Animal Biochromes and Structural Colours, Second Edition: Physical, Chemical, and Distributional and Physiological Features of Coloured Bodies in the Animal World

By Denis L. Fox

This title is part of UC Press's Voices Revived program, which commemorates University of California Press’s mission to seek out and cultivate the brightest minds and give them voice, reach, and impact. Drawing on a backlist dating to 1893, Voices Revived makes high-quality, peer-reviewed scholarship accessible once again using print-on-demand technology. This title was originally published in 1976.
This title is part of UC Press's Voices Revived program, which commemorates University of California Press’s mission to seek out and cultivate the brightest minds and give them voice, reach, and impact. Drawing on a backlist dating to 1893, Voices Revived

• Language: English
• Publisher: University of California Press
• Release date: Dec 22, 2023
• ISBN: 9780520339422

Denis L. Fox

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ANIMAL BIOCHROMES AND

STRUCTURAL COLOURS

ANIMAL BIOCHROMES

AND

STRUCTURAL COLOURS

PHYSICAL, CHEMICAL,

DISTRIBUTIONAL & PHYSIOLOGICAL FEATURES

OF

COLOURED BODIES IN THE

ANIMAL WORLD

Second Edition

BY

DENIS L. FOX

Professor Emeritus of Marine Biochemistry,

Scripps Institution of Oceanography of the University of California

Ni les recherches spéciales, ni les vues générales, ne suffisent isolément à constituer aucune science; c’est par leur alliance, par leur union, qu’elle se fond et se développe. CLAUDE BERNARD

UNIVERSITY OF CALIFORNIA PRESS

BERKELEY LOS ANGELES LONDON

University of California Press

Berkeley and Los Angeles, California

University of California Press, Ltd.

London, England

Copyright © 1976 by The Regents of the University of California

ISBN: 0-520-02347-1

Library of Congress Catalog Card Number: 72-89801

Printed in the United States of America

TO

MY LATE PARENTS

Who gave more than ever they received

AND TO

MY WIFE, MY COLLEAGUES AND STUDENTS

Who have helped me more than they can know

AND

IN MEMORY OF OUR SON STEVE (1935-1954),

a summertime assistant keeper in

the San Diego Zoo bird department.

CONTENTS 1

CONTENTS 1

TEXT-FIGURES

TABLES

PREFACE TO SECOND EDITION

PREFACE TO FIRST EDITION

CHAPTER I INTRODUCTION

CHAPTER II PHYSICAL AND CHEMICAL FOUNDATIONS OF COLOUR

A. General considerations

B. Whiteness

CHAPTER IV TYNDALL SCATTERING AND DIFFRACTION COLOURS

CHAPTER V IRIDESCENT COLOURS

CHAPTER VI CAROTENOIDS

A. Chemical structure

B. Chemical properties; general methods of separation and analysis

C. Carotenoids in the animal phyla‡‡

PROTOZOA

SPONGES

COELENTERATES

WORMS AND BRYOZOA***

ECHINODERMS

MOLLUSCS

, , ARTHROPODS

, CHORDATES

D. Formation of vitamin A

E. Natural deposits in the environment

F. Summary

CHAPTER VII CHROMOLIPOIDS OR LIPOFUSCINES

CHAPTER VIII QUINONES

A. Naphthoquinones

B. Anthraquinones

CHAPTER IX ANTHOCYANS AND FLAVONES

A. Anthocyans COMMON SOURCES, PHYSICAL AND CHEMICAL PROPERTIES

OCCURRENCE IN ANIMALS

B. Flavones COMMON SOURCES, P H Y S I C A L AND C H E M I C A L PROPERTIES

OCCURRENCE IN ANIMALS

CHAPTER X INDOLE PIGMENTS: INDIGOIDS AND MELANINS

A. Indigoids

B. Melanins

DISTRIBUTION

CHEMICAL PROPERTIES

EXTRACTION AND DETERMINATION

DISPOSITION IN TISSUES AND TISSUE DERIVATIVES

TYROSINASE

OTHER PROTEIN CATABOLITES

CHAPTER XI TETRAPYRROLES: PORPHINS AND BILINS

A. General biochemical considerations

B. Porphins

HAEM PROTEINS IN ORGANISMS

MISCELLANEOUS PORPHYRINS

C. Bilins

CHAPTER XII FLAVINS OR LYOCHROMES

A. Occurrence, properties and distribution

B. Summary

CHAPTER XIII PURINES AND PTERINS

A. Purines

B. Pterins

CHAPTER XIV SOME MISCELLANEOUS ANIMAL BIOCHROMES

BIBLIOGRAPHY

INDEX OF SUBJECTS

INDEX OF AUTHORS

SUPPLEMENT

SUPPLEMENTARY INDEX OF SUBJECTS

SUPPLEMENTARY INDEX OF AUTHORS

TEXT-FIGURES

TEXT-FIGURES

FIGURES IN SUPPLEMENT

i Two anthraquinones encountered in nature 383

2 Some unusual apo-carotenoids 394

3 Carotenoid composition of young P. pulchrum 403

4 Structural formulae of some carotenoids 410

5 Fall and rise of flamingo blood-carotenoid levels 413

6 Possible manner and sites of chemical equilibria! binding 418

7 Possible manner and route of formation of polar carotenoids 419

TABLES

TABLES IN SUPPLEMENT

PREFACE TO SECOND EDITION

Were a survey undertaken of the faunal kingdom for deciding on the class exhibiting the greatest beauty of form and color, it would be natural to anticipate a fair range of choices among individual judges, based in no small degree upon each one’s comparative interest, familiarity and personal involvement with animals belonging within certain taxonomic categories.

Human subjectivity, being what it is, must in the end confer added richness to the unavoidable comparisons which daily confront the naturalist, or groups of them.

Close or personal association with researches across a generous spectrum of animal types should, one might assume, contribute toward increased objectivity in a comparative survey. It is not as though one were confronted with the solemn and ultimately fateful judgment which the Greek shepherd Paris was called upon to render concerning the comparative pulchritude of the three goddesses Hera, Minerva and Aphrodite.

In the first instance the inquiry posed to a naturalist is academic, and his choice concerns nobody save himself. Moreover, a reasonable breadth of appreciation may confer upon such a judge the ability to apply several kinds of special qualifications leading to gratifying, plural decisions. Thus, even such possible candidates as the scorpion, octopus or baboon should enjoy their own finite chances for placement.

Retrospective reflection recalls to mind personal association, at various times across the years, with a considerable variety of animal forms. Pigmentary and other studies in the laboratory have been concerned with unicellular phytozoans, sponges, coelenterates (anemones, corals, jellyfish and siphono- phores), various worms (flatworms, sipunculids, and annelids), echinoderms (sea-stars, sea-urchins and sea-cucumbers), bivalved, gastropod and cephalopod molluscs, arthropods (crustaceans and schemochromes of a few insects), cyclostomes, teleostean and elasmobranch fishes, reptiles and birds.

In an early paragraph of the Introduction to the first edition of this book, it was pointed out that, despite the great diversity in organismic form, based upon incalculable numbers of different, successive linkages between the amino acids constituting the proteins of species, there exist far fewer classes (hardly exceeding a dozen) of biochrome to fulfill the diversified coloration and patterning seen in the animal kingdom.

Hence the appreciation of the comparative biochemist, who must of necessity also be a naturalist, is led into expanding realms, each with its own charm.

In what I write below, I confess at the outset to having exercised somewhat arbitrary options in the presentation of examples in a supplementary section. Attempts at an all-inclusive revision would have stretched the task beyond the reasonable proportions chosen to fulfill the basic purposes set forth in the beginning.

During the two decades since the first printing of this book by the Cambridge University Press, studies of animal biochromes have become steadily more enticing and more intense, while the significance of the subject has deepened in several disciplines of biology. As a consequence, reviews of specific areas have appeared within the wide boundaries of the field, complementing our understanding and appreciation of biological pigments in general.

But in the period since 1966, when the first edition of this book became exhausted in supply, there have been many expressions of hope from friends and fellow scientists that a succeeding edition or reprinting of the book might be expected. Several publishers have communicated similar gratifying wishes, accompanied by invitations.

The appearance or particular anticipation of new books or extended review treatments relating to animal biochromy have admittedly contributed to reasons for my delay in undertaking another immersion in the task. I had, moreover, published numerous research papers, and chapters or other reviews, within this general field of reference, which postponed further my taking up this assignment once again.

Since the primary purpose of the work is to present to students and other interested readers a survey of the fundamental physical and chemical features responsible for color manifestation in animals, but not to cover in toto recent advances relating to the identity and metabolism of all newly studied biochromic representatives, I readily agreed, with two provisos, to the reprinting of the first edition of Animal Biochromes, now by the University of California Press. The first condition was that the author be allowed to screen the whole current text with some care, attempting to delete errors and to modify certain passages scattered through its pages. The second reservation involved the inclusion of a supplemental section, to be inserted at the end of the book, attempting to bring up to date at least a few of the more recent findings relating to certain of the more conspicuous and important animal pigments, most notably among the carotenoids, tetrapyrroles and melanins. The Supplement includes its own alphabetically tabulated bibliography and its separate indices by subject and author. Hopefully these latter arrangements may combine to keep the book within reasonable access of all who may desire to own a copy, while not placing readers to excessive inconvenience through the inclusion of both an original and a newer additional set of references and indices.

It will be noticed that, in the Supplement to this Second Edition, some minor changes occur, for example, in alternative conventional spelling of certain words, and the newly adopted designation of nm (= nannometers) replacing the older equivalent mu (= millimicra) for reference to the Amax (= maximal light-absorption locus) of a pigment in a reference solvent. Structural formulae are presented in greatly simplified, modern form.

Of any avid readers and uncited authors alike I would ask forgiveness if, in the compelling interest of spatial economy while seeking to indicate instances of more recent and important findings regarding animal biochromes, I may have emphasized references to written reviews, themselves citing many original sources, and to reports of investigations wherein I shared first-hand experience.

Again, I thank many friends and colleagues, including officers and staff members of the University of California Press, for the numerous kindnesses and assistance accorded me toward the completion of this Second Edition.

PREFACE TO FIRST EDITION

The chromatology of living organisms is an expanding realm and an increasingly alluring meeting-ground for biologists of diverse interests. During recent years investigations concerning the colours and pigments of animals have advanced, materially and through numerous channels, our knowledge and appreciation of this field. A diversity of emphasis has been represented, in accordance with the various chief interests or viewpoints of contributors. Need for a survey of the general subject of animal pigments has been expressed by numerous colleagues and other scientific friends of the author, and the writing was undertaken at the original suggestion of Dr Hans Winterstein, of Istanbul, and Professor James Gray, of Cambridge University.

Granted that one who undertakes to write about the physical and biochemical features of animal colours has accepted a task that is not easy, he nevertheless finds himself at once in an enviable company of biologists and chemists who share his own love of the subject and its ramifications. Hence mutual aid is afforded in gradually advancing our understanding of the physiological significance of chromogenic substances in organisms or in certain of their products. The biochemist or physiologist, concerned with the physical and chemical nature, origin, fate and metabolic significance of coloured materials, looks to anatomists, cytologists and pathologists for information concerning occurrence, distribution and special circumstances applying to the appearance of colour in tissues and structures; he gains from the geneticist knowledge relating to the heritability of pigmentary and other colour manifestations, while the lore of systematists, naturalists and experimental biologists augments his understanding of environmental and adaptive chromatic responses.

In the present work my attempt has been to gather together facts and possible correlations regarding colours and coloured compounds of animals to serve the interests of students and investigators in various fields of biology, physicists and chemists with biological leanings, and especially those whose work in the field of biochemistry might derive benefit from a summary treatment of certain animal pigments of established or potential physiological bearing.

Any consideration of the various factors which may confer colour upon animals leads at once to the need for distinguishing between two fundamentally different sources from which colour may arise, namely, chemical or pigmentary compounds on the one hand, and on the other, purely physical structures of such fine dimensions as to interrupt visible light waves and disintegrate the spectrum. Hence no general treatment of the sources of coloration in animals would be complete, nor could it satisfy the average biologist, if the discussion were limited to the purely chemical origins of colour. It should be emphasized, furthermore, that the dark melanins, the yellow, orange or red carotenoids, as well as haem compounds, often play important roles in the emphasis or profound modification of physical colours. Some early chapters have therefore been devoted to a consideration of physical types and specific examples of structural colours of animals before undertaking the survey of pigmentary compounds. The complexity of animal biochromy has rendered a certain number of multiple citations and cross-references unavoidable.

So extensive is the existing literature dealing with various phases of pigmentation and structural coloration in animals that I have not attempted to cover it completely. Nor has there been a purpose of multiplying extensively the examples within a limited group of animals (e.g. Tyndall blues in birds which typify a given colour manifestation). Rather, I have tried to give emphasis to numerous researches and some of the outstanding reviews which bear upon the principal physical, chemical, metabolic and distributional phases under discussion. Wherever possible, I have followed a natural urge to share with my readers the results of first-hand experiments, as well as personal observations in the laboratory, the field, the zoo, and to a limited extent the museum.

The principal groups of animal pigments have been classified according to their chemical constitution. Within such a scheme consideration has been given, in approximate sequence, to the chemical nature, occurrence, distribution in phyla and in tissues, and metabolic or physiological significance of the various compounds. Purely chemical discussions have been made relatively brief and orientative, aided by formulae in many instances; exhaustive chemical treatment has been omitted in view of the excellent publications by Zechmeister (1934), Mayer & Cook (1943), and others. Discussion of adaptive coloration among animals has been limited to only a few comments, since this subject has been admirably covered by such authors as Cott (1940), Sumner (1940b), Parker (1948) and, of the earlier workers, Poulton (1890), Beddard (1892) and Biedermann (1914).

A degree of guidance in the present treatment has been available in reviews by Lederer (1935, 1938, 1940), in the work of Verne (1926a, 1930), and in the still older volume by Miss Newbigin (1898).

If the assembled information and references should prove serviceable as a guide to our better understanding of animal pigments and as a stimulus to further research in the subject in time to come, the writer will feel that his efforts have been generously rewarded. To such ends, discussion, comments and constructive criticism will be welcome.

It is a pleasure to record some grateful acknowledgements. I owe thanks to more friends and colleagues than can be listed here. They are aware and I am appreciative of the help and encouragement that they have rendered. Foremost among those to whom I feel especial gratitude is my friend and colleague the late Professor Francis B. Sumner, who first aroused my interest in natural pigments; with him I have shared an interest in many problems of animal pigmentation; I have benefited by critical discussions with him and by collaborative research. Likewise, I am appreciative of numerous enjoyable and stimulating discussions and joint research with Dr Carl F. A. Pantin, F.R.S., of the Department of Zoology, Cambridge University; to Professor James Gray, F.R.S., of the same Department, to the late Professor Sir F. Gowland Hopkins, F.R.S., of the Sir William Dunn Institute of Biochemistry at Cambridge, to Dr Wesley R. Coe, Professor Emeritus of Zoology from Yale University, and Dr Gilberto G. Villela at the Instituto Oswaldo Cruz, Rio de Janeiro, I also express my gratitude for information and friendly encouragement. The kindness and courtesies of Professor Jean Verne of the Faculty of Medicine, Paris, and of Dr Edgar Lederer of the University of Parisare acknowledged with pleasure. Grateful appreciation is extended also to Professor Clyde W. Mason, of Cornell University, for his interest and helpful correspondence.

I am indebted to Mrs Belle J. Benchley, Executive Secretary of the San Diego Zoological Society, to Mrs Emily Burlingame, formerly of the Zoological Hospital, to Mr K. C. Lint, of the bird department, to Mr C. B. Perkins, in charge of reptiles at the San Diego Zoo and co-member of the Research and Hospital Committee of the Zoo, and to the late Mr Clinton G. Abbott, Director, and Mr Charles F. Harbison, curator of the entomological collection at the San Diego Museum of Natural History, for valuable information of many kinds and for opportunities to examine and in some instances to retain useful specimens and materials.

For their encouragement and material support, I owe a lasting debt of gratitude to two foundations. The Rockefeller Foundation, in 1938-9 and in 1949, afforded unique opportunities for travel, study and research abroad, in the general field of plant and animal pigments. The John Simon Guggenheim Memorial Foundation, in 1945-6, rendered support to provide a maximum degree of freedom for preparing the manuscript.

To the following among my associates and students I wish to extend special thanks: Miss Elizabeth M. Kampa for executing many of the drawings, charts, and chemical formulae and Table 1 in its revised form, for reading much of the manuscript and for help with the bibliography; Dr Sheldon C. Crane for help with the proof-reading, for copying some of the chemical formulae, for help in copying Table 1, and for aid with the bibliography; Mr Sam Hinton for copying Figs. 2, 3, 14, 21 and 22a; Dr Bayard H. McConnaughey for Figs. 28-33; Miss Leia Mae Jeffrey for help in verifying the bibliography and textual references thereto.

To a number of friends in Cambridge I remain especially indebted, for their never-failing interest, patience and helpful suggestions, notably in the summer of 1949. Of these I wish especially to name Dr C. F. A. Pantin, Dr J. Eric Smith, Dr V. H. Booth, Dr Sidney Smith, Dr Joseph Needham and Dr M. G. M. Pryor.

To Mr R. J. L. Kingsford, Secretary, and his associates in the Cambridge University Press I am deeply grateful for innumerable courtesies and the painstaking care and thoughtfulness with which they have accorded their co-operation in the publication of this work.

This monograph owes much to the willing co-operation of others. Its shortcomings remain mine alone. DENIS L FOX

LA JOLLA, CALIFORNIA, AND CAMBRIDGE, ENGLAND

July and August 1949

CHAPTER I

INTRODUCTION

The elements which constitute the vast realm of colour in nature have, from the earliest days of man, arrested his attention and incited his wonder. Anyone who undertakes the preparation of a treatise on colour in the living world is necessarily confronted with the task of keeping separate the deeply aesthetic and otherwise subjective aspects from the purely physical, chemical and metabolic sides of the topic.

Beyond the universal attraction which colours have always held for primitive peoples, artists, laymen and scientists alike, chromatology is an expanding branch of science, and finds increasingly extensive use as well in such applied fields as medicine, many branches of agriculture, modern warfare, and numerous industries. The physician, who bases many of his diagnostic conclusions upon the colours of tissues and body fluids, is scrutinizing the empirical signposts of various normal or aberrant metabolic functions; likewise the grower has long learned to recognize stages of ripeness or reproductive maturity in his living products by viewing the kind and intensity of their colours, while the experimental or practical geneticist, when concerned with any of various colour manifestations, is in reality studying the heritability of certain kinds of biochemical processes.

Over half a century ago, Miss Newbigin (1898) suggested the significance and potential importance of colour in the functional economy of the organism. She wrote, in part, If we once realize that the colour of an organism is not an isolated characteristic forced upon it, as it were, from without, but may be merely the outward expression of its constitution, we may surely hope not only to be delivered from many laborious hypotheses as to the use of colour in particular cases, but also may perhaps learn something of the physiology of colour.’ Further, she continued, ..there are three reasons why it is desirable that the biologist should concern himself with colour in organisms. The first is the conspicuousness of colour phenomena in a merely objective survey of animals and plants; the second is the relation of these colours to current theories of evolution; and the third is their importance in comparative physiology.’ It is the latter aspect which denotes the field to which this book is devoted.

Probably the most recent general work dealing with the pigmentary colours in animals appeared in the volume by Verne (1926a). Here, we read in his preface, " I believe it unnecessary to insist here upon the importance of the study of pigments. Whatever the branch of biology cultivated, one is bound to become interested in them. The systematist, the physiologist, the anatomist, the cytologist, the chemist, the geneticist, the pathologist must at some time consider, from diverse angles, the pigmentary question. Conceiving this general interest, a pigment represents, according to our definition, only a naturally coloured stage of the metabolism of compounds resulting from reactions, normal or pathological, of living substance. Beginning with this stage, easy to observe because of its physical character, it would seem that one might more favourably penetrate the secrets of this metabolism’ (transí.).

The late Professor F. B. Sumner, long actively engaged in studies of adaptive concealment effected by animals through colour changes, once wrote a useful article entitled: ‘Color and pigmentation: why they should interest us as biologists’ (1937). In it he called attention to the fact that colours and colour-patterns occupy a prominent position in the conspicuous aspects of animals, and that they are often of great importance to biologists in distinguishing between closely related species. He laid stress also upon the chromatic features of animals in relation to the prevailing colours of their habitats, and to climatic factors. Finally, he drew attention to the importance of colour in diverse branches of biology, and concluded, in agreement with others, ‘From whatever direction we approach the study of life, we cannot escape the phenomena of color’.

A vast amount of study has been devoted to secondary values of structural and pigmentary colours in animals. Adaptive coloration, serving in both advertisement and concealment, has long been imitated by man for like purposes. But the comparative primary attributes, physical, chemical and metabolic, of coloured molecules and polymolecular coloured structures, constitute the intended scope of the present work. Secondary biological adaptive values of colour as such can be given only brief consideration here. The reader’s search for far-reaching treatment of chromatic adaptation will be amply rewarded by consulting references such as those given in the preface.

The general make-up and biochemical reactions of protoplasm in the most diverse living organisms are profoundly similar in many respects. Basic factors in the emergence of the vast assortment of living organisms known to-day have lain in the innumerable permutations of linkages between two dozen or more amino-acids in the constitution of countless different species of protein molecules. With coloured molecules the story is simpler, for there exist far fewer classes of pigments of biochemical origin than there are protein variants to go round. So a multitude of organisms again manifest their fundamental chemical similarity by sharing the property of synthesizing coloured molecules of certain classes. Failing such synthetic capacity, some share the compounds by passing them from one to another, i.e. from prey to predator, in varying quantities and stages of alteration. Out of the community of pigments, shades and patterns manifested by diverse organisms, all grades of similarities and degrees of contrast must and do emerge among the possessors themselves and between these and their inanimate environment, of which latter the variations in light and colour are more restricted. Thus we witness to-day many instances of animals’ colours and patterns which simulate certain backgrounds so closely as to render detection nearly impossible; some lizards, fishes, crustaceans and cephalopods even possess the ability to alter their chromatic patterns readily in the direction of concealment against given backgrounds. Again, colour, pattern or shape may have become sufficiently associated with the possession of stings, acrid secretions, etc., in certain insects that predators avoid them. Naturalists refer to such cases as warning characters. Certain edible insect species may survive largely because of chance mutations in colour, pattern or shape (or all three in varying degrees), rendering them closely similar to injurious or distasteful species, and we have then instances of so-called protective mimicry. There is, indeed, extensive survival among species whose conspicuousness is not noticeably offset by any means of defence against enemies. Such animals may succeed by fleetness of escape, by sheer weight of numbers, by frequency or high fecundity of reproductive periods, or by any of a number of other biological adjustments.

Those who look askance upon the idea of survival value in adaptive coloration seem to have confused the issue with early conceptions among some of the older naturalists involving a kind of supposed ‘planning’ or ‘purposefulness’ on the part of those animals which exhibit the ability to so modify their appearance.

In the course of evolution there have occurred mutations involving the localization of certain quantities and kinds of carotenoid and melanin pigments within specialized photo-receptive areas. In some groups these areas have evolved into highly specialized eyes, whose value in finding food and in avoiding enemies is amply manifest. Similarly, many species have inherited and elaborated upon an original mutational ability to mobilize these same pigments and others into aggregates within specialized cells of the integument. Here the pigmentary material may rest in a relatively stable condition, or it may be somewhat concentrated in integumentary products such as hair, scales, feathers, chitin and the like. In certain seeing animals, the degree of disperson of pigment within specialized chromatophore cells of the integument is altered rapidly under nervous control. It is furthermore known that, in some fishes, reptiles, crustaceans and cephalopods, there are direct physiological and even biochemical relationships between the eyes and the physical status and quantities of chromatophore pigments. Of great interest in this connexion are Sumner’s researches (1945) on differences in relative quantities of melanin and of guanine in fishes exposed to different photic environments.

We are aware that the potentially functional eyes of fishes living at great ocean depths can serve the owners’ economy to only a slight degree, if in some instances at all. It is also certain that extravagant coloration in the same and other numerous benthic species can fulfil no useful purpose, per se, in the natural habitat. Nor is conspicuous coloration of recognizable value, per se, in many species inhabiting lighted regions. But in spite of all this, nobody will deny the manifest usefulness and survival value that has resulted from the evolution of eyes. Similarly, no one who has given study and thought to the great weight of evidence (see Cott, 1940) can doubt the important role of chromatic adaptation in the preservation of certain animal species against capturé by eyed predators.

Further advances in this general field of study will be possible only with an accurate knowledge of the chemical and physical systems which give rise to colour in animals, and with the use of discriminating terms for the classification of the various coloured bodies. The purposes of this book are to review and classify this knowledge.

In the first place, colour in animals concerns two distinct levels of morphology. Thus the chemical or pigmental colours involve the absorption of one or more fractions of white light by the electronic activities of special molecular types. So-called physical or structural coloration, on the other hand, is evoked by materials which, although they may possess no pigmentary constituents, involve ultra-microscopic dimensions lying within the range of visible light (i.e. 0-4-0-7u), and thus disintegrating the component rays.

For reasons explained below, I refer to the true pigmentary compounds of organisms as biochromes, while the several classes of physical structure capable of producing colour are designated as schemochromes.¹ In this book the schemochromes will be discussed in Chapters III, IV and V, while the longer treatment of the various types of pigmentary molecules will occupy the remaining nine chapters. It should be emphasized at once, however, that the production of conspicuous schemochromic colours depends upon underlying light-absorbing deposits of biochromic material, and that this is usually melanin.

To return to the classes of coloured compounds, Sumner (1937) pointed out that the word ‘pigment’ is frequently applied to a material which, by design or by chance, imparts the quality of colour to something else, whether manufactured or naturally occurring. A discriminating scientific term to include the natural colouring matters has become very desirable, especially to the biochemist, whose interests are concerned with the origin and metabolic significance of this diverse group of compounds. The self-explanatory word biochrome, with its cognates biochromic and biochromy, has therefore been proposed (Fox, 19446) and has been employed in other publications. It is not anticipated that the appearance of the new term in the title of the present book will have mystified many readers before they have opened it.

We classify the great chemically and physiologically heterogeneous group of enzymes, vitamins and hormones under the collective term of biocatalysts, which have in common their biological origin and their property of regulating various physiological functions. The biochromes comprise a group of coloured molecules whose chemical and physiological heterogeneity is as great as that existing among the biocatalysts. Like the latter, the biochromes are similar in two common characteristics, namely, their origin and occurrence in living organisms and their possession of another fundamental chemical property, i.e. a degree of molecular resonance (see below) resulting in selective absorption of light waves within the visible spectrum. White, and likewise black, biological products provide exceptions to the second distinctive property, but their inclusion among the biochromes appears to be reasonable, and will be discussed later.

We shall, in later pages, consider molecular resonance, visible as colour, called forth by varying kinds and degrees of chemical unsaturation or unfulfilled valency. " In many instances, unsaturated chromophoric groupings may impart both colour and increased reactivity or chemical instability to the same molecule. Such compounds may therefore assume more readily important biochemical roles (e.g. carotenoids, tetrapyrroles, flavins, some pterins and certain quinones) or may constitute representative by-products of special metabolic processes (e.g. bile pigments, melanins, indoles and certain pterins). Colour and biochemical activity are, in such instances, two interlocked effects of the same fundamental molecular phenomenon* (Fox, 1944b).

In recent years, biochromic compounds have received increasing recognition as physiologically active products of living tissue. Some biochromes are also biocatalysts, e.g. chlorophyll, cytochrome and other haem compounds, lactoflavine, xanthopterin and certain carotenoids.

Table i (end of book) presents, in relatively abbreviated form, a survey of the physical colours and the more common pigments encountered in the animal kingdom. This tabulation makes no pretence to completeness, but has for its aim a kind of representative panorama, suggesting the nearly universal distribution of some structural or pigmentary chromogens, in contrast with the very limited or sometimes apparently haphazard occurrence of others.

We may readily observe, for example, the wide occurrence of structural whites, contrasting noticeably with the more limited distribution of interference-iridescence, and especially with the relative paucity of so-called Tyndall blues. Again, turning now to the pigments proper, the conspicuous carotenoids show universal distribution among animal groups, shared by the less noticeable chromolipoids and porphyrins. The dark melanins exhibit relatively few gaps, while the rare but showy naphtho- and anthraquinones are limited to single respective animal groups within widely separated phyla. While of wide, perhaps universal, occurrence in living organisms, the flavins are encountered in very low concentrations, and rarely contribute a conspicuous yellow colour to an animal’s tissues.

A word of explanation is in order regarding unavoidable gaps and other irregularities in the table. In the first place, the list of animal phyla has been left incomplete because the inclusion of such groups as the Ctenophora (comb-jellies), Nemathelminthes (roundworms, free-living and parasitic), Trochelminthes (rotifers), Chaetognatha (arrow-worms), and Brachiopoda (lamp-shells) would have resulted merely in the multiplication of empty spaces in the table, since so little is known regarding the biochromes of these phyla. On the other hand, it has been desirable, for fairly obvious reasons, to expand a few of the higher phyla into certain of their constitutent classes.

The + signs following symbols designate relative degrees of richness or conspicuousness of the particular colour-yielding factor in the corresponding structure, whether it happens to be visible or internally concealed. Examples are the relatively rich carotenoid stores in the feathers of certain birds or in the digestive gland of the octopus, conspicuous manifestations of certain pterins in the wings of some Lepidoptera and small but extractable yields of these compounds in, say, the urine or liver of mammals. A parenthetic (+) indicates relatively rare, small or sometimes questionable amounts of a substance.

In some instances the + sign alone is given, to indicate that a substance has been reported as present in animals belonging to the group in question, having left unspecified the tissues investigated, or having analysed whole, small organisms. This designation will be observed notably in the chromolipoid column, and to a considerable extent in the porphyrins, flavins and purines. Yellow-brown or dark chromolipoids, chemically distinct from carotenoids and from melanins, may be seen in the reserve fat of many species, and in intracellular lipoid plastids of all species examined. For these reasons, the presence of chromolipoids has been assigned, with a high probability of veracity, to all phyla listed in the table. The same applies to at least one set of compounds in the porphyrin series, i.e. the cytochromes, whose presence has been detected in all species of animals reportedly investigated. The incidence of porphyrins therefore appears in each group, a mere + sign sufficing to indicate that cytochromes are the only representatives of the series actually certain to be found. A similar situation probably applies to the flavins, but it seemed that such an assumption should be treated with a little more restraint at the moment, since published investigations with respect to these compounds are somewhat more limited. Meanwhile, the very wide occurrence of flavins, and the importance of riboflavin in cellular oxidative processes, renders the distribution of this group of biochromes somewhat parallel to, and perhaps equally as arresting as, that of the cytochromes.

Complete gaps occur in some of the spaces provided. These do not necessarily mean that a biochrome is lacking in the particular group, but merely that its detection has not, to the writer’s knowledge, been reported. No negative symbols are risked.

There may arise some question regarding the apparently arbitrary inclusion of the white or silvery purines, such as guanine, among the true biochromes or pigments, rather than with the structural whites which include the calcareous and siliceous skeletal materials. This has been done because the metabolically significant purines are often seasonal in their deposition and resulting conspicuousness (e.g. in the nuptial periods of some insects and amphibians, wherein definite sexual differences in purine metabolism are apparent), and often contribute to the contrasting coloration of an animal’s integument. Again, the purines are related chemically to the pterin biochromes, with which they often occur side by side. The universal presence of purines in nucleoproteins leads to the inclusion of the former in each animal group, although present for the most part, not as visible, deposited material, but internally, in a soluble or colloidal condition. Ideally, it should be admitted that the crystalline purines belong in both groups of colourproducing factors, since their silvery or spectral aspects are manifestations of their physical disposition, as are the blues of some fishes and reptiles, in whose integument turbid, light-scattering layers of guanine overlie strata of dark melanin. Conversely, these same blues, listed under structural colours, could not be manifest were it not for the absorbing light-screen of melanin underlying the various light-scattering media. Likewise, most of the iridescent colours of animals are brought out in their full brilliance by virtue of melanin under-layers. Pigmentary substances of several kinds may in this way implement the manifestation of structural coloration.

The final column, devoted to miscellaneous biochromes, is far short of a complete survey, but is intended to list a number of the more outstanding unusual examples.

1 ¹ This term, from the Greek oxua (= form), was suggested by Professor Ivan M. Linforth of the Department of Classics, University of California.

CHAPTER II

PHYSICAL AND CHEMICAL FOUNDATIONS

OF COLOUR

We view opaque substances by the incident light which is reflected from their surfaces, while transparent objects are seen partly by such reflexion and in part by transmitted light. Colours (referred to in the broadest sense, to include white, black and the numerous intermediate grey tones) manifested by the majority of natural objects have their origin in the absorption of those lightrays which do not emerge by reflexion or transmission (see Wood, 1934; Sumner, 1937; Brode, 1939). A black substance absorbs light of all wavelengths equally and completely; degrees of equal but incomplete absorption of all wave-lengths yield greys, while equal and complete reflexion of the whole spectrum is characteristic of white substances. True colours result from selective absorption of incident light, which, penetrating the surface of the object to a greater or less degree, strikes minute internal structures, subsequently undergoing reflexions, refractions or diffractions, or encountering electronic vibrational fields involving critical frequencies. The unabsorbed residue of light which emerges is of a colour complementary to that which has been spent. For a thorough and attractive presentation of light and its resolution into its wave-components, readers are referred especially to R. W. Wood’s volume, Physical Optics (1934).

The chief physical events which affect incident light as it impinges upon or penetrates ordinary classes of surfaces may be placed in summary form as follows:

(1) Simple reflexion of unbroken white light, provided by many small crystals (either per se, as by calcium carbonate, lead sulphate, etc., or through separation by air-bubbles, as in snow), smooth surfaces, amorphous or other fine powders and colloidal systems in mass.

(2) Refraction into spectral colours by prismatic structures, which retard light-velocity in a progressively increasing degree from the longest (red) to the shortest (violet) wave-lengths. Rainbows are brought about through refraction of light, with internal reflexions, by water droplets.

(3) Scattering, with degrees of plane-polarization, brought about by discontinuities within otherwise simple transparent media. As examples we may consider the blue colour of the iris or of fine smoke, effected by colloidal suspensions of extremely fine particles; the blue of the sky is the result of light-scattering by atmospheric gases and water, as well as by minute suspended solids. Wood (1934) gives additional examples of various liquids, crystals and other solids which may produce scattering under certain conditions to give many different colours. A sufficient increase in the size of the responsible bodies yields merely reflexion of white light in different degrees, or, if the particles possess a colour of their own, then the emergent light will be of the colour complementary to that absorbed.

(4) Diffraction into spectra by gratings, minute openings, or fine striae, which bend light in increasing degrees in proportion to wave-length, i.e. least in the violet and most in the red range. One may find examples of diffraction colours in the rich spectral fraction reflected from the surface of a black phonograph record, or in light seen through very small apertures.

(5) Interference, notably by thin films or laminations, which yield iridescent or changeable colours as a result of asynchrony between the wave-trains reflected from the upper and the lower surfaces of the layers. Examples of this may be seen in the surface of a soap bubble, the strand of a spider’s web, Newton’s rings, produced by a thin layer of oil upon water, or by two pieces of flat glass pressed closely together enclosing an air film, mucous secretions of some molluscs, and the nacreous depositions in their shells.

(6) Absorption, (a) Panchromatic. Light may enter a large dark space through a very small aperture, or may be directed into a mass of microstructures (e.g. charcoal, platinum black or vertical crystals of condensed tellurium), in either case undergoing internal reflexions or refractions to a point of complete or nearly complete extinction, with conversion of the light energy into heat or kinetic motion. Edser (1902) discusses this as calorescence. Total absorption of this kind is exercised by a black body, while grey aspects arise by the reflexion of some of the unfractionated light; a shiny black appearance is produced by some reflexion from a smooth surface.

(7) Selective. The colour of a compound depends upon the selective absorption of incident light within definite wave-lengths, the unabsorbed residue being reflected or transmitted to the eye. This capacity to absorb light selectively is due, in turn, to varying kinds and degrees of chemical unsaturation, or unfulfilled valencies embodied in so-called chromophores or colour-carrying groups within the molecule. These areas of special molecular activity arise from reduction in the relative speed or frequency of motion on the part of one or more pairs of the compound’s many vibrating valenceelectrons. Sufficient modification in the speed of electronic vibration imparts to the whole molecule a degree of special vibratory motion or chemical resonance. And this phenomenon, in the modern view, is the basis of colour manifestation (see Pauling, 1939; Wheland, 1944). A molecule in a resonating condition absorbs, by internal frictional forces, that critical fraction of incident light whose frequency corresponds with the resonance frequency peculiar to the compound. The light thus absorbed is converted into heat, and the residual portion which emerges is of a complementary colour. Thus, if a compound possesses resonance characteristics such that only the shortest visible light-waves are subject to interference and resulting absorption, the violet and parts of the blue spectral region will be screened out, and the compound will appear yellow. Red substances, having slightly longer resonance values, similarly extinguish light in the blue and green, while blue or green compounds result from cancellation of light in the red or orange realm. Black compounds, as we have observed, absorb all light equally and completely, whereas white compounds are characterized by such resonance frequencies as to absorb light only in the invisible regions of the spectrum.

Brode (1943) points out that, as the region of absorption is moved through the visible spectrum from the short (violet) to the long or red end, the succession of deepening colours produced is as follows:

GREENISH YELLOW -> YELLOW -> ORANGE -> RED -> PURPLE -> BLUE -> GREEN.

He reminds us that these colours are not of spectral purity, but include all of the visible field except the absorbed fraction. The yellows, for example, transmit all or much of the visible light except violet and blue; and purple

compounds transmit a combination of blue and red. The observed colour of a compound depends upon the wave-length of the dominant or maximally transmitted fraction of light.

The chart shown in Fig. 1 is a diagram of the approximate ranges of the colours in the visible spectrum. Fig. 2 outlines the regions of absorption which result in the manifestation of the numerous complementary colours. Fig. 3 illustrates the relative sensitivity (at low intensities) of the human eye to the various spectral colours, with a maximum in the green at about 555 mu, and with limits at about 400 and 700 mu, in the extremes of violet and red respectively.

It is probable that eyes and the biochromes involved in vision originated in animals living beneath the surface of the sea. There, the visible fraction of sunlight which penetrates most deeply shows a maximum intensity through a fairly broad band in the blue-to-green portion of the spectrum. In turbid waters, the maximum is not only greatly reduced, but undergoes a shift from the blue into the green region, corresponding rather well with the maximum of the visibility curve (Hulbert, 1926; Walls, 1942; Sverdrup, Johnson & Fleming, 1942).

Since a consideration of the various aspects of animal biochromes will constitute the major portion of this book, structural coloration in animals, involving examples from items (1), (3) and (5) of the foregoing list, will be discussed in the succeeding pages before treating the more extensive information regarding selective absorption, or chemical coloration (Sumner, 1937) listed under item (6b) above.

Before proceeding to the numerous examples of structural colours among animals, let us summarize, in a way, the classes of these schemochromes by elaborating somewhat on an illustration given by Miss Newbigin (1898). The possible modifications of a single substance may be brought out in a number of physical ways without altering its chemical properties, to provide examples of the different classes of physical chromatic effects. Thus ordinary glass in the form of a thin plate is colourless and transparent, showing some surface reflexion) but if it be viewed through its long diameter, or if it be resolved into many fine needles and these be set on end and thus viewed in mass, parallel to their long axes, a considerable degree of selective absorption can be appreciated; glass heated and blown into very thin films may be observed to display the iridescence which is due to interference. When glass is cut to give prismatic edges, we see refraction of light into rainbow colours; if, instead, a series of very fine, closely set parallel lines be cut in the surface of the material, the resulting grating provides the spectral colours of diffraction. If now reduced to fine powder, the same substance appears white by scattered reflexion of whole light; finally, if one were to reduce the size of such powdered particles to dimensions of between 0-4 and 0-7u, and were to suspend these in air, the blue colour of light-scattering with plane-polarization would be manifested.

CHAPTER III

STRUCTURAL COLOURS OF ANIMALS

A. General considerations

in an earlier review (1936ff) the author recalled the fact that colours manifested by animals may be either structural or pigmentary. Animal pigments may furthermore be exogenous or endogenous, i.e. derived from plant food, whether directly or indirectly, or arising from biochemical processes characteristic of the animal’s metabolism.

With the several principal kinds of structural coloration clearly in mind, and with a degree of experience and familiarity regarding structural and pigmentary manifestations in nature generally, it is possible to distinguish readily the basis of a colour in the majority of examples encountered.

A survey of the animal world yields many examples of three, and possibly four, of the principal schemochromatic or structurally coloured surfaces, or the combined effects of two or more physical factors in the manifestation of colour. These three chief types of structural colours are the various whites of total reflexion, the Tyndall blues of scattered reflexion, and the iridescent aspects arising from interference of light by successive surfaces of thin layers. Diffraction by fine striations, ridges or pits in the surface of a structure may be responsible for brilliant spectral flashes sometimes observable in the reflexion of a direct beam of light. Numerous instances are encountered wherein a schemochrome co-exists with a true pigment, the combination producing a resultant colour different from that which would have arisen from the presence of either factor alone, e.g. green resulting from Tyndall blue and carotenoid yellow.

The display of colours through purely structural, non-pigmentary arrangements within various naturally occurring materials was recognized long ago. But until the latter part of the last century relatively little insight had been gained as to the architectural features of surfaces or other structures responsible for colour production in animals. The colours were merely referred to in passing as ‘optical’ or even as ‘prismatic’ in origin. Numerous illustrative examples were given of optical coloration, but penetrative investigations as to the ultimate causative agencies were generally lacking.

Within the past five decades, however, some careful investigators, including physicists, chemical microscopists and histologists, have brought fruitful efforts to bear upon the elucidation of the various fundamental bases of structural coloration. The abounding literature of such investigations deals chiefly with various colours encountered in birds’ feathers, in the wings of Lepidoptera, the elytra or wing-cases of beetles, and the general integument of these and other insects. Notable contributions are those of Poulton (1887), Mayer (1897), Strong (1902, 1903), Mallock (1911), Michelson (1911), Biedermann (1914), Lord Rayleigh (1919), Onslow (1921), von Geldern (iguanid lizards, 1921), the younger Lord Rayleigh (1923), Süffert (1924), Frank & Ruska (1939), Frank (1939), Anderson & Richards (1942), Wigglesworth (1948), and an outstanding series of investigations by Mason (1923 a, b, 1924, 1926, 1927 a, 1929). Further reference will be made to the researches reported by some of the foregoing investigators. First it will be well to consider some of the methods which may be applied to the recognition of structural colours. To determine whether a given chromatic effect arises from the presence of an actual pigment or is due to one or more special architectural features, several physical and chemical tests may be applied. Molecular compounds which absorb certain rays of the visible spectrum and reflect or transmit other components will be affected little if at all by alterations in orientation or other physical conditions, but may be extracted or modified chemically and chromatically by treatment with special reagents. Conversely, the structural colours undergo no far-reaching or permanent changes through exposure to solvents or other non-destructive chemicals, since the actual material in question is commonly constituted of relatively inert or refractory products such as chitin, keratinaceous deposits, melanin, etc. Rather, the structural colours are susceptible to destruction or modification by any mechanical treatment involving alteration of the purely optical system or systems.

Series of distinctive tests for structural colours are given by various authors, and these have been summarized notably by Mason (1926). They may be listed in two general categories, as follows:

(a) Visual or photometric observations

(1) Multiple iridescent colours and specular or non-specular blues and greens are the most commonly occurring ones. Iridescent colours are changeable with the angle of incidence; these and the other structural colours vanish, commonly giving place to brown or grey, when the material is examined by transmitted instead of by reflected light.

(2) All constituents of the incident white light are accounted for, whether in the reflected, scattered or transmitted fractions.

(ft) Physical and chemical treatment

(1) The hue is modified by mechanical pressure, distortion, swelling or shrinking.

(2) Characteristic colours are respectively either (a) destroyed, or (ft) greatly altered by rendering the system optically homogeneous, or nearly so, through permeation with fluid of refractive index equal, or nearly equal, to that of the structure under investigation. These effects are reversible, the original colour being restored upon removal of the foreign fluid.

(3) The colour is not removed or extinguished by bleaching or by any chemical treatment which does not effect destruction, distortion or alterations in thickness of the material.

(4) In instances of purely structural colours, no pigment, save often brown or black, is extractable by solvents or demonstrable by reagents. When melanin is removed by mechanical or chemical means, the undamaged structural blue or iridescent colours in the residual material may be re-emphasized by substituting a new dark background such as Indian ink. Green structures often yield a yellow (carotenoid) pigment to appropriate solvents, whereupon the residual complementary structural blue is manifested, in place of green, by the now carotenoid-free material.

Generally speaking, the converse of the foregoing will be characteristic of true pigments, and distinctions may be emphasized by additional specific chemical and spectroscopic examinations applying to particular compounds.

While the various classes of biochromes will be discussed in considerable detail in later sections, it is still to be remembered that certain of these pigmentary compounds often play conspicuous roles emphasizing or modifying a structural colour. Melanin, for example, plays a widely spread synergistic part in the manifestation of structural blues and greens, by serving as an underlying dark screen for absorbing all light save the scattered fraction. The same dark compound brings out the brilliant changeable iridescent and metallic colours which characterize the interference of light by layers of thin superposed laminations. To employ a commonplace comparison, most of us have been impressed with the increased conspicuousness and splendour of iridescent oil-films when these lie on turbid water or upon a wet, black pavement. Reference has already been made to the conversion of structural blue to green by passage of the scattered light-rays through depositions of yellow carotenoids.

The manifestation of green colour through combined physical and chemical factors is by no means the universal agency of greenness among animals, but is of frequent occurrence, notably in the feathers of birds and in reptilian scales. There are, on the other hand, a few actual green pigments, e.g. in green feathers of the touraco, in some egg-shells, in certain aquatic invertebrates, and in the skin and skeletons of certain belonid and cottid fishes. Again, there are purely structural, non-pigmentary greens and blue-greens, arising from interference of light by multiple thin laminae, encountered especially in the elytra or wing-covers of many beetles, and in the integument of certain flies and wasps. Examples of the foregoing will be given below.

We shall consider in order the several classes of schemochromes as exhibited by animal species. These are: the whites of totally scattered reflexion, the blues of partially reflected scattering, the iridescent and ‘metallic’ colours of interference, and the less frequently noticed spectral effects of diffraction of direct light-beams by minute striations or natural gratings.

B. Whiteness

The various white aspects, resulting from simple, scattered reflexion, arise commonly as effects of the heterogeneous orientation of colloidally dispersed matter, or from solid colourless materials in relatively concentrated, thick layers. In the colloidal class are included gas-in-solid, gas-in-liquid, liquidin-liquid and solid-in-liquid systems.

White, gas-in-solid systems are widely exemplified by white feathers and by white hair, including the fur of polar and of albino mammals, etc. Microscopically, a strand of brown hair shows the long pith cells filled with brown, diffuse melanin, while in a white hair the pith is a column of froth, whose air-bubbles are separated by solid, keratinaceous cell detritus. The white appearance of the body and tentacles of certain otherwise pigmented sea anemones may be due partly, but by no means wholly, to a somewhat similar condition, although here the system is a gas-in-liquid one. The effect can be produced artificially in anemones by injecting into the gastrovascular cavity a dilute solution of hydrogen peroxide; the enzyme catalase in the animal promptly begins to decompose the peroxide, setting free very fine bubbles of oxygen whose presence then gives rise to white areas in the lumen of both scapus and tentacles, especially at and near the tips of the latter. After some of these fine, white-reflecting aggregates have coalesced to give larger bubbles, these may be set free by squeezing the tentacle at the tip, rupturing the wall. In the plant kingdom, many white blossoms and similar areas on leaves arise also from the presence of finely dispersed intercellular vesicles of air, separating colourless material. These gaseous bubbles may be seen readily in the so-called aerenchyma of petals under the lower power of the microscope. Pressure on the cover-glass squeezes the tissue free from air, which coalesces into large bubbles, while the tissues become transparent.

Turning to white liquid-in-liquid colloidal systems (emulsions), we find that these are widely distributed throughout the animal kingdom. To cite a few examples, fat or protein (or both) in pale integument or in milk, reserve lipid deposits and the like are outstanding emulsions. Serous membranes, such as those of the swim-bladder of fishes and the peritoneum of many animals, likewise connective tissue, white muscle and much nerve tissue might be added to this list, although some of the latter may actually partake more of the characteristics of the solid-in-liquid class.

In the class of white solids are some inorganic compounds, e.g. silica in diatom tests and in skeletons of hexactinellid sponges, calcium carbonate in Foraminifera, in calcareous sponges, corals, echinoderms, countless mol- luscan shells, crustacean carapaces, the shell material of birds’ eggs, etc. Calcium phosphate is present in the skeletons of vertebrate animals. Organic compounds, which will be discussed in greater detail in the pigment section, are responsible for some white-reflecting surfaces; examples are crystalline guanine and similar purines in the leucophores of many invertebrates, fishes, amphibians, and in the scales of reptiles and the wings of Lepidoptera. The shining white ventral skin of many fishes and amphibians are especially rich in such material. White crystalline uric acid is present in the endoderm of the plumose anemone Metridium senile (Fig. 4), and probably in other coelenterates. The chemically allied pterins, e.g. leucopterin, in the white markings of butterflies’ wings are additional examples. Elastin proteins constitute the sclerotic coat of the eye, while keratins form the material of nails or claws, horns, hoofs and beaks, all of which structures, in the absence of any true pigment, appear white, palely translucent, or faintly yellow.

Mason has devoted considerable study and discussion to the whiteness of animals, with particular reference to feathers (1923 a) and insects (1926). He reminds us that white is produced when light is reflected and refracted at the surfaces of very small, optically heterogeneous bodies or spaces, whether the material involved is coloured or not. Opaque brown or black compounds, such as melanin, are exceptions, since their presence always confers resulting degrees of brown or grey. The barbules of numerous typical white feathers, Mason points out, are without significant internal structure, and may be rendered immediately transparent and nearly invisible to the eye if immersed in media such as balsam or cresol, since fluids of refractive index

closely approximating that of keratin (»=1*54) eliminate the reflexion and refraction of light from the vast number of minute surfaces which are ordinarily responsible for the white appearance.

Similarly, exposure of the barbs or shafts of many white feathers to such fluids as cresol gradually effaces the reflected white, rendering the structure transparent as the medium in this case slowly replaces the air in the innumerable enclosed pores of the layer of so-called box-cells or alveolar cells.

Mason (1926) discusses the different grades of whiteness which may be observed, notably among insects. He points out that nearly all white appearances in insects are to be explained on the basis of unordered reflecting and refracting surfaces of very low transparency. Such structures may be deprived of their white reflectivity and rendered optically homogeneous by replacement of the air, as in feathers, with a fluid of refractive index closely approximating that of the white material. If