The Southern Monarch butterfly, Danaus erippus (= Danaus plexippus erippus), is found in tropical and subtropical latitudes of South America, including Brazil, Uruguay, Paraguay Argentina, Bolivia, Chile and Peru. The larvae feed on a variety of species, including milkweeds such as Asclepias curassavica. I took this photograph in Buenos Aires in the Spring (March 2012) and did not know what type of Monarch it was at the time. It appears that there are number of different Danaus erippus varieties; this looks to be Danais erippus ab. larensis Köhler, 1929, TL: Argentina (see: http://www.butterfliesofamerica.com/L/danaus_erippus_types.htm).
You can tell the difference the Southern Monarch and its northern cousin, The Monarch butterfly (Danaus plexippus) by the two white dots which appear within the orange cells on the upper side of the forewing, visible in the photo.