October 14, 1920] 



NATURE 



217 



ing- the brilliant gold beetle Flusiotis resplendens, 

 no effect is at first seen, except perhaps a slight 

 dulling of the lustre. The golden surface, scarred 

 and scratched by the polishing crystals, remains 

 unaltered for a period, which appears to depend 

 on the thickness of the cuticle. Then, at a 

 certain depth, the gold is suddenly replaced by 

 a metallic magenta, and where this is seen to 

 shine through a very thin film of gold, a silvery- 

 blue effect is produced. On continuing to polish, 

 the magenta rapidly disappears, revealing the 

 black underlying chitin. These changes may be 

 closely imitated by gently polishing a thin film of 

 gold deposited electrolytically on copper. Similar 

 effects are also seen when a copper sheet is gently 

 heated in the steam-oven until films of oxide are 

 formed. It is worth mentioning that Mallock has 

 shown, by means of polishing experiments, that 

 the coloured films of steel oxide are not due to 

 interference in the manner usually supposed. 



In the case of these beetles it may be objected 

 that the colour of a thin film would change if its 

 thickness were reduced by polishing in the above 

 way. The change of colour, however, would be 

 gradual, and not abrupt as is the case. More- 

 over, the sequence of colours would be different 

 from that which actually occurs, as, for instance, 

 when bluish-green and green wing-cases change 

 to crimson or scarlet. 



Some Iridescent Colours which Depend on 

 Moisture. 



The small beetles, forming the group known as 



Tortoise Beetles, throw considerable light on the 



question of surface colours by their behaviour in 



various fluids. Many of these beetles are golden 



when alive, or when preserved in spirit, but brown 



when dry. The colour, however, returns on 



soaking them in alcohol, but not in all other 



organic fluids. Section 4 (Fig. i) is cut from 



a wing-case which has a thin cuticle, c, and is 



in other respects almost exactly like that of 



M ordinary scaleless beetle that retains its 



lour after drA'ing. Moreover, the colour 



lines not disapjjear under pressure, but here 



I he resemblance ceases, for, curiously enough, 



tlie golden glitter remains, even after the cuticle 



has been polished away, so long as the surface 



is kept moist. In fact, almost the whole wing- 



' asc may be ground away before the metallic 



lustre disappears. It is obvious, from this 



■ urious behaviour, that the whole thickness of the 



surface layer of chitin is concerned, when in a 



nioist condition, in reflecting a lustre, somewhat 



in the same way as pebbles on a beach glitter in 



■ I' sunshine when washed by an advancing wave. 



ven vertical cracks in the chitin are seen to have 



ilden sides. The reason for this behaviour is 



:v:\r. When dry, aqueous alcohol and certain 



fluids can penetrate the cuticle, but other organic 



fluids cannot. The colour does not return immedi- 



■'■ly after immersing the beetle, but it must soak, 



cause the protecting cuticle is pervious only with 



me difficulty.^ For the same reason, once it is 



NO. 2659, VOL. 106] 



wet, the colour takes some time to disappear, 

 because the cuticle prevents the moisture from 

 evaporating, as it must no doubt do when the 

 beetle is alive. The fact that the whole thickness 

 of the chitin layer reflects metallic colour suggests 

 that, in scaleless beetles, colour may be due to a 

 skin of chitin having the same properties as that 

 of the Tortoise Beetles, but with the surface so 

 polished that moisture is not necessary for the 

 development of the colour. 



Some of the most interesting beetles of this 

 group are those which, though colourless when 

 dry, develop a brilliant iridescence on being 

 moistened with a wet brush. There is a re- 

 markable bug with these properties, named 

 Pycanum rubens, which is a bright apple-green 

 when alive or in spirit, but a dull purple-brown 

 when dry. Experiments were made in various 

 fluids, and it was found that in alcohol, which 

 was completely dehydrated by metallic calcium, 

 immersion produced no colour. But the slightest 

 trace of water caused the colour to return, and 

 dilute acid had the effect of making it more or less 

 permanent. Pressure experiments showed that 

 there was no surface colour due to a molecular 

 structure, because the colour completely disap- 

 peared, to return again as soon as the pressure 

 was removed, just as the colours of thin films 

 should do. These and other observations made 

 it probable that the colour arose in a thin mem- 

 brane with, like gelatin, a specific power of absorb- 

 ing water, but not other organic solvents. When 

 dry, the membrane must be too uneven to cause 

 regular reflection, but as soon as it is swollen with 

 water it can give the colours of thin films. 



The third example of this type of iridescence 

 was found in certain metallic ticks first described 

 by Prof. Nuttall. When dry, they are a dull 

 ochre, and in most anhydrous fluids a matt silver, 

 like freshly cut aluminium, but in aqueous fluids 

 they show bright metallic colours, both green and 

 red. Reference to a section of the scutum, 6 

 (Fig. i), shows that a protective cuticle, c, covers 

 a thick layer of chitin interpenetrated by innumer- 

 able tiny air-canals, h. It is probable that, when 

 dry, the white light reflected from this air renders 

 all structural colours invisible ; but when an 

 aqueous fluid, which can penetrate the cuticle, 

 fills these canals, the dark background produced 

 clearly shows up the colours which may, or may 

 not, be due to an absorbent membrane, as in the 

 case of P. rubens. 



In the cour.se of these investigations many other 

 very interesting iridescent structures were ex- 

 amined, for a description of which there is here 

 no space. Many of these revealed no adequate 

 colour-producing structure, and they present a 

 most interesting field for further research. Among 

 the most striking objects are the green metallic 

 wings of certain bees and dragon-flies, such as 

 some of the Carpenter bees, and Neurobasis 

 chinensis. These very brilliant and thin structures 

 behaved in many ways like the wing-cases of 

 scaleless beetles. There arc also the iridescent 



