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Studies in the Theory of Descent, Volume II
Studies in the Theory of Descent, Volume II
I. Larva and Imago vary in Structure independently of each other
It would be meaningless to assert that the two stages above mentioned were completely independent of one another. It is obvious that the amount of organic and living matter contained in the caterpillar determines the size of the butterfly, and that the quantity of organic matter in the egg must determine the size of the emergent larva. The assertion in the above heading refers only to the structure; but even for this it cannot be taken as signifying an absolute, but only a relative independence, which, however, certainly obtains in a very high degree. Although it is conceivable that every change of structure in the imago may entail a correlative change of structure in the larva, no such cases have as yet been proved; on the contrary, all facts indicate an almost complete independence of the two stages. It is quite different with cases of indirect dependence, such, for example, as are brought about by ‘nurse-breeding.’ This phenomenon is almost completely absent in Lepidoptera, but is found in Diptera, and especially in Hymenoptera in every degree. The larvæ of ichneumons which live in other insects, require (not always, but in most instances) that the female imago should possess a sharp ovipositor, so that in this case also the structure and mode of life of the larva influences the perfect insect. This does not depend, however, on inherent laws of growth (correlation), but on the action of external influences, to which the organism endeavours to adapt itself by natural selection.
I will now let the facts speak for themselves.
It is shown by those species in which only one stage is di- or polymorphic that not every change in the one stage entails a corresponding change in the other. Thus, in all seasonally dimorphic species we find that the caterpillars of butterflies which are often widely different in the colour and marking of their successive generations are absolutely identical. On the other hand, many species can be adduced of which the larvæ are dimorphic whilst the imagines occur only in one form (compare the first and second essays in this volume).
There are however facts which directly prove that any one stage can change independently of the others; I refer to the circumstance that any one stage may become independently variable – that the property of greater variability or of greater constancy by no means always occurs in an equal degree in all the three stages of larva, pupa, and imago, but that sometimes the caterpillar is very variable and the pupa and imago quite constant. On the other hand, all three stages may be equally variable or equally constant, although this seldom occurs.
If variability is to be understood as indicating the period of re-modelling of a living form, whether in its totality or only in single characters or groups of characters, from the simple fact of the heterochronic variability of the ontogenetic stages, it follows that the latter can be modified individually, and that the re-modelling of one stage by no means necessarily entails that of the others. It cannot however be doubted that variability, from whatever cause it may have arisen, is in all cases competent to produce a new form. From the continued crossing of variable individuals alone, an equalization of differences must at length take place, and with this a new, although not always a widely deviating, constant form must arise.
That the different stages of development of a species may actually be partly variable and partly constant, and that the variable or constant character of one stage has no influence on the other stages, is shown by the following cases, which are, at the same time, well adapted to throw light on the causes of variability, and are thus calculated to contribute towards the solution of the main problem with which this investigation is concerned.
When, in the following pages, I speak of variability, I do not refer to the occurrence of local varieties, or to variations which occur in the course of time, but I mean a high degree of individual variability – a considerable fluctuation of characters in the individuals of one and the same district or of the same brood. I consider a species to be constant, on the other hand, when the individuals from a small or large district differ from one another only to a very slight extent. Constant forms are likewise generally, but not invariably, such as are poor in local varieties, whilst variable forms are those which are rich in such variations. Since the terms “variable” and “constant” are but relative, I will confine myself to the most extreme cases, those in which the individual peculiarities fluctuate within very wide or very narrow limits.
As no observations upon the degree of variability shown by a species in the different stages of its development were available, I was obliged to fall back upon my own, at least so far as relates to the larval and pupal stages, whilst for the imaginal stage the wide experience of my esteemed friend Dr. Staudinger has been of essential service to me.
Let us in the first place confine our attention to the three chief forms which every Lepidopteron presents, viz. larva, pupa, and imago. With respect to the constancy or variability of these three forms, we actually find in nature all the combinations which are theoretically conceivable.
(1.) There are species which possess a high degree of constancy in all three stages, such, for example, as Limenitis Camilla, Pieris Brassicæ,1 Sphinx Ligustri, and Euchelia Jacobææ.
(2.) There are species showing a high degree of variability in all three stages. This case must be of rare occurrence, as I am only able to adduce Araschnia Prorsa-Levana, a fact which arises from the circumstance that the pupal stage is, as a rule, but seldom variable.
(3.) There are species which are variable in two stages and constant in the third. To this class, for example, belongs Smerinthus Tiliæ, of which the larva and imago are very variable, whilst the pupa is quite constant. The same is the case with Lasiocampa Pini, the well-known fir moth. Many butterflies show this same phenomenon in other combinations, such, for instance, as Vanessa Urticæ and Polychloros, in which the larva and pupa are very variable, and the imago very constant. In a less degree the same is also the case with Vanessa Atalanta, whilst in Pieris Napi the pupa and imago are variable, and the caterpillar remarkably constant, this likewise being the case with the local form Bryoniæ, which, according to my theory, is to be regarded as the parent form of Napi (See Part I. of the present volume).
(4.) There are species which are constant in two stages, and variable only in the third. Thus, a few species can be found in which the larva and pupa are constant and the imago variable. This is the case with Saturnia Yamamai, the imago of which is well known to present numberless shades of colour, varying from light yellow to greyish black, whilst the green caterpillar shows only slight individual differences of marking, and scarcely any differences of colour. The pupa of this species is quite constant. Arctia Caja and Hebe, and Chelonia Plantaginis belong to this same category.
There are a very large number of species which possess very constant imagines and pupæ, but extremely variable larvæ. The following are the cases known to me: —Macroglossa Stellatarum, Fuciformis and Bombyliformis; Chærocampa Elpenor, Celerio, and Nerii; Deilephila Galii, Livornica, Hübn., Hippophaës, Vespertilio, and Zygophylli; Sphinx Convolvuli; Acherontia Atropos; Smerinthus Ocellatus and Tiliæ; Callimorpha Hera; Cucullia Verbasci and Scrophulariæ.
Cases in which the variability depends entirely upon the pupa, while the larva and imago are extremely constant, are of great rarity. Vanessa Io is a case in point, the pupa being light or dark brown, or bright golden green, whilst in the two other stages scarcely any light shades of colour or variations in the very complicated marking are to be met with.
The facts thus justify the above view that the individual stages of development change independently – that a change occurring in one stage is without influence on the preceding and succeeding stages. Were this not the case no one stage could possibly become variable without all the other stages becoming so. Did there exist a correlation between larvæ, pupæ, and imagines of such a nature that every change in the larva entailed a corresponding change in the imago, as soon as a large number of larval characters became fluctuating (i. e. as soon as this stage became variable), a large number of imaginal characters would necessarily also become fluctuating (i. e. this stage would also become correspondingly variable).
There is one other interpretation which might perhaps be attempted from the point of view of the old doctrine of species. It might be said that it is a special property of certain larval or imaginal markings to be variable whilst others are constant, and since the larval and imaginal markings of a species are generally quite distinct, it may easily happen that a butterfly possessing markings having the property of constancy may belong to a caterpillar having variable markings.
There is a soul of truth underlying this objection, since it is true that the various forms of markings which occur in Lepidoptera apparently reach different degrees of constancy. If we speak of the constancy or variability of a species, a different meaning is attached to these expressions according as we are dealing e. g. with a species of Sphinx or a species of Arctia. That which in the latter would be estimated as a high degree of constancy, in the former would be taken as a considerable amount of variability. It is of interest, in connection with the question as to the causes of constancy, to note that the power of any form of marking to attain to a high degree of constancy is by no means inversely proportional to the complication of the marking, as would have been expected à priori.
Thus, the species of Sphinx and of allied genera possess on their fore-wings, which are mostly coloured with a mixture of dull grey, white and black, an exceedingly complicated arrangement of lines which, in constant species, show a high degree of uniformity: on the other hand, the checquered fore-wings of our Arctiidæ, which are far more coarsely marked, always show, even in the most constant species, well-marked individual differences. The different types of marking must therefore be measured by different standards.
But in granting this, we decidedly refute the statement that constancy and variability are inherent properties of certain forms of marking.
This reasoning is based on the simple fact that a given type of marking comprises both species of great constancy and of (relatively) great variability.
Thus, the fore-wings of Sphinx Ligustri and S. Convolvuli are extremely constant, whilst the very similarly marked Anceryx (Hyloicus) Pinastri is exceedingly variable. Similarly Deilephila Euphorbiæ is known by its great variability of colouring and marking, whilst D. Galii, which resembles this species so closely as to be sometimes confounded with it, possesses a high degree of constancy, and further, the Corsican and Sardinian D. Dahlii is very variable. Among the family Arctiidæ, Callimorpha Hera and the Alpine Arctia Flavia are cases of constancy, whilst A. Caja, which is so similar to the last species, is so generally variable that two perfectly identical specimens can scarcely be found together.
The same can be shown to hold good for the markings of caterpillars. Thus, the larva of D. Dahlii shows very considerable variability, whilst that of D. Galii is very constant in marking (disregarding the ground-colour). So also the larva of Vanessa Urticæ is very variable and that of V. Antiopa very constant, &c.
The great differences with respect to constancy or variability which are displayed by the different stages of one and the same species, must therefore find their explanation elsewhere than in the type of the marking itself. The explanation must be found in the circumstance that each stage changes independently of the others, and at different periods can enter a new phase of variability.
We are here led in anticipation to the main question: – Are changes produced by internal or external causes? is it the physical nature of the organism which is compelled to become remoulded spontaneously after the lapse of a certain period of time? or does such modification only occur when produced directly or indirectly by the external conditions of life?
In the cases before us the facts undoubtedly indicate a complete dependence of the transformations upon external conditions of life.
The independent appearance of variability in the separate stages of the metamorphosis might, however, be regarded as only apparent. It might still be attempted to attribute the changes to a purely inherent cause, i. e., to a phyletic vital force, by assuming that the latter acts periodically in such a manner that at first one and then the following stage becomes variable, until finally the entire species is transformed.
There is but little to be said in reply to this if we once take refuge in entirely unknown forces, the operation of which can be arbitrarily conceived to be either constant or periodic.
But granting that such a transforming power exists and acts periodically, the variability must always pass over the different stages in a fixed direction, like a wave over the surface of water – imago, pupa, and larva, or larva, pupa, and imago, must successively become variable. Cases like that of Araschnia Prorsa, in which all three stages are variable, may certainly be thus explained, but those instances in which the larva and imago are extremely variable, and the pupa quite constant, are entirely inexplicable from this point of view.
The latter can, however, be very simply explained if we suppose the changes to be dependent upon external influences. From this standpoint we not only see how it is possible that an intermediate stage should remain uninfluenced by the changes which affect the two other stages, but we can also understand why it should just be the pupal stage that plays this part so frequently. If we ask why most pupæ are constant and are relatively but very slightly variable, the answer will be found in the facts that all pupæ which remain concealed in the earth or inside plants (Sesiidæ), or which are protected by stout cocoons, show complete constancy, whilst any considerable amount of variability occurs only in those pupæ which are suspended or openly exposed. This is closely connected with a fact to which I have called attention on a former occasion,2 viz., that dimorphism occurs in certain pupæ, but only in those which are openly exposed and which are therefore visible to their foes. I am only acquainted with such cases among the pupæ of butterflies, and it is likewise only among these that I have found any considerable amount of variability.
Facts of this kind indicate that Nature does not uselessly sport with forms, but that at any rate changes of this sort result from external influences. The greater frequency of variability among larvæ and its comparative rarity in imagines is also undoubtedly in favour of this view.
It has already been shown that species with variable larvæ and constant imagines are extremely common, but that those with constant larvæ and variable imagines are very rare. This confirms the conclusions, already drawn above, first, that the variability of the imago cannot owe its existence to the variability of the larvæ, and secondly, that the causes which produce variability affect the larval condition more commonly than that of the imago.
Where can these causes be otherwise sought than in the external conditions of life, which are so widely different in the two stages, and which are much more variable for the larva than for the imago?
Let us take the species of one genus, e. g. those of Deilephila. The imagines of our European species – as far as we know – all live in precisely the same manner; they all fly at twilight,3 showing a preference for the same flowers and very often frequenting the same spots, so that in the haunts of one species the others are almost always to be met with, supposing them to occur in the same locality. They conceal themselves by day in similar places, and are attacked by similar foes.
It is quite different with the caterpillars. These, even in the case of the most closely allied species, live under different conditions, as appears from the fact that they feed on different plants. The latter can, however, produce changes both directly and indirectly. The larvæ may acquire adaptive colours and markings, and these would vary in accordance with the colour and structure of the food-plant; or they may become brightly coloured as a sign of distastefulness in cases where they are inedible. Then again the colour of the soil on which the larvæ live would act upon their colours making these adaptive. Certain habits of the caterpillars may also be dependent upon the nature of their food-plants. Thus, e. g. Deilephila Hippophaës feeds only at night, and conceals itself by day under moss and among the leaves at the base of the food-plant; but D. Euphorbiæ could not acquire such a habit, because Euphorbia Cyparissias generally grows on arid soil which is poor in vegetation, and which therefore affords no concealment, and furthermore, because a caterpillar, as long as it continues to feed, cannot, and as a matter of fact does not, ever wander far from its food-plant. A habit of concealment by burying in the earth also, such for example as occurs in Acherontia Atropos, could not be acquired by D. Euphorbiæ, because its food-plant generally grows on hard, dry, and stony ground.
In addition to these considerations, the foes would be different according as the caterpillar lived on plants which formed dense thickets covering large extents of the shore (Hippophae) or grew isolated on dry hillocks and declivities where the herbage was scanty or altogether absent; or again, according as the insect, in conjunction with such local differences, fed by day or had acquired the habit of feeding only by night. It must in fact be admitted that new and improved adaptations, or, in more general terms, that inducements to change, when depending on the environment, must be more frequently dissimilar for larvæ than for the imagines. We must accordingly expect to find actual change, or that condition of variability which may be regarded as initiative to change, occurring more commonly in larvæ than in perfect insects.
Since facts are in complete accordance with the results of these à priori considerations we may also venture to conclude that the basis of the considerations is likewise correct, viz., the supposition that the changes of colour and marking in caterpillars, pupæ, and imagines result from external influences only.
This must not be taken as signifying that the single stages of the larval development are also only able to change through the action of external influences. The larval stages are correlated with each other, as has already been shown (see the previous essay): new characters arise in the adult caterpillar at the last stage and are then gradually transferred back to the younger stages quite independently of external influences, this recession being entirely brought about by the laws of correlation. Natural selection here only exerts a secondary action, since it can accelerate or retard this transference, according as the new characters are advantageous or disadvantageous to the younger stages.
Now as considerable individual differences appear in the first acquisition of a new character with respect to the rapidity and completeness with which the individuals acquire such a character, the same must obtain for the transference of an improvement acquired in the last stage to the next younger stage. The new character would be acquired by different individuals in different degrees and at different rates – it would have, to a certain extent, to struggle with the older characters of the stage; in brief, the younger stage would become variable.
Variability of this kind might well be designated as secondary, in contradistinction to primary variability; the latter (primary) depends upon an unequal reaction of the individual organisms to external influences, the former (secondary) results from the unequal strength and rate of the action of the innate laws of growth governing the organism. In both cases alike exceeding variability may occur, but the causes producing this variability are dissimilar.
The different stages of larval development would thus frequently display independent variability in a manner similar to the pupal or imaginal stages, since they can show individual variability while the other stages of development remain constant. This appearance of independent variability in the different stages of the larval development, however, is in truth deceptive – we have here in fact a kind of wave of variability, which passes downwards through the developmental stages, becoming gradually weaker, and finally dying out completely.
In accordance with this, we very frequently find that only the last or two last stages are variable, while the younger stages are constant. Thus in Macroglossa Stellatarum, the larvæ are constant in the first, second, and third stages, but become variable in the fourth, and in the fifth stage first show that high degree of variability which has already been described in detail (See. Pl. III., Figs. 3–12). The larvæ, of Vanessa Cardui also, according to my notes, are extremely constant in the first four stages in spite of their complicated marking, but become variable in the fifth stage, although to no very great extent.
In Smerinthus Tiliæ, Ocellatus and Populi also, the greatest larval variability is shown only in the last stage, the preceding stages being very constant. These cases by no means depend upon the marking of the young stages being simpler and therefore being less capable of varying. The reverse case also occurs. In a somewhat similar manner as the young of the tapir and wild hog are striped, while the adult animals are plainly coloured, the young caterpillars of Saturnia Yamamai possess longitudinal black lines on a yellow ground, while as early as in the second stage a simple green colour appears in the place of this complicated but perfectly constant marking. If the young stages are so frequently constant, this rather depends upon the fact that the transference of a new character to these stages not only takes place gradually, but also with continually diminishing energy, in a manner somewhat similar to physical motion, which continually diminishes in speed by the action of resistance till it is completely arrested. This constancy of the younger stages may further be due to the circumstance that the characters would only be transferred when they had become fixed in the last stage, and were consequently no longer variable. The transferred characters may thus have acquired a greater regularity, i. e. a less degree of variability, than they possessed at their first origination. Extensive investigations in this special direction must be made if the precise laws, in accordance with which the backward transference of new characters takes place, are to be discovered. By such researches only should we arrive with certainty at the causes which determine the lesser variability of the young larval stages.
It may also occur that the early stages are variable, whilst the later stages are constant, although this case appears to happen less frequently. Thus, the caterpillars of Gastropacha Quercifolia vary considerably in the second stage but are constant at a later period, and the same is the case with Spilosoma Urticæ, which in the second stage may be almost considered to be dimorphic, but which subsequently becomes constant.
Cases in which the first stage is variable appear to be of the least frequent occurrence. I know of only one such instance, viz., Anceryx Pinastri, of which the newly hatched larvæ (Pl. VI., Fig. 53) show considerable differences in the brownish-black crescentic spots. The second (Fig. 54), third, and fourth stages are then tolerably constant, while the fifth stage again is very variable.
An instance of this kind can be easily explained by two waves of variation, the first of which now affects only the first stage, while the second has just commenced to affect the fifth stage. Such a supposition is not opposed to any theoretical considerations, but rather has much probability in its favour, since we know that species are from time to time subject to be remodelled; and further, that the coalescence of several stages of phyletic development in the ontogeny of one and the same species (see p. 226, development of the genus Deilephila) shows that during the backward transference of one character, new characters may appear in the last stage of the ontogeny, and indeed very frequently at a time when the next youngest character has not been transferred back so far as to the first stage.
That this secondary variability is to a certain extent brought about by the conflict between the old and new characters, the latter striving to suppress the former, is shown by the caterpillar of Saturnia Carpini which I have observed for many years from this point of view, and than which I do not know a more beautiful illustration.
When these larvæ leave the egg they are black, but in the adult state are almost bright green – this at least being the case in a local form which, from the district in the vicinity of Genoa where it is found, I will designate as the var. Ligurica. Now whilst these two extreme stages of development are relatively constant, the intermediate stages show a variability which becomes greater the nearer the last stage is approached, this variation in the marking depending simply on the struggle between the green colour and the more anciently inherited black. In this manner there arises, especially in the fourth stage of the German local form, an incredible mixture of the most diverse markings, all of which can, however, be very easily explained from the foregoing point of view.
The simpler and, as I am inclined to believe, the older form of the transformation is presented to us in the local variety Ligurica. In the last stage, when 7.5 centimeters long, this form is of a beautiful bright green colour without any trace of black marking4 (Pl. VIII., Fig. 77). The colour of the six orange warts which are situated on each segment is also similar in all specimens, so that this stage is perfectly constant.
Our German S. Carpini shows different characters in the fifth stage. It is true that individual specimens occur which are entirely green without any black, but these are rare; the majority possess a more or less broad black ring encircling the middle of each segment (Pl. VIII., Figs. 78 and 79). Those specimens in which the black ring has become broken up into large or small spots surrounding the base of the warts constitute intermediate forms (Fig. 80). The last stage of the German local form, unlike that of the Genoese local form, is therefore very variable.
The two forms, moreover, do not simply differ in being more or less advanced in phyletic development, but also in several other points. As it is of great theoretical interest to show that a species can develop local differences only in the stage of larva, I will here subjoin the plain facts.
The differences consist in that the Genoese local form goes through five moults whilst the German local form, like most caterpillars, has only four moults. Further, in the Genoese form the light green, which is also possessed by the German form in the fourth stage, when it once appears, is retained to the end of the larval development, whilst in the fifth stage of the German form this colour is replaced by a dull greyish-green (compare Figs. 77 and 78). There is further a very considerable difference in the earlier stages which shows that the phyletic transforming process has taken a quite independent course in the two forms. Since the struggle between the green and black – retaining this idea – appears to be quite finished in the last stage of the Genoese form, we should expect that the new colour, green, would now also have encroached further upon the younger stages than in the German form. Nevertheless, this is not the case, but quite the reverse happens, the black maintaining its ground longer in the Italian than in the German form.
In the Genoese form the two first stages are completely black, and in the third stage an orange-yellow lateral stripe first appears. In the German form this stripe appears in the second stage, and there is not subsequently added, at least on the middle segments, a yellow border surrounding some of the warts of the median series. In the third stage, however, the yellow (which is but the precursor of the later green colour) becomes further extended, so that the caterpillars often appear of an orange colour, some or all of the warts and certain spots and stripes only being black (Figs. 66 and 68). The warts are also often yellow while the ground remains in most part black – in brief, the bright colour is in full struggle with the black, and an endless series of variations is the result of this conflict, whilst in the corresponding stage of the Genoese form almost complete constancy prevails.
This constancy remains also in the following (fourth) stage, the caterpillar still being deep black, only the yellow (sulphur-coloured) lateral stripe, which has now become brighter, indicating the impending change (Fig. 67). This takes place in the fifth stage, in which the ground-colour suddenly becomes bright green, the black remaining at most only in traces on the anterior edges of the segments.
This is the same marking as is shown by the fourth stage of the German form, only in this case individuals quite destitute of black do not occur. In many specimens indeed black forms the ground-colour, the green only appearing in certain spots (Figs. 71 to 75); in others the green predominates, and these two extremes are connected by innumerable intermediate forms, so that this stage must be regarded as the most variable of all.
The sixth stage of the Genoese and the fifth of the German form have already been compared together. The results may be thus tabulated: —
A. German form. B. Genoese form.
Stage I. 9 days. Black; constant. 9 days. Black; constant.
Stage II. 8 days. Black, with orange-yellow lateral stripe; variable. Black, with yellow; very variable. 11 days. Black; constant.
Stage III. 5 days (in some cases as much as 16 days). 12 days. Black, with orange-yellow lateral stripes; constant.
Stage IV. 16 days (in some cases only 5 days). Bright green and black mixed; very variable. 6 days. Black, with bright yellowish lateral stripe; constant.
Stage V. 6 days (frequently longer). Dark green, with or without black bands; variable. 6 days. Bright green, small traces of black; variable.
Stage VI. Pupation. 18 days. Bright green, without any black; constant.
Stage VII. Pupation.
From this comparison we perceive that the process of transformation has at least become preliminarily concluded in the Genoese form. Why the backward transference of the newly-acquired character to the young stages has not yet occurred, or, at least, why it is not in progress, does not appear; neither can it be stated whether this will take place later, although we may venture to suppose that such will be the case. At first sight but a relatively short time appears necessary for the single stage V., which is still in a state of fluctuation (variable), to become constant by continued crossing, like all the other stages.
That the transformation is still in full progress in the German form, is shown by the fact that in this case all the stages are variable with the exception of the first – the second stage being only variable to a small extent, the third to a much greater extent, and the fourth to the highest degree conceivable, whilst the fifth and last stage is again less variable – so that the greatest struggle between the old and new characters takes place in the fourth stage.
Among the innumerable variations presented by this last stage a complete series of transitional forms can be arranged so as to show the gradual conquest of the black by the green, and thus indicating, step by step, the course which the latter colour has taken.
In the blackest specimens there is nothing green but the lateral (infra-spiracular) line which was yellow in the preceding stage, and a crescent-shaped streak at the base of the middle warts together with a still smaller crescent at the base of the upper warts (Figs. 71 and 81). These spots become extended in lighter specimens and approximate so as to leave only narrow black bridges, a third spot being added at the posterior edge of the warts (Figs. 72 and 82). The three spots then extend on all sides, still leaving for a long period narrow black lines at the boundaries where their growth has caused them to abut. In this manner there frequently arises on the green ground a true hieroglyphic-like marking (Figs. 85 and 86). Finally the black disappears from the anterior edge and diminishes on the middle line of the back where it still partly remains as a T-shaped figure (Figs. 73 and 74), although generally replaced elsewhere by the green with the exception of small residues.
One point remained for a long time inexplicable to me, viz., the change of the light green into dark grey-green which appeared in the last stage in connection with a total change of the black marking.
Supposing that new characters are actually acquired only in the last stage, and that from this they are transferred to the younger stages, we should expect to find completely developed in the last stage the same colouring and markings as are possessed more or less incompletely in the fourth stage. Now since the developmental tendency to the removal of black and to the predominance of green – if we may thus venture to express it – is obvious in the fourth stage, we may expect to find in the fifth stage a bright green ground-colour, either without any mixture of black or with such black spots and streaks as were retained in the fourth stage as residues of the original ground-colour. But instead of this the fifth stage shows a dark green colour, and a more or less developed black marking which cannot in any way be derived from that of the fourth stage.
The Genoese local form observed last year first gave me an explanation to the extent that in this form the last stage is actually only the potential penultimate stage, or, more correctly expressed, that the same characters which at present distinguish the last stage of this form, are already more or less completely transferred to the penultimate stage.
The apparently paradoxical behaviour of the German form can be explained by supposing that before the pure bright green had become completely transferred to the penultimate stage a further change appeared in the last stage, the green ground-colour becoming darker, and black transverse bands being formed. The marking of the last stage would then be regarded as the reverse of that of the preceding stage; the absence of black would be the older, simple black spots at the base of the warts the next in succession, and a connected black transverse band the most advanced state of the development.
Whether this explanation is correct, and if so, what causes have produced the second change, may perhaps be learnt at some future time by a comparison with the ontogeny of other Saturniidæ; in the meantime this explanation receives support from another side by the behaviour of the Genoese local form. If the last stage of the German form has actually commenced to be again re-modelled, then this variety is further advanced in phyletic development than the Genoese form; and this corresponds entirely with the theory that in the former the light colour (the orange considered as preliminary to the transformation into green) has already been carried down into the second stage, whilst in the Genoese variety even in the fourth stage only the first rudiments of the colour-transformation show themselves.
The Genoese form is to a certain extent intermediate between the German form of Saturnia Carpini and the nearly related S. Spini, a species inhabiting East Germany. In this latter the larvæ, even in the adult state, are completely black with yellow warts. This form of caterpillar must therefore be regarded as phyletically the oldest, and this very well agrees with the character of the moth, which differs essentially from S. Carpini only in not being sexually dimorphic. In Carpini the male possesses a far more brilliant colouring than the female, the latter agreeing so completely with the female of Spini that it can hardly be distinguished therefrom, especially in the case of the somewhat larger South European specimens of the last species. Now as the more simple colouring of the female must in any case be regarded as the original form, we must consider Spini, both sexes of which possess this colouring, to be phyletically the older form, and Carpini, the male of which has become differently coloured, must be considered as the younger type. This completely accords with the characters of the larvæ.
I must here mention that I have also asked myself the question whether the variations of the different larval stages are connected together as cause and effect – whether the lightest specimens of the fifth stage may perhaps not also have been the lightest individuals of the third and fourth stages.
Such relationship is only apparent between the third and fourth stages; the darkest larvæ of the third stage become the darker varieties of the fourth stage, although it is true that the lighter forms of the third sometimes also become dark varieties in the fourth stage. Between the fourth and fifth stages there is scarcely any connection of this kind to be recognized. Thus, the darkest varieties of the fourth stage sometimes become the lightest forms of the fifth stage, whilst in other cases from the lightest individuals of the fourth stage there arise all the possible modifications of the fifth stage. Further details may be omitted: the negative result cannot cause any surprise, as it is a necessary consequence of the continued crossing that must take place.
We thus see that the three chief stages of development (larva, pupa, and imago) actually change in colour independently of each other, the single stages of the larval development being however in greater dependence upon one another, and being connected indeed in such a manner that a new character cannot be added to the last stage without being transferred in the course of time to the preceding stage, and at a later period from this again even to the youngest stage, supposing it not to be previously delayed in the course of its transference by unknown opposing forces. On this last point, however, the facts at present available do not admit of any certain decision.
But why do the individual larval stages behave in this respect so very differently to the chief stages of the whole development? why are the former so exactly correlated whilst the latter are not? If new characters have a general tendency to become transferred to the younger ontogenetic stages, why are not new imaginal characters first transferred to the pupa, and finally to the larva?
The answer to these questions is not far to find. The wider two stages of a species differ in structure, the less does correlation become possible; the nearer the two stages are morphologically related, the more powerful does the action of correlation become. It is readily conceivable that the more widely two succeeding stages deviate in structure and mode of life, the less possible does it become for characters to be transferred from one to the other. How is it possible, for example, that a new character in the proboscis or on the wings of a butterfly can be transferred to the caterpillar? If such correlation existed it could only manifest itself by some other part of the caterpillar changing in correspondence with the change of the proboscis or wings of the butterfly. That this is not the case has, in my opinion, been conclusively shown by all the foregoing considerations respecting the independent variability of the chief stages of the metamorphosis.
There are, moreover, an endless number of facts which prove the independence of the individual stages of development – I refer to the multitudinous phenomena presented by metamorphosis itself. The existence of that form of development which we designate as metamorphosis is alone sufficient to prove incontestably that the single stages are able to change independently of one another to a most remarkable extent.
If we now ask the question: how has the so-called “complete” metamorphosis of insects arisen? the answer can only be: through the gradual adaptation of the different stages of development to conditions of life which have continually deviated more and more widely from each other.5
But if individual stages of the post-embryonic development can finally attain to such complete diversity of structure as that of the larva and imago through gradual adaptations to continually diverging conditions of life, this shows that the characters acquired by the single stages are always only transferred to the same stages of the following generation, whilst the other stages remain uninfluenced thereby. This depends upon that form of heredity designated by Darwin “inheritance at corresponding periods of life,” and by Haeckel “homochronic heredity.”
II. Does the Form-relationship of the Larva coincide with that of the Imago?
Having thus established the independence in the variability of the individual stages of metamorphosis, I will now turn to the consideration of the question as to how far a parallelism is displayed in the phyletic development of these stages. Is there a complete congruence of form-relationship between larvæ on the one hand and imagines on the other? does the classification founded on the morphology of the imagines agree with that based on the morphology of the larvæ or not?
If, according to Claus,6 we divide the order Lepidoptera into six great groups of families, it is at once seen that these groups, which were originally founded exclusively on imaginal characters, cannot by any means be so clearly and sharply defined by the larval characters.
This is certainly the case with the Geometræ, of which the larvæ possess only ten legs, and on this account progress with that peculiar “looping” movement which strikes even the uninitiated. This group, which is very small, is however the only one which can be founded on the morphology of the larvæ; it comprises only two nearly related families (Phytometridæ and Dendrometridæ), and it is not yet decided whether these should not be united into one group comprising the family characters of the whole of the “loopers.”
Neither the group of Micro-lepidoptera, nor those of the Noctuina, Bombycina, Sphingina, and Rhopalocera, can be based systematically on larval characters. Several of these groups are indeed but indistinctly defined, and even the imagines present no common characteristics by which the groups can be sharply distinguished.
This is well shown by the Rhopalocera or butterflies. These insects, in their large and generally brilliantly coloured wings, which are usually held erect when at rest, and in their clubbed antennæ, possess characters which are nowhere else found associated together, and which thus serve to constitute them a sharply defined group.7 The caterpillars, however, show a quite different state of affairs. Although the larval structure is so characteristic in the individual families of butterflies, these “larval-families” cannot be united into a larger group by any common characters, and the “Rhopalocera” would never have been established if only the larvæ had been known. It is true that they all have sixteen legs, that they never possess a Sphinx-like horn, and that they are seldom hairy, as is the case with many Bombycidæ,8 but these common negative characters occur also in quite distinct groups.
In the butterflies, therefore, a perfect congruence of form-relationship does not exist, inasmuch as the imagines constitute one large group of higher order whilst the larvæ can only be formed into families. If it be admitted that the common characters of butterflies depend on their derivation from a common ancestor, the imagines must have retained certain common characters which enable them to be recognized as allies, whilst the larvæ have preserved no such characters from the period at which the families diverged.
Without going at present into the causes of these phenomena I will pass on to the consideration of further facts, and will now proceed to investigate both the form-relationships within the families. Here there can be no doubt that in an overwhelmingly large majority of cases the phyletic development has proceeded with very close parallelism in both stages; larval and imaginal families agree almost completely.
Thus, under the group Rhopalocera there is a series of families which equally well permit of their being founded on the structure of the larva or on that of the imago, and in which the larvæ and imagines therefore deviate from one another to the same extent. This is the case, for instance, with the families of the Pieridæ, Papilionidæ, Danaidæ, and Lycænidæ.
But there are also families of which the limits would be very different if the larvæ were made the basis of the classification instead of the butterflies as heretofore. To this category belongs the sub-family Nymphalinæ. Here also a very characteristic form of caterpillar indeed prevails, but it does not occur in all the genera, being replaced in some by a quite different form of larva.
In the latest catalogue of Diurnal Lepidoptera, that of Kirby (1871), 112 genera are comprised under this family. Of these most of the larvæ possess one or several rows of spines on most or on all the segments, a character which, as thus disposed, is not met with in any other family.
This character is noticeable in genera 1 to 90, if, from those genera of which the larvæ are known, we may draw a conclusion with reference to their allies. I am acquainted with larvæ of genus 2, Agraulis, Boisd. (Dione, Hübn.); of genus 3, Cethosia, Fabr.; 10, Atella, Doubl.; 12, Argynnis, Fabr.; 13, Melitæa,9 Fabr.; 19, Araschnia, Hübn.; 22, Vanessa, Fabr.; 23, Pyrameis, Hübn.; 24, Junonia, Hübn.; 31, Ergolis, Boisd.; 65, Hypolimnas, Hübn. (Diadema, Boisd.); 77, Limenitis, Fabr.; 81, Neptis, Fabr.; 82, Athyma, Westw.; and finally with those of genus 90, Euthalia, Hübn. – which, according to Horsfield’s figures, possess only two rows of spines, these being remarkably long and curved, and fringing both sides. It may be safely assumed that the intermediate genera would agree in possessing this important character of the Nymphalideous larvæ, viz., spines.
After the genus 90 there are 22 more genera, and these are spineless, at least in the case of the two chief genera, 93, Apatura, and 104, Nymphalis. Of the remainder I know neither figures nor descriptions.10 In the two genera named the larvæ are provided with two or more spine-like tentacles on the head, and the last segment ends in a fork-like process directed backwards. The body is otherwise smooth, and differs also in form from that of the larvæ of the other Nymphalinæ, being thickest in the middle, and tapering anteriorly and posteriorly; neither is the form cylindrical, but somewhat flattened and slug-shaped. If therefore we were to arrange these butterflies by the larvæ instead of by the imagines, these two genera and their allies would form a distinct family, and could not remain associated with the 90 other Nymphalideous genera.
We have here a case of incongruence; the imagines of the genera 1–90 and 91–112 are more closely allied than their larvæ.
From still another side there arises a similar disagreement. The larvæ of the genera Apatura and Nymphalis agree very closely in their bodily form and in their forked caudal appendage with the caterpillars of another sub-family of butterflies, the Satyrinæ, whilst their imagines differ chiefly from those of the latter sub-family in the absence of an enlargement of certain veins of the fore-wings, an essential character of the Satyrinæ.
This double disagreement has also been noticed by those systematists who have taken the form of the caterpillar into consideration. Thus, Morris11 attempted to incorporate the genera Apatura and Nymphalis into the family Libytheidæ, placing the latter as transitional from the Nymphalidæ to the Satyridæ. But although the imagines of the genera Apatura, Nymphalis, and Libythea may be most closely related – as I believe they actually are – the larvæ are widely different, being at least as different as are those of Apatura and Nymphalis from the remaining Nymphalinæ.
Now if we could safely raise Apatura and Nymphalis into a distinct family – an arrangement which in the estimation of Staudinger12 is correct – and if this were interpolated between the Satyridæ and Nymphalidæ, such an arrangement could only be based on the larval structure, and that of the imagines would thus remain unconsidered, since no other common characters can be found for these two genera than those which they possess in common with the other Nymphalideous genera.
The emperor-butterflies (Apatura), by the ocelli of their fore-wings certainly put us somewhat in mind of the Satyrinæ, in which such spots are always present; but this character does not occur in the genus Nymphalis, and is likewise absent in most of the other genera of this group. The genus Apatura shows in addition a most striking similarity in the markings of the wings to the purely Nymphalideous genus Limenitis, and it is therefore placed, by those systematists who leave this genus in the same family, in the closest proximity to Limenitis. This resemblance cannot depend upon mimicry, since not only one or another but all the species of the two genera possess a similar marking; and further, because similarity of marking alone does not constitute mimicry, but a resemblance in colour must also be added. The genus Limenitis actually contains a case of imitation, but in quite another direction; this will be treated of subsequently.
It cannot therefore be well denied that in this case the larvæ show different relationships to the imagines.
If the “natural” system is the expression of the genetic relationship of living forms, the question arises in this and in similar cases as to whether the more credence is to be attached to the larvæ or to the imagines – or, in more scientific phraseology, which of the two inherited classes of characters have been the most distinctly and completely preserved, and which of these, through its form-relationship, admits of the most distinct recognition of the blood-relationship, or, inversely, which has diverged the most widely from the ancestral form? The decision in single instances cannot but be difficult, and appears indeed at first sight impossible; nevertheless this will be arrived at in most cases as soon as the ontogeny of the larvæ, and therewith a portion of the phylogeny of this stage, can be accurately ascertained.
As in the Rhopalocera most of the families show a complete congruence in the form-relationship of the caterpillars and perfect insects, so a similar congruence is also found in the majority of the families belonging to other groups. Thus, the two allied families of the group Sphingina can also be very well characterized by their larvæ;13 both the Sphingidæ and the Sesiidæ possess throughout a characteristic form of larva.
Of the group Bombycina the family of the Saturniidæ possess thick cylindrical caterpillars, of which the segments are beset with a certain number of knob-like warts. It is true that two genera of this family (Endromis and Aglia) are without these characteristic warts, but the imagines of these genera also show extensive and common differences from those of the other genera. A distinct family has in fact already been based on these genera (Endromidæ, Boisd.). Thus the congruence is not thereby disturbed.
So also the families Liparidæ, Euprepiidæ, and Lithosiidæ appear sharply defined in both forms; and similar families occur likewise under the Noctuina, although in this group the erection of families presents great difficulties owing to the near relationship of the genera, and is always to some extent arbitrary. It is important, however, that it is precisely the transitional families which present intermediate forms both as larvæ and as imagines.
Such an instance is offered by the Acronyctidæ, a family belonging to the group Noctuina. The imagines here show in certain points an approximation to the group Bombycina; and their larvæ, which are thickly covered with hairs, likewise possess the characteristics of many of the caterpillars of this group.14
A second illustration is furnished by the family Ophiusidæ, which is still placed by all systematists under the Noctuina, its affinity to the Geometrina, however, being represented by its being located at the end of the Noctuina. The broad wings and narrow bodies of these moths remind us in fact of the appearance of the “geometers;” and the larvæ, like the imagines, show a striking resemblance to those of the Geometrina in the absence of the anterior abdominal legs. For this reason Hübner in his work on caterpillars has termed the species of this family “Semi-Geometræ.”
All these cases show a complete congruence in the two kinds of form-relationship; but exceptions are not wanting. Thus, the family Bombycidæ would certainly never have been formed if the larval structure only had been taken into consideration, since, whilst the genera Gastropacha, Clisiocampa, Lasiocampa, Odonestis, and their allies, are thickly covered with short silky hairs disposed in a very characteristic manner, the caterpillars of the genus Bombyx, to which the common silkworm, B. Mori, belongs, are quite naked and similar to many Sphinx-caterpillars (Chærocampa). Are the imagines of the genera united under this family, at any rate morphologically, as unequally related as their larvæ? Whether it is correct to combine them into one family is a question that does not belong here; we are now only concerned with the fact that the two stages are related in form in very different degrees.
An especially striking case of incongruence is offered by the family Notodontidæ, under which Boisduval, depending only on imaginal characters, united genera of which the larvæ differed to a very great extent. In O. Wilde’s work on caterpillars this family is on this account quite correctly characterized as follows: – “Larvæ of various forms, naked or with thin hairs, sixteen or fourteen legs.”15 In fact in the whole order Lepidoptera there can scarcely be found associated together such diverse larvæ as are here placed in one imago-family; on one side the short cylindrical caterpillars of the genus Cnethocampa, Steph. (C. Processionea, Pithyocampa, &c.), which are covered with fine, brittle, hooked hairs, and are very similar to the larvæ of Gastropacha with which they were formerly united; and on the other side there are the naked, humped, and flat-headed larvæ of the genus Harpyia, Ochs., with their two long forked appendages replacing the hindmost pair of legs, and the grotesquely formed caterpillars of the genera Stauropus, Germ., Hybocampa, Linn., and Notodonta, Ochs.
The morphological congruence between larvæ and imagines declares itself most sharply in genera, where it is the rule almost without exception. In this case we can indeed be sure that a genus or sub-genus founded on the imagines only will, in accordance with correct principles, present a corresponding difference in the larvæ. Had the latter been known first we should have been led to construct the same genera as those which are now established on the structure of the imagines, and these, through other circumstances, would have stood in the same degree of morphological relationship as the genera founded on the imagines. There is therefore a congruence in a double sense; in the first place the differences between the larvæ and imagines of any two genera are equally great, and, in the next place, the common characters possessed by these two stages combined cause them to form precisely the same groups defined with equal sharpness; the genera coincide completely.
So also the butterflies of the sub-family Nymphalinæ can well be separated into genera by the characters of the larvæ, and these, as far as I am able to judge, would agree with the genera founded on the imagines.
The genus Melitæa, for example, can be characterized by the possession of 7–9 fleshy tubercles bearing hairy spines; the genus Argynnis may be distinguished by always having six hairy unbranched spines on each segment, and the genus Cethosia by two similar spines on each segment; the genus Vanessa shows sometimes as many as seven branched spines; and the genus Limenitis never more than two branched blunt spines on each segment, and so forth. If we go further into details it will be seen that the most closely related imagines, as might indeed have been expected, likewise possess the most nearly allied larvæ, whilst very small differences between the imagines are also generally represented by corresponding differences in the larvæ. Thus, for instance, the genus Vanessa of Fabricius has been divided into several genera by later authors. Of these sub-genera, Grapta, Doubl. (containing the European C. – album, the American Fabricii, Interrogationis, Faunus, Comma, &c.), is distinguished by the fact that the larvæ not only possess branched spines on all the segments with the exception of the prothorax, but these spines are also present on the head; in the genus Vanessa (sensû strictiori), Doubl., the head and prothorax are spineless (e. g. V. Urticæ); in the tropical genus Junonia, Hübn., which was also formerly (Godart, 181916) united with Vanessa, the larvæ bear branched spines on all the segments, the head and prothorax included.
It is possible to go still further and to separate two species of Vanessa as two new genera, although they have hitherto been preserved from this fate even by the systematists most given to “splitting.” This decision is certainly justifiable, simply because these species at present stand quite alone, and the practical necessity of forming a distinct genus does not make itself felt, and this practical necessity moreover frequently comes into conflict with scientific claims: science erects a new genus based on the amount of morphological difference, it being quite immaterial whether one or many species make up this genus; such an excessive subdivision is, however, a hindrance to practical requirements, as the cumbrous array of names thereby becomes still further augmented.
The two species which I might separate from Vanessa on the ground of their greater divergence, are the very common and widely distributed V. Io and Antiopa, the Peacock Butterfly and the Camberwell Beauty. In the very remarkable pattern of their wings, both show most marked characteristics; Io possesses a large ocellus on each wing, and Antiopa has a broad light yellow border which is not found in any other species of Vanessa. There can be no doubt but that each of these would have been long ago raised into a genus if similarly marked species of Vanessa occurred in other parts of the world, as is the case with the other species of the genus. Thus, it is well known that there is a whole series of species resembling our V. Cardui, and another series resembling our V. C. – album, the two series possessing the same respective types of marking; indeed on these grounds the sub-genera Pyrameis and Grapta have been erected.17
I should not have considered it worth while to have made these remarks if it had not been for the fact that the caterpillars of V. Io and V. Antiopa differ in small particulars from one another and from the other species of the genus. These differences relate to the number and position of the spines, as can be seen from the following table: —
Species of the Genus Vanessa, Fabr.
This character of the number of spines will not be considered as too unimportant when we observe how perfectly constant it remains in the nearly allied species. This is the case in the three consecutive forms, Urticæ, Polychloros, and Ichnusa. Now when we see that two species which differ in their imaginal characters present correspondingly small differences in their larvæ, this exact systematic congruence indicates a completely parallel phyletic development.
Exceptions are, however, to be met with here. Thus, Hübner has united one group of the species of Vanessa into the genus Pyrameis just mentioned, on account of certain characteristic distinctions of the butterflies. I do not know, however, how this genus admits of being grounded on the structure of the larvæ; the latter, as appears from the above table, agree exactly in the number and position of the spines with the caterpillars of Vanessa (sensû strictiori), nor can any common form of marking be detected which would enable them to be separated from Vanessa.
Still more striking is the incongruence in the genus Araschnia, Hübn. (A. Prorsa-Levana), which, like the genus Pyrameis, is entirely based on imaginal characters. This is distinguished from all the other sub-genera of the old genus Vanessa by a small difference in the venation of the wings (the discoidal cell of the hind-wings is open instead of closed). Now it is well-known that in butterflies the wing-venation, as most correctly shown by Herrich-Schäffer, is the safest criterion of “relationship.” It thus happens that this genus, typified by the common Levana, is in Kirby’s Catalogue separated from Vanessa by two genera, and according to Herrich-Schäffer18 by forty genera! Nevertheless, the larvæ agree so exactly in their spinal formula with Grapta that we should have no hesitation in regarding them as a species of this sub-genus. It appears to me very probable that in this case the form-relationship of the caterpillar gives more correct information as to the blood-relationship of the species than that of the imago – in any case the larvæ show a different form-relationship to the imagines.
Just as in the case of butterflies there are many genera of Sphingidæ which can be based on the structure of the larvæ, and which agree with those founded on the imagines.
Thus, the genus Macroglossa is characterized by a straight anal horn, a spherical head, and by a marking composed of longitudinal stripes, these characters not occurring elsewhere in this combination. The nearly allied genus Pterogon, on the other hand, cannot be based on the larvæ only, since not only is the marking of the adult larva very distinct in the different species, but the anal horn is present in two species, whilst in a third (P. Œnotheræ) it is replaced by a knob-like eye-spot. The genus Sphinx (sensû strictiori) is distinguished by the simple, curved caudal horn, the smooth, egg-shaped head and smooth skin, and by a marking mainly composed of seven oblique stripes. The genus Deilephila is distinguished from the preceding by a dorsal plate, situated on the prothorax and interrupting the marking, as well as by the pattern, which here consists of a subdorsal line with ring-spots more or less numerous and developed; the skin also is rough, “shagreened,” although it must be admitted that there are exceptions (Vespertilio). The genus Chærocampa admits also of being based on the form-relationship of its caterpillars, although this is certainly only possible by disregarding the marking and taking alone into consideration the peculiar pig-like form of the larvæ. The genus Acherontia, so nearly related to Sphinx, possesses in the doubly curved caudal horn a character common to the genus (three species known19). Finally may be mentioned the genus Smerinthus, of which the larvæ, by their anteriorly tapering form, their shagreened skin and almost triangular head with the apex upwards, their simply curved anal horn, and by their seven oblique stripes on each side, constitute a genus as sharply defined as that formed by the moths.
Although in all the systematic divisions hitherto treated of there are cases where the form-relationship of the larva does not completely coincide with that of the imago, such incongruences are of far more frequent occurrence in the smallest systematic group, viz. species.