Mimicry in Butterflies/Chapter 10

It is clear from the last few chapters that the theory of mimicry in butterflies with its interpretation of the building up of these likenesses by means of natural selection in the form of predaceous birds and other foes is open to destructive criticism from several points of view. The evidence from mimicry rings makes it almost certain that in some cases the resemblance must be founded on an initial variation of such magnitude that the mimic could straightway be confused with the model. Till the mimic can be mistaken for the model natural selection plays no part. The evidence from breeding suggests strongly that in certain cases (e.g. Papilio polytes) the likeness arose in the form in which we know it to-day. In such cases there is no reason for supposing that natural selection has had anything to do with the formation of the finished mimic. Considerations of this nature may be said to have destroyed the view, current until quite recently, that in the formation of a mimetic resemblance the exclusive agent was natural selection. During the past few years it has come to be admitted by the staunchest upholders of the theory of mimicry that natural selection would not come into play until the would-be mimic was sufficiently like the model to be confused with it under natural conditions. The part now often attributed to natural selection is to put a polish on the resemblance and to keep it up to the mark by weeding out those which do not reach the required standard. It is supposed that if natural selection ceases to operate the mimetic resemblance is gradually lost owing to the appearance of variations which are no longer weeded out. An interesting case has recently been brought forward by Carpenter and explained on these lines: The Nymphaline Pseudacraea eurytus is a polymorphic species found in Central Africa. In Uganda it occurs in several distinct forms which were originally supposed to be distinct species. Three of these forms bear a marked resemblance to three species of the Acraeine genus Planema.

These different species occur round Victoria Nyanza and also on some of the islands in the lake. Some interesting points are brought out by a comparison of the occurrence and variation of the species on the mainland with what is found on Bugalla Island in the Sesse Archipelago. On the mainland the Pseudacraeas are abundant but the Planemas even more so, outnumbering the former by about 5:2. Moreover, it is rare to find individuals more or less intermediate between the three forms, though they are known to occur. On Bugalla Island, however, a different state of things is found. The Pseudacraeas are very abundant, whereas the Planemas, owing doubtless to the scarcity of their food plant, are relatively rare, and are very greatly outnumbered by the Pseudacraeas. At the same time the proportion of transitional forms among the Pseudacraeas is definitely higher than on the mainland. These facts are interpreted by Carpenter as follows:—

On the mainland where the models are abundant there is a vigorous action on the part of natural selection. The mimetic forms have a strong advantage and the non-mimetics have been gradually weeded out. But on the island, where the Pseudacraeas outnumber the models, the advantage obtained through mimicry is not so great. The so-called transitional forms are little, if at all, worse off than those closely resembling the scarce models, and consequently have as good a chance of surviving as any of the typical mimetic forms. On the mainland, however, the enemies of Pseudacraea are well acquainted with the Planema models which are here common, and discriminate against individuals which are not close mimics of the Planemas. The result is that on the mainland transitional forms are scarcer than on the island. Natural selection maintains a high standard for the mimetic likeness on the mainland owing to the abundance of the model; but when the model is scarce the likeness ceases to be kept up to the mark strictly, and tends to become lost owing to the appearance of fresh variations which are no longer weeded out.

Here it should be stated that the various Pseudacraeas form a population in which the different forms mate freely with one another. In the few breeding experiments that Dr Carpenter was able to make he found that obscura could produce terra, and that tirikensis was able to give obscura, the male in each case being, of course, unknown. Far too little work has as yet been done on the genetics of these various forms, and it would be rash to make assumptions as to the nature of the intermediates until the method of experimental breeding has been more extensively employed in analysing their constitution. Possibly it is not without significance that the abundance or scarcity of the obscura form runs parallel with the abundance or scarcity of the intermediates. It suggests that the intermediates are heterozygous in some factor for which the typical obscura is homozygous, and the fact that the intermediates are more numerous than obscura is what is to be looked for in a population mating at random. This case of the polymorphic Pseudacraea eurytus is one of the greatest interest, but it would be hazardous to draw any far-reaching deductions from such facts as are known at present. When the genetics of the various typical forms and of the intermediates has been worked out it will be disappointing if it does not throw clear and important light on these problems of mimetic resemblance.

As the result of modern experimental breeding work it is recognised that an intermediate form between two definite varieties may be so because it is heterozygous for a factor for which one variety is homozygous and which is lacking in the other—because it has received from only one parent what the two typical varieties receive from both parents or from neither. Its germ cells, however, are such as are produced by the two typical forms, and the intermediate cannot be regarded as a stage in the evolution of one variety from the other. In these cases of mimicry the existence of intermediate forms does not entail the deduction that they have played a part in the evolution of one pattern from another under the influence of a given model. It is quite possible that the new mimetic pattern appeared suddenly as a sport and that the intermediates arose when the new form bred with that which was already in existence. But before we are acquainted with the genetic relationships between the various forms, both types and intermediates, speculation as to their origin must remain comparatively worthless.

In this connection a few words on another source of variation may not be out of place. The patterns of butterflies are often very sensitive to changes in the conditions to which they are exposed during later larval and pupal life. Many moths and butterflies in temperate climates are double brooded. The eggs laid by the late summer brood hatch out, hibernate in the larval or pupal state, and emerge in the following spring. This spring brood produces the summer brood during the same year. In these cases it often happens that the two broods differ in appearance from one another, a phenomenon to which the term "Seasonal Dimorphism" has been applied. A well-marked instance is that of the little European Vanessid, Araschnia levana. The so-called levana form which emerges in the spring is a small black and orange-brown butterfly (Pl. VI, fig. 10). From the eggs laid by this brood is produced another brood which emerges later on in the summer, and is, from its very different appearance, distinguished as the prorsa form (Pl. VI, fig. 9). It is very much darker than the spring form and is characterised by white bands across the wings. The eggs laid by the prorsa form give rise to the levana form which emerges in the following spring. It has been shewn by various workers, and more especially by the extensive experiments of Merrifield, that the appearance of the levana or the prorsa form from any batch of eggs, whether laid by prorsa or levana, is dependent upon the conditions of temperature under which the later larval and early pupal stages are passed. By cooling appropriately at the right stage levana can be made to produce levana instead of the prorsa which it normally produces under summer conditions. So also by appropriate warming prorsa will give rise to prorsa. Moreover, if the conditions are properly adjusted an intermediate form porima can be produced, a form which occurs occasionally under natural conditions. The pattern is, in short, a function of the temperature to which certain earlier sensitive stages in this species are submitted. What is true of A. levana is true also of a number of other species. In some cases temperature is the factor that induces the variation. In other countries where the year is marked by wet and dry seasons instead of warm and cold ones moisture is the agent that brings about the change. In some of the South African butterflies of the genus Precis the seasonal change may be even more conspicuous than in A. levana. In Precis octavia, for example, the ground colour of the wet season form is predominantly red, while in the dry season form of the same species the pattern is different, blue being the predominating colour (cf. Pl. VI, figs. 11 and 12). Such examples as these are sufficient to shew how sensitive many butterflies are to changes in the conditions of later larval and earlier pupal life. The variations brought about in this way are as a rule smaller than in the examples chosen, but in no case are they known to be inherited, and in no case consequently could variation of this nature play any part in evolutionary change. Before any given variation can be claimed as a possible stage in the development of a mimetic likeness satisfactory evidence must be forthcoming that it is not of this nature, but that it is transmissible and independent of climatic and other conditions.

Many species of butterflies, especially such as are found over a wide range, exhibit minor varieties which are characteristic of given localities. These minor varieties may be quite small. In Danais chrysippus, for example, African and Asiatic specimens can generally be distinguished. On examples from India a small spot is seen just below the bar on the fore wing and on the inner side of it. Eastwards towards China this spot tends to become larger and confluent with the white bar, giving rise to an L-shaped marking; westwards in Africa the spot tends to disappear altogether. The existence of such local races has been used as an argument for the hereditary transmission of very small variations—in the present instance the size of a small white spot. For if it can be supposed that small differences of this nature are always transmitted, it becomes less difficult to imagine that a mimetic resemblance has been brought about by a long series of very small steps. But before this can be admitted it is necessary to shew by experiment that the size of this spot is independent of environmental conditions, both climatic and other. Apart from temperature and moisture it is not improbable that the formation of pigment in the wings may depend in some degree upon the nature of the food. The larvae of D. chrysippus feed upon various Asclepiads, and it is at any rate conceivable that the pigment formation, and consequently the details of pattern, may be in slight measure affected by the plant species upon which they have fed. The species of food plants are more likely to be different at the extremities of the range of a widely distributed form like D. chrysippus, and if they are really a factor in the pattern it is at the extremities that we should expect to find the most distinct forms. Actually we do find this in D. chrysippus, though it does not, of course, follow that the cause suggested is the true one, or, if true, the only one. Of the nature of local races too little at present is known to enable us to lay down any generalization. We must first learn by experiment how far they remain constant when transported from their own environment and bred in the environment under which another distinct local race is living. The behaviour of the transported race under the altered conditions would help us in deciding whether any variation by which it is characterised had a definite hereditary basis or was merely a fluctuation dependent upon something in the conditions under which it had grown up. The decision as to whether it is hereditary or not must depend upon the test of breeding, through which alone we can hope to arrive at a satisfactory verdict upon any given case.

The particular geographical variation which has just been considered happens to be a small one. But it may happen that a geographical variety is much more distinct. Indeed it is not impossible that butterflies which are at present ranked as distinct species may prove eventually to be different forms of the same species. Especially is this likely to be true of many forms in South America, of which Bates long ago remarked "that the suspicion of many of the species being nothing more than local modifications of other forms has proved to be well founded." Since Bates' day more material has been forthcoming and it has been shewn that certain colour schemes are characteristic of distinct geographical regions in South America where they may occur in species belonging to very different genera and families. In Central America, for example, the pattern common to many species is determined by horizontal and oblique black bands on a bright fulvous brown ground, with two broken yellow bars towards the tip of the fore wing. The general type is well shewn by Mechanitis saturata and the female of Dismorphia praxinoe (Pl. X, figs. 7 and 3). Belonging to this pattern group are a number of different species belonging to various families, including several Heliconines and Ithomiines, Pierids such as Dismorphia and Perrhybris, Nymphalines of the genera Eresia and Protogonius, and other forms. In Eastern Brazil the predominant pattern is one characterised by a yellow band across the hind wing and a white or yellow apical fore wing marking (cf. Pl. XV, figs. 3 and 8). Here also, with the exception of the Perrhybris, all the various genera which figured in the last group are again represented. It is true that the members of this second group are regarded as belonging to different species from those of the first group, but as species here are made by the systematist chiefly, if not entirely, on the colour pattern this fact may not mean much. Passing now to Ega on the Upper Amazons the general ground colour is a deep chestnut purple and the apical area of the fore wings presents a much mottled appearance (cf. Pl. XV, figs. 4 and 9). In this group again we find represented the different genera found in the other groups, the only notable absentees being Eresia and Perrhybris. Lastly in Ecuador, Peru, and Bolivia the general pattern scheme consists of orange-tawny markings on a black ground (cf. Pl. XV, figs. 5 and 10). This group differs somewhat in composition from the preceding in that it contains no Pierid and no Danaid. On the other hand its numbers have been strengthened by the accession of a Papilio, an Acraea, and two species of the Satyrid genus Pedaliodes. Certain writers have seen in the theory of mimicry the only explanation of these peculiar geographical pattern groups. The fashion is in each case set by the most abundant form, generally an Ithomiine of the genus Melinaea. The rest are mimics of this dominant species, either in the Batesian or Müllerian sense. Batesian mimics are such genera as Dismorphia and Protogonius, to which there are no reasons for attributing disagreeable properties. Of the nature of Müllerian mimics on the other hand are the various Heliconines and Ithomiines which enter into the combination. In each case the whole assemblage is a great "mimicry ring," of which the pattern is dictated by the Ithomiine that predominates in point of numbers. It is, however, very doubtful whether this can be accepted as a satisfactory explanation. The four groups which we have considered are all characterised by a peculiar and distinctive coloration, and in each case the pattern must on the theory of mimicry be regarded as a highly efficient warning pattern. One or other of these patterns must doubtless be looked upon as the most primitive. If so the question at once arises as to why a distasteful genus should change from one efficient warning pattern to another quite distinct one. If the newer pattern affords better protection we should expect it to have spread and eventually to have ousted the older one. That it has not done so must probably be attributed to the old pattern being as efficient as the new one. But if this is so we are left without grounds for assuming the change to have been brought about by natural selection through the agency of enemies to whom warning colours appeal. For natural selection can only bring about a change that is beneficial to the species. Hence we must suppose the change on the part of the dominant model to have been independent of natural selection by enemies, and due to some condition or set of conditions of which we are ignorant. It is not inconceivable that the new colour scheme was associated with some physiological peculiarity which was advantageous to the species in its altered surroundings. If so natural selection may have favoured the new variety, not because of its colour scheme, but owing to the underlying physiological differences of which the pattern is but an outward sign. And if this could happen in one species there seems to be no reason why it should not happen in others. The weak point of the explanation on the mimicry hypothesis is that it offers no explanation of the change in the so-called dominant Ithomiine pattern as we pass from one region to another. Whatever the cause of this change may be there would appear to be nothing against it having also operated to produce similar changes in other unrelated species, in which case the mimicry hypothesis becomes superfluous. It is not unlikely that the establishing of these new forms was due to natural selection. If they were associated with physiological peculiarities better adapted for their environment it is reasonable to suppose that natural selection would favour their persistence as opposed to the older type until the latter was eliminated. But such action on the part of natural selection is quite distinct from that postulated on the mimicry hypothesis. On the one view the colour itself is selected because it is of direct advantage to its possessor; on the other view the colour pattern is associated with a certain physiological constitution which places the butterflies possessing it at an advantage as compared with the rest.

It is, nevertheless, possible that mimicry may have played some part in connection with establishing the new colour pattern in some of these South American species. For if the new pattern had become established in the predominant distasteful species, and if some of the members of a palatable form (e.g. Protogonius) were to shew a variation similar to that already established in the distasteful species, and if further there be granted the existence of appropriate enemies, then it would be almost certain that the newer form in palatable species would eventually replace the older form. In such a case the part played by natural selection would be the preservation of a chance sport which happened to look like an unpalatable form. There is no reason for regarding the change as necessarily brought about by the gradual accumulation of a long series of very small variations through the operation of natural selection.