Stage 6.—A single row of ring-spots replaces the subdorsal line.D. Galii,Vespertilio, andMauritanicarepresent this stage at the conclusion of theirontogeny.95
Stage 7.—A double row of ring-spots. OnlyD. Dahlii,Euphorbiæ, andNicæaattain to this highest stage ofDeilephilamarking, the two firstspecies in the fourth stage, andNicæain 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,NicæaandEuphorbiæhave advanced to the seventh phyletic stage,ZygophylliandHippophaësonly to the third, and some specimens ofZygophyllito 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,Galiiin its last ontogenetic stage reaches thesixthphyletic stage; in its penultimate stage it reaches thefifthphyletic stage; and in its third stage; thefourthphyletic stage is represented, so that little imagination is required to anticipate that in the second stage thethirdorsecondphyletic 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.
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 ofD. Galii; that in others certain steps may be omitted, as withD. 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 ofD. Hippophaës, there may be drawn what to me appears to be a very important conclusion, viz. that the ring-spots ofDeilephilafirst 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 AmericanD. Lineata, and perhaps also those ofZygophylliandLivornica. The few observations on the development ofD. Galiialready recorded give support to thisview, 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 generaChærocampaandDeilephila. 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 theposteriorsegments. InDeilephila, on the other hand, a single ring-spot is formed on the penultimate segment bearing the caudal horn, and this is repeated on theanteriorsegments by secondary transference. With respect to the origination of the ring-spot also, there is a distinction between this genus andChærocampa, 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 inDeilephilathe 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 theprimaryring-spots can be drawn from the manner in which thesecondaryring-spots are formed.Because inHippophaësthe 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. InChærocampaalso, 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 genusDeilephilahave 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 theadultlarvæ ofEuphorbiæ, and still less frequently inNicæa, whilst they must be expected to be of more common occurrence inVespertilio, and also, as has already been seen, inDahlii. In this last species, as also inVespertilio, the completely-developed subdorsal line is still present in the third stage, whilst it is possessed byEuphorbiæonly 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 ofDahliifound 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 laststage ofVespertiliothis line appears still more frequently, whilst inEuphorbiæit 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. OfNicæaI 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 theseventhphyletic stage,e.g.D. Euphorbiæ, not unfrequently show variations corresponding to thesixthstage,i.e.only one instead of two rows of ring-spots—the upper and first-appearing series. On the other hand, forms reverting to thefifthphyletic stage (ring-spots with connecting subdorsal line) occur but very rarely. I have never met with such cases in adult living caterpillars ofD. 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 adultstate, belong to thesixthphyletic stage, not unfrequently show the characters of thefifthstage more or less developed, as, for example,D. Vespertilio.96
The caterpillars of this genus are very similar in appearance, and all possess extremely simplemarkings. 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 ofSmerinthusknown 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.
The light green eggs are nearly spherical, andafter 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 darkergreen 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.
After another period of 4–5 days the third moult takes place, and there now commences a dimorphism which will perhaps be better designated as variability, since the two extremes are connected by transitional forms. The majority of the larvæ have, as in the preceding stage, pure white oblique stripes, but many of them possess a blood-red spot on the anterior side of the stripes, this spot showing all gradations in size and depth of colour between maximum development and a mere trace. Special interest attaches to these spots, as they are the first rudiments of the coloured border of the oblique stripes which occurs in so manySphinxcaterpillars.
In the fifth stage—the last of the larval development—thered spots become more strongly pronounced. Among eighty caterpillars from one brood there were about twenty without any red whilst the remainder were ornamented with more or less vivid blood-red spots, often large and irregular in form. In some specimens the spots had become drawn out intolines,98forming a coloured edge to the oblique white stripes, similar to that possessed by the larva ofSphinx Ligustri. The caterpillar is thus represented in many figures, but generally the coloured stripe is made too regular, as in reality it is always irregularly defined above, and never so sharp and even as inSphinx Ligustri. The character is here obviously not yet perfected, but is still in a state of development.
From green spherical eggs there emerged larvæ 6.5 millimeters in length without any markings. They were of a light greenish-white, the large head and long caudal horn being of the same colour. The posterior boundary of the segments appears as a light shining ring (Pl. VI. Fig. 55).
The characteristic markings of the genus appear on the following day without the occurrence of any moult: seven oblique white stripes arise from near the dorsal line, and extend along the sides in a direction parallel to that of the horn. On the three front segments they are represented only bythree small white spots (Fig. 56). The caterpillar likewise possesses a marking of which the adult species of the genus retain only a trace, viz., a well-developed, pure white subdorsal line, which is crossed by the six anterior oblique stripes, and uniting with the upper part of the seventh extends to the caudal horn.
I long believed that the markings described were first acquired in the second stage, as I was possessed with the generally accepted idea that the changes of form and colour in insects could only occur at the period of ecdysis. I at first thought that the moult had escaped my notice, and I was only undeceived by close observation of individual specimens.
Second Stage.
The first moult took place after five days, the larvæ being 1.4 centimeters in length. Only unimportant changes of marking are connected therewith. The subdorsal line loses much in thickness and definition, and the first and last of the oblique stripes become considerably broader than the intermediate ones (Fig. 57). The green ground colour and also the stripes acquire a yellowish hue.
On the other hand, there occur changes in form. The head, which was at first rounded, becomes of the characteristic triangular shape, with the apex upwards, common to all the species of the genus,and at the same time acquires two white lines, which unite above at the apex of the angle. The shagreening of the skin now also takes place, and the red spot at the base of the horn is formed.
There appears to be at this stage a general tendency for the suffusion of red, the thoracic legs also becoming of this colour.
Third Stage.
The second moult occurs after six or eight days, the marking only changing to the extent of the subdorsal line becoming still more indistinct. This line can now only be distinctly recognized on the three front segments in a few individuals, whilst in the majority it is completely absent. Sometimes the ferruginous red spots on the oblique stripes now appear, but this character is not completely developed till the fifth stage. Out of about ninety bred specimens in which I followed the entire development, only one possessed such spots, and these were situated on both sides of the sixth segment.
Fourth Stage.
The third moult, which takes place after another period of six days, is not associated with any change of marking.
In this stage also I observed in one specimen (not the one just mentioned) the ferruginous spots, and again only on the sixth segment. On account of the theoretical conclusions which may be drawnfrom this localization of the spots—supposing it to be of general occurrence—it becomes of importance to institute observations with different broods, so as to investigate their first appearance, frequency, and local limitation. It appears to me very probable that, with respect to frequency and time of appearance, there would be great differences, since, in the last stage, it is just this character which shows a great variability. It would be more remarkable if it should be established that the first appearance of the spots was always limited to a certain segment; and there would then be a great analogy with the first appearance of the eye-spots inChærocampaand the ring-spots inDeilephila.
Fifth Stage.
The adult caterpillar does not differ in marking to any considerable extent from the preceding stages. The first and last stripes do not appear larger than the intermediate ones, as the latter now increase in size. Many specimens were entirely without red spots; in others they were present, but were small and inconspicuous, whilst in others again there were two spots, one above the other, of a vivid ferruginous red, these coalescing in some cases, and thus forming one spot of a considerable size. I have never seen these spots formed into a regular, linear, coloured border to the white oblique stripes—as occasionally happens inTiliæ—either in living specimens, blown larvæ, or in figures.
The green eggs much resemble those ofPopuli, as also do the newly hatched caterpillars, which, as in the case of this last species, are entirely without markings. As withPopuli, the markings are formed in the course of the first stage, and are distinctly visible before the first moult. The long caudal horn is of a red colour.
After two to three days the caterpillars moult, their length then being one centimeter; the seven beautiful oblique white stripes, and the fine white subdorsal line, are more strongly pronounced, the latter becoming broader in front. They differ fromPopuliin having the oblique stripes united in the dorsal line.
The second moult occurs after another three days, and brings no important change; only the fine subdorsal line becoming somewhat fainter. Neither is the third moult, which takes place four days later, associated with the appearance of any essentially new character. The oblique stripes remain as before, but their upper portions now stand on a somewhat darker green ground-colour, whilst the subdorsal line vanishes, leaving distinct traces only on the three or four front segments.
The fourth moult follows after a period of seven days, and my bred larvæ underwent scarcely any alteration in marking. Only small differences in coloration became perceptible in the head andhorn, these changing to bluish. Specimens occur, although but rarely, which show in this last stage red spots in the vicinity of the oblique stripes, just in the same manner as withPopuli, in which species, however, they occur more commonly. I only once found an adult larva ofOcellatuspossessing reddish-brown spots above and below the obliquestripes,99exactly as in one of the specimens figured by Rösel.100
In this stage also there remains almost always on the three to six front segments, a more or less distinct residue of the subdorsal, which extends backwards from the head as a whitish line intersecting the foremost oblique stripes. (Fig. 70,Pl. VII.)
From the meagre materials furnished by these three obviously nearly related species, we may at least conclude that, with respect to marking, three stages of development can be distinguished:—(1) Simple (green) coloration without marking; (2) subdorsal lines crossed by seven pairs of oblique stripes; (3) more or less complete absence of the subdorsal lines, the oblique stripes remaining, and showing a tendency to become edged with a red border.
Which of the three species is the oldest I will not attempt to decide. If we might venture to form any conclusion from the frequency of the red spots,Tiliæwould be the youngest,i.e., the species which has made the farthest advance. But this does not agree with the fact that the oblique stripes appear somewhat later in this species. Both these distinctions are, however, too unimportant to enable us to build certain conclusions on them. Neither does a comparison of the adultlarvæ with other species ofSmerinthusfurnish any further information of importance.
Of the genusSmerinthus, Latr., thirty species were catalogued byGray,101of which I am only acquainted with the larvæ of eight (five European, and three North American). None of these in the last stage possess a complete subdorsal line together with oblique stripes. Neither, on the other hand, do any of them show a more advanced stage of development in having the red spots constantly formed into coloured border-stripes. We must therefore admit that they have all reached nearly the same stage of phyletic development. On turning to the doubtfully placed genusCalymnia, Boisduval, which is represented in Gray by only one species, figured byWestwood102as aSmerinthus, we first meet with an older stage of development of the genus.
The adult caterpillar ofC. Panopus, from the East Indies, possesses, in addition to the oblique stripes, a completely developed subdorsalline,103and thus corresponds to the first stage ofS. Populi. This species may possibly retain in its ontogeny a stage in which the oblique stripes are also absent, whilst the subdorsal line is present. From the early disappearance of the subdorsal line in the species ofSmerinthus, we may venture to conclude that this character appeared at an early stage of the phylogeny, whilst the oblique stripes represent a secondary form of marking, as shall be further establishedsubsequently.104
The adult larvæ of five species are known, and to these I can now add a sixth. In Gray the genus contains twenty-sixspecies.105I cannot find any figures or descriptions of the young stages of these caterpillars, and I have myself only observed the complete ontogeny of one species.
By placing a captured femaleM. Stellatarumin a capacious breeding-cage, in the open air, I was enabled to procure eggs. The moth hovered about over the flowers, and laid its small, grass-green, spherical eggs (partly when on the wing), singly, on the leaves, buds, and stalks ofGalium Mollugo. Altogether 130 were obtained in threedays.106
First Stage.
After about eight days the caterpillars emerge. They are only two millimeters in length, and are at first yellowish, but soon become green, set with small single bristles, and they possess a short greenish caudal horn, which afterwards becomes black. The head is greenish-yellow. The young larvæ are entirely destitute of marking. (Pl. III., Fig. 1).
Second Stage.
The first moult takes place after four days, the caterpillar now acquiring the marking which it essentially retains to pupation.
Fine white subdorsal and spiracular lines appear, and at the same time a dark green dorsal line, which, however, does not arise from the deposition of pigment, as is generally the case, but from a division in the folds of the fatty tissue along this position. (Fig. 2,Pl. III.)
The colour is now dirty green in all specimens, the skin being finely shagreened.
Third Stage.
The second moult, occurring after another period of four days, does not bring any change ofmarking, the colour only becoming somewhat darker. Length, twelve millimeters.
Fourth Stage.
The third moult (after another four days) likewise brings only a change of colouring, which is of such a nature that the caterpillar becomes dimorphic. At the same time that peculiar roughening of the skin takes place which, in the case ofChærocampa, was designated as “shagreening.” The colour is now light grass-green in some specimens, and dark green in others; in these last the subdorsal line is edged above with dark brown, and the spiracles are also of this colour. Length, seventeen millimeters.
Fifth Stage.
Four days later, after the fourth ecdysis, the dimorphism becomes a polymorphism. Five chief types can bedistinguished:—
Variety I.—Light green (Fig. 7,Pl. III.); dorsal line, blackish-green, strongly marked; subdorsal line broad, pure white, edged above with dark green; spiracular line, chrome-yellow; horn, black, with yellow tip and blue sides. Spiracles, blackish-brown, with narrow yellow border; legs, and extremities of prolegs, vermilion-red.
Variety II.—Blackish-brown (Fig. 6,Pl. III.); head and prothorax, yellowish-brown; markings the same as above.
Variety III.—Blackish-green or greenish-black (Figs. 10 and 11,Pl. III.); subdorsal line with blackish-green border above, gradually passing into a light green ground-colour; spiracular line, chrome-yellow; head and prothorax, greenish-yellow.
Variety IV.—Light green (Figs. 4 and 12,Pl. III.); dorsal line quite feeble; subdorsal broad, only faintly edged with dark green; subspiracular line, faint yellowish; head and prothorax, green.
Variety V.—Brownish-violet (Fig. 8,Pl. III.); the black dorsal line on a reddish ground either narrow or broad.
From these five varieties we see that the different types do not stand immediately next to one another; they are, in fact, connected by numerous transitional forms, the ground-colour varying greatly, being dark or light, yellowish or bluish. (CompareFigs. 4, 5, 7, and 12.) The markings remain the same in all, but may be of very different intensities. The dorsal line is often only very feebly indicated, and the subdorsal line is frequently but faintly edged; the latter is also sometimes deep black above and bordered rather darkly beneath, the sides then being of a dark green, often with blackish dots on the yellow spiracular line (Fig. 5,Pl. III.), this likewise being frequently edged with black. Only the horn and legs are alike in all forms. The green ground-colour passes into blackish-green, greenishor brownish-black, and again, from reddish-brown to lilac (Fig. 3), this last being the rarest colour.
The designation “polymorphism” may here appear very inapplicable, since we have no sharply distinct forms, but five very variable ground-colours connected by numerous intermediate modes of coloration. Should, however, the term “variability” be suggested, I am in possession of an observation which tends to show that the different colours have to a certain extent become fixed. I found a brown caterpillar, the five front segments of which were light green on the left side, and the fifth segment brown and green mixed (Fig. 9,Pl. III.). Such parti-coloration can evidently only appear where we have contending characters which cannot become combined; just as in the case of hermaphrodite bees, where one half of a segment is male and the other half female, the two characters never becoming fused so as to produce a truly intermediateform.107From this observation, I conclude that some of the chiefvarieties ofStellatarumhave already become so far removed from one another that they must be regarded as intermediate fixed forms, the colours of which no longer become fused together when they occur in one individual, but are developed in adjacent regions. Other facts agree with this conclusion. Thus, among the 140 adult larvæ which I bred from the batch of eggs above mentioned, the transition forms were much in the minority. There were forty-nine green and sixty-three brown caterpillars, whilst only twenty-eight were more or less transitional.
On these grounds I designate the phenomenon as “polymorphism,” although it may not yet have reached, as such, its sharpest limits. This would be brought about by the elimination of the intermediateforms.108
Immediately before pupation, all the caterpillars, both green and brown, acquire a lilac coloration. The fifth stage lasts seven days, and the whole larval development twenty-three days, the period from the deposition of the eggs to the appearance of the moth being only thirty-one days.
I have treated of the polymorphism ofStellatarumin detail, not only because it has hitherto remained unknown, and an analysis of such cases has been completelyignored,109but more particularly because, it appears to me, that important conclusions can be drawn therefrom. Moreover,such an extreme multiplicity of forms is interesting, since, so far as I know, polymorphism to this extent has not been observed in any insect.
The theoretical bearing of this polymorphism will be treated of subsequently. It is not in any way connected with a more advanced development of the markings, sinceM. Stellatarumshows in this respect a very low state of development. This species displays only two stages:—(1), complete absence of all markings; and (2), a simple subdorsal, with dorsal and spiracular lines. We must therefore admit that the phyletic development of the markings has for a long time remained at a standstill, or, what expresses the same thing, that the marking which the adult larva now possesses is extremely old.
In order to complete my observations onM. Stellatarum, I now add some remarks on the pupa, the colour variations of which it appeared of importance to investigate, owing to the extraordinary variability of the caterpillar. The pupa varies but very slightly; the ochreous yellow ground-colour sometimes passes into reddish, and sometimes into greenish; the rather complicated blackish-brown marking of streaky lines is very constant, especially on the wing portions, being at most only more or less strongly pronounced. The minute colour variations of the pupa therefore have no connection with the colour of the caterpillar, both green and brown larvæ furnishing sometimesreddish-yellow and sometimes greenish-yellow pupæ.
The comparison ofM. Stellatarumwith the other known species of the genus, brings scarcely any addition to our knowledge of the phyletic development. Thus, the two European species of which the caterpillars are known, viz.M. FuciformisandBombyliformis,110show essentially the same markings asStellatarum, the chief element being a well-developed subdorsal line. The IndianM. Gilia, Herrich-Schäf., possesses also thisline,111and, together with the East IndianM. Corythus,Walk.,112has oblique stripes in addition; the stripes do not, however, cross this line, but commence underneath it, and probably originated at a later period than the subdorsal line. Should this be the case, we must regardM. Corythusas representing a later phyletic stage. According to Duponchel’s figures, in bothM. FuciformisandBombyliformissmall oblique stripes (red) occur near the spiracles, but these have nothing to do with the oblique stripes ofM. Giliajust mentioned, as they run in a contrary direction. Of the two European species, I haveonly seen the living caterpillar ofFuciformis, and this possessed no oblique stripes.
To these five species I am now enabled to add a sixth, viz.Macroglossa Croatica,113a species inhabiting Asia Minor and Eastern Europe, of which a specimen and notice were kindly forwarded to me by Dr. Staudinger. The adult caterpillar much resembles that ofM. Stellatarumin form and marking, but the subdorsal line appears much less distinctly defined, and the dorsal and spiracular lines seem to be entirely absent. The colour is generally green, but varies to red, and the subdorsal is more distinct and sharper in the young than in the adult larva. The markings of this species do not therefore in any way surpass those ofStellatarum, but are, on the contrary, muchsimpler.114
Although I am acquainted with only a small portion of the developmental history of a single species of this genus, I will here proceed to record this fragment, since, taken in connection with two other species, it appears to me sufficient to determine, at least broadly, the direction of development which this genus has taken.
The adult larva, as made known by many, and for the most part good figures, has very complicated markings, which do not seem derivable from any of the elements of marking in theSphingidæhitherto considered. I was therefore much surprised at finding a young caterpillar of this species, only twelve millimeters in length, of a light green colour, without any trace of the subsequent latticed marking, and with a broad white subdorsal line extending along all the twelve segments. (Pl. VII., Fig. 63). Judging from the size and subsequent development, this caterpillar was probably in the third stage.
The same colouring and marking remained during the following (fourth) stage; but in the position occupied by the caudal horn in otherSphingidæ, there could now be observed the rudiment of a future ocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The subdorsal line disappears suddenly in the fifth stage, when the larva becomes dark green (rarely) or blackish-brown; the latticed marking and the small oblique stripes are also acquired, together with the beautifully developed eye-spots, consisting of a yellow mirror with black nucleus and ground-area (Pl. VII., Fig. 65).
The North AmericanPterogon GauræandP. Abboti116also show markings precisely similar tothose of this European species in the adult state; but in the two former the markings are of special interest as indicating the manner in which the primary Sphinx-marking has become transformed into that of the apparently totally different adultP. Œnotheræ.P. Gauræis green, with a complicated latticed marking, which closer observation shows to arise from the dorsal line being resolved into small black dots, whilst the subdorsal line is broken up into black, white-bordered triangles. This caterpillar therefore gives fresh support to the remarkable phenomenon that the animals as well as the plants of North America are phyletically older than the European fauna and flora, a view which also appeared similarly confirmed byDeilephila Lineata, the representative form ofD. Livornica. In entire accordance with this is the fact that the larva ofP. Gauræis without the eye-spot on the eleventh segment, and instead thereof still shows the original although small caudal horn. The perfect insect also resembles ourP. Œnotheræin colour and marking, but not in the form of the wings.
That the caterpillars of the genusPterogonoriginally possessed the caudal horn we learnfromP. Gorgoniades, Hübn.,117a species now inhabiting south-east Russia, and for a knowledge of which I am indebted to Dr. Staudinger’s collection. There are in this about eight blown specimens, from 3.7 to 3.9 centimeters in length, which show a marking, sometimes on a red and sometimes on a green ground, which unites this species with the young form ofP. Œnotheræ, viz., a broad white subdorsal line, extending from the small caudal horn to the head. In addition to this, however, the caterpillar possesses an extraordinarily broad white red-bordered infra-spiracular line, a fine white dorsal stripe, and a similar line between the subdorsal and spiracular,i.e.a supra-spiracular line.
The caterpillars in Staudinger’s collection, notwithstanding their small size, all belong to the last stage, as the moth itself does not measure more than 2.6 centimeters in expanse, and is therefore among the smallest of the knownSphingidæ. This species has therefore in the adult condition a marking very similar to that ofŒnotheræwhen young—it bears toŒnotheræthe same relationship thatDeilephila Hippophaësdoes toD. Euphorbiæ, only in the present case the interval between the two species is greater.Gorgoniadesis obviously a phyletically older species, as we perceive from the marking and from the possession of a horn.We certainly do not yet know whetherŒnotheræpossesses a horn in its earliest stages, although in all probability it does so; in any case the ancestor ofŒnotheræhad a horn, since the closely alliedP. Gaurænow possesses one.
We thus see that also in the genusPterogonthe marking of the caterpillars commences with a longitudinal line formed from the subdorsal; an infra-spiracular or also a supra-spiracular line (Gorgoniades) being added. A latticed marking is developed from the linear marking by the breaking up of the latter into spots or small patches, which finally (inŒnotheræ) become completely independent, their connection with the linear marking being no longer directly perceptible.
Of this genus (in the narrow sense employed by Gray) I have only been able, in spite of all trouble, to obtain fertile eggs of one species. The females cannot be induced to lay in confinement, and eggs can only be obtained by chance.
I long searched in vain the literature of this subject for some account of the young stages of these caterpillars, and at length found, in a note to Rösel’s work, an observation of Kleemann’s on the young forms ofSphinx Ligustri, which, although far from complete, throws light on certain points.
From a female ofS. LigustriKleemann obtained 400 fertile eggs. The caterpillars on emergingare “at first entirely light yellowish-green, but become greener after feeding on the fresh leaves;” the horn is also at first light green, and then becomes “darker.” The young larvæ spin webs, by which they fasten themselves to the leaves of their food-plant (this, so far as I know, has not been observed in any species ofSphingidæ). They moult four times, the border round the head and the purple stripes appearing after the third moult, these stripes “having previously been entirely white.” The ecdyses follow at intervals of about six days, increasing to about ten days after the fourthmoult.118
From this short account we gather that in the third stage the marking consists of seven oblique white stripes, which acquire coloured edges in the fourth stage, a fact which I have myself frequently observed. On the most important point Kleemann’s observations unfortunately give no information—the presence or absence of a subdorsal line in the youngest stages. That he does not mention this character, can in no way be considered as a proof of its actual absence. I am rather inclined to believe that it is present in the first, and perhaps also in the second stage. There occur, however, species of the genusSphinx(sensû strictiori) which possess a subdorsal line when young, as I think may be certainly inferred from the factthat the remains of such a line are present in the adult larva ofS. Convolvuli.
This conclusion becomes still more certain on comparing the markings with those of a nearly allied genus; without such comparison the separation of the genusMacrosila, Boisd., fromSphinxis scarcely justifiable. If to these two genera we addDolba, Walk., andAcherontia, Ochs., we must be principally struck with the great similarity in the markings, which often reaches to such an extent that the differences between two species consist entirely in small shades of colour, while the divergence of the moths is far greater.
Of the genera mentioned, I am acquainted altogether with fourteen species of caterpillars:—Macrosila Hasdrubal,Rustica,119andCingulata;119Sphinx Convolvuli,Ligustri,Carolina,119Quinquemaculata,119Drupiferarum,119Kalmiæ,119andGordius;119Dolba Hylæus;119Acherontia Atropos,Styx,120andSatanas.120With one exception all these caterpillars possess oblique stripes of the nature of those of theSmerinthuslarvæ, and most of them are without any trace of a subdorsal line; one species—the North AmericanM. Cingulata—has a completely developed subdorsal; and the typical European species,S. Convolvuli, has a rudimentarysubdorsal line. The ground-colour in most of these species is of the same green as that of the leaves of their food-plants; some are brown,i.e.earth-coloured, and in these the markings do not appear so prominently; others again possess very striking colours (A. Atropos), the oblique stripes in these cases being very vivid. OnlyM. Hasdrubal121separates itself completely from thissystem of classification, since this species is deep black with narrow yellow rings, the horn and last segment being red.
The large and most striking caterpillar ofM. Hasdrubalis the same which Wallace has made use of for his theory of the brilliant colours of caterpillars. The explanation of the origin of this widely divergent mode of marking could only be furnished by the ontogeny, in which one or another of the older phyletic stages will certainly have been preserved.
Strictly speaking the same should be said of the other species—nevertheless their comparison with the so similarly markedSmerinthinæ, together with the circumstance that in certain species a subdorsal line can be traced, makes it appear correct to suppose that here also the subdorsal was the primary marking, this line being subsequently entirely replaced by the oblique stripes. TheSphinginæwould therefore be a younger group than theSmerinthinæ, a conclusion which is borne out by the fact that in the former the oblique stripes have reached a higher development, being always of two, and sometimes even of three colours (S. Drupiferarum, white, red, black), whilst in the species ofSmerinthusthey only occasionally possess uniformly coloured borders.
Although this genus is not admitted into most of the European catalogues—the solitary European species representing it being referred to the genusSphinx,Linn.122—its separation fromSphinxappears to me to be justified, not because of the striking differences presented by the moths, but because the caterpillars, judging from the little we know of them, likewise show a similar degree of difference.
I have frequently succeeded in obtaining fertile eggs ofAnceryx Pinastriand I will now give the developmental history of this caterpillar, which has already been figured with great accuracy in Ratzeburg’s excellent work on forest insects. Rösel was acquainted with the fact that the “pine moth” laid its eggs singly on the needles of the pine in June and July, and he described them as “yellowish, shining, oval, and of the size of a millet seed.”
On emerging, the caterpillars are six millimeters in length, of a light yellow colour, the head shining black with a yellow clypeus. The caudal horn, which is forked at the tip, is also at first yellowish, but soon becomes black. No particular marking is as yet present, but a reddish stripe extends along the region of the dorsal vessel, and the course of the spiracles is also marked by an orange-red line. (Fig. 53, A & B,Pl. VI.)
As soon as the young larvæ are filled with food they acquire a greenish streak. The first moult occurs after four days, and immediately after this there is still an absence of distinct markings, with the exception of a greenish-white spiracular line. In the course of some hours, however, the original light green ground-colour becomes darker, and at the same time a sharp, greenish-white subdorsal line appears, together with a parallel line extending above the spiracles, which, inPterogon Gorgoniades, has already been designated as the “supra-spiracular.” The dorsal line is absent:the head is light green, with two narrow blackish-brown lines surrounding the clypeus; the horn and thoracic legs are black; claspers, reddish green; length, twelve to thirteen millimeters. (Fig. 54.)
Third Stage.
After another period of four days the second moult occurs, neither colour nor marking being thereby affected. Only the horn, now no longer forked, becomes brownish with a black tip. The young caterpillars are now, as before, admirably adapted to the pine needles, on which they feed by day, and from which they can only be distinguished with difficulty.
Fourth Stage.
The third moult also brings no essential change. The ground-colour and marking remain the same, only the spiracles, which were formerly dull yellowish, are now of a vivid brick-red. The horn becomes yellowish-red at the base.
Fifth Stage.
The marking is only completely changed in the fifth and last stage. A broad reddish-brown dorsal line replaces the subdorsal, more or less completely. The supra-spiracular line also becomes broken up into numerous short lengths, whilst the green ground-colour in some specimens becomes more orless replaced by a brownish shade extending from the back to the sides. Horn, black; the upper part of the first segment with a corneous plate, similar to that of theDeilephilalarvæ.
This stage is very variable, as shown by the figures in various works. The variations arise on the one hand from the struggle between the green ground-colour and the reddish-brown extending from above, and, on the other hand, from a more or less complete disappearance of the associated longitudinal lines. The latter are sometimes completely retained, this being the case in a caterpillar figured by Hübner (Sphinges, III.,LegitimæC, b), where both the subdorsal and supra-spiracular lines are continuous from segment 11 to segment 1, an instance which may perhaps be regarded as a reversion to the primary form.
The entire change of the marking from the fourth to the fifth stage depends upon the fact that the young larvæ resemble theneedlesof the pine, whilst the adults are adapted to thebranches. I shall return to this later.
The ontogeny ofA. Pinastrimakes us acquainted with three different forms of marking: (1) simple coloration without marking; (2) a marking composed of three pairs of parallel longitudinal lines; (3) a complicated marking, arising from the breaking up of the last and the addition of a darker dorsal line.
Of the fourteen species placed by Gray in thegenusAnceryx, I find, in addition to the one described, notices of only twocaterpillars:—
A. Coniferarum,123a North American species, lives onPinus Palustris, and was figured by Abbot and Smith. Colour and marking very similar toA. Pinastri.
A. Ello,Linn.,124according to the authority of Mérian, is described byClemens125as dark brown, “with a white dorsal line, and irregular white spots on the sides.” It lives on a “species ofPsidiumorGuava.”
Most of the species ofAnceryxappear to live onConiferæ, to which they show a general and decided adaptation. In the absence of decisive information, I partly infer this from the names, asAnceryx Juniperi(Africa). It has long been known that in ourA. Pinastrithe mixture of brown and fir-green, interspersed with conspicuous irregular light yellowish and white spots, causes the adult larva to present a very perfect adaptation to its environment. Of this caterpillar Rösel states:—“After eating it remains motionless, and is then difficult to see, because it is of the same colour as its food, since its brown dorsal line has almost the colour of the pine twigs; and whois not familiar with the fact that beneath the green needles there is also much yellow to be found?”
This adaptation to the needles and twigs obviously explains why this caterpillar in the adult condition is so far removed from those of the genusSphinx, while the moths are so nearly related that they were only separated as a distinct genus when we became acquainted with a large number of species.