This is a very unusual (dexiine?) tachinid with plumose aristae, deep gena, tiny calyptrae & a straight median vein. See also pe-taxon #10 – a very similar tachinid with a brown body colour.
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.