore, a somewhat more perfect repetition of the fronteye-spots.82
Group3.—In the species of this group the eye-spots are repeated on all the segments. I am acquainted with seven suchChærocampalarvæ, of whichC. Bisecta,Horsfield,83shows some affinity to the foregoing group, since the eye-spots on segments 6–11 have not yet attainedfull perfection. InC. Odenlandiæ,Fabr.,84and inC. AlectofromIndia,85the eye-spots appear to be perfectly alike on all the segments; whilst inC. Acteus,Cram.,86and in the North AmericanC. Tersa87(Pl. IV., Fig. 35) they are smaller on the other segments than on the fourth; and inC. Celerio, Linn., fromIndia,88the size of the spots diminishes from the head to the tail.
In this group also the subdorsal line is retained in a very variable degree. In some species it appears to have completely vanished (C. Acteus,Celerio); in others it is present as a light stripe extending along all the segments (C. Alecto); whilst in others it is retained as a broad white stripe, which extends only to the fourth segment (C. Tersa,Fig. 35). In species possessing eye-spots, the subdorsal line is thus a very variable character. It is, however, an interesting fact that even in the present group, which has made the greatest step forward, the subdorsal line is of general occurrence, because the eye-spots in all these species may have almost a similar development to those ofElpenorandPorcellus. The ontogeny of the tropical species would alonegive a definite reply on this point, but unfortunately we are not acquainted with any of the young forms, so that we can but presume that some of them at least would show only in the first stage the simple subdorsal line without eye-spots; that in the second stage the primary pairs of eye-spots would be formed on the fourth and fifth segments, whilst the transference of these spots to the remaining segments would take place in the last stage.
The foregoing assumption is based immediately on the ontogeny ofElpenorandPorcellus; it is supported by the considerable size attained by the eye-spots in many species of the third group, and would receive additional confirmation by observations on the IndianC. Celerio, supposing that Horsfield’s statements do not arise from a confusion of species. This skilful observer, who was the first to breed systematically a large number of tropical larvæ, has given a figure of the Indian caterpillar ofC. Celerio, according to which this species possesses eye-spots on all the segments from the fourth to the tenth. The European form of this same species has eye-spots only on segments four and five, a fact which does not appear to have been known to Horsfield, as no mention of it is made in his notice of the Indian species. If the caterpillar figured is really that ofCelerio, which I consider to be by no means improbable, not only is it thus shown that in thespecies of the third group the ocelli on the hind segments have a secondary origin through a repetition of the primary ones of the front segments, but we can also establish that the same species in two different regions may arrive at two different phyletic stages.
If, finally, we sum up the facts taught by the ontogeny of the two German species, and the adult forms of the other species, we can form therefrom a tolerably complete picture of the course of development of the genusChærocampa. Of the four phyletic stages indicated by the ontogeny ofElpenorandPorcellus, three still form the terminus of the development of existing species. The great differences among the caterpillars of this genus can be very simply explained on the view that they stand at different levels of phyletic development; some species having remained far behind (Group 1), others having advanced further (Group 2), and others having reached the highest point of development (Group 3). The fact that the species of the third group are only tropical accords well with this view, since many facts prove that phyletic development proceeds more rapidly in the tropics than in temperate climates.
The striking markings of theChærocampalarvæ may, in brief, be stated to originate from a local transformation of two portions of the subdorsal line into eye-spots, and the subsequenttransference of these two primary ocelli to the other segments. The eye-spots always originate on segments four and five, and from these the transference mostly occurs backwards, although in certain cases it takes place at the same time forwards. Herein,i.e.in the origin of the eye-spots, there lies a great distinction between the genusChærocampaand the genusDeilephila, with which it was formerly associated, and in which the origin of a very similar kind of marking can be traced to quite another source.
I am acquainted with the caterpillars of nine European and one North American species, these differing in marking to such a wonderful extent that they appear to offer at first sight but little hope of being able to trace them to a common form. These ten species can be separated, according to their markings, into five groups, which I will briefly define before entering upon their ontogeny.
The first group consists of three species, and comprises the commonest and most widely-ranging of all the European species,Deilephila Euphorbiæ, as well asD. Dahliifrom Sardinia and Corsica, andD. Nicæa, a species of very restricted range, which appears to occur only in one small district on the French coast of the Mediterranean. Thesethree species agree in marking to the extent of their possessing in the adult form two rows of ring-spots on each side, whilst the subdorsal line is completely absent.
The second group, consisting also of three species, shows a great resemblance toEuphorbiæ, but has only one row of ring-spots. It containsD. Vespertilio,D. Galii, and the AlgerianD. 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 ofD. Zygophylli, which occurs on the shores of the Caspian Sea, and the North AmericanD. 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 byD. 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 groupsare 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 beingHippophaës, the second that ofZygophylli, the third that ofLivornica, the fourth that ofGalii, and the fifth and youngest that ofEuphorbiæ. Only in this last am I acquainted with thecompletedevelopment 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.
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 ofEuphorbia Cyparissias. The eggs much resemble those ofChærocampa 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 theleaf, 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 theEuphorbialeaves 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 forChærocampa, 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 underneaththis line on the abdominal legs. These dots are not only of value as a character for differentiating the generaDeilephilaandChærocampa, 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 spiracularline89appears 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.
InFig. 41the 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 amongthemselves.90Thus, 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. Inone 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 inEuphorbiæis 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 ofDeilephila.
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.”
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 ofEuphorbiæ. 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 toEuphorbiæ; 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 ofGalii),appears to me to be of but very little importance, as I shall now proceed to show from the development ofD. Vespertilio.
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 onEpilobium 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 isno 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 beeasily 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.
The markings of this species appear to be developed in a precisely similar manner to those ofD. 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 ofVespertilio.
I found two specimens in the fourth stage in the Upper Engadine. One of these (Fig. 45)was already of a dark, blackish-greenground-colour91with 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 figuredthe 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,92in his well-known work, takes no notice of Rösel’s observation, but simply describes the caterpillar ofGaliias “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 amongthe 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 inPl. 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 ofD. 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 ofD. Nicæa(Fig. 51,Pl. VI.).I have never seen a caterpillar ofGaliiwhich 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.
Of the North AfricanD. Mauritanica, which likewise belongs to theGaliigroup, I have not been able to obtain specimens or figures of the younger stages. The adult caterpillar is very similar to that ofEuphorbiæ, 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 theLivornicagroup.
This, the only European species here to be considered, possesses almost the same markings asGaliiin 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, althoughBoisduval93also figures a light green specimen; from which it may be inferred, from analogy withGaliiandVespertilio, 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 ofVespertiliorepresented 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 speciesD. Lineata, Fabr., andD. Zygophylli, Ochs., the former being the North American representative of ourD. Livornica, but differing in remaining permanently at the fourth stage of this last species. I am acquainted withD. Lineataonly 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 excellenceof 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 ourLivornica.
This species, which is the next allied form toD. 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 ofDeilephila, 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 maysuppose 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.
This is the only representative of the fifth and oldest group known to me. The moth resemblesD. Euphorbiæto 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 samegreyish-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 inFig. 59for 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 thenucleus,94can be recognized on close inspection.
Specimens in which the spots extend from the eleventh to the seventh segment are of morefrequent 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 thesesecondaryring-spots resulting from transference developed? They do not, as in the case of theprimaryeye-spots of theChærocampinæ, 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 twocrescents 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 ofD. 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 ofD. 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 ofD. Hippophaësis 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 ofHippophaës, 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.
Regarding only theadultlarvæ of the species ofDeilephila, 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 ofD. EuphorbiæandD. 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 thefirst 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 alsoGalii).
Stage 3.—The subdorsal line bears a ring-spot on the penultimate segment; the other markings as in the last stage.D. Hippophaësonly 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. Zygophylliand the North AmericanD. Lineatabelong here.
Stage 5.—Closed ring-spots are situated on the subdorsal line. Of the known species, onlyD. Livornicaconcludes its development at this phyletic stage.