Darwinism (1889) Part 17

The skunk as an example of warning coloration--Warning colours among insects--b.u.t.terflies--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 cla.s.s 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 a.s.sociates, 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 cla.s.s 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 ill.u.s.trating 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 b.u.t.terflies--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 b.u.t.terflies 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 was.h.i.+ngs.

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 b.u.t.terflies whose bodies had been devoured. Mr. Belt once observed a pair of birds capturing insects for their young; and although the Heliconidae swarmed in the vicinity, and from their slow flight could have been easily caught, not one was ever pursued, although other b.u.t.terflies did not escape. His tame monkey also, which would greedily munch up other b.u.t.terflies, 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 Danaidae. The Hon. Mr. Justice Newton, who a.s.siduously collected and took notes upon the Lepidoptera of Bombay, informed Mr.

Butler of the British Museum that the large and swift-flying b.u.t.terfly 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.[92]

Besides these three families of b.u.t.terflies, there are certain groups of the great genus Papilio--the true swallow-tailed b.u.t.terflies--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 Heliconidae. 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 b.u.t.terflies 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 s.e.xual 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 s.e.xual 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 pa.s.sed 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 b.u.t.terflies, 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.[93] 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 caeruleocephala, 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.[94]

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, b.u.t.terflies, 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.[95]

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 b.u.t.terflies, 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.[96]

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 quant.i.ty of green liquid, of an acid and disagreeable smell similar to that of the spurge milk, only worse.[97]

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.

_Mimicry._

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 b.u.t.terflies of the Amazon, and, on his return home, gave the first rational explanation of it.[98] 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 ill.u.s.trative of "warning coloration." But along with them other b.u.t.terflies 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 Euterpe, 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 s.e.xes. 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-sh.e.l.l, 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.

[Ill.u.s.tration: FIG. 23.--Methona psidii (Heliconidae). Leptalis orise (Pieridae).]

Besides these Pieridae, Mr. Bates found four true Papilios, seven Erycinidae, three Castnias (a genus of day-flying moths), and fourteen species of diurnal Bombycidae, all imitating some species of Heliconidae which inhabited the same district; and it is to be especially noted that none of these insects were so abundant as the Heliconidae 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 Heliconidae 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 b.u.t.terflies in the same country. It is, therefore, clear that if any other b.u.t.terflies 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 Heliconidae[99] all over tropical America, the vast number of their genera and species, and their marked distinctions from all other b.u.t.terflies, it follows that they const.i.tute 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 larvae 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 b.u.t.terflies. 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 distinguis.h.i.+ng mark, by which the devourers of b.u.t.terflies in general might learn that these particular b.u.t.terflies were uneatable; and every variation leading to such distinction, whether by form, colour, or mode of flight, was preserved and acc.u.mulated by natural selection, till the ancestral Heliconoids became well distinguished from eatable b.u.t.terflies, 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 larvae feeding on many plants and acquiring different habits; while the b.u.t.terflies 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 Pieridae, inhabiting the same district, happened to be sufficiently like some of the Heliconidae to be occasionally mistaken for them. These, of course, survived while their companions were devoured. Those among their descendants that were still more like Heliconidae 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 Heliconidae, and along with them species of Leptalis (Pieridae), 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 Heliconidae in order to devour the former and reject the latter. The Pieridae would, however, usually be less numerous, because their larvae are often protectively coloured and therefore edible, while the larvae of the Heliconidae are adorned with warning colours, spines, or tubercles, and are uneatable. It seems probable that the larvae and pupae of the Heliconidae 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 larvae are distasteful while the perfect insects are eatable, but I believe none in which the reverse is the case. The larvae of the Pieridae 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 larvae 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 acc.u.mulation 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 b.u.t.terflies, brought from tropical America by amateurs, are to be found some species of the mimicking Pieridae, Erycinidae, or moths, and the mimicked Heliconidae, 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 Muller, writing from Brazil to Professor Meldola, says, "One of the most interesting of our mimicking b.u.t.terflies is Leptalis melite. The female alone of this species imitates one of our common white Pieridae, 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."[100] 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 Pieridae 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 Pieridae which the female Leptalis melite very nearly resembles. The case, however, is interesting as showing that the b.u.t.terflies 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 Danaidae take the place of the Heliconidae of America, in their abundance, their conspicuousness, their slow flight, and their being the subjects of mimicry. They exist under three princ.i.p.al forms or genera. The genus Euploea is the most abundant both in species and individuals, and consists of fine broad-winged b.u.t.terflies 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.[101] Several species of Diadema, a genus of b.u.t.terflies 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 Agaristidae. These are usually adorned with the most brilliant colours or conspicuous markings, they fly slowly in forests among the b.u.t.terflies 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 Liparidae (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.

[Ill.u.s.tration: FIG. 24.--Opthalmis lincea (Agaristidae). Artaxa simulans (Liparidae).]

In Africa exactly similar phenomena recur, species of Papilio and of Diadema mimicking Danaidae or Acraeidae 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 Panopaea (another genus of Nymphalidae), three of Melanitis (Eurytelidae), and two of Papilio, resemble with equal accuracy some species of Acraea.[102] He has also independently observed the main facts on which the explanation of the phenomenon rests,--the unpleasant odour of the Danais and Acraea, extending to their larvae and pupae; their great abundance, slow flight, and disregard of concealment; and he states that while lizards, mantidae, and dragonflies all hunt b.u.t.terflies, and the rejected wings are to be found abundantly at some of their feeding-places, those of the two genera Danais and Acraea were never among them.

The two groups of the great genus Papilio (the true swallow-tailed b.u.t.terflies) 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 b.u.t.terflies are mimicked by Pieridae 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 b.u.t.terfly 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 b.u.t.terfly, 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.[103]

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.[104]

_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.