Darwinism (1889) Part 19

The two species figured, though belonging to such distinct and even remote genera, have acquired almost identical tints and markings so as to be deceptively alike. The surface of the wings is, in both, transparent yellowish, with black transverse bands and white marginal spots, while both have similar black-and white-marked bodies and long yellow antennae. Dr. Muller states that they both show a preference for the same flowers growing on the edges of the forest paths.[105]

We will now proceed to give the explanation of these curious similarities, which have remained a complete puzzle for twenty years.

Mr. Bates, when first describing them, suggested that they might be due to some form of parallel variation dependent on climatic influences; and I myself adduced other cases of coincident local modifications of colour, which did not appear to be explicable by any form of mimicry.[106] But we neither of us. .h.i.t upon the simple explanation given by Dr. Fritz Muller in 1879.

His theory is founded on the a.s.sumed, but probable, fact, that insect-eating birds only learn by experience to distinguish the edible from the inedible b.u.t.terflies, and in doing so necessarily sacrifice a certain number of the latter. The quant.i.ty of insectivorous birds in tropical America is enormous; and the number of young birds which every year have to learn wisdom by experience, as regards the species of b.u.t.terflies to be caught or to be avoided, is so great that the sacrifice of life of the inedible species must be considerable, and, to a comparatively weak or scarce species, of vital importance. The number thus sacrificed will be fixed by the quant.i.ty of young birds, and by the number of experiences requisite to cause them to avoid the inedible species for the future, and not at all by the numbers of individuals of which each species consists. Hence, if two species are so much alike as to be mistaken for one another, the fixed number annually sacrificed by inexperienced birds will be divided between them, and both will benefit.

But if the two species are very unequal in numbers, the benefit will be comparatively slight for the more abundant species, but very great for the rare one. To the latter it may make all the difference between safety and destruction.

To give a rough numerical example. Let us suppose that in a given limited district there are two species of Heliconidae, one consisting of only 1000, the other of 100,000 individuals, and that the quota required annually in the same district for the instruction of young insectivorous birds is 500. By the larger species this loss will be hardly felt; to the smaller it will mean the most dreadful persecution resulting in a loss of half the total population. But, let the two species become superficially alike, so that the birds see no difference between them.

The quota of 500 will now be taken from a combined population of 101,000 b.u.t.terflies, and if proportionate numbers of each suffer, then the weak species will only lose five individuals instead of 500 as it did before.

Now we know that the different species of Heliconidae are not equally abundant, some being quite rare; so that the benefit to be derived in these latter cases would be very important. A slight inferiority in rapidity of flight or in powers of eluding attack might also be a cause of danger to an inedible species of scanty numbers, and in this case too the being merged in another much more abundant species, by similarity of external appearance, would be an advantage.

The question of fact remains. Do young birds pursue and capture these distasteful b.u.t.terflies till they have learned by bitter experience what species to avoid? On this point Dr. Muller has fortunately been able to obtain some direct evidence, by capturing several Acraeas and Heliconidae which had evidently been seized by birds but had afterwards escaped, as they had pieces torn out of the wing, sometimes symmetrically out of both wings, showing that the insect had been seized when at rest and with the two pairs of wings in contact. There is, however, a general impression that this knowledge is hereditary, and does not need to be acquired by young birds; in support of which view Mr. Jenner Weir states that his birds always disregarded inedible caterpillars. When, day by day, he threw into his aviary various larvae, those which were edible were eaten immediately, those which were inedible were no more noticed than if a pebble had been thrown before the birds.

The cases, however, are not strictly comparable. The birds were not young birds of the first year; and, what is more important, edible larvae have a comparatively simple coloration, being always brown or green and smooth. Uneatable larvae, on the other hand, comprise all that are of conspicuous colours and are hairy or spiny. But with b.u.t.terflies there is no such simplicity of contrast. The eatable b.u.t.terflies comprise not only brown or white species, but hundreds of Nymphalidae, Papilionidae, Lycaenidae, etc., which are gaily coloured and of an immense variety of patterns. The colours and patterns of the inedible kinds are also greatly varied, while they are often equally gay; and it is quite impossible to suppose that any amount of instinct or inherited habit (if such a thing exists) could enable young insectivorous birds to distinguish all the species of one kind from all those of the other.

There is also some evidence to show that animals do learn by experience what to eat and what to avoid. Mr. Poulton was a.s.sured by Rev. G.J.

Bursch that very young chickens peck at insects which they afterwards avoid. Lizards, too, often seized larvae which they were unable to eat and ultimately rejected.

Although the Heliconidae present, on the whole, many varieties of coloration and pattern, yet, in proportion to the number of distinct species in each district, the types of coloration are few and very well marked, and thus it becomes easier for a bird or other animal to learn that all belonging to such types are uneatable. This must be a decided advantage to the family in question, because, not only do fewer individuals of each species need to be sacrificed in order that their enemies may learn the lesson of their inedibility, but they are more easily recognised at a distance, and thus escape even pursuit. There is thus a kind of mimicry between closely allied species as well as between species of distinct genera, all tending to the same beneficial end. This may be seen in the four or five distinct species of the genus Heliconius which all have the same peculiar type of coloration--a yellow band across the upper wings and radiating red stripes on the lower,--and are all found in the same forests of the Lower Amazon; in the numerous very similar species of Ithomia with transparent wings, found in every locality of the same region; and in the very numerous species of Papilio of the "Aeneas" group, all having a similar style of marking, the resemblance being especially close in the females. The very uniform type of colouring of the blue-black Euplaeas and of the fulvous Acraeas is of the same character.[107] In all these cases the similarity of the allied species is so great, that, when they are on the wing at some distance off, it is difficult to distinguish one species from another. But this close external resemblance is not always a sign of very near affinity; for minute examination detects differences in the form and scalloping of the wings, in the markings on the body, and in those on the under surface of the wings, which do not usually characterise the closest allies. It is to be further noted, that the presence of groups of very similar species of the same genus, in one locality, is not at all a common phenomenon among unprotected groups. Usually the species of a genus found in one locality are each well marked and belong to somewhat distinct types, while the closely allied forms--those that require minute examination to discriminate them as distinct species--are most generally found in separate areas, and are what are termed representative forms.

The extension we have now given to the theory of mimicry is important, since it enables us to explain a much wider range of colour phenomena than those which were first imputed to mimicry. It is in the richest b.u.t.terfly region in the world--the Amazon valley--that we find the most abundant evidence of the three distinct sets of facts, all depending on the same general principle. The form of mimicry first elucidated by Mr.

Bates is characterised by the presence in each locality of certain b.u.t.terflies, or other insects, themselves edible and belonging to edible groups, which derived protection from having acquired a deceptive resemblance to some of the inedible b.u.t.terflies in the same localities, which latter were believed to be wholly free from the attacks of insectivorous birds. Then came the extension of the principle, by Dr. F.

Muller, to the case of species of distinct genera of the inedible b.u.t.terflies resembling each other quite as closely as in the former cases, and like them always found in the same localities. They derive mutual benefit from becoming, in appearance, one species, from which a certain toll is taken annually to teach the young insectivorous birds that they are uneatable. Even when the two or more species are approximately equal in numbers, they each derive a considerable benefit from thus combining their forces; but when one of the species is scarce or verging on extinction, the benefit becomes exceedingly great, being, in fact, exactly apportioned to the need of the species.

The third extension of the same principle explains the grouping of allied species of the same genera of inedible b.u.t.terflies into sets, each having a distinct type of coloration, and each consisting of a number of species which can hardly be distinguished on the wing. This must be useful exactly in the same way as in the last case, since it divides the inevitable toll to insectivorous birds and other animals among a number of species. It also explains the fact of the great similarity of many species of inedible insects in the same locality--a similarity which does not obtain to anything like the same extent among the edible species. The explanation of the various phenomena of resemblance and mimicry, presented by the distasteful b.u.t.terflies, may now be considered tolerably complete.

_Mimicry in other Orders of Insects._

A very brief sketch of these phenomena will be given, chiefly to show that the same principle prevails throughout nature, and that, wherever a rather extensive group is protected, either by distastefulness or offensive weapons, there are usually some species of edible and inoffensive groups that gain protection by imitating them. It has been already stated that the Telephoridae, Lampyridae, and other families of soft-winged beetles, are distasteful; and as they abound in all parts of the world, and especially in the tropics, it is not surprising that insects of many other groups should imitate them. This is especially the case with the longicorn beetles, which are much persecuted by insectivorous birds; and everywhere in tropical regions some of these are to be found so completely disguised as to be mistaken for species of the protected groups. Numbers of these imitations have been already recorded by Mr. Bates and myself, but I will here refer to a few others.

In the recently published volumes on the Longicorn and Malacoderm beetles of Central America[108] there are numbers of beautifully coloured figures of the new species; and on looking over them we are struck by the curious resemblance of some of the Longicorns to species of the Malacoderm group. In some cases we discover perfect mimics, and on turning to the descriptions we always find these pairs to come from the same locality. Thus the Otheostethus melanurus, one of the Prionidae, imitates the malacoderm, Lucidota discolor, in form, peculiar coloration, and size, and both are found at Chontales in Nicaragua, the species mimicked having, however, as is usual, a wider range. The curious and very rare little longicorn, Tethlimmena aliena, quite unlike its nearest allies in the same country, is an exact copy on a somewhat smaller scale of a malacoderm, Lygistopterus amabilis, both found at Chontales. The pretty longicorn, Callia albicornis, closely resembles two species of malacoderms (Silis chalybeipennis and Colyphus signaticollis), all being small beetles with red head and thorax and bright blue elytra, and all three have been found at Panama. Many other species of Callia also resemble other malacoderms; and the longicorn genus Lycidola has been named from its resemblance to various species of the Lycidae, one of the species here figured (Lycidola belti) being a good mimic of Calopteron corrugatum and of several other allied species, all being of about the same size and found at Chontales. In these cases, and in most others, the longicorn beetles have lost the general form and aspect of their allies to take on the appearance of a distinct tribe.

Some other groups of beetles, as the Elateridae and Eucnemidae, also deceptively mimic malacoderms.

Wasps and bees are often closely imitated by insects of other orders.

Many longicorn beetles in the tropics exactly mimic wasps, bees, or ants. In Borneo a large black wasp, whose wings have a broad white patch near the apex (Mygnimia aviculus), is closely imitated by a heteromerous beetle (Coloborhombus fasciatipennis), which, contrary to the general habit of beetles, keeps its wings expanded in order to show the white patch on their apex, the wing-coverts being reduced to small oval scales, as shown in the figure. This is a most remarkable instance of mimicry, because the beetle has had to acquire so many characters which are unknown among its allies (except in another species from Java)--the expanded wings, the white band on them, and the oval scale-like elytra.[109] Another remarkable case has been noted by Mr. Neville Goodman, in Egypt, where a common hornet (Vespa orientalis) is exactly imitated in colour, size, shape, att.i.tude when at rest, and mode of flight, by a beetle of the genus Laphria.[110]

The tiger-beetles (Cicindelidae) are also the subjects of mimicry by more harmless insects. In the Malay Islands I found a heteromerous beetle which exactly resembled a Therates, both being found running on the trunks of trees. A longicorn (Collyrodes Lacordairei) mimics Collyris, another genus of the same family; while in the Philippine Islands there is a cricket (Condylodeira tricondyloides), which so closely resembles a tiger-beetle of the genus Tricondyla that the experienced entomologist, Professor Westwood, at first placed it in his cabinet among those beetles.

[Ill.u.s.tration: FIG. 26.--Mygnimia aviculus (Wasp). Coloborhombus fasciatipennis (Beetle).]

[Ill.u.s.tration: FIG. 27.

a. Doliops sp. (Longicorn) mimics Pachyrhynchus...o...b..fae, (b) (a hard curculio).

c. Doliops curculionoides mimics (d) Pachyrhynchus sp.

e. Scepastus pachyrhynchoides (a gra.s.shopper), mimics (f) Apocyrtus sp. (a hard curculio).

g. Doliops sp. mimics (h) Pachyrhynchus sp.

i. Phoraspis (gra.s.shopper) mimics (k) a Coccinella.

All the above are from the Philippines. The exact correspondence of the colours of the insects themselves renders the mimicry much more complete in nature than it appears in the above figures.]

One of the characters by which some beetles are protected is excessive hardness of the elytra and integuments. Several genera of weevils (Curculionidae) are thus saved from attack, and these are often mimicked by species of softer and more eatable groups. In South America, the genus Heilipus is one of these hard groups, and both Mr. Bates and M.

Roelofs, a Belgian entomologist, have noticed that species of other genera exactly mimic them. So, in the Philippines, there is a group of Curculionidae, forming the genus Pachyrhynchus, in which all the species are adorned with the most brilliant metallic colours, banded and spotted in a curious manner, and are very smooth and hard. Other genera of Curculionidae (Desmidophorus, Alcides), which are usually very differently coloured, have species in the Philippines which mimic the Pachyrhynchi; and there are also several longicorn beetles (Aprophata, Doliops, Acronia, and Agnia), which also mimic them. Besides these, there are some longicorns and cetonias which reproduce the same colours and markings; and there is even a cricket (Scepastus pachyrhynchoides), which has taken on the form and peculiar coloration of these beetles in order to escape from enemies, which then avoid them as uneatable.[111]

The figures on the opposite page exhibit several other examples of these mimicking insects.

Innumerable other cases of mimicry occur among tropical insects; but we must now pa.s.s on to consider a few of the very remarkable, but much rarer instances, that are found among the higher animals.

_Mimicry among the Vertebrata._

Perhaps the most remarkable cases yet known are those of certain harmless snakes which mimic poisonous species. The genus Elaps, in tropical America, consists of poisonous snakes which do not belong to the viper family (in which are included the rattlesnakes and most of those which are poisonous), and which do not possess the broad triangular head which characterises the latter. They have a peculiar style of coloration, consisting of alternate rings of red and black, or red, black, and yellow, of different widths and grouped in various ways in the different species; and it is a style of coloration which does not occur in any other group of snakes in the world. But in the same regions are found three genera of harmless snakes, belonging to other families, some few species of which mimic the poisonous Elaps, often so exactly that it is with difficulty one can be distinguished from the other. Thus Elaps fulvius in Guatemala is imitated by the harmless Pliocerus equalis; Elaps corallinus in Mexico is mimicked by the harmless Homalocranium semicinctum; and Elaps lemniscatus in Brazil is copied by Oxyrhopus trigeminus; while in other parts of South America similar cases of mimicry occur, sometimes two harmless species imitating the same poisonous snake.

A few other instances of mimicry in this group have been recorded. There is in South Africa an egg-eating snake (Dasypeltis scaber), which has neither fangs nor teeth, yet it is very like the Berg adder (Clothos atropos), and when alarmed renders itself still more like by flattening out its head and darting forward with a hiss as if to strike a foe.[112]

Dr. A.B. Meyer has also discovered that, while some species of the genus Callophis (belonging to the same family as the American Elaps) have large poison fangs, other species of the same genus have none; and that one of the latter (C. gracilis) resembles a poisonous species (C.

intestinalis) so closely, that only an exact comparison will discover the difference of colour and marking. A similar kind of resemblance is said to exist between another harmless snake, Megaerophis flaviceps, and the poisonous Callophis bivirgatus; and in both these cases the harmless snake is less abundant than the poisonous one, as occurs in all examples of true mimicry.[113]

In the genus Elaps, above referred to, the very peculiar style of colour and marking is evidently a "warning colour" for the purpose of indicating to snake-eating birds and mammals that these species are poisonous; and this throws light on the long-disputed question of the use of the rattle of the rattlesnake. This reptile is really both sluggish and timid, and is very easily captured by those who know its habits. If gently tapped on the head with a stick, it will coil itself up and lie still, only raising its tail and rattling. It may then be easily caught. This shows that the rattle is a warning to its enemies that it is dangerous to proceed to extremities; and the creature has probably acquired this structure and habit because it frequents open or rocky districts where protective colour is needful to save it from being pounced upon by buzzards or other snake-eaters. Quite parallel in function is the expanded hood of the Indian cobra, a poisonous snake which belongs also to the Elapidae. This is, no doubt, a warning to its foes, not an attempt to terrify its prey; and the hood has been acquired, as in the case of the rattlesnake, because, protective coloration being on the whole useful, some mark was required to distinguish it from other protectively coloured, but harmless, snakes.

Both these species feed on active creatures capable of escaping if their enemy were visible at a moderate distance.

_Mimicry among Birds._

The varied forms and habits of birds do not favour the production among them of the phenomena of warning colours or of mimicry; and the extreme development of their instincts and reasoning powers, as well as their activity and their power of flight, usually afford them other means of evading their enemies. Yet there are a few imperfect, and one or two very perfect cases of true mimicry to be found among them. The less perfect examples are those presented by several species of cuckoos, an exceedingly weak and defenceless group of birds. Our own cuckoo is, in colour and markings, very like a sparrow-hawk. In the East, several of the small black cuckoos closely resemble the aggressive drongo-shrikes of the same country, and the small metallic cuckoos are like glossy starlings; while a large ground-cuckoo of Borneo (Carpococcyx radiatus) resembles one of the fine pheasants (Euplocamus) of the same country, both in form and in its rich metallic colours.

More perfect cases of mimicry occur between some of the dull-coloured orioles in the Malay Archipelago and a genus of large honey-suckers--the Tropidorhynchi or "Friar-birds." These latter are powerful and noisy birds which go in small flocks. They have long, curved, and sharp beaks, and powerful grasping claws; and they are quite able to defend themselves, often driving away crows and hawks which venture to approach them too nearly. The orioles, on the other hand, are weak and timid birds, and trust chiefly to concealment and to their retiring habits to escape persecution. In each of the great islands of the Austro-Malayan region there is a distinct species of Tropidorhynchus, and there is always along with it an oriole that exactly mimics it. All the Tropidorhynchi have a patch of bare black skin round the eyes, and a ruff of curious pale recurved feathers on the nape, whence their name of Friar-birds, the ruff being supposed to resemble the cowl of a friar.

These peculiarities are imitated in the orioles by patches of feathers of corresponding colours; while the different tints of the two species in each island are exactly the same. Thus in Bouru both are earthy brown; in Ceram they are both washed with yellow ochre; in Timor the under surface is pale and the throat nearly white, and Mr. H.O. Forbes has recently discovered another pair in the island of Timor Laut. The close resemblance of these several pairs of birds, of widely different families, is quite comparable with that of many of the insects already described. It is so close that the preserved specimens have even deceived naturalists; for, in the great French work, _Voyage de l'Astrolabe_, the oriole of Bouru is actually described and figured as a honey-sucker; and Mr. Forbes tells us that, when his birds were submitted to Dr. Sclater for description, the oriole and the honey-sucker were, previous to close examination, considered to be the same species.

_Objections to the Theory of Mimicry._

To set forth adequately the varied and surprising facts of mimicry would need a large and copiously ill.u.s.trated volume; and no more interesting subject could be taken up by a naturalist who has access to our great collections and can devote the necessary time to search out the many examples of mimicry that lie hidden in our museums. The brief sketch of the subject that has been here given will, however, serve to indicate its nature, and to show the weakness of the objections that were at first made to it. It was urged that the action of "like conditions,"

with "accidental resemblances" and "reversion to ancestral types," would account for the facts. If, however, we consider the actual phenomena as here set forth, and the very constant conditions under which they occur, we shall see how utterly inadequate are these causes, either singly or combined. These constant conditions are--

1. That the imitative species occur in the same area and occupy the very same station as the imitated.

2. That the imitators are always the more defenceless.

3. That the imitators are always less numerous in individuals.

4. That the imitators differ from the bulk of their allies.

5. That the imitation, however minute, is _external_ and _visible_ only, never extending to internal characters or to such as do not affect the external appearance.

These five characteristic features of mimicry show us that it is really an exceptional form of protective resemblance. Different species in the same group of organisms may obtain protection in different ways: some by a general resemblance to their environment; some by more exactly imitating the objects that surround them--bark, or leaf, or flower; while others again gain an equal protection by resembling some species which, from whatever cause, is almost as free from attack as if it were a leaf or a flower. This immunity may depend on its being uneatable, or dangerous, or merely strong; and it is the resemblance to such creatures for the purpose of sharing in their safety that const.i.tutes mimicry.

_Concluding Remarks on Warning Colours and Mimicry._