While examining what I thought was a Peruvian Mesembrinella (Calliphoridae, Mesembrinellinae) I realized that I was in fact looking at a tachinid – with a normal metathoracic spiracle and no pale hairs under the scutellum. Then when I examined my trap of Mesembrinella I found 2 more from French Guiana. They key out to Scotiptera (Dexinae) and on checking with Monty Wood it seems that there is a very closely related genus called Daetaleus that might be incorporated into Scotiptera sometime in the future anyway. I will investigate whether mine are either of the known species (purpureus Aldrich, 1924 and azurea Curran, 1934 (=Zuanalia)).
EDIT (16/10/2010): Monty has suggested I check Dexia basalis Walker in the BMNH next time I am there because it is a similar species. Daeteleus purpureus apparently has a darkened costa, while azurea lacks this feature.