Darwinism (Wallace)/Chapter IX
WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration—Warning colours among insects—Butterflies—Caterpillars—Mimicry—How mimicry has been produced—Heliconidae—Perfection of the imitation—Other cases of mimicry among Lepidoptera—Mimicry among protected groups—Its explanation—Extension of the principle—Mimicry in other orders of insects—Mimicry among the vertebrata—Snakes—The rattlesnake and the cobra—Mimicry among birds—Objections to the theory of mimicry—Concluding remarks on warning colours and mimicry.
We have now to deal with a class of colours which are the very opposite of those we have hitherto considered, since, instead of serving to conceal the animals that possess them or as recognition marks to their associates, they are developed for the express purpose of rendering the species conspicuous. The reason of this is that the animals in question are either the possessors of some deadly weapons, as stings or poison fangs, or they are uneatable, and are thus so disagreeable to the usual enemies of their kind that they are never attacked when their peculiar powers or properties are known. It is, therefore, important that they should not be mistaken for defenceless or eatable species of the same class or order, since in that case they might suffer injury, or even death, before their enemies discovered the danger or the uselessness of the attack. They require some signal or danger-flag which shall serve as a warning to would-be enemies not to attack them, and they have usually obtained this in the form of conspicuous or brilliant coloration, very distinct from the protective tints of the defenceless animals allied to them.
The Skunk as illustrating Warning Coloration.
While staying a few days, in July 1887, at the Summit Hotel on the Central Pacific Railway, I strolled out one evening after dinner, and on the road, not fifty yards from the house, I saw a pretty little white and black animal with a bushy tail coming towards me. As it came on at a slow pace and without any fear, although it evidently saw me, I thought at first that it must be some tame creature, when it suddenly occurred to me that it was a skunk. It came on till within five or six yards of me, then quietly climbed over a dwarf wall and disappeared under a small outhouse, in search of chickens, as the landlord afterwards told me. This animal possesses, as is well known, a most offensive secretion, which it has the power of ejecting over its enemies, and which effectually protects it from attack. The odour of this substance is so penetrating that it taints, and renders useless, everything it touches, or in its vicinity. Provisions near it become uneatable, and clothes saturated with it will retain the smell for several weeks, even though they are repeatedly washed and dried. A drop of the liquid in the eyes will cause blindness, and Indians are said not unfrequently to lose their sight from this cause. Owing to this remarkable power of offence the skunk is rarely attacked by other animals, and its black and white fur, and the bushy white tail carried erect when disturbed, form the danger-signals by which it is easily distinguished in the twilight or moonlight from unprotected animals. Its consciousness that it needs only to be seen to be avoided gives it that slowness of motion and fearlessness of aspect which are, as we shall see, characteristic of most creatures so protected.
Warning Colours among Insects.
It is among insects that warning colours are best developed, and most abundant. We all know how well marked and conspicuous are the colours and forms of the stinging wasps and bees, no one of which in any part of the world is known to be protectively coloured like the majority of defenceless insects. Most of the great tribe of Malacoderms among beetles are distasteful to insect-eating animals. Our red and black Telephoridae, commonly called "soldiers and sailors," were found, by Mr. Jenner Weir, to be refused by small birds. These and the allied Lampyridae (the fireflies and glow-worms) in Nicaragua, were rejected by Mr. Belt's tame monkey and by his fowls, though most other insects were greedily eaten by them. The Coccinellidae or lady-birds are another uneatable group, and their conspicuous and singularly spotted bodies serve to distinguish them at a glance from all other beetles.
These uneatable insects are probably more numerous than is supposed, although we already know immense numbers that are so protected. The most remarkable are the three families of butterflies—Heliconidae, Danaidae, and Acraeidae—comprising more than a thousand species, and characteristic respectively of the three great tropical regions—South America, Southern Asia, and Africa. All these butterflies have peculiarities which serve to distinguish them from every other group in their respective regions. They all have ample but rather weak wings, and fly slowly; they are always very abundant; and they all have conspicuous colours or markings, so distinct from those of other families that, in conjunction with their peculiar outline and mode of flight, they can usually be recognised at a glance. Other distinctive features are, that their colours are always nearly the same on the under surface of their wings as on the upper; they never try to conceal themselves, but rest on the upper surfaces of leaves or flowers; and, lastly, they all have juices which exhale a powerful scent, so that when one kills them by pinching the body, the liquid that exudes stains the fingers yellow, and leaves an odour that can only be removed by repeated washings.
Now, there is much direct evidence to show that this odour, though not very offensive to us, is so to most insect-eating creatures. Mr. Bates observed that, when set out to dry, specimens of Heliconidae were less subject to the attacks of vermin; while both he and I noticed that they were not attacked by insect-eating birds or dragonflies, and that their wings were not found in the forest paths among the numerous wings of other butterflies whose bodies had been devoured. Mr. Belt once observed a pair of birds capturing insects for their young; and although the Heliconidæ swarmed in the vicinity, and from their slow flight could have been easily caught, not one was ever pursued, although other butterflies did not escape. His tame monkey also, which would greedily munch up other butterflies, would never eat the Heliconidae. It would sometimes smell them, but always rolled them up in its hand and then dropped them.
We have also some corresponding evidence as to the distastefulness of the Eastern Danaidæ. The Hon. Mr. Justice Newton, who assiduously collected and took notes upon the Lepidoptera of Bombay, informed Mr. Butler of the British Museum that the large and swift-flying butterfly Charaxes psaphon, was continually persecuted by the bulbul, so that he rarely caught a specimen of this species which had not a piece snipped out of the hind wings. He offered one to a bulbul which he had in a cage, and it was greedily devoured, whilst it was only by repeated persecution that he succeeded in inducing the bird to touch a Danais.
Besides these three families of butterflies, there are certain groups of the great genus Papilio—the true swallow-tailed butterflies—which have all the characteristics of uneatable insects. They have a special coloration, usually red and black (at least in the females), they fly slowly, they are very abundant, and they possess a peculiar odour somewhat like that of the Heliconidæ. One of these groups is common in tropical America, another in tropical Asia, and it is curious that, although not very closely allied, they have each the same red and black colours, and are very distinct from all the other butterflies of their respective countries. There is reason to believe also that many of the brilliantly coloured and weak-flying diurnal moths, like the fine tropical Agaristidae and burnet-moths, are similarly protected, and that their conspicuous colours serve as a warning of inedibility. The common burnet-moth (Anthrocera filipendula) and the equally conspicuous ragwort-moth (Euchelia jacobeae) have been proved to be distasteful to insect-eating creatures.
The most interesting and most conclusive example of warning coloration is, however, furnished by caterpillars, because in this case the facts have been carefully ascertained experimentally by competent observers. In the year 1866, when Mr. Darwin was collecting evidence as to the supposed effect of sexual selection in bringing about the brilliant coloration of the higher animals, he was struck by the fact that many caterpillars have brilliant and conspicuous colours, in the production of which sexual selection could have no place. We have numbers of such caterpillars in this country, and they are characterised not only by their gay colours but by not concealing themselves. Such are the mullein and the gooseberry caterpillars, the larvae of the spurge hawk-moth, of the buff-tip, and many others. Some of these caterpillars are wonderfully conspicuous, as in the case of that noticed by Mr. Bates in South America, which was four inches long, banded across with black and yellow, and with bright red head, legs, and tail. Hence it caught the eye of any one who passed by, even at the distance of many yards.
Mr. Darwin asked me to try and suggest some explanation of this coloration; and, having been recently interested in the question of the warning coloration of butterflies, I suggested that this was probably a similar case,—that these conspicuous caterpillars were distasteful to birds and other insect-eating creatures, and that their bright non-protective colours and habit of exposing themselves to view, enabled their enemies to distinguish them at a glance from the edible kinds and thus learn not to touch them; for it must be remembered that the bodies of caterpillars while growing are so delicate, that a wound from a bird's beak would be perhaps as fatal as if they were devoured. At this time not a single experiment or observation had been made on the subject, but after I had brought the matter before the Entomological Society, two gentlemen, who kept birds and other tame animals, undertook to make experiments with a variety of caterpillars.
Mr. Jenner Weir was the first to experiment with ten species of small birds in his aviary, and he found that none of them would eat the following smooth-skinned conspicuous caterpillars—Abraxas grossulariata, Diloba cæaeruleocephala, Anthrocera filipendula, and Cucullia verbasci. He also found that they would not touch any hairy or spiny larvae, and he was satisfied that it was not the hairs or the spines, but the unpleasant taste that caused them to be rejected, because in one case a young smooth larva of a hairy species, and in another case the pupa of a spiny larva, were equally rejected. On the other hand, all green or brown caterpillars as well as those that resemble twigs were greedily devoured.
Mr. A.G. Butler also made experiments with some green lizards (Lacerta viridis), which greedily ate all kinds of food, including flies of many kinds, spiders, bees, butterflies, and green caterpillars; but they would not touch the caterpillar of the gooseberry-moth (Abraxas grossulariata), or the imago of the burnet-moth (Anthrocera filipendula). The same thing happened with frogs. When the gooseberry caterpillars were first given to them, "they sprang forward and licked them eagerly into their mouths; no sooner, however, had they done so, than they seemed to become aware of the mistake that they had made, and sat with gaping mouths, rolling their tongues about, until they had got quit of the nauseous morsels, which seemed perfectly uninjured, and walked off as briskly as ever." Spiders seemed equally to dislike them. This and another conspicuous caterpillar (Halia wavaria) were rejected by two species—the geometrical garden spider (Epeira diadema) and a hunting spider.
Some further experiments with lizards were made by Professor Weismann, quite confirming the previous observations; and in 1886 Mr. E.B. Poulton of Oxford undertook a considerable series of experiments, with many other species of larvae and fresh kinds of lizards and frogs. Mr. Poulton then reviewed the whole subject, incorporating all recorded facts, as well as some additional observations made by Mr. Jenner Weir in 1886. More than a hundred species of larvae or of perfect insects of various orders have now been made the subject of experiment, and the results completely confirm my original suggestion. In almost every case the protectively coloured larvae have been greedily eaten by all kinds of insectivorous animals, while, in the immense majority of cases, the conspicuous, hairy, or brightly coloured larvae have been rejected by some or all of them. In some instances the inedibility of the larvae extends to the perfect insect, but not in others. In the former cases the perfect insect is usually adorned with conspicuous colours, as the burnet and ragwort moths; but in the case of the buff-tip, the moth resembles a broken piece of rotten stick, yet it is partly inedible, being refused by lizards. It is, however, very doubtful whether these are its chief enemies, and its protective form and colour may be needed against insectivorous birds or mammals.
Mr. Samuel H. Scudder, who has largely bred North American butterflies, has found so many of the eggs and larvae destroyed by hymenopterous and dipterous parasites that he thinks at least nine-tenths, perhaps a greater proportion, never reach maturity. Yet he has never found any evidence that such parasites attack either the egg or the larva of the inedible Danais archippus, so that in this case the insect is distasteful to its most dangerous foes in all the stages of its existence, a fact which serves to explain its great abundance and its extension over almost the whole world.
One case has been found of a protectively coloured larva,—one, moreover, which in all its habits shows that it trusts to concealment to escape its enemies—which was yet always rejected by lizards after they had seized it, evidently under the impression that from its colour it would be eatable. This is the caterpillar of the very common moth Mania typica; and Mr. Poulton thinks that, in this case, the unpleasant taste is an incidental result of some physiological processes in the organism, and is itself a merely useless character. It is evident that the insect would not conceal itself so carefully as it does if it had not some enemies, and these are probably birds or small mammals, as its food-plants are said to be dock and willow-herb, not suggestive of places frequented by lizards; and it has been found by experiment that lizards and birds have not always the same likes and dislikes. The case is interesting, because it shows that nauseous fluids sometimes occur sporadically, and may thus be intensified by natural selection when required for the purpose of protection. Another exceptional case is that of the very conspicuous caterpillar of the spurge hawk-moth (Deilephila euphorbiae), which was at once eaten by a lizard, although, as it exposes itself on its food-plant in the daytime and is very abundant in some localities, it must almost certainly be disliked by birds or by some animals who would otherwise devour it. If disturbed while feeding it is said to turn round with fury and eject a quantity of green liquid, of an acid and disagreeable smell similar to that of the spurge milk, only worse.
These facts, and Mr. Poulton's evidence that some larvae rejected by lizards at first will be eaten if the lizards are very hungry, show that there are differences in the amount of the distastefulness, and render it probable that if other food were wanting many of these conspicuous insects would be eaten. It is the abundance of the eatable kinds that gives value to the inedibility of the smaller number; and this is probably the reason why so many insects rely on protective colouring rather than on the acquisition of any kind of defensive weapons. In the long run the powers of attack and defence must balance each other. Hence we see that even the powerful stings of bees and wasps only protect them against some enemies, since a tribe of birds, the bee-eaters, have been developed which feed upon them, and some frogs and lizards do so occasionally.
The preceding outline will sufficiently explain the characteristics of "warning coloration" and the end it serves in nature. There are many other curious modifications of it, but these will be best appreciated after we have discussed the remarkable phenomenon of "mimicry," which is bound up with and altogether depends upon "warning colour," and is in some cases the chief indication we have of the possession of some offensive weapon to secure the safety of the species imitated.
This term has been given to a form of protective resemblance, in which one species so closely resembles another in external form and colouring as to be mistaken for it, although the two may not be really allied and often belong to distinct families or orders. One creature seems disguised in order to be made like another; hence the terms "mimic" and mimicry, which imply no voluntary action on the part of the imitator. It has long been known that such resemblances do occur, as, for example, the clear-winged moths of the families Sesiidae and Aegeriidae, many of which resemble bees, wasps, ichneumons, or saw-flies, and have received names expressive of the resemblance; and the parasitic flies (Volucella) which closely resemble bees, on whose larvae the larvae of the flies feed.
The great bulk of such cases remained, however, unnoticed, and the subject was looked upon as one of the inexplicable curiosities of nature, till Mr. Bates studied the phenomenon among the butterflies of the Amazon, and, on his return home, gave the first rational explanation of it. The facts are, briefly, these. Everywhere in that fertile region for the entomologist the brilliantly coloured Heliconidae abound, with all the characteristics which I have already referred to when describing them as illustrative of "warning coloration." But along with them other butterflies were occasionally captured, which, though often mistaken for them, on account of their close resemblance in form, colour, and mode of flight, were found on examination to belong to a very distinct family, the Pieridae. Mr. Bates notices fifteen distinct species of Pieridae, belonging to the genera Leptalis and Enterpe, each of which closely imitates some one species of Heliconidae, inhabiting the same region and frequenting the same localities. It must be remembered that the two families are altogether distinct in structure. The larvae of the Heliconidae are tubercled or spined, the pupae suspended head downwards, and the imago has imperfect forelegs in the male; while the larvae of the Pieridae are smooth, the pupae are suspended with a brace to keep the head erect, and the forefeet are fully developed in both sexes. These differences are as large and as important as those between pigs and sheep, or between swallows and sparrows; while English entomologists will best understand the case by supposing that a species of Pieris in this country was coloured and shaped like a small tortoise-shell, while another species on the Continent was equally like a Camberwell beauty—so like in both cases as to be mistaken when on the wing, and the difference only to be detected by close examination. As an example of the resemblance, woodcuts are given of one pair in which the colours are simple, being olive, yellow, and black, while the very distinct neuration of the wings and form of the head and body can be easily seen.
Besides these Pieridæ, Mr. Bates found four true Papilios, seven Erycinidæ, three Castnias (a genus of day-flying moths), and fourteen species of diurnal Bombycidæ, all imitating some species of Heliconidæ which inhabited the same district; and it is to be especially noted that none of these insects were so abundant as the Heliconidæ they resembled, generally they were far less common, so that Mr. Bates estimated the proportion in some cases as not one to a thousand. Before giving an account of the numerous remarkable cases of mimicry in other parts of the world, and between various groups of insects and of higher animals, it will be well to explain briefly the use and purport of the phenomenon, and also the mode by which it has been brought about.
How Mimicry has been Produced.
The fact has been now established that the Heliconidæ possess an offensive odour and taste, which lead to their being almost entirely free from attack by insectivorous creatures; they possess a peculiar form and mode of flight, and do not seek concealment; while their colours—although very varied, ranging from deep blue-black, with white, yellow, or vivid red bands and spots, to the most delicate semitransparent wings adorned with pale brown or yellow markings—are yet always very distinctive, and unlike those of all the other families of butterflies in the same country. It is, therefore, clear that if any other butterflies in the same region, which are eatable and suffer great persecution from insectivorous animals, should come to resemble any of these uneatable species so closely as to be mistaken for them by their enemies, they will obtain thereby immunity from persecution. This is the obvious and sufficient reason why the imitation is useful, and therefore why it occurs in nature. We have now to explain how it has probably been brought about, and also why a still larger number of persecuted groups have not availed themselves of this simple means of protection.
From the great abundance of the Heliconidæ all over tropical America, the vast number of their genera and species, and their marked distinctions from all other butterflies, it follows that they constitute a group of high antiquity, which in the course of ages has become more and more specialised, and owing to its peculiar advantages has now become a dominant and aggressive race. But when they first arose from some ancestral species or group which, owing to the food of the larvæ or some other cause, possessed disagreeable juices that caused them to be disliked by the usual enemies of their kind, they were in all probability not very different either in form or coloration from many other butterflies. They would at that time be subject to repeated attacks by insect-eaters, and, even if finally rejected, would often receive a fatal injury. Hence arose the necessity for some distinguishing mark, by which the devourers of butterflies in general might learn that these particular butterflies were uneatable; and every variation leading to such distinction, whether by form, colour, or mode of flight, was preserved and accumulated by natural selection, till the ancestral Heliconoids became well distinguished from eatable butterflies, and thenceforth comparatively free from persecution. Then they had a good time of it. They acquired lazy habits, and flew about slowly. They increased abundantly and spread all over the country, their larvæ feeding on many plants and acquiring different habits; while the butterflies themselves varied greatly, and colour being useful rather than injurious to them, gradually diverged into the many coloured and beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridæ, inhabiting the same district, happened to be sufficiently like some of the Heliconidæ to be occasionally mistaken for them. These, of course, survived while their companions were devoured. Those among their descendants that were still more like Heliconidæ again survived, and at length the imitation would become tolerably perfect. Thereafter, as the protected group diverged into distinct species of many different colours, the imitative group would occasionally be able to follow it with similar variations,—a process that is going on now, for Mr. Bates informs us that in each fresh district he visited he found closely allied representative species or varieties of Heliconidæ, and along with them species of Leptalis (Pieridæ), which had varied in the same way so as still to be exact imitations. But this process of imitation would be subject to check by the increasing acuteness of birds and other animals which, whenever the eatable Leptalis became numerous, would surely find them out, and would then probably attack both these and their friends the Heliconidæ in order to devour the former and reject the latter. The Pieridæ would, however, usually be less numerous, because their larvæ are often protectively coloured and therefore edible, while the larvæ of the Heliconidæ are adorned with warning colours, spines, or tubercles, and are uneatable. It seems probable that the larvæ and pupæ of the Heliconidæ were the first to acquire the protective distastefulness, both because in this stage they are more defenceless and more liable to fatal injury, and also because we now find many instances in which the larvæ are distasteful while the perfect insects are eatable, but I believe none in which the reverse is the case. The larvæ of the Pieridæ are now beginning to acquire offensive juices, but have not yet obtained the corresponding conspicuous colours; while the perfect insects remain eatable, except perhaps in some Eastern groups, the under sides of whose wings are brilliantly coloured although this is the part which is exposed when at rest.
It is clear that if a large majority of the larvæ of Lepidoptera, as well as the perfect insects, acquired these distasteful properties, so as seriously to diminish the food supply of insectivorous and nestling birds, these latter would be forced by necessity to acquire corresponding tastes, and to eat with pleasure what some of them now eat only under pressure of hunger; and variation and natural selection would soon bring about this change.
Many writers have denied the possibility of such wonderful resemblances being produced by the accumulation of fortuitous variations, but if the reader will call to mind the large amount of variability that has been shown to exist in all organisms, the exceptional power of rapid increase possessed by insects, and the tremendous struggle for existence always going on, the difficulty will vanish, especially when we remember that nature has the same fundamental groundwork to act upon in the two groups, general similarity of forms, wings of similar texture and outline, and probably some original similarity of colour and marking. Yet there is evidently considerable difficulty in the process, or with these great resources at her command nature would have produced more of these mimicking forms than she has done. One reason of this deficiency probably is, that the imitators, being always fewer in number, have not been able to keep pace with the variations of the much more numerous imitated form; another reason may be the ever-increasing acuteness of the enemies, which have again and again detected the imposture and exterminated the feeble race before it has had time to become further modified. The result of this growing acuteness of enemies has been, that those mimics that now survive exhibit, as Mr. Bates well remarks, "a palpably intentional likeness that is perfectly staggering," and also "that those features of the portrait are most attended to by nature which produce the most effective deception when the insects are seen in nature." No one, in fact, can understand the perfection of the imitation who has not seen these species in their native wilds. So complete is it in general effect that in almost every box of butterflies, brought from tropical America by amateurs, are to be found some species of the mimicking Pieridæ, Erycinidæ, or moths, and the mimicked Heliconidæ, placed together under the impression that they are the same species. Yet more extraordinary, it sometimes deceives the very insects themselves. Mr. Trimen states that the male Danais chrysippus is sometimes deceived by the female Diadema bolina which mimics that species. Dr. Fritz Müller, writing from Brazil to Professor Meldola, says, "One of the most interesting of our mimicking butterflies is Leptalis melite. The female alone of this species imitates one of our common white Pieridæ, which she copies so well that even her own male is often deceived; for I have repeatedly seen the male pursuing the mimicked species, till, after closely approaching and becoming aware of his error, he suddenly returned." This is evidently not a case of true mimicry, since the species imitated is not protected; but it may be that the less abundant Leptalis is able to mingle with the female Pieridæ and thus obtain partial immunity from attack. Mr. Kirby of the insect department of the British Museum informs me that there are several species of South American Pieridæ which the female Leptalis melite very nearly resembles. The case, however, is interesting as showing that the butterflies are themselves deceived by a resemblance which is not so great as that of some mimicking species.
Other Examples of Mimicry among Lepidoptera.
In tropical Asia, and eastward to the Pacific Islands, the Danaidæ take the place of the Heliconidæ of America, in their abundance, their conspicuousness, their slow flight, and their being the subjects of mimicry. They exist under three principal forms or genera. The genus Euplæa is the most abundant both in species and individuals, and consists of fine broad-winged butterflies of a glossy or metallic blue-black colour, adorned with pure white, or rich blue, or dusky markings situated round the margins of the wings. Danais has generally more lengthened wings, of a semitransparent greenish or a rich brown colour, with radial or marginal pale spots; while the fine Hestias are of enormous size, of a papery or semitransparent white colour, with dusky or black spots and markings. Each of these groups is mimicked by various species of the genus Papilio, usually with such accuracy that it is impossible to distinguish them on the wing. Several species of Diadema, a genus of butterflies allied to our Vanessas, also mimic species of Danais, but in this case the females only are affected, a subject which will be discussed in another chapter.
Another protected group in the Eastern tropics is that of the beautiful day-flying moths forming the family Agaristidæ. These are usually adorned with the most brilliant colours or conspicuous markings, they fly slowly in forests among the butterflies and other diurnal insects, and their great abundance sufficiently indicates their possession of some distastefulness which saves them from attack. Under these conditions we may expect to find other moths which are not so protected imitating them, and this is the case. One of the common and wide-ranging species (Opthalmis lincea), found in the islands from Amboyna to New Ireland, is mimicked in a wonderful manner by one of the Liparidæ (the family to which our common "tussock" and "vapourer" moths belong). This is a new species collected at Amboyna during the voyage of the Challenger, and has been named Artaxa simulans. Both insects are black, with the apex of the fore wings ochre coloured, and the outer half of the hind wings bright orange. The accompanying woodcuts (for the use of which I am indebted to Mr. John Murray of the Challenger Office) well exhibit their striking resemblance to each other.
In Africa exactly similar phenomena recur, species of Papilio and of Diadema mimicking Danaidæ or Acræidæ with the most curious accuracy. Mr. Trimen, who studied this subject in South Africa, has recorded eight species or varieties of Diadema, and eight of Papilio, which each mimic some species of Danais; while eight species or varieties of Panopæa (another genus of Nymphalidæ), three of Melanitis (Eurytelidæ), and two of Papilio, resemble with equal accuracy some species of Acræa. He has also independently observed the main facts on which the explanation of the phenomenon rests,—the unpleasant odour of the Danais and Acræa, extending to their larvæ and pupæ; their great abundance, slow flight, and disregard of concealment; and he states that while lizards, mantidæ, and dragonflies all hunt butterflies, and the rejected wings are to be found abundantly at some of their feeding-places, those of the two genera Danais and Acræa were never among them.
The two groups of the great genus Papilio (the true swallow-tailed butterflies) which have been already referred to as having the special characteristics of uneatable insects, have also their imitators in other groups; and thus, the belief in their inedibility—derived mainly from their style of warning coloration and their peculiar habits—is confirmed. In South America, several species of the "Aeneas" group of these butterflies are mimicked by Pieridæ and by day-flying moths of the genera Castnia and Pericopis. In the East, Papilio hector, P. diphilus, and P. liris, all belonging to the inedible group, are mimicked by the females of other species of Papilio belonging to very distinct groups; while in Northern India and China, many fine day-flying moths (Epicopeia) have acquired the strange forms and peculiar colours of some of the large inedible Papilios of the same regions.
In North America, the large and handsome Danais archippus, with rich reddish-brown wings, is very common; and it is closely imitated by Limenitis misippus, a butterfly allied to our "white admiral," but which has acquired a colour quite distinct from that of the great bulk of its allies. In the same country there is a still more interesting case. The beautiful dark bronzy green butterfly, Papilio philenor, is inedible both in larva and perfect insect, and it is mimicked by the equally dark Limenitis ursula. There is also in the Southern and Western States a dark female form of the yellow Papilio turnus, which in all probability obtains protection from its general resemblance to P. philenor. Mr. W.H. Edwards has found, by extensive experiment, that both the dark and yellow females produce their own kinds, with very few exceptions; and he thinks that the dark form has the advantage in the more open regions and in the prairies, where insectivorous birds abound. But in open country the dark form would be quite as conspicuous as the yellow form, if not more so, so that the resemblance to an inedible species would be there more needed.
The only probable case of mimicry in this country is that of the moth, Diaphora mendica, whose female only is white, while the larva is of protective colours, and therefore almost certainly edible. A much more abundant moth, of about the same size and appearing about the same time, is Spilosoma menthrasti, also white, but in this case both it and its larva have been proved to be inedible. The white colour of the female Diaphora, although it must be very conspicuous at night, may, therefore, have been acquired in order to resemble the uneatable Spilosoma, and thus gain some protection.
Mimicry among Protected (Uneatable) Genera.
Before giving some account of the numerous other cases of warning colours and of mimicry that occur in the animal kingdom, it will be well to notice a curious phenomenon which long puzzled entomologists, but which has at length received a satisfactory explanation.
We have hitherto considered, that mimicry could only occur when a comparatively scarce and much persecuted species obtained protection by its close external resemblance to a much more abundant uneatable species inhabiting its own district; and this rule undoubtedly prevails among the great majority of mimicking species all over the world. But Mr. Bates also found a number of pairs of species of different genera of Heliconidæ, which resembled each other quite as closely as did the other mimicking species he has described; and since all these insects appear to be equally protected by their inedibility, and to be equally free from persecution, it was not easy to see why this curious resemblance existed, or how it had been brought about. That it is not due to close affinity is shown by the fact that the resemblance occurs most frequently between the two distinct sub-families into which (as Mr. Bates first pointed out) the Heliconidæ are naturally divided on account of very important structural differences. One of these sub-families (the true Heliconinæ) consists of two genera only, Heliconius and Eueides, the other (the Danaoid Heliconinæ) of no less than sixteen genera; and, in the instances of mimicry we are now discussing, one of the pairs or triplets that resemble each other is usually a species of the large and handsome genus Heliconius, the others being species of the genera Mechanitis, Melinæa, or Tithorea, though several species of other Danaoid genera also imitate each other. The following lists will give some idea of the number of these curious imitative forms, and of their presence in every part of the Neotropical area. The bracketed species are those that resemble each other so closely that the difference is not perceptible when they are on the wing.
In the Lower Amazon region are found—
In Central America are found—
In the Upper Amazon region—
In New Grenada—
(?) Mechanitis sp.
(?) Mechanitis mothone.
In Rio Janeiro—
In South Brazil—
Besides these, a number of species of Ithomia and Napeogenes, and of Napeogenes and Mechanitis, resemble each other with equal accuracy, so that they are liable to be mistaken for each other when on the wing; and no doubt many other equally remarkable cases are yet unnoticed.
The figures above of the fore and hind wings of two of these mimicking species, from Dr. Fritz Müller's original paper in Kosmos, will serve to show the considerable amount of difference, in the important character of the neuration of the wings, between these butterflies, which really belong to very distinct and not at all closely allied genera. Other important characters are—(1) The existence of a small basal cell in the hind wings of Ituna which is wanting in Thyridia; (2) the division of the cell between the veins 1b and 2 of the hind wings in the former genus, while it is undivided in the latter; and (3) the existence in Thyridia of scent-producing tufts of hair on the upper edge of the hind wing, while in Ituna these are wanting; but in place of them are extensible processes at the end of the abdomen, also emitting a powerful scent. These differences characterise two marked subdivisions of the Danaoid Heliconinae, each containing several distinct genera; and these subdivisions are further distinguished by very different forms of larvae, that to which Ituna belongs having from two to four long threadlike tentacles on the back, while in that containing Thyridia these are always absent. The former usually feed on Asclepiadeae, the latter on Solanaceae or Scrophulariaceae.
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. Müller states that they both show a preference for the same flowers growing on the edges of the forest paths.
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. But we neither of us hit upon the simple explanation given by Dr. Fritz Müller in 1879.
His theory is founded on the assumed, but probable, fact, that insect-eating birds only learn by experience to distinguish the edible from the inedible butterflies, and in doing so necessarily sacrifice a certain number of the latter. The quantity 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 butterflies 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 quantity 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 butterflies, 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 butterflies till they have learned by bitter experience what species to avoid? On this point Dr. Müller 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 butterflies there is no such simplicity of contrast. The eatable butterflies 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 assured 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. 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 butterfly 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 butterflies, or other insects, themselves edible and belonging to edible groups, which derived protection from having acquired a deceptive resemblance to some of the inedible butterflies 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. Müller, to the case of species of distinct genera of the inedible butterflies 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 butterflies 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 butterflies, 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 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. 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, attitude when at rest, and mode of flight, by a beetle of the genus Laphria.
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.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. 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 pass 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. 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.
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 illustrated 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 constitutes mimicry.
Concluding Remarks on Warning Colours and Mimicry.
Colours which have been acquired for the purpose of serving as a warning of inedibility, or of the possession of dangerous offensive weapons, are probably more numerous than have been hitherto supposed; and, if so, we shall be able to explain a considerable amount of colour in nature for which no use has hitherto been conjectured. The brilliant and varied colours of sea-anemones and of many coral animals will probably come under this head, since we know that many of them possess the power of ejecting stinging threads from various parts of their bodies which render them quite uneatable to most animals. Mr. Gosse describes how, on putting an Anthea into a tank containing a half-grown bullhead (Cottus bubalis) which had not been fed for some time, the fish opened his mouth and sucked in the morsel, but instantly shot it out again. He then seized it a second time, and after rolling it about in his mouth for a moment shot it out again, and then darted away to hide himself in a hole. Some tropical fishes, however, of the genera Tetrodon, Pseudoscarus, Astracion, and a few others, seem to have acquired the power of feeding on corals and medusae; and the beautiful bands and spots and bright colours with which they are frequently adorned, may be either protective when feeding in the submarine coral groves, or may, in some cases, be warning colours to show that they themselves are poisonous and uneatable.
A remarkable illustration of the wide extension of warning colours, and their very definite purpose in nature, is afforded by what may now be termed "Mr. Belt's frog." Frogs in all parts of the world are, usually, protectively coloured with greens or browns; and the little tree-frogs are either green like the leaves they rest upon, or curiously mottled to imitate bark or dead leaves. But there are a certain number of very gaily coloured frogs, and these do not conceal themselves as frogs usually do. Such was the small toad found by Darwin at Bahia Blanca, which was intense black and bright vermilion, and crawled about in the sunshine over dry sand-hills and arid plains. And in Nicaragua, Mr. Belt found a little frog gorgeously dressed in a livery of red and blue, which did not attempt concealment and was very abundant, a combination of characters which convinced him that it was uneatable. He, therefore, took a few specimens home with him and gave them to his fowls and ducks, but none would touch them. At last, by throwing down pieces of meat, for which there was a great competition among the poultry, he managed to entice a young duck into snatching up one of the little frogs. Instead of swallowing it, however, the duck instantly threw it out of its mouth, and went about jerking its head as if trying to get rid of some unpleasant taste.
The power of predicting what will happen in a given case is always considered to be a crucial test of a true theory, and if so, the theory of warning colours, and with it that of mimicry, must be held to be well established. Among the creatures which probably have warning colours as a sign of inedibility are, the brilliantly coloured nudibranchiate molluscs, those curious annelids the Nereis and the Aphrodite or sea-mouse, and many other marine animals. The brilliant colours of the scallops (Pecten)and some other bivalve shells are perhaps an indication of their hardness and consequent inedibility, as in the case of the hard beetles; and it is not improbable that some of the phosphorescent fishes and other marine organisms may, like the glow-worm, hold out their lamp as a warning to enemies. In Queensland there is an exceedingly poisonous spider, whose bite will kill a dog, and cause severe illness with excruciating pain in man. It is black, with a bright vermilion patch on the middle of the body; and it is so well recognised by this conspicuous coloration that even the spider-hunting wasps avoid it.
Locusts and grasshoppers are generally of green protective tints, but there are many tropical species most gaudily decorated with red, blue, and black colours. On the same general grounds as those by which Mr. Belt predicted the inedibility of his conspicuous frog, we might safely predict the same for these insects; but we have fortunately a proof that they are so protected, since Mr. Charles Home states that one of the bright coloured Indian locusts was invariably rejected when offered to birds and lizards.
The examples now given lead us to the conclusion that colours acquired for the purpose of serving as a danger-signal to enemies are very widespread in nature, and, with the corresponding colours of the species which mimic them, furnish us with a rational explanation of a considerable portion of the coloration of animals which is outside the limits of those colours that have been acquired for either protection or recognition. There remains, however, another set of colours, chiefly among the higher animals, which, being connected with some of the most interesting and most disputed questions in natural history, must be discussed in a separate chapter.
- Nature, vol. iii. p. 165. Professor Meldola observed that specimens of Danais and Euplaea in collections were less subject to the attacks of mites (Proc. Ent. Soc., 1877, p. xii.); and this was corroborated by Mr. Jenner Weir. Entomologist, 1882, vol. xv. p. 160.
- See Darwin's Descent of Man, p. 325.
- Transactions of the Entomological Society of London, 1869, p. 21.
- Ibid., p. 27.
- Nature, vol. iii. p. 147.
- Stainton's Manual of Butterflies and Moths, vol. i. p. 93; E. B. Poulton, Proceedings of the Zool. Soc. of London, 1887, pp. 191-274.
- See Transactions of the Linnean Society, vol. xxiii. pp. 495-566, coloured plates.
- These butterflies are now divided into two sub-families, one of which is placed with the Danaidæ; but to avoid confusion I shall always speak of the American genera under the old term Heliconidæ.
- R. Meldola in Ann. and Mag. of Nat. Hist., Feb. 1878, p. 158.
- See Trans. Linn. Soc., vol. xxv. Wallace, on Variation of Malayan Papilionidæ; and, Wallace's Contributions to Natural Selection chaps. iii. and iv., where full details are given.
- See Trans. Linn. Soc., vol. xxvi., with two coloured plates illustrating cases of mimicry.
- Edwards's Butterflies of North America, second series, part vi.
- Professor Meldola informs me that he has recorded another case of mimicry among British moths, in which Acidalia subsericata imitates Asthena candidata. See Ent. Mo. Mag., vol. iv. p. 163.
- From Professor Meldola's translation of Dr. F. Müller's paper, in Proc. Ent. Soc. Lond., 1879, p. xx.
- Island Life, p. 255.
- This extension of the theory of mimicry was pointed out by Professor Meldola in the paper already referred to; and he has answered the objections to Dr. F. Müller's theory with great force in the Annals and Mag. of Nat. Hist., 1882, p. 417.
- Godman and Salvin's Biologia Centrali-Americana, Insecta, Coleoptera, vol. iii. part ii., and vol. v.
- Trans. Ent. Soc., 1885, p. 369.
- Proc. Cambridge Phil. Soc., vol. iii. part ii., 1877.
- Compte-Rendu de la Société Entomologique de Belgaue, series ii., No. 59, 1878.
- Nature, vol. xxxiv. p. 547.
- Proceedings of the Zool. Soc. of London, 1870, p. 369.
- The Naturalist in Nicaragua, p. 321.
- Mr. Belt first suggested this use of the light of the Lampyridae (fireflies and glow-worms)—Naturalist in Nicaragua, p. 320. Mr. Verrill and Professor Meldola made the same suggestion in the case of medusae and other phosphorescent marine organisms (Nature, vol. xxx. pp. 281, 289).
- W.E. Armit, in Nature, vol. xviii. p. 642.
- Proc. Ent. Soc., 1869, p. xiii.