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Thanks a lot Chris and Theo for your answers. Yes, I agree about the the poor visibility of the keying features, but this happens almost always in photos of living insects.
T. coerulescens was my guess too; in my opinion legs are much darker in T. conspersa, but maybe this is not a diagnostic character.
Matteo's guess was T. alpina, but I read on Cerretti's thesis about the uncertain status of alpina, Theo's statement seems the perfect balance.