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Hello, I´m sorry I can´t help, but I can add their separation (my translation may need corrections):

** in Engel (1938), for "groups of species"
- all terguites with "yellow" (light) hairs wirh some longer black hairs, last terguites often with witish hairs. Also sternites light coloured excepting the first (dark brown) -> B. medius group)
- last 3-4 terguites with "dense brownblack" hairs and a white haired central longitudinal stripe that in males could be not evident. Sternites with black-brown hairs -> (B. discolor)

** In Faune de France (Séguy, 1926)
- femora with dense black basal hairs, abdomen: apex and sternites with black or brown hairs -> B. discolor
- femora with a few hairs (pilosité médiocre), abdomen with light colour hairs -> B. medius

Kind regards