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Odile,

I have my reference material now. This fly is Coilometopia trimaculata (Fabricius) - Richardiidae; the species is widespread in northern S. America. The head and wing are illustrated in "Genera Insectorum" 113, by Hendel, 1911. One striking characteristic of this species can just be seen in your photos - a longitudinal black band down the middle of the yellow scutellum (the rest of the thorax being yellow also).

Eric