01-03-2013 Probable id as Argia sordida, based on comparing photos on Observado.org by Tom Kompier of A. sordida:
http://observado.org/soort/photos/93740
and A. modesta:
http://observado.org/soort/photos/93715

04/02/2008
I will try and work out which species are possible here, using Lencioni. Difficult as states are mentioned, but I still don't know anything about habitat or altitude. In cases where also Mato Grosso or amazonian states are mentioned, I have decided they are not an option, because it might be a lowland or dry land species.... but I don't know if I am correct.
Argia claussenii - Minas Gerais (Argentina, Uruguay, Paraguay) - might not be an atlantic rainforest species.
Argia croceipennis - RJ, SP, SC, RGdS (Uruguay, Paraguay, Argentina)
Argia lilacina - MT, MG, SP (Paraguay, Uruguay, Argentina) - same as for A. claussenii
Argia modesta - MG, SP, RJ (Brazil only)
(Argia mollis)
Argia sordida - MG, SP, RJ (Brazil only)
Argia tamoyo - MT, MG, RJ (Bolivia) - doubtful?

Based on this list, I would say the best contenders are A. croceipennis, A. modesta and A. sordida