NATURE 



289 



THURSDAY, JULY 24, 1890. 



T' 



THE COLOURS OF ANIMALS. 



The Colours of Animals: their Meaning and Use 

 especially considered in the case of Insects. By 

 Edward Bagnall Poulton, M.A., F.R.S., &c. With 

 Chromolithograph Frontispiece and Sixty-six Figures 

 in Text. (London : Kegan Paul, Trench, Triibner, 

 and Co., Limited, 1890.) 

 HIS new volume of the International Scientific 

 Series gives an excellent summary of the most 

 recent researches as to the varied uses of the colours of 

 animals, and more especially of those admirable observa- 

 tions and experiments on variable protective colouring 

 with which Mr. Poulton's name is associated, and which 

 mark an era in this branch of natural history. The 

 main outlines of the subject are so well known, both to 

 naturalists and to general readers, that it will only be 

 necessary here to indicate some of the more important 

 of the matters now first treated in a popular work, and 

 to make a i&s'i remarks on some of the more difficult 

 problems discussed in the volume. 



The first chapter gives a short but very clear statement 

 of the physical cause of animal colours, and contains 

 some valuable observations on the effect of thin films of 

 air or of hquids in the production of iridescent colours. 

 In some cases dried insects lose some of their metallic 

 colours, but these reappear when the specimen is dipped 

 ^ in water. Even living beetles have been observed to 

 k lose their lustre after hybernation, and to regain it after 

 a drinking water. Then we have a sketch of the general 

 I uses of colour to animals, and it is shown that the 

 frequent dark colour of arctic insects has probably a 

 physiological use in enabhng them to absorb as much 

 heat as possible during the brief period of their existence 

 under an arctic sun. This is supported by some direct 

 observations ; but the further suggestion that the white 

 colour of so many arctic birds and mammals has also a 

 physiological use in checking the loss of heat through 

 radiation is less satisfactory. Not only is there no 

 evidence to show that the loss of animal heat is at all 

 influenced by the colour of the fur or feathers, but it is 

 evident that the same result could be brought about by a 

 very slight increase in the texture or thickness of the 

 covering, such as actually occurs in all arctic animals. 

 In the seventh chapter there is a very interesting dis- 

 cussion on the way in which the white winter coat of 

 arctic animals is produced, and it is shown that in the 

 American arctic hare the brown hairs of the summer 

 coat turn white at the tips, while a quantity of new white 

 hairs grow among them, producing at once the thicken- 

 ing of the coat for warmth and the change of colour for 

 protection. That this last is the only function of the 

 colour is well indicated by the case of the raven, which 

 is found in the extreme north of the polar regions, even 

 during the most intense colds of winter, wherever the 

 reindeer and musk-sheep range. Yet it is here as black 

 as elsewhere, although the occasional occurrence of pied 

 and even of white ravens in various parts of Europe and 

 America shows that a white race could be produced if 

 NO. 1082, VOL. 42] 



that colour was of any advantage to the bird in its arctic 

 habitat. 



Two chapters are devoted to a subject which Mr. 

 Poulton has made especially his own, the variable protec- 

 tive colouring of insects. This was first noticed by the 

 late Mr. T. W. Wood, the well-known natural history 

 artist who furnished many of the best illustrations for Dar- 

 win's " Expression of the Emotions in Man and Animals," 

 and the result of his experiments were brought before the 

 Entomological Society of London in 1867. Since then a 

 few other observations have been made by sever.1l 

 naturalists, but little was known of the extent or of the 

 exact causes of the adaptation till Mr. Poulton carried 

 out his experiments for several years in succession, and on 

 so extensive a scale that in one year over 700 larvae of the 

 small tortoiseshell butterfly {Vanessa urticce) were ob- 

 served under various surroundings, and the colours of 

 the resulting chrysalides recorded. In this way pup^e 

 were obtained varying from black to nearly white or 

 metallic golden colours, in each case corresponding 

 more or less closely to the coloured surfaces on which 

 they were suspended. By changing the coloured sur- 

 roundings at different stages of the process, and by 

 blinding some of the larvae, it was ascertained that the 

 period of susceptibility is the quiescent stage just before 

 the change to the pupa state, and that in this case vision 

 has nothing to do with the change of colour. By a 

 number of ingenious experiments, it was ascertained that 

 the whole surface of the skin is sensitive to the action of 

 variously-coloured light, and the effect on the pupa-skin 

 is produced, not directly, as by some photographic action, 

 but by a physiological process acting through the nervous 

 system. In some cases even the cocoons spun by the 

 larvae are modified by the surrounding colours ; and still 

 more curious changes are effected in the larva itself 

 when, as in so many cases, the same species feeds on 

 several plants having differently-coloured leaves. Even 

 the presence of numerous dark twigs has been shown to 

 cause a corresponding change of colour in the larva of 

 the peppered moth {Amphidasis betularia). These two 

 chapters afford a beautiful example of a very difficult and 

 interesting inquiry leading to an explanation of some of 

 the most curious colour-phenomena in the animal king- 

 dom. Mr. Poulton points out the essential difference 

 between this mode of colour-adjustment and that of the 

 chameleon, and of some Crustacea, frogs, and fishes, which 

 can rapidly adjust their colours to new surroundings, in 

 the following passage : — 



"The essential difference between the two kinds of 

 adjustment is that, in the one case, the pigmented part 

 of certain cells contracts in obedience to nervous stimuli, 

 and thus alters the general appearance ; while in the 

 other case the coloured part is actually built up of the 

 appropriate tint, or loses its colour altogether and be- 

 comes transparent in obedience to the same stimuli. 

 The frog or fish has a series of ready-made screens 

 which can be shifted to suit the environment ; the insect 

 has the power of building up an appropriate screen. In 

 many cases, however, the green colour of caterpillars is 

 due to the ready-made colour of the blood, which be- 

 comes effective when pigment is removed from the super- 

 ficial cells, but which disappears when the latter are 

 rendered opaque. Here, however, the superficial cells 

 form the screen which has to be built up, or from which 

 the colour must be dismissed ; and in certain species 



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