From Henry Walter Bates 11 March 1867
Royal Geographical Society | 15, Whitehall Place, S.W.
March 11 1867
My dear Mr Darwin
I promised when you were here to look up a few cases of sexual ornamentation &c in insects & send the particulars to you.1 Here they are.
1. Gay-coloured butterflies.
The tropical American genus Epicalia is a good case. The males are amongst the most gaudy of all butterflies; the females are generally very plain. I know both sexes of 12 species: in one both sexes are similar in pattern & colours & this pattern is that of the great majority of the females of the genus; in 9 other species the two sexes are so different that Entomologists formerly placed them in separate genera, but the male of one of the 9 is totally different in colours & pattern from the other 8 males although as gaily coloured as they; in the remaining 2 species both males & females are gaily coloured but males more so than females.2
I think this case will interest you; the fact of the females of 10 species being of the same type of colouration gives us a clue to the ancestral type, from which the males have diverged by sexual selection, & this type is that of both sexes in several allied genera found in various parts of the world. The fact, again, of 2 species having females gaily-coloured & very different from the females of the rest of the species points to the inheritance of gay colours being partaken of by both sexes in the two species whilst in the rest male has inherited male colours & female female. The males & females in this genus do not frequent separate haunts as in many other genera; but the females fly slower & nearer the ground than the males. The facts afforded by Epicalia are not isolated in Entomology— other genera & natural groups of species within genera afford similar illustrations.3
In some genera when the males are much more gaily coloured than their females the males are immeasurably more numerous than the females & spend most of their days in the open sunlight whilst their obscurer partners are confined to the shade of the woods (Genera, Catagramma, Eunica, Megistanis &c).4 In other cases the females have clearly diverged from the Common type of coloration while the males have remained unchanged in this respect; groups of species of Pieris come under this category.5
2 Armature of males.
I have looked through my collection of horned genera of Lamellicorn beetles; viz, Copris, Phanæus and Onthophagus and find 5 specimens with their cephalic horns broken or edge of clypeus chipped. As we generally select perfect specimens in collecting this is only an indication of what might be found if attention was drawn to the subject. The male horns & thoracic bosses are so wonderfully developed in many species that they must have been drawn out by a long course of Natural Selection & therefore must be of some use to the species; but no one has yet recorded, to my knowledge, a case of males fighting: true, the species come out of their holes only at night.6
Excessive variability in size of horns & bosses I find is the rule, but not a rule without exceptions. For instance, the most wonderfully horned species of Onthophagus in the world viz. O. rangifer of the Zambezi region, shows in abt. 100 specimens I have inspected (collected at random by a non-Entomologist) shew no imperfectly developed males; the female is without armature in this species.7
The rule in Onthophagus is, however, a gradual degradation of horns &c. from fully-developed males down to males so degenerate that they are scarcely to be distinguished from females.
Another fact is that the species in Onthophagus cannot be naturally classed according to the horns of the male: in each natural group of species there are species with one cephalic horn & others with a pair, like the bull.
In Phanæus & Copris there are species with horned females scarcely to be distinguished from degenerate males.8
If you wish to ask me any more special questions about these matters, I shall be glad to try to answer them
Yours sincerely | H W Bates
Wallace brought forward your enquiry regarding gay caterpillars before the Entomological Society last Meeting & several practical men are looking out for explanations.9
Sexual ornamentation of insects: coloration of Epicalia genus [of tropical S. American butterflies];
horned genera of lamellicorn beetles [see Descent 1: 370, 388].
Wallace brought CD’s question about gay-coloured caterpillars before the Entomological Society. Members now seeking explanations.
Please cite as
Darwin Correspondence Project, “Letter no. 5438,” accessed on 20 January 2017, http://www.darwinproject.ac.uk/DCP-LETT-5438