In addition to the drawing of a caterpillar which CD intends to use,
HWB sends information on differences of colour and pattern between the sexes of species of Papilio.
Argynnis diana and A. sagana have females that are brightly coloured, but these may be cases of protective mimicry.
Royal Geographical Society | 15, Whitehall Place, S.W.
April 20 1868
My dear Mr Darwin
I have found amongst my old papers a rude drawing of the conspicuous caterpillar which you intend to mention so enclose it.f1 I believe it is natural size; the black is not deep enough, otherwise I think the colour is accurate
The species of Æneas group of Papilio in which the sexes show no difference in colours or pattern aref2
P. Montezuma males simply have deeper & richer ground
P. Panthonus colour than females
P. Ascanius— I see no difference in tint of ground colour.
P. Ascanius in both sexes is handsome; I think more conspicuous & gay in colouring than any of the other four species here mentioned. The whole Æneas group is remarkable for its rich, strongly-constratedf3 & conspicuous colouring; but the rule is—less richness of tint in ♀ than in ♂, although the ♀ has often more variety of colour. It is very clear there is no adaptation of female colouring to disguise in this group.
The females of Argynnis Diana & A. Sagana are of olive-brown or blackish colours relieved by lighter marks.f4 Regarded absolutely one cannot consider them less beautiful than their males; but there is no doubt, here, adaptation for purposes of disguise has taken place; Entomologists of the countries where they are found alone could say what they imitate; I have no idea myself; but A. Sagana ♀ is coloured much like many common butterflies of Adolias group found in some countries.f5
Yours sincerely | H W Bates
A capital suggestion that of H. Doubleday!f6