This is a rather dubious tachinid – with its unbent median vein and very shallow subscutellum – but something about it reminds me of a tachinid. The plumose anyennae and deep gena might suggest Dexiinae. See also pe-taxon #29.
EDIT (09/10/2010): After showing this to Adrian Pont he thinks that they most resemble muscids but will do further research and report back.
EDIT (6/01/2011): Claudio Carvalho has confirmed this as a Souzalopesmyia sp. – many thanks to him and Adrian for looking at them.