Key to the Madagascar species of Betasyrphus

Note that the males of B. stuckenbergi are not yet known and therefore are not included in the key. However the very long antennae will presumably also be present in the male sex and key out correctly. Both species have a completely yellow face, while a large proportion of the other Afrotropical species have a dark knob or median vitta on the face. For more distinctive characters and separation of the Oriental and Palaearctic B. serarius see Tab. 1.

1 Antennae with basoflagellomere (3rd joint) very elongated, three times longer than wide (measurements on female holotype and paratype; only other Afrotropical species with very long antennae is B. inflaticornis (Bezzi, 1915), not known from Madagascar and differentiated by a dark facial knob). Wing hyaline without dark maculae. Anterior part of wing yellowish. Abdomen appearing almost black with only vague maculae on tergite 3, on tergite 2 missing (description from females only)........................................................................................ B. stuckenbergi Keiser 1971

- Antennae with basoflagellomere (3rd joint) one and a half to two times as long as wide. Wing hyaline usually with distinct brownish macula (clearly developed in most males, weaker and sometimes missing in the females and part of the males), see Fig. 12. Fasciae on tergites 2+3 distinct and widely separated from their anterior margin............... ........................................................................................................................................................... B. keiseri sp. nov.