September 30, 1920] 



NATURE 



149 



The Iridescent Colours of Insects. 



By H. Onslow 



Thin Films." 

 of the iridescent 



I. — The Colours of 



IT is strange that the cause 

 hues of insects and other animals should to 

 a great extent still remain one of the unexplained 

 problems of optics : theories have been advanced 

 without end, but so far not one that is completely 

 satisfactory. It is very significant that Prof. R. W. 

 Wood, in speaking of certain metallic films the 

 bright colours of which may be due to an ex- 

 ' cdingly fine state of division, remarks: "There 

 .ippears to be a large number of cases in which 

 brilliant colours are shown which cannot be 

 xplained by any of the common laws of optics 



ith which we are familiar. As far as I am 

 aware, no very satisfactory explanation has ever 

 been given of the colours of certain feathers and 

 butterflies, and I strongly suspect there is some 

 action of absorbing matter, in a state of very fine 

 division, upon light waves, which is not yet com- 

 pletely understooid." 



This opinion is given in spite of the fact that 

 no less an authority than Michelson declares that 

 all the colours in question are surface colours ; 

 that is to say, they are due to selective metallic 

 reflection, like the coloured surfaces of aniline 

 ,, dyes and metals. Nothing further is required to 

 1. show how chaotic and contradictory is the present 



ate of knowledge on this subject. 



A discussion of the merits of the rival theories 

 would not here be in place. ^ There will not, 

 however, be much danger in venturing to predict 

 that the almost infinite variety of iridescent 

 colours depends upon every possible factor which 



ri produce such colours. Neglecting the metallic 



ms of Prof. Wood, just mentioned, and ana- 



L.'^ous cases, it is clear that the colours of insects 

 niist be caused in one of the following ways : — 



fi) Pigmentation. 



(2) Interference of light by "thin films" (as in 

 the case of soap-bubbles). 



(3) Diffraction of light by "gratings." 



(4) Dispersion of light by prisms. 



I f.S) The scattering of light by small particles 

 I (as in the blue of the sky). 



^ (6) Selective metallic reflection (as in metals and 

 I aniline dyes). 



In dealing with iridescent colours, the first 



possibility may be neglected, though pigments, 

 \ such as yellow, arc often found combined with 



structural colours, to form green and so forth. 



The Interference of Light hy "Thin Films." 

 This is well known to be the cause of the colour 

 of soap-bubbles and oil films, .nnd it certainly 

 tteems to offer one of the Ix-st oxplan.Ttions of the 

 Mridcsrence of insects. The late F.ord Rnylrigh has 

 |»hown how the objections of Michelson may be 

 Jmet by postulating films of a peculiar structure. 



• "On ■ PtrioHic SirKtur* In Cttu'ta ln<«ct S<r»l»«, iikI th« OiUM of 

 Mr ItolrKcni Colours." Bjr H. Orwiow. K<W (Mfor* ih* Royal ScciMy 

 I Jani<»ry ig, ttttn 



NO. 2657, VOL. 106] 



Also Biedermann has shown that when iridescent 

 scales are placed in a highly refractive fluid, all 

 colours disappear. This could not possibly occur 

 if the colour were due to a substance resembling 

 aniline dyes, for in these circumstances its 

 body-colour would inevitably be seen by trans- 

 mitted light. If, however, the colour were due to 

 thin films of air separated by layers of chitin, this 

 loss of colour is exactly what would be expected 

 when the air-spaces were filled by a liquid ; for the 

 periodic structure would become a continuous 

 medium. Moreover, Mallock has described how, 

 by applying gentle pressure to scales, the colour 

 fades, sometimes tending to return when the pres- 

 sure is released. Naturally, if there are plates of 

 chitin separated by air-films, such pressure w'ould 

 alter the spacing between the plates, and thus 

 cause the colour to fade. 



Since the minute structure of iridescent scales, 

 etc., had never been examined, the present writer 



Fig. I. — Omitkefiltra ^stidfin j. Body yellow, wings emermld-green 

 And black, (i naiural size.) 



has carried out an extensive microscopical inves- 

 tigation in order to discover whether any light 

 could be thrown on the question by this means. 

 The varied types of the different structures found 

 fully justified the expectation that a number of 

 factors were involved, but though the colours of 

 many scales could be accounted for by well-known 

 laws, in other cases no explanation appeared 

 adequate. A few instances will therefore be 

 selected to illustrate the main types of iridescent 

 colours found in insects. 



Morpho menelaus. — The brilliant blue-green of 

 this wonderful insect is well known. It is given by 

 two layers of scales — section la, Fig. 2, the pale 

 blue upper layer, which shows the anomaly of ap- 

 pearing blue both by transmitted and by reflected 

 light (Natitre, vol. ciii., p. 84, April, 1919), and 

 section ib, Fig. 2, the deep blue lower layer of 

 scales, which bears a very curious periodic struc- 

 ture. This structure, best seen in transverse sec- 

 tions of the scales, is shown in Fig. 2, tb. The films 

 of transparent chitin, a, here appear as pillars, and 

 between them there arc films of air, af. Seen in 

 longitudinal section, these pillars become long, 



