Dear Sir,

With reference to your article in last weeks “Nature” it appears to me, that in some cases the forewings of our British Butterflies, are either conspicuously marked underneath, or are transparent, so that the bright colour of the upper surface shows through when the insects are flying.¹

The undersurface of the hinder wings, is generally very soberly coloured in these instances, and (if my impression be correct) I think that when the insect is settled, the bright portion of the undersurface of the forewing, is hidden, or covered, by the dull part of the hind wing

If this be so, the insects when flying would be aided in finding each other, by being able to see bright colours both from above and from below: while this would not prevent complete protection when the butterfly was at rest.

Cases in point, are, the Red Admiral; the peacock, (to a less degree); the painted Lady; the large & small Tortoiseshell; the small Copper; the clouded Yellow; the pale clouded Yellow; the small Heath; the Grayling; the Meadow Brown; and the Orange Tip.²

It seems strange that some of our brilliant Butterflies, like the Red Admiral, should be so conspicuous when flying, & when therefore most exposed to the attacks of birds; and so dully coloured when they are at rest and so little exposed— If the bright colours were warnings that the species was uneatable, one would expect the undersurfaces to be similarly coloured.

It may be that the butterflies are on the wing chiefly at times when birds are not so. Or that the bright colours are warnings to certain foes, while the dull ones are protections against others. Or again, the ready recognition of the butterflies by each other, may be so advantageous for the continuation of the species, as to outweigh the risk of capture by birds.

Some butterflies—the metallic looking fritillaries for instance—appear most attractive when the wings are closed. possibly in such cases the sexes seek each other when one or the other is at rest.

The brighter colours of the White Admiral & the Swallow tail³ are transparent, so that the Insects are readily seen both from above & below when flying.

Believe me to be, dear Sir | yours obediently | J Innes Rogers.

Ch. Darwin Esq F.R.S.