The colours of animals : their meaning and use, especially considered in the case of insects

Front matter

THE

INTERNATIONAL SCIENTIFIC SERIES.

Each Book Complete in One Volume. Crown 8vo. cloth, 5s. unless otherwise described.

Preface

I have adopted a general title, ‘ The Colours of Animals,’ in order to indicate the contents of this volume, although the vast majority of the examples are taken from insects, and indeed almost invariably from a single order, the Lepidoptera. The examples are, however, employed merely to illustrate principles which are of wide application.

I have purposely abstained from multiplying instances when a little observation or even reflection will supply them in large numbers. For example, the ordinary Protective Resemblances of mammals and birds are barely alluded to, on this account. On the other hand, more difficult problems, such as the change of colour in arctic mammals, or the meaning of the colours of birds’ eggs, are treated at far greater length. My object in both cases is the same : to stimulate observation in a subject which will amply repay investigation, from the scientific value of the results, and the never-failing interest and charm of the inquiry.

Vlll

Variable Protective Resemblance in insects is treated in considerable detail, for the reasons given above, and because much of the work is so recent that no complete account can be found outside the original memoirs.

My chief object has been to demonstrate the utility of colour and marking in animals. In many cases I have attempted to prove that Natural Selection has sufficed to account for the results achieved ; and I fully believe that further knowledge will prove that this principle explains the origin of all appearances except those which are due to the subordinate principle of Sexual Selection, and a few comparatively unimportant instances which are due to Isolation or to Correlation of Growth.

In support of these views I have endeavoured to bring together a large amount of experimental evidence in favour of the theories as to the various uses of colour. Further experiments are still greatly needed.

In the chapters on ‘ Sexual Selection ’ I have argued in favour of Darwin’s views, and have attempted to defend them against recently published attacks.

At the conclusion of the volume I have brought forward a detailed classification of the various uses of colour, in which new, and, I believe, more convenient terms are suggested. Definitions and examples are also given in the classification, wffiich is, in fact, a brief abstract of the whole book.

PREFACE

IX

I have to thank the Councils of various scientific societies for the courteous permission to copy figures from their respective publications. The figures in the coloured plate are copied from the plates accompanying Mr. Roland Trimen’s paper in the ‘ Transactions of the Linnean Society,’ vol. xxvi. pp. 497-522. Figures 18, 19, 20, 21, and 22 are copied from the plate accompanying Mr. R. Rowdier Sharpe’s paper in the ‘ Proceedings of the Zoological Society,’ 1873, pp. 414 et seqq. Figures 3, 4, 5, 6, 7, 8, 11, 14, 58, 60, 61, 62 are copied from the plates and woodcuts accompanying my papers in the £ Transactions of the Entomological Society,’ 1884, 1885, 1887, and 1888. Figures 25, 26, and 27 are copied from the plate accompanying Mrs. Barber’s paper in the ‘ Transactions of the Entomological Society,’ 1874, pp. 519 et seqq. Figures 29 and 30 are copied from the plate and woodcuts accompanying my paper in the * Philosophical Transactions of the Royal Society,’ vol. 178 (1887), B, pp. 311-441. Figures 15, 16, 53, 54, 63, 64, 65, 66 are copied from the woodcuts and plates accompanying G. W. and E. G. Peckham’s paper in the ‘ Occasional Papers of the Natural History Society of Wisconsin,’ vol. i. (1889), Milwaukee. Figures 55 and 56 are copied from the plates accompanying Professor Weismann’s ‘ Studies in the Theory of Descent,’ translated by Professor Meldola. Figure 10 is copied from one of the plates accompanying Dr. Wilhelm Muller’s ‘ Sudamerikanische Nymphaliden-

X raupen ’ (‘ Zoologische Jalirbiicher,’ J. W. Spengel, Jena, 1886). Figure 42 is copied from Yogt (‘The Natural History of Animals’: English translation: Blackie and Son). Figures 44 and 45 are copied from the plates accompanying Curtis’s ‘ British Lepidoptera.’ The remaining figures are original. Figure 17 was kindly lent me by Dr. A. B. Wallace, to whom it had been sent by Mr. Wood-Mason. In preparing the drawings of the original figures I have been greatly assisted by my wife, my sister Miss L. S. Poulton, Miss Horman Fisher, Mr. Alfred Sich, Mr. Alfred Robinson, and especially by Miss Cundell.

I have almost invariably referred to original papers from which facts or conclusions have been adopted ; so that any reader having access to a scientific library may easily gain possession of further details. Not wishing to overburden the book with such notes, I have abstained from referring constantly to my own papers, although most of the examples are taken from them. A list of my papers which deal with the colours of insects is therefore printed below.

‘ Transactions Entomological Society,’ London, 1884, pp. 27-60 „ „ „ „ 1885, „ 281-829

„ „ „ „ 1886, „ 187-179

„ „ „ „ 1887, „ 281-321

„ „ „ „ 1888, „ 515-606

‘ Philos. Trans. Eoyal Society,’ vol. 178 (1887), B, pp. 311-441 Abstract of the above in ‘Proceedings Eoyal Society,’ 1887, vol. xlii. pp. 94-108

‘ Proceedings Eoyal Society,’ 1885, vol. xxxviii. pp. 269-315 „ „ „ 1886, vol. xl. pp. 135-173

‘ Proceedings Zoological Society,’ 1887, pp. 191-274

PREFACE

XI

Short papers or notes (exclusive of those which are mere abstracts of the above) : —

1 Proceedings Entomological Society,’ London, 1887, pp. 1-li

„ „ pp. viii-x

,, ,, ,, xxvii-xxviii

„ 1889 „ xxxvii-xl

1 Journal of the Victoria Institute,’ 1888, vol. xxii., ‘ On Mimicry.’

It is my pleasant duty to thank many friends for their kind assistance. I owe to Professor Meldola more than I can possibly express : his writings first induced me to enter upon this line of investigation, and I have had the benefit of his great experience and wise advice during the whole of the time that I have been at work. Nearly every subject touched upon in this volume has been discussed with him.

Professor Westwood has always been most kind in helping me with the literature of the subject, with which he has so intimate an acquaintance, and in giving me the free use of the Hope collection at Oxford. Professor E. Ray Lankester has read the proof-sheets dealing with the classifications of the uses of colour, and has offered valuable suggestions. Several beautiful examples were suggested to me by Professor C. Stewart. Dr. Gunther, Mr. Roland Trimen, Mr. Oldfield Thomas, Mr. R. Rowdier Sharpe, Mr. F. E. Beddard, Mr. W. W. Fowler, and Mr. A. PI. Cocks have been very kind in answering questions upon their special subjects. Sir John Conroy has

XU kindly helped me in explaining the physical questions involved in the first chapter. I am especially pleased to speak of the help received from my former pupils Mr. W. Garstangand Mr. R. C. L. Perkins, who have supplied many valuable instances, which are specified in the volume, where other kind assistance is also duly acknowledged.

Although I have ventured to disagree with my friend Dr. A. R. Wallace upon the subject of 1 Sexual Selection,’ I wish to acknowledge how very much I owe to his writings, which I have very frequently quoted. I have also made great use of the late Thomas Belt’s extremely interesting and suggestive ‘ Naturalist in Nicaragua.’

Among recent papers I wish especially to mention that by G. W. and E. G. Peckham, of Milwaukee, U.S.A. The minute observation of these authors upon the courtship of spiders of the family At ticks is a model for investigation in a subject which has never before been attacked systematically.

Above all, I should wish to acknowledge, although I can never fully express, the depth of my indebtedness to the principles which first made Biology a science, the principles enunciated by Charles Darwin. It is common enough nowadays to hear of new hypotheses which are believed (by their inventors) to explain the fact of evolution. These hypotheses are as destructive of one another as they are supposed to be of Natural Selection, which remains as the one

PREFACE

Xlll solid foundation upon which evolution rests. I have wished to express this conviction because my name has been used as part of the support for an opposite opinion, by an anonymous writer in the ‘Edinburgh Review.’ 1 In an article in which unfairness is as conspicuous as the prejudice to which it is due, I am classed as one of those ‘ industrious young observers ’ who ‘ are accumulating facts telling with more or less force against pure Darwinism.’ 2 On the strength of this and other almost equally strange evidence, the Reviewer triumphantly exclaims, ‘ Darwin, the thanes fly from thee ! ’ In view of this public mention of my name, I may perhaps be excused for making the personal statement that any scientific work which I have had the opportunity of doing has been inspired by one firm purpose — the desire to support, in however small a degree, and to illustrate by new examples, those great principles which we owe to the life and writings of Charles Darwin, and especially the preeminent principle of Natural Selection.

E. B. P.

1 Edinburgh Review. Article V. April 1888, pp. 407-47.

2 p. 443. The bias of the writer appears in a most singular manner upon this page. In the short space of seventeen lines the following adjectives are divided between five writers and their works : — industrious, illustrious, gifted, well-read, acute, intelligent, brilliant, thoughtful. I need hardly say that all five writers are believed by the Reviewer to oppose the theory of Natural Selection.

Contents

CHAPTER PAGE

I. The Physical Cause of Animal Colours 1

II. The Uses of Colour 12

CHAPTER I Colours due to absorption

The colours of animals are produced in various ways. By far the commonest method is the absorption of certain elements of light by means of special substances which are called pigments, or colouring matters. The colour of each pigment is due to those elements of the light which it does not absorb, and which can therefore emerge and affect the eye of the spectator. Black is, of course, caused by the absorption of all the constituents of light, so that nothing reaches the eye. The colour of red pigment, like that of red glass, depends upon the fact that red is less absorbed than any other element of the light which passes through. If a sheet of red glass be placed upon white paper, the light traverses the glass, is reflected from the surface of the paper, re-traverses the glass, and emerges. Similarly, in painting, bright effects are produced by covering a surface of Chinese white with the desired colour. The light passing twice through the thickness of the colour, absorption is far more complete than when only one thickness is traversed, as in a piece of red glass held up to the light. Absorption being more complete, the red colour is deeper. Animal pigments are nearly always twice traversed by the light, and therefore a very thin layer produces a considerable effect.

Animal colours are therefore generally due to precisely the same optical principle which causes the colour of a wall-paper, a carpet, or a picture. Certain transparent animals are, however, for the most part coloured by light which passes but once through them, upon the same principle as the colours of a stainedglass window. The beautiful transparent blue of many pelagic animals, such as the Portuguese Man-of-war ( Physalia ), is caused in this way.

It would be out of place to discuss the details of the causes of colour by absorption. I may, however, mention that vibrations of very different rates are started in the luminiferous ether by the sun, the electric light, &c. A certain series of these vibrations causes the effect of white light when it falls on our retina ; but there are vibrations above and below this visible series — vibrations which we cannot see. We can, however, prove their existence in other ways ;

— - and it is certain that some animals can see vibrations which do not affect our eyes.1 The slowest vibrations that we can see, produce the effect of red, the most rapid the effect of violet, while the intermediate vibrations cause the other well-known colours of the rainbow or the spectrum.

The absorption of certain elements of light therefore means the disappearance of ethereal vibrations with a certain speed. It is believed that these vibrations disappear because their motion has been communicated to the particles of the absorbing body. It is also believed that these particles are in a state of constant vibration, and that the vibrations of ether, which are timed to those of the body, are used up in increasing the motion of the latter.

A white appearance due to light being scattered

The production of white is due to a different principle, for we know that when light passes through a body without any absorption, the body is transparent and invisible rather than white. When all the light passes through, the body is completely invisible. Whiteness is due to reflection of the whole visible series of vibrations, unaccompanied by the absorption of a part of them, as in the production of colours. But regular reflection, viz. reflection from

1 Sir John Lubbock, The Senses of Animals, Chapter X. (International Scientific Series).

a polished surface like that of a mirror, does not cause whiteness : it renders the surface itself invisible, but produces images of surrounding objects. A white appearance is produced by irregular reflection, which causes the light to he scattered or reflected in all directions. To produce such a result there must he an immense number of surfaces in an immense number of different directions. If a coloured substance be reduced to powder of various degrees of fineness, the colour will diminish in intensity, and the whiteness will increase, according to the fineness of the powder ; this is because the number of reflecting surfaces is increased, while the thickness of the grams is diminished. This will be clear from the following consideration. When a beam of light falls on a sheet of glass, a known fraction (about 4 per cent.) of the light is reflected back from the first surface : the larger portion, however, enters the glass, and, after suffering a certain amount of absorption, reaches the second surface and is again partially reflected. If the glass be powdered, the number of surfaces will be so immensely increased that all the light will be reflected by a small thickness of the powder. The light reflected from the second surface of each grain of coloured glass will still be coloured by absorption, but not sufficiently to produce any visible results, when the thickness of the grain is very small.

Reflection is the immediate cause of whiteness, and the amount of reflection is due to the difference between the refractive powers (viz. the power of changing the direction of rays of light) possessed by the grains of glass and the substance, such as air or water, which lies between them. Thus the refractive powers of glass and water are much nearer than those of glass and air: hence a dry powder will reflect far more than a wet one, and will appear much whiter.

To take a few familiar examples : snow is white, because of the minute globules of air which refract very differently from the crystals between which they are entangled ; ice, on the other hand, is transparent. If snow be compressed the air is driven out, and the mass becomes transparent ; if ice be powdered it becomes white like snow. The froth of a coloured liquid is not coloured like the latter, but is white. Milk and fat are white because light is scattered from the surfaces of the countless oil globules, which refract very differently from the substance which lies between them. The surface of well-polished glass is almost invisible, because it reflects regularly, but a scratched surface is very visible, because there are surfaces in many different directions, which therefore scatter the light, while the far more numerous surfaces of ground glass scatter the light far more effectually and produce a white appearance.

The white markings of animals are produced in various ways. White hairs and feathers owe their appearance, like snow, to the number of minute bubbles of gas ■which are contained in their interstices. Fat is also made use of to give a white appearance ; and the same result may be obtained by the presence of minute granules, probably akin to pigment, but differing widely from it in optical properties, in that no absorption takes" place.

Colours due to thin plates

It has been stated already that when light traverses a sheet of glass surrounded by air, a certain proportion of it is reflected back at the first surface and a certain proportion at the second surface. The light will be reflected in the same direction from both surfaces. It is believed that the vibrations of ether, some of which affect us as light, are in the form of undulations of different lengths ; if, therefore, the sheet of glass be sufficiently thin, some of the undulations reflected from the second surface will interfere with those started ^from the first surface. This will happen'when ther sheet is of such a thickness that the wave of light reflected from the second surface is half an undulation behind that reflected from the first surface ; for then the two sets of undulations will be in opposite directions, and will therefore neutralise each other.

This will be quite clear if we apply the same reasoning to those visible undulations from which the name itself has been borrowed — the waves on the surface of water. If a set of ripples is started by the motion of an object in still water, and then another set is started from another object moved, so that the ripples succeed each other at exactly the same rate as the preceding set, and if the second set is begun when the first has advanced half a complete ripple (viz. a movement up and down), it is clear that the. upward movement of the second will correspond to the downward movement of the first and vice versa, so that, if the objects are so placed that the two sets of ripples are traversing the same sheet of water, they will neutralise and destroy each other.

If we compare a number of sheets of glass which are successively thinner and thinner, interference will first occur among the longest undulations of light, because half an undulation will of course require a greater distance (or thickness) than when the undulations are shorter. As thinner and thinner sheets are examined interference will gradually pass through the whole spectrum from red to violet, destroying sets of waves with shorter and shorter undulations. The colour seen in each case will be due to the other sets of waves which are not destroyed.

The amount of reflection, and therefore of interference and of colour produced, depends upon the difference between the refractive power of the thin sheet and the substance on each side of it.

Such interference colours are seen in a soap-bubble, and the colours change as the bubble becomes larger and the film thinner : they differ, too, on the various parts of the bubble, because the thinness also varies. A bubble of melted glass may be blown thin enough to produce the same effects, which are also well seen when a thin layer of air is enclosed between two sheets of glass or between the plates of some crystals, or when a drop of oil is allowed to spread out into a thin film on the surface of water. When a substance has a laminated structure, and sufficiently thin films are enclosed between the laminae, very marked effects are seen. Thus the metallic appearance of the laminated flakes which are formed on the surface of glass which has been long buried in the earth, is accounted for. If these brilliant flakes are wetted the colour fades away, because the thin films of air between the laminae are displaced by water, with a refractive power much nearer to that of the glass, and the amount of reflected light is therefore diminished.

Interference colours due to thin films are certainly very important among animals, but the extent to which they occur is imperfectly known. The iridescent colours of many beetles’ wings are probably due to thin films of air included between layers of a horny consistence. Such colours are unaltered in dried specimens. In other cases the chinks between the layers are kept open by films of less powerfully refractive liquids. When the tissue becomes dry the films evaporate and the colour disappears. We must suppose that the denser layers come together,

THE PHYSICAL CAUSE OP ANIMAL COLOURS 9

obliterating the chinks and excluding the air ; otherwise the colours would be more brilliant than ever, because the refractive power of air is even lower than that of the liquids. The brilliant metallic appearance of many chrysalides, especially in the genus Vanessa , is caused by a large number of films of liquid enclosed between the laminae of the dense outer layer. If the pupa be kept in spirit or water the colour remains, but disappears directly it is dry, although it can be renewed any number of times by wetting. This may even occur in a living animal, for Dr. Sharp has just directed my attention to an interesting observation made by Dr. Nickerl, who found that a brilliantly golden beetle ( Cardbus auronitens) lost all its lustre after hybernating in captivity, but entirely regained it after drinking some water.

Colours due to diffraction

When white light falls upon a surface on which there are a number of fine parallel grooves the reflected light appears coloured, the colour varying with the angle at which the light falls on the surface, and with the angle at which it is seen. This is due to the light reflected from different portions of the surface having different distances to travel before reaching the observer : and when (as occurs when the grooves are very close together) these differences amount to half a complete undulation for any particular length of vibration, interference is caused, and the vibration of that particular rate is wanting from the reflected light, which therefore appears coloured.

Opinions differ as to the relative importance of animal colours due to thin plates and to diffraction. Many which were believed to result from the latter are in all probability due to the former. The iridescent colours on the inner surface of many shells (mother-of-pearl) are at any rate partially caused by diffraction, for an accurate cast of the surface exhibits traces of the colours.1 The shell is, however, a laminated structure, and the colours may therefore in part be caused by thin plates.

Colours due to refraction (prismatic colours)

When light passes through a wedge-shaped transparent substance (or prism) with greater refractive power than the surrounding medium, it is bent in the same direction at both surfaces, but its different constituents are bent unequally. The slowest vibrations (red) are bent least, the most rapid (violet) most ; and when the substance possesses a sufficiently high refractive power, all the colours of white light are seen arranged like the rays of a fan in the order of their rates of vibration. Prismatic colours like those of the diamond are due to refraction.

1 Professor C. Stewart informs me that he has repeated Brewster’s original experiment, upon which the above statement depends. He found that the colour was due to a thin layer of shell which had been stripped off and adhered to the surface of the wax.

It is doubtful how far the colours of animals are caused by this principle ; but Dr. Gadow has given strong reasons for supposing that the metallic colours of birds’ feathers are produced in this way,1 and there are scales on the wings-cases of certain beetles ( Pachyrhynchus ) which also may owe their colours to refraction.

All these causes of animal colours may be conveniently grouped under two heads — (1) pigmentary, and (2) structural. The first head includes colours caused by absorption, and the effects produced vary with the chemical nature of the substance (pigment). The second head includes the colours or appearances produced in all other ways, the efficient cause being the structure of the substance rather than its chemical nature.

1 Proc Zool. Soc. 1882, pp. 409 et seq.

CHAPTEE II

I. Non-significant colours

Colour, as such, is not necessarily of any value to an organism. Organic substances frequently possess a chemical and physical structure which causes certain light-waves to be absorbed ; or the elements of tissues may be so arranged that light is scattered, or interference colours are produced. Thus blood is red, fat is white, and the external surface of the airbladder in certain fishes has a metallic lustre, like silver. In such cases there is no reason why we should inquire as to the use or meaning of the colour in the animal economy; the colour, as such, has no more meaning than it has in a crystal of sulphate of copper or iron. Such colours are the incidental results of chemical or physical structure, which is valuable to the organism on its own account. This argument will he still further enforced if we remember that the colours in question are, strictly speaking, not colours at all. Blood and fat are so constituted that they will be red and white, respectively, in the presence of

THE USES OE COLOUR light, but they cannot be said to possess these colours in their normal position, buried beneath the opaque surface of an animal.

The existence of non- significant colours is, nevertheless, most important, for they form the material out of which natural or sexual selection can create significant colours. Thus, the colour of blood may be made use of for ‘ complexion,’ while fat may be employed to produce white markings, as in certain insect larvte. The yellow, brown, and red fatty matters of the connective tissue are accumulated beneath the skin in patches, so as to produce patterns.

All colour originally non-significant

All animal colour must have been originally non-significant, for although selective agencies have found manifold uses for colour, this fact can never have accounted for its first appearance. It has, however, been shown that this first appearance presents no difficulty, for colour is always liable to occur as an incidental result. This is even true of the various substances vdiich seem to be specially set apart for the production of colour in animals ; for pigments occur abundantly in the internal organs and tissues of many forms. The brilliant colours of some of the lower organisms are probably also non-significant. In all higher animals, however, the colours on the surface of the body have been significant for a vast period of time, so that their amount, their arrangement in patterns, their varying tints, and their relation to the different parts of the body, have all been determined by natural selection through innumerable generations. Because the origin of all pigments is to be found in the incidental result of the chemical and physical nature of organic compounds, it by no means follows that incidental or non- significant colours would have appeared at all on the surface of most animals. And we find as a matter of fact that such colours tend to disappear altogether, directly they cease to be useful, as in cave-dwelling animals. On the other hand, the non -significant colour of blood or of fat would persist undimiuished in such forms.

Colours may be destroyed by natural selection

Just as natural selection may develop an appearance which harmonises with the surroundings, out of the material provided by non-significant colour, the same agency may lead to the disappearance of the latter when it impedes the success of an animal in the struggle for existence. Thus the red colour of blood has disappeared in certain transparent fishes, which are thereby concealed from their enemies. Among the manifold possible variations of nature is that of a fish with colourless blood, which can, nevertheless, efficiently perform all the duties of this fluid. While such a variation would be no advantage to the great majority of vertebrates, it would be very beneficial to a fish which was already difficult to detect on the surface of the ocean on account of its transparency.

II. Significant colours

Colours may be useful in many ways, and are therefore always liable to be turned to account in one direction or another. They may be of direct physiological value to the organism, or may assist in the struggle for existence by deluding other species, or by aiding the individuals of the same species, or they may be intimately connected with courtship.

1. The Direct Physiological Value of colour

The colour of chlorophyll, which causes the green appearance of vegetation, must be intimately connected with the important changes which take place in this substance in the presence of light. It is well known that under these circumstances carbon dioxide (popularly called * carbonic acid ’) can be split up, and its carbon made to unite with the elements of water, forming organic substance. Although this process has been much studied it is still very imperfectly understood. It is clear, however, that the colour of chlorophyll, involving the special absorption of certain light-waves, has some direct bearing upon the changes which occur.

No equally clear instance has been proved to occur in the animal kingdom, except in those few forniB which resemble plants in possessing chlorophyll. Dr. Hickson, however, believes that among corals ‘ the most widely distributed colours will eventually be proved to be allied to chlorophyll . . . and perform a very similar if not precisely identical physiological function.’ It is much to be desired that this interesting suggestion, which Dr. Hickson supports by many arguments, may be thoroughly tested as soon as possible.1