400 MR. E. B. POULTON ON THE COLOUR-RELATION BETWEEN EXPOSED 
The resemblance to brass or gold can hardly be of value, because the only one of these 
substances which occurs in nature is not sufficiently abundant to offer a model for 
imitation which is likely to be of any service to the insect ; and the same objection 
holds good against any metal or metallic sulphide, although, as far colour or lustre is 
concerned, the resemblance to such a substance as iron pyrites would be admirably 
adapted for protective purposes. 
But it is hardly enough to say that the gilded appearance is unlike anything 
which is usually of interest to insect-eating animals. It is certainly necessary in 
addition to point to some substance in the surroundings which is also of no interest, 
and which the pupae are protected by resembling. And such a substance is, doubtless, 
found in the glittering mineral mica, which is often metallic and golden in 
appearance, and which is very widespread and abundant. The shape of the 
chrysalis of the Yanessidae is very angular, and strongly resembles a mineral surface, 
and the usual appearance of the scattered golden patches on a grey ground is exactly 
the effect produced by the manner in which the flakes of mica occur scattered among 
other less brilliant minerals in granite and other rocks. It has been shown in 
the experiments that the excessively golden appearance is only produced in normal 
pupae when the surroundings are correspondingly brilliant, and such a stimulus would 
of course be provided by an unusual abundance of mica flakes, or of exceptionally 
large crystals of this mineral. Furthermore it has been shown that the tint of the 
lustrous pupae varies with the colour of the surroundings, being generally silvery 
when white paper was used, and golden when a gilt surface was employed. Hence 
the various tints of mica, white and silvery or dark and golden, would produce the 
corresponding protective shades of colour on the pupae. When the brilliant lustre of 
mica or other minerals of recently fractured and exposed rock-surfaces is dimmed by 
the process of weathering and growth of lichens a grey colour is produced which 
would act as a stimulus for the darker varieties of pupa, -while the darkest would 
be formed in the deep shadow of irregular cavities and furrows. Hence the pupae of 
the Yanessidae have two chief varieties to correspond with the two chief conditions of 
their mineral surroundings—the brilliant exposed and the grey weathered conditions— 
while any intermediate result would be formed by any intermixture of the stimuli. 
In this country we do not see the brilliant metallic pupae of V. urticce , because 
in our moist climate the rock-surfaces become grey and weathered almost directly, and 
because man has offered such facilities for pupation by the erection of walls and 
houses, which also quickly become grey or are built of some colour {e.g., red brick) 
which probably does not act as a stimulus. But so perfect is the protection in the 
natural state that it is extremely rare to find the pupae of these most abundant 
insects anywhere except upon walls and houses, which, being plane surfaces, do not 
conceal the angular forms of the pupae. But the very shape which renders them 
conspicuous on these artificial mineral surroundings is eminently protective against 
almost any natural surface of rock. The susceptibility of the species remains, as 
