The rheas are ratites (flightless birds, with unkeeled sterna) in the genus Rhea, native to South America. There are two existing species: the Greater or American Rhea and the Lesser or Darwin's Rhea. The genus name was given in 1752 by Paul Mohring and adopted as the English common name. Mohring's reason for choosing this name, from the Rhea of classical mythology, is not known. Depending on the South American region, the rhea is known locally as nandu guazu (Guarani, meaning big spider, most probably en relation to their habit of opening and lowering alternatively wings when they run), ema (Portuguese), suri (Quechua), or choique (Mapudungun).