1887.] on Oilded Chrysalides. 35 



exposed to any one colour the more does the pupa follow its influence. 

 Particoloured pupae were not obtained, thus probably pointing 

 towards the action of the nervous system rather than towards the 

 direct action of light on or through the skin itself. 



6. The Nature of the Effects produced. — The colouring matter of 

 the dark pupae is contained in a thin superficial layer of the cuticle ; 

 below this is a thicker layer divided into exceedingly delicate lamelljB 

 between which fluids are present, and the latter form the thin plates 

 which, by causing interference of light, produce the brilliant metallic 

 appearance. The thinner upper layer being dark, acts as a screen in 

 the dark pupae. Precisely the same metallic appearances are caused 

 by the films of air between the thin plates of glass which are formed 

 on the surface of bottles long exposed to earth and moisture. Both have 

 the same spectroscopic characters and the same transmitted colours 

 (complementary to those seen by reflection). The brilliancy of the 

 cuticle can be preserved in spirit for any length of time; it disappears on 

 drying, but can be renewed on wetting (this had been previously kno vn), 

 and the colours are seen to change during the process of drying, and 

 when the cuticle is pressed, for the films are thus made thinner. The 

 same lamellated layer exists in non-metallic pupae, and is used as a 

 reflector for transparent colouring matter contained in its outer 

 lamellae. Thus the structure which rendered possible the brilliant 

 effects due to interference, probably existed long before these special 

 effects were obtained, and was used for a different purpose. 



7. The Biological Value of the Gilded Appearance. — It is probable 

 that the gilded pupee of Yanessidae resemble glittering minerals such as 

 mica (which is very common in many places) ; their shape is very angu- 

 lar, and like that of minerals : conversely the grey pupae resemble grey 

 and weathered rock-surfaces, and the two conditions of rock would them- 

 selves act as a stimulus for the production of pupae of corresponding 

 colour. The power was probably gained in some dry hot country, 

 where mineral surfaces do not weather q[uickly. Once formed it may 

 be used for other purposes, and in certain species is probably a 

 warning to the enemies that the insect is inedible. It is interesting 

 to note how the Yanessidae, primarily coloured so as to resemble 

 mineral surroundings, are modified for pupation on plants. Thus 

 Vanessa lo has a green form which is produced among leaves ; 

 V. atalanta has no green form, and spins together the leaves for 

 concealment, bat both these species commonly pupate freely exposed 

 on mineral surfaces ; V. urticse has neither the green form nor the 

 habit, and it has a strong disinclination to pupate on its food-plant, 

 as many observations concurred in proving. 



III. Experiments upon Vanessa atalanta. — This species was also 

 made brilliantly golden or dark-coloured by the use of appropriate 

 surroundings in the larval condition. 



lY. Experiments upon Papilio machaon. — This species, like P. poly- 

 damiis (Fritz Miiller) has no power of being influenced by surroundinof 

 colours. A brown pupa was obtained on the food-plant, and many 



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