Darwinism (1889) Part 20

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. .h.i.therto 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 ill.u.s.tration 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 compet.i.tion among the poultry, he managed to entice a young duck into s.n.a.t.c.hing 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 sh.e.l.ls 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 phosph.o.r.escent 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 gra.s.shoppers 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 b.u.t.terflies 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 b.u.t.terflies 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 ill.u.s.trating cases of mimicry.]

[Footnote 103: Edwards's _b.u.t.terflies 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. Muller'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. Muller's theory with great force in the _Annals and Mag. of Nat. Hist._, 1882, p. 417.]

[Footnote 108: G.o.dman 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 Societe Entomologique de Belgaue_, series ii., No. 59, 1878.]

[Footnote 112: _Nature_, vol. x.x.xiv. 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 phosph.o.r.escent marine organisms (_Nature_, vol. x.x.x. pp. 281, 289).]

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

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

CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF s.e.x

s.e.x colours in the mollusca and crustacea--In insects--In b.u.t.terflies and moths--Probable causes of these colours--s.e.xual selection as a supposed cause--s.e.xual coloration of birds--Cause of dull colours of female birds--Relation of s.e.x colour to nesting habits--s.e.xual colours of other vertebrates--s.e.xual selection by the struggles of males--s.e.xual characters due to natural selection--Decorative plumage of males and its effect on the females--Display of decorative plumage by the males--A theory of animal coloration--The origin of accessory plumes--Development of accessory plumes and their display--The effect of female preference will be neutralised by natural selection--General laws of animal coloration--Concluding remarks.

In the preceding chapters we have dealt chiefly with the coloration of animals as distinctive of the several species; and we have seen that, in an enormous number of cases, the colours can be shown to have a definite purpose, and to be useful either as a means of protection or concealment, of warning to enemies, or of recognition by their own kind.

We have now to consider a subordinate but very widespread phenomenon---the differences of colour or of ornamental appendages in the two s.e.xes. These differences are found to have special relations with the three cla.s.ses of coloration above referred to, in many cases confirming the explanation already given of their purport and use, and furnishing us with important aid in formulating a general theory of animal coloration.

In comparing the colours of the two s.e.xes we find a perfect gradation, from absolute ident.i.ty of colour up to such extreme difference that it is difficult to believe that the two forms can belong to the same species; and this diversity in the colours of the s.e.xes does not bear any constant relation to affinity or systematic position. In both insects and birds we find examples of complete ident.i.ty and extreme diversity of the s.e.xes; and these differences occur sometimes in the same tribe or family, and sometimes even in the same genus.

It is only among the higher and more active animals that s.e.xual differences of colour acquire any prominence. In the mollusca the two s.e.xes, when separated, are always alike in colour, and only very rarely present slight differences in the form of the sh.e.l.l. In the extensive group of crustacea the two s.e.xes as a rule are identical in colour, though there are often differences in the form of the prehensile organs; but in a very few cases there are differences of colour also. Thus, in a Brazilian species of sh.o.r.e-crab (Gelasimus) the female is grayish-brown, while in the male the posterior part of the cephalo-thorax is pure white, with the anterior part of a rich green. This colour is only acquired by the males when they become mature, and is liable to rapid change in a few minutes to dusky tints.[118] In some of the freshwater fleas (Daphnoidae) the males are ornamented with red and blue spots, while in others similar colours occur in both s.e.xes. In spiders also, though as a rule the two s.e.xes are alike in colour, there are a few exceptions, the males being ornamented with brilliant colours on the abdomen, while the female is dull coloured.

_s.e.xual Coloration in Insects._

It is only when we come to the winged insects that we find any large amount of peculiarity in s.e.xual coloration, and even here it is only developed in certain orders. Flies (Diptera), field-bugs (Hemiptera), cicadas (h.o.m.optera), and the gra.s.shoppers, locusts, and crickets (Orthoptera) present very few and unimportant s.e.xual differences of colour; but the last two groups have special musical organs very fully developed in the males of some of the species, and these no doubt enable the s.e.xes to discover and recognise each other. In some cases, however, when the female is protectively coloured, as in the well-known leaf-insects already referred to (p. 207), the male is smaller and much less protectively formed and coloured. In the bees and wasps (Hymenoptera) it is also the rule that the s.e.xes are alike in colour, though there are several cases among solitary bees where they differ; the female being black, and the male brown in Anthophora retusa, while in Andraena fulva the female is more brightly coloured than the male. Of the great order of beetles (Coleoptera) the same thing may be said.

Though often so rich and varied in their colours the s.e.xes are usually alike, and Mr. Darwin was only able to find about a dozen cases in which there was any conspicuous difference between them.[119] They exhibit, however, numerous s.e.xual characters, in the length of the antennae, and in horns, legs, or jaws remarkably enlarged or curiously modified in the male s.e.x.

It is in the family of dragonflies (order Neuroptera) that we first meet with numerous cases of distinctive s.e.xual coloration. In some of the Agrionidae the males have the bodies rich blue and the wings black, while the females have the bodies green and the wings transparent. In the North American genus Hetaerina the males alone have a carmine spot at the base of each wing; but in some other genera the s.e.xes hardly differ at all.

The great order of Lepidoptera, including the b.u.t.terflies and moths, affords us the most numerous and striking examples of diversity of s.e.xual colouring. Among the moths the difference is usually but slight, being manifested in a greater intensity of the colour of the smaller winged male; but in a few cases there is a decided difference, as in the ghost-moth (Hepialus humuli), in which the male is pure white, while the female is yellow with darker markings. This may be a recognition colour, enabling the female more readily to discover her mate; and this view receives some support from the fact that in the Shetland Islands the male is almost as yellow as the female, since it has been suggested that at midsummer, when this moth appears, there is in that high lat.i.tude sufficient twilight all night to render any special coloration unnecessary.[120]