1887.] between certain Pupce and their Surroundings. 103 



vol. 38, 1885, p. 279), and show an upper thin layer, and a lower much 

 thicker, finely laminated layer, which is also striated vertically to the 

 surface. With Professor Clifton's kind assistance I have been able to 

 show that the appearances follow from interference of light, due to 

 the presence of films of liquid between the lamellae of the lower layer. 

 The microscope shows brilliant red and green tints by reflected light, 

 while in transmitted light the complementary colours are distinct, 

 but without brilliancy. The latter colours are seen to change when 

 pressure is applied to the surface of the cuticle, and when the process 

 of drying is watched under the microscope, owing in both cases to the 

 liquid films becoming thinner. In the dry cuticle the solid lamellae 

 probably come into contact, and prevent the admission of air, which, 

 if present, would cause even greater brilliancy than liquid. The 

 spectroscope shows broad interference bands in the transmitted light, 

 which change their position on altering the angle of incidence of the 

 light which passes through the cuticle. Precisely similar colours, 

 metallic on reflection, non-metallic and with the complementary tints 

 on transmission, with the same spectroscopic appearances and changes 

 induced by the same means, are seen in the surface films which are 

 formed on bottle glass after prolonged exposure to earth and moisture. 

 In the alternating layers of the pupa the chitinous lamellae are of higher, 

 the liquid films of lower refractive index, hence water or alcohol pro- 

 duce brilliant appearances, while liquids of higher refractive indices 

 produce less effect* 



It is very interesting to note that this most specialised means of 

 producing colour is probably derived in the most simple manner from 

 the ordinary lamellated layer of other non-metallic pupae (e.g., 

 P. machaon) in which the lamellae merely act as reflectors, so that the 

 pupa is brightly coloured by absorption due to pigment contained in 

 the outer lamellae only, and hence traversed twice by a large part of 

 the incident light. 



The dark pupae of V. urticce, and the dark parts of the brilliant 

 pupae, contain abundant pigment in the upper thinner layer only, 

 which therefore acts as a screen, and shuts off light from the lamel- 

 lated layer below, thus preventing the metallic appearance. In the 

 brilliant pupae this layer is transparent, and of a bright yellow colour, 

 and doubtless assists in producing the yellowness of the golden 

 appearance by absorption of light. The two layers are of different 

 chemical constitution, for the upper will not stain in logwood, while 

 the lower does so without difficulty. 



9. The Biological Value of the Gilded Appearance. — Mr. T. W. Wood 

 suggested that the appearance was so essentially unlike anything 

 usually found in the organic kingdoms as to protect the organisms 

 possessing it. Others have thought that it has the value of a warning 

 colour, indicating an unpleasant taste. It is probable that it is now 



