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Letter 5438

Bates, H. W. to Darwin, C. R.

11 Mar 1867

    Summary Add

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    Sexual ornamentation of insects: coloration of Epicalia genus [of tropical S. American butterflies];

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    horned genera of lamellicorn beetles [see Descent 1: 370, 388].

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    Wallace brought CD's question about gay-coloured caterpillars before the Entomological Society. Members now seeking explanations.

Transcription

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. 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.

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.

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). 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.

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.

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.

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.

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.

    Footnotes Add

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    f1 5438.f1
    CD had called on Bates the evening of Monday 18 February 1867 during his recent visit to London (see letter to A. R. Wallace, 23 February 1867); CD wrote notes on their discussion of lamellicorn beetles, Epicalia (a butterfly genus), and the sphinx moth (see DAR 81: 14--15). CD had also been corresponding with Alfred Russel Wallace about whether the colour of butterflies and their larvae was due to sexual selection or to the natural selection of protective coloration (see letter from A. R. Wallace, 2 March [1867] and nn. 2 and 3).
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    f2 5438.f2
    The genus Epicalia has been subsumed within the genera Catonephele and Nessaea in the family Nymphalidae, whose classification is still debated; recently the genera Catonephele and Nessaea were placed in a subfamily Biblidinae (see Wahlberg et al. 2003). Bates described an Epicalia species in Bates 1863, 2: 52.
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    f3 5438.f3
    CD included Bates's information on Epicalia in Descent 1: 388--9. A draft for CD's discussion in Descent is in DAR 81: 182. For CD's explanation of the variation in colours in Epicalia and some other Lepidoptera, see Descent 1: 399--415, 419--20.
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    f4 5438.f4
    Without referring to these three particular genera, Bates described these habitat preferences and the relative numbers of male and female butterflies in Bates 1863, 2: 227--8. See Descent 1: 309.
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    f5 5438.f5
    Pieris, or whites, are in family Pieridae. CD referred to the brightly coloured females in some species of Pieridae in Descent 1: 413.
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    f6 5438.f6
    Lamellicorn beetles (family Scarabaeidae) include several horned genera. CD included figures of a male and female of species of Copris, Phanaeus, and Onthophagus in Descent 1: 369. He discussed the question of whether males used their horns in fighting, noting Bates's failure to find sufficient evidence (ibid, p. 371). CD's report of Bates's similar information from 18 February 1867 is in DAR 81: 14. For Bates's later publication on his lamellicorn collections, see Bates 1886--90.
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    f7 5438.f7
    CD discussed the `excessive variability' of horns in species of lamellicorn in Descent 1: 370--1; he also mentioned Bates's comment on Onthophagus rangifer, but added that Bates's later research contradicted his original finding. The beetle known as O. rangifer is now usually considered to belong to the genus Proagoderus, or alternatively to the subgenus Proagoderus of the genus Onthophagus (personal communication, Clarke Scholtz).
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    f8 5438.f8
    CD reported Bates's information on the number of cephalic horns in Onthophagus, and also noted that males of Phanaeus lancifer, as well as males of other Phanaeus and Copris species, had horns only slightly less developed than the females in Descent 1: 370.
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    f9 5438.f9
    Bates refers to Wallace's query regarding colourful caterpillars made at the 4 March 1867 meeting of the Entomological Society of London (see letter from A. R. Wallace, 24 February [1867] and nn. 2--4 and 9); see also Descent 1: 417. CD had initially asked Bates why he thought some caterpillars were conspicuous (see letter to A. R. Wallace, 23 February 1867).
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