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Jere and Igor,
Thanks for your helpful responses. Okay, I have had another look at the fly and I cannot get a match with any of the species described in Fonseca. The only possibility might be S loewi (now also listed as a synonym of contristans)! However I cannot detect any obvious modification of the mid tarsus apical segment. So a question, is the lateral flattening of the apical segment slight and not very obvious? If this is the case, then I think my fly is likely to be Fonseca's S loewi. (now S contristans)

The fly certainly has extensive white dusting over the frons, thorax and abdomen. Does S contristans have such dusting?

Another feature of my fly, is that the face is actually glistening bright white, rather than metallic. I moved the light around to see this effect, which I had previously not noted.

I need a good reference collection! :D

Sorry I am getting very confused. But I now think this fly is S contristans (named as S loewi in Fonseca 1978) :@

Any comments?