Chapter IX: Warning Coloration and Mimicry (2)

One of the characters by which some beetles are protected is excessive hardness of the elytra and integuments. Several genera of weevils (Curculionidae) are thus saved from attack, and these are often mimicked by species of softer and more eatable groups. In South America, the genus Heilipus is one of these hard groups, and both Mr. Bates and M. Roelofs, a Belgian entomologist, have noticed that species of other genera exactly mimic them. So, in the Philippines, there is a group of Curculionidae, forming the genus Pachyrhynchus, in which all the species are adorned with the most brilliant metallic colours, banded and spotted in a curious manner, and are very smooth and hard. Other genera of Curculionidae (Desmidophorus, Alcides), which are usually very differently coloured, have species in the Philippines which mimic the Pachyrhynchi; and there are also several longicorn beetles (Aprophata, Doliops, Acronia, and Agnia), which also mimic them. Besides these, there are some longicorns and cetonias which reproduce the same colours and markings; and there is even a cricket (Scepastus pachyrhynchoides), which has taken on the form and peculiar coloration of these beetles in order to escape from enemies, which then avoid them as uneatable.[111] The figures on the opposite page exhibit several other examples of these mimicking insects.

Innumerable other cases of mimicry occur among tropical insects; but we must now 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.[112] Dr. A.B. Meyer has also discovered that, while some species of the genus Callophis (belonging to the same family as the American Elaps) have large poison fangs, other species of the same genus have none; and that one of the latter (C. gracilis) resembles a poisonous species (C. intestinalis) so closely, that only an exact comparison will discover the difference of colour and marking. A similar kind of resemblance is said to exist between another harmless snake, Megaerophis flaviceps, and the poisonous Callophis bivirgatus; and in both these cases the harmless snake is less abundant than the poisonous one, as occurs in all examples of true mimicry.[113]

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

_Mimicry among Birds._

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

More perfect cases of mimicry occur between some of the dull-coloured orioles in the Malay Archipelago and a genus of large honey-suckers--the Tropidorhynchi or "Friar-birds." These latter are powerful and noisy birds which go in small flocks. They have long, curved, and sharp beaks, and powerful grasping claws; and they are quite able to defend themselves, often driving away crows and hawks which venture to approach them too nearly. The orioles, on the other hand, are weak and timid birds, and trust chiefly to concealment and to their retiring habits to escape persecution. In each of the great islands of the Austro-Malayan region there is a distinct species of Tropidorhynchus, and there is always along with it an oriole that exactly mimics it. All the Tropidorhynchi have a patch of bare black skin round the eyes, and a ruff of curious pale recurved feathers on the nape, whence their name of Friar-birds, the ruff being supposed to resemble the cowl of a friar. These peculiarities are imitated in the orioles by patches of feathers of corresponding colours; while the different tints of the two species in each island are exactly the same. Thus in Bouru both are earthy brown; in Ceram they are both washed with yellow ochre; in Timor the under surface is pale and the throat nearly white, and Mr. H.O. Forbes has recently discovered another pair in the island of Timor Laut. The close resemblance of these several pairs of birds, of widely different families, is quite comparable with that of many of the insects already described. It is so close that the preserved specimens have even deceived naturalists; for, in the great French work, _Voyage de l'Astrolabe_, the oriole of Bouru is actually described and figured as a honey-sucker; and Mr. Forbes tells us that, when his birds were submitted to Dr. Sclater for description, the oriole and the honey-sucker were, previous to close examination, considered to be the same species.

_Objections to the Theory of Mimicry._

To set forth adequately the varied and surprising facts of mimicry would need a large and copiously 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.[114]

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.[115] 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.[116]

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.[117]

* * * * *

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.

FOOTNOTES:

[Footnote 92: _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.]

[Footnote 93: See Darwin's _Descent of Man_, p. 325.]

[Footnote 94: _Transactions of the Entomological Society of London_, 1869, p. 21.]

[Footnote 95: _Ibid._, p. 27.]

[Footnote 96: _Nature_, vol. iii. p. 147.]

[Footnote 97: 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.]

[Footnote 98: See _Transactions of the Linnean Society_, vol. xxiii. pp. 495-566, coloured plates.]

[Footnote 99: These butterflies are now divided into two sub-families, one of which is placed with the Danaidae; but to avoid confusion I shall always speak of the American genera under the old term Heliconidae.]

[Footnote 100: R. Meldola in _Ann. and Mag. of Nat. Hist._, Feb. 1878, p. 158.]

[Footnote 101: See _Trans. Linn. Soc._, vol. xxv. Wallace, on Variation of Malayan Papilionidae; and, Wallace's _Contributions to Natural Selection_ chaps. iii. and iv., where full details are given.]

[Footnote 102: See _Trans. Linn. Soc._, vol. xxvi., with two coloured plates illustrating cases of mimicry.]

[Footnote 103: Edwards's _Butterflies of North America_, second series, part vi.]

[Footnote 104: 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.]

[Footnote 105: From Professor Meldola's translation of Dr. F. Müller's paper, in _Proc. Ent. Soc. Lond._, 1879, p. xx.]

[Footnote 106: _Island Life_, p. 255.]

[Footnote 107: 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.]

[Footnote 108: Godman and Salvin's _Biologia Centrali-Americana, Insecta, Coleoptera_, vol. iii. part ii., and vol. v.]

[Footnote 109: _Trans. Ent. Soc._, 1885, p. 369.]

[Footnote 110: _Proc. Cambridge Phil. Soc._, vol. iii. part ii., 1877.]

[Footnote 111: _Compte-Rendu de la Société Entomologique de Belgaue_, series ii., No. 59, 1878.]

[Footnote 112: _Nature_, vol. xxxiv. p. 547.]

[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]

[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]

[Footnote 115: 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).]

[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]

[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]