2l6 



NATURE 



[October 14, 1920 



and transmitted colours disappear when scales are 

 immersed in fluids of a highly refractive index, 

 and that all colours vanish when scales are sub- 

 jected to pressure, as could scarcely be the case if 

 the colours were due to some molecular structure, 

 such as a film of aniline dye. 



Now in the case of scaleless beetles and in many 

 other iridescent structurep, including bees and 

 dragon-flies with bright, metallic wings, (i) the 

 colour does not disappear on exerting pressure; (2) 

 the reflected colour does not disappear on immer- 

 sion in fluids of high refractive index, even when 

 penetration is facilitated by a vacuum ; and (3) 

 the transmitted colour, so far as this can be seen, 

 also persists. It is worth noting that the data 

 which Michelson relies on to show the similarity 

 in the behaviour of polarised light, when reflected 

 from iridescent insect structures and films of 

 aniline dye, fit the wing-cases of beetles much 

 more closely than they do the wings of butterflies 

 or the feathers of birds. 



Sections i and 2 of Fig. i show typical irides- 

 cent wing-cases. Section i is that of the common 

 green Rose Beetle {Cetonia aurata) and section 2 

 that of a beetle with a peculiar sheen {Anoniala dus- 

 sumieri), due to the numerous dome-shaped pro- 

 tuberances, b, seen in section. The surface of the 

 chitin is protected by a thin cuticle, c, o-^fi thick, 

 but there appears to be no structure likely to 

 produce colour. It is clear that this film cannot 

 cause the colour, because the wing must have a 

 protective sheath of some sort, otherwise the 

 colour would disappear as soon as the surface 

 came into optical contact with a refractive fluid. 

 Consequently it is very important to determine 

 exactly at what depth the colour-producing layer 

 is situated, in order to decide whether there is 

 room for an adequate structure to exist above it. 

 For this purpose a wing-case was carefully 

 polished under the microscope with a paste of the 

 finest carborundum and cedar-wood oil. On re- 

 moving the surface-layer, a very remarkable 

 change of colour was observed to have taken 

 place. Sections were cut from suitable portions 

 of the polished area, the exact colour and position 

 of which could be determined. A composite 

 picture of three different sections is shown in 3 

 (Fig. i). To the extreme right of section o the 

 cuticle, c, is untouched, but further on it is partly 

 removed, and the lowest point seems to have been 

 reached near b, in section n. Somewhere here the 

 colour changes to magenta, but as soon as the 

 irregular dark line dividing the cuticle from the 

 lower layers is passed all colour vanishes, and the 

 black chitin is exposed, as at a, in section m. 

 Thus it appears that this line- running underneath 

 the cuticle is the seat of the colour-producing 

 layer, and its extreme depth is about 05^ below 

 the surface. This figure, which was obtained with 

 the microscope, was checked by an indirect method 

 of measurement, and the two results were found 

 to agree with sufficient accuracy. 



There appears to be a choice between two alter- 

 natives : (i) the colour may be due to a single 

 NO. 2659, VOL. 106] 



thin film, which muSt lie under a protective sheath 

 of some description, or (2) it may be produced by 

 a layer having properties similar to an aniline 

 dye. Although it is possible to show that under 

 suitable conditions single films could, by inter- 

 ference, produce colours as bright as those shown 

 by beetles, the second alternative appears the 

 more probable. This is principally because of the 

 exceedingly small distance between the surface of 

 the wing-case and the lowest limit of the colour- 

 producing layer. Within the space of 0-5/1, room 



Fig. 1. 



/, Surface layer, or '• Emailschichi" of Cetonia aurata. 

 Cy surface cuticle. 



2, V^ing'CASc of Ati^mata dussumztri. c, cuticle; i, knobs or 

 bosses. 



5, Composite section of Pliisiotis respUndens after polishing. 

 »«, underlying chitin : «, magenta portion : o^ unpolished portion ; 

 a, black chitin ; b, magenta film ; c, surface cuticle. 



4, Wing-case of Thlaspidomtrrfiha baiyii. c, surface cuticle. 

 St^^^^ °^ ChrysocJiioris awea, f, imbricated scales. 



6, Scutum of lick Amhlyomma hetiraeum. c, surface cuticle; 

 h, black hair-like canals filled with air. 



These sections were all drawn tothe'scale fx=t mm. with Zeits 

 2 mm. apochromat, N.A. i'4, and Comp. Oc. 



must be made both for the protective cuticle and 

 for the thin film itself. This might, indeed, be 

 just possible, but is unlikely, because of the 

 peculiar sequence of the colours caused by reduc- 

 ing the thickness of the iridescent film in polishing. 

 This appearance is difficult to describe, and 

 must be seen to be appreciated fully. On polish- 



