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I anwser to myself : I found (in Seguy's Faune de France, Diptères Brachycères) that Ilione trifaria had a synonym called Tetanocera unipunctata Macquart, 1849. If I correctly understand Ilione unipunctata is the same as this Tetanocera unipunctata and is now considered as a good species. Since Ilione trifaria is quite different from my fly (last antennal segment shorter and rounder, wing spots, habitus...), I think I. unipunctata may also be excluded. Therefore I can't see a better option than I. lineata.