From H. W. Bates   29 March 1867

My Dear Mr Darwin

I was interrupted in the examination of horned-beetles required to answer your last question by the arrival of a large addition to my collection of them from Paris.¹ The development of horns & excrescences & the sexual differences in this respect are so wonderfully diversified that I think of tabulating the species & furnishing you with results, if I think they are worth your having. Meantime I will try to answer your other questions.

First, the poser of the Lady Darwinian.² It is a very fair question & ought to be answered. There are no doubt great gaps in the gradation of forms, as now existing, between the ordinary white Pierid Leptalis & its highly specialised congeners which mimic Heliconidæ. But we must not exaggerate the width of these gaps, nor think that all the existing links are represented on the plates to my Linnæan paper.³ There are a good many species in collections bridging over the differences between the extreme forms in the genus. Some of these do not wear the livery of any Heliconids, although the wings are elongated like them, departing in this character from the Pierid type. They are forest insects, fly slowly & are rare; how they escape extermination I cannot say. This much however may be said that the undersurface of their wings is coloured & marked something like a dead leaf & this would probably stand them in some stead, as their wings are closed in repose, & they fly very little. (These species are Leptalis Licinia, Psamathe, Kollari Nemesis &c).⁴ Amongst the Leptalids mimicking Heliconids there is a very considerable diversity of coloration, for the various species are adapted to almost all the extreme forms of Heliconidæ. I have shown also in my paper that one Leptalis mimics not a Heliconid but a member of a totally different group, which nevertheless has a general resemblance in form & colours to the Heliconidæ.⁵

Now let us put all these facts together. We see that the weak, struggling Leptalids manage to escape extermination by various disguises; some perhaps more effective than others, for I ought to have mentioned that one Leptalis, at least, is an abundant insect (L. Eumelia, a perfect imitation of a very common Ithomia, I. Eurimedia).⁶ There can be no difficulty, therefore, in understanding how at all previous periods in the history of the group, the species of Leptalis have found some means of escaping extinction, even though the forms had not reached the extreme divergence from the family type which they have now attained. It must be remembered that the Leptalids are inhabitants of humid forests where none, or scarcely any, of their Family allies live. The proper places for Pieridæ are open sunny grass lands; the Leptalidæ therefore, in intruding into the forest, encountered unusual difficulties & it became a necessity that they should become greatly modified if they were to maintain their ground at all. First their wings became elongated— this I believe was totally independent of adaptation to the long winged Heliconids—it may be connected with relaxation of muscular development—for many forest genera of butterflies are long-winged. Being long-winged & sporting into various colours—not at first the gayer colours of Heliconids—accidental resemblances of their varieties to some other object, no matter what, saved them from extinction. This continued for countless generations & in various parts of the great Tropical forest until the astonishing mimicry of Heliconidæ was brought about.⁷

Another question you asked was whether any female Vanessa or Machaon group of Papilio existed differing in colours from its partner & from the usual gay colouring of its genus.—⁸

I believe none is known. If you extended the search to the next allied genus (to Vanessa) Junonia, I could give you a string of cases of sexual disparity in colour. There are all grades of disparity among the species of Junonia (1) Male & female alike.— (2) male & female same colours but male brighter (3) male & female different in colours but a certain resemblance betraying their relationship (4) male & female so different that no one would judge them to belong to one & the same species.⁹

Yours sincerely | H W Bates

I know of no case of male monkeys fighting together.¹⁰