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well there is this http://www.mapres.../2960.html

second part due some tome next year (much delayed as is way with these things)

otherwise these old difficult papers/books in German

Becker, T. (1906) Usia Latr. Berliner Entomologische Zeitschrift, 50[1905], 193–228.
Engel, E.O. (1932) Bombyliidae. In: Lindner, E., ed., Die Fliegen der palaearktischen Region, p. 49–96. Vol. 4, pt. 3. E. Sch-weizerbart, Stuttgart.
Paramonov, S.J. (1947) Uebersicht der mit der Gattung Usia Latr. (Bombyliidae, Diptera) naechstverwandten Gattungen. Eos, 23, 207–20.
Paramonov, S.J. (1950) Bestimmungstabelle der Usia-Arten der Welt (Bombyliidae, Diptera). Eos, 26, 341–78.