My dear M^r. Darwin,

I have traced (from the plate in Guenée’s “Phalénites”) the enclosed outline of the Moth that I had in my mind as possessing so beautiful an under-surface. You will gather an idea of its colouring from what I have written on and about the tracing.¹ The grand ocellus on the forewing is a marvellous feature, when one reflects that there is no trace whatever of it on the upper side! Unfortunately, the insect is a ♀; or what a case it would have been had the ♂ only possessed so magnificent an ornament. (Query:— May not female animals sometimes be provided with special ornaments to attract the males, and may not sexual selection operate in their case as well as in that of males?) This is what Guenée says about Gastrophora: vol. IX, p. 187— “J’établis ce genre sur une magnifique espèce australienne, dont je ne possède malheureusement qu’un seul sexe. Il est probable que le mâle fournit des caractères encore plus tranchés”.² He places the genus in the family Œnochromidæ of the “Phalénites” (= our Geometræ).³

The two following Geometræ (also figured by Guenée) have the under surface more conspicuously coloured than the upper, viz: Hypographa Phlegetonaria Guén. (Tasmania), belonging to the same Family as the beautiful Gastrophora,⁴ and Hyperythra limbolaria, Guén. (Bengal), belonging to the Family Ennomidæ (the “Thorn” Moths of English collectors).⁵ In the first of these, the dull-grey with darker striæ of the upper side is varied beneath with white lines and cloudings which bring the dark streaks out into strong relief: the sexes are quite similar. The second, Hyperythra limbolaria, is of the usual orange-yellow of the “Thorns” on the superior surface, but the outer margin of both wings on the underside has a broad irregular bordering of deep-red. (Fortunately enough, this species affords a capital instance of the female being apparently much more abundant than the male, Guenée observing (IX, p. 101) “Elle parait très-commune aux Indes et j’en ai vu une grande quantité, mais jamais un seul mâle”.⁶ Indeed he says of the whole genus, in noting some diversities of structure between the sexes, p. 100 “Ces différences sont d’autant plus essentielles à noter, que l’on trouve dans les envois quatre ou cinq femelles au moins contre un seul mâle”.)⁷

Among English Geometræ, several species of Ennomos (such as E. illunaria) and Fidonia piniaria occur to me as having the undersurface more variegated than the upper.⁸ Fidonia piniaria is much given to holding the wings quite erect over its back, retaining them in that position for a considerable time. I have just referred to Guenée’s notes on the genus Fidonia, & find him observing this habit of F. piniaria. He says: “Le papillon porte, au repos, les ailes relevées comme les Diurnes”.⁹ I have noticed many Geometræ who carry the wings in the same manner just after settling, but they soon let them sink to a level with the body. At the Cape,¹⁰ a near ally of Fidonia piniaria, apparently undescribed, holds its wings erect almost as long as the English species is in the habit of doing. Both sexes do this, but I am not sure whether the ♀s retain the position as long as the ♂s.

Turning to the Noctuæ,¹¹ I find that Guenée figures the undersides of three species presenting a more conspicuous appearance than the upper. These are Lygniodes hypoleuca, Guén. (Bengal), Hypopyra unistrigata, Guén. (Bengal), and Entomogramma pardus (Senegal)¹² The first (♂) is glossy brownish-black above, but white, with two brownish bands beneath: in the ♀ the contrast of the two surfaces is much less striking, the upper side being duller & browner & the under of an impure whitish. The second is dull ochrey-grey, with darker & lighter streaks above; while the underside is bright orange-red, with black stripes. The third (which I have taken commonly at Natal)¹³ is pale, dull greyish-ochreous above, with a darker border; but beneath is ochre-yellow with numerous black-spots. All three are quadrifid Noctuæ,¹⁴ the first belonging to the family Ophideridæ, & the 2^nd. & 3^rd. to the Hypopyridæ.¹⁵ Entomogramma pardus is a day-flying moth, and I have noticed that both it and another much larger species of the same family (with a bright red-lead underside Hypopyra Capensis, Herr.– Schäff.) have a habit, when settled on the ground or herbage, of now and then suddenly lifting the wings slightly, so as to afford one a glimpse of the bright colouring of the abdomen and underside. These Quadrifid Noctuæ (though by no means all of the section) seem to approach the Geometræ in having the posterior wings almost wholly exposed when at rest and marked with a continuation of the lines and colouring of the anterior wings on both surfaces.

I have only one example of each sex of Erebia (Oreina) Ligea, but I find that the ♂ does want the whitish pupil of the ocelli on the upper surface, but underneath (though the ocelli are smaller than in the ♀) the pupils are distinct. The same thing is observable in the allied Erebia Euryale (also European).¹⁶ In the genus Satyrus it seems often the case that the ocelli of the ♂ are less distinct than those of the ♀; I find this observable in S. Hermione, Alcyone, Briseis and Arethusa (all European).¹⁷

As regards the Hesperiidæ that settle with all the wings expanded, usually on the underside of leaves, the only species with which I am acquainted (in nature) that have anything remarkable about their under-surface are the Natalian Nisoniades Ophione and Caprona Canopus both of which have the hindwings white.¹⁸ It may be advantageous to conceal the conspicuous whiteness, though this could as well be effected by settling on the upperside of leaves; but by choosing the underside of a leaf it would seem that these active insects evade actual pursuers or watchful foes very effectually. Under the shade of foliage, too, their fully-expanded wings, usually of sombre colours, would less readily attract the notice of enemies on the ground.

I hope that these notes may prove of use, imperfect as they are. I shall bear the subject in mind, and acquaint you with anything that I chance upon in connection with it.

Very truly yours, | Roland Trimen