Type species: Sphinx equestris Fabricius, 1793, by subsequent designation by Kirby, 1892, Synonymic Cat. Lepid. Heterocera, 1: 659 (but cited as Sphinx nessus Drury). Kirby cited as type species Sphinx nessus Drury, 1773, a nominal species not originally included in Theretra. At the same time, Kirby placed nessus as a senior synonym of equestris, a nominal species originally included in Theretra. Under the Code, Article 69(a)(iv), this designation constitutes a fixation of the originally included nominal species as the type species.
Zolotuhin & Ryabov (2012) considered Theretra to be a “heterogeneous conglomerate” in which “at least 4 lineages are joined in this polyphyletic group”. They then divided Theretra into four separate genera by the simple expedient of reinstating three of its junior synonyms and assigning some (but not all) of the current species of Theretra among them. Under this interpretation, Theretra is monobasic, containing only the type-species, T. nessus. Oreus Hübner (with Hathia Moore as a junior synonym) is reinstated for the “grey” Theretra clotho group, characterized primarily by an apical field of deciduous “caltrap” cornuti on the phallus. Gnathostypsis Wallengren is reinstated for “large and robust species with lineal fore wing pattern, rather oblique, and with developed pattern of the hind wing including transversal fasciae and red spots sometimes covering the whole wing surface”; i.e., those species with a red hindwing (T. alecto, T. capensis, T. pallicosta and T. suffusa). Finally, Florina Tutt is reinstated for the small to medium-sized “stripy” species (T. hausmanni, T. japonica, T. lycetus, T. oldenlandiae and T. silhetensis; “T. pinastrina Martyn, 1797” was also included even though Kitching & Cadiou, 2000 demonstrated that this taxon was both attributable to Butler (1876) and is a junior synonym of Hippotion balsaminae Walker). Finally, Theretra insignis and Theretra turneri were stated to belong to Hippotion, and T. insularis “Is a member of Cechetra gen. nov…. if not Oreus”.
There are certainly issues regarding the current concept of Theretra; a number of species may currently be included that do not belong there, but there is also a case to be made for a number of species currently placed in other genera to be included within Theretra. However, the division of Theretra and consequent changes to the species-level taxonomy proposed by Zolotuhin & Ryabov is considered inadequate for the following reasons:
1. Although Theretra is rather heterogeneous morphologically, that alone is no basis for asserting the genus is polyphyletic. Many taxa are morphologically heterogeneous yet monophyletic (although in the case of Theretra, it is accepted that this does have yet to be demonstrated).
2. The diagnosis of Oreus is based largely on the shape of the harpe and the presence of a field of deciduous cornuti on the phallus. However, these cornuti also occur in the Cechenena aegrota species group. They are also present in T. insularis, which was suggested to belong to Cechetra, even though these cornuti are lacking in members of this genus.
3. No evidence was presented for the monophyly of the reinstated Gnathostypsis and Florina.
4. A significant number of species were not allocated to any of the four generic concepts, e.g. the T. cajus species group, T. castanea, the T. jugurtha group and T. orpheus species group.
Overall, there is no doubt that a revision of Theretra is required, and satisfactory resolution will probably only come from a careful phylogenetic analysis combining morphology and DNA sequence data (and not just DNA barcodes) not jist of the genus itself, but of the entire subtribe Choercampina. The division of Theretra by Zolotuhin & Ryabov does not represent an improvement on the current taxonomy and consequently, it is not adopted here.