The rheas are ratites (flightless birds with unkeeled sterna) in the genus Rhea, native to South America. There are two extant species: the Greater or American Rhea and the Lesser or Darwin's Rhea. The genus name was given in 1752 by Paul Möhring and adopted as the English common name. Möhring's named the rhea based on the Greek Titan Rhea, whose name is derived from the Greek Rhea (῾Ρέα) by metathesis from έρα "ground". This was fitting, the rhea being a flightless ground bird. Depending on the South American region, the rhea is known locally as ñandú guazu (Guaraní, meaning big spider, most probably in relation to their habit of opening and lowering alternatively wings when they run), ema (Portuguese), suri (Quechua), or choique (Mapudungun).