The species is easily confused with the giant swallowtail, which it closely resembles in both larval and adult stages.

The hind wings are predominantly brown, toothed and have a long caudal process.

The species occurs from Texas and the West Indian Islands southwards to Buenos Aires, but is wanting on the Lesser Antilles.

— autocles R. & J., occurring from Texas to Nicaragua, has no cell-spot; the yellow areas are pale.

— nealces R. & J. is distributed from Nicaragua to North-West Ecuador and eastward, to Trinidad and the Lower Orinoco.

The forewing has always a cell-spot: the yellow tint is somewhat deeper than in autocles, but less deep than in the next subspecies; very common in Bogoti-collections.

is a large form, which inhabits the Middle and Lower Amazon and the eastern slopes of Ecuador, Peru and Bolivia; the submarginal spots of the forewing are almost always absent.

— brasiliensis R: & J. , from Brazil, Paraguay and North Argentina, is often still larger than cinyras; the cell-spot of the forewing is absent or small, the first spot near the apex of the wing is mostly large and produced into a point.

Adult Thoas swallowtails fly year round in the tropics, feeding on nectar of a variety of flowers, including Lantana, Stachytarpheta, and Bougainvillea among other species.