The extraordinary difference existing between the sexes of exotic Lepidoptera, and particularly among the Butterflies, (_Papilionidae_ Lin.) is a subject which hitherto has received but little attention; nor am I aware of any entomological writer who has described those characters which absolutely distinguish the sexes: characters which, I am persuaded, will hereafter be found of the first importance in a natural arrangement of these insects. But in the prosecution of this desirable object, the naturalist, as far as regards foreign Lepidoptera, will have to encounter serious obstacles; many individuals must be examined of each species, and some of these dissected. It falls to the lot of few to pursue their inquiries in the native regions of these insects. Collections in this country are very few, and some of these are not always open to the scientific labourer; neither can specimens be sacrificed for dissection, where there are not more than two or three individuals of a species. This is in general a very rare insect; observed for the first time by Dr. Langsdorff and myself early in June (the tropical autumn), in a wood adjoining the Organ Mountains at Rio de Janeiro. From its local abundance, we were able to ascertain the sexes. The two upper figures are of the female, and the lower of the male insect. * * * * * Pl. 125 [Illustration] PAPILIO Nerius. * * * * * GENERIC CHARACTER.--See Pl. 92. * * * * * SPECIFIC CHARACTER. _P. (Gr. Ecaud.) Alis nigris, fascia communi maculisque viridibus; posticis dentatis breviter caudatis; his subtus fascia subargentea, marginali, nervis divisa._ Godart. P. (Gr. Ecaud.) Wings black, with spots and a common band of green; posterior wings dentated, obsoletely tailed, beneath with a silvery marginal band, divided by the nerves. P. Nireus. _Fab. Sys. Ent._ 3. _p._ 36. _Godart Ency. Meth._ 9. 1. _p._ 48. _Drury_ 2. _pl._ 4. _fig._ 1. 2. _Cramer_, _p._ 187. A. B. (_mas._) _pl._ 378. F. G. (_fem._) * * * * * I have figured this insect, principally because it will fully illustrate the first section (_a._) in the arrangement of this beautiful family proposed at plate 92. The two divisions there adopted, after the manner of Linnaeus, (_Graeci_ et _Trojani_) I am fully aware, are purely artificial; but the facility this distribution will give to the student, in searching after a particular species, is so obvious, that it need hardly be pointed out.