Oligeriops Hull

Reemer, Menno & Stahls, Gunilla, 2013, Generic revision and species classification of the Microdontinae (Diptera, Syrphidae), ZooKeys 288, pp. 1-213 : 50-51

Oligeriops Hull Figs 249-253

Oligeriops Hull, 1937a: 26. Type species: Microdon chalybeus Ferguson, 1926a: 176, by original designation.

Description.

Body length: 7-10 mm. Dark-coloured, stout-legged flies with oval abdomen and moderately long antennae. Head about as wide as thorax. Face convex; wider than an eye. Lateral oral margins produced. Vertex flat. Occiput wide over entire length, narrowest point halfway. Eye bare. Eye margins in male not converging at level of frons, with mutual distance around 4 times as large as width of antennal fossa. Antennal fossa about as wide as high. Antenna longer than distance between antennal fossa and anterior oral margin; basoflagellomere longer than scape; with dorsal margin curved dorsad, more or less sickle-shaped; bare. Postpronotum pilose. Scutellum semicircular; without calcars. Anepisternum weakly sulcate; pilose, with small bare part on ventral half. Anepimeron entirely pilose. Katepimeron convex; with wrinkled texture; bare. Wing: vein R4+5 with posterior appendix; vein M1 more or less straight, perpendicular to vein R4+5; postero-apical corner of cell r4+5 rectangular, with small appendix; crossvein r-m located between basal 1/6 of cell dm. Abdomen oval, about twice as long as wide. Tergites 3 and 4 fused. Sternite 1 pilose. Male genitalia: phallus not or little projecting beyond apex of hypandrium, slightly bent dorsad, shallowly furcate, with both processes about equally long; epandrium without ventrolateral ridge; surstylus unfurcate.

Diagnosis.

Vein R4+5 with posterior appendix. Postpronotum pilose. Abdomen oval. Anepisternum largely pilose, at most with small bare part on ventral half. Basoflagellomere sickle-shaped: dorsal margin curved upward.

Discussion.

Hull (1937a) described Oligeriops as a genus, with only Microdon chalybeus Ferguson, 1926 included, without indicating its diagnostic generic characters. Hull (1949) used the reduced size of the eyes (due to widened occiput and gena) and the sickle-shaped antenna as key characters. Thompson and Vockeroth (1989) list Oligeriops as synonym of Microdon . Cheng and Thompson (2008) express their doubts about ranking Oligeriops as a genus, while referring to the antennae of Australian Microdon species as illustrated in Ferguson (1926b). These illustrations show that other species originally described in Microdon also have a curved basoflagellomere, just like Microdon chalybeus Ferguson, 1926, but nevertheless these species were not included in Oligeriops by Hull (1937a, 1949). Cheng and Thompson (2008) state that 'Whether these other species have reduced eyes remains to be seen!'. However, as Ferguson (1926a, b) already noticed, the four species he described are all ‘close’ and 'very similar’. Examination of type specimens, additional material and original descriptions, has confirmed this, and has made clear that all five species presently included in Oligeriops have reduced eyes and sickle-shaped basoflagellomeres indeed. Based on these and other morphological similarities, there is no doubt that they are closely related.

Oligeriops does not fit into the concept of Microdon s.s. as defined in the present paper. In addition to the reduced size of the eye and the curved basoflagellomere, the following characters distinguish Oligeriops from Microdon : anepisternum almost entirely pilose, at most with small bare part ventrally; propleuron bare; postero-apical corner of cell r4+5 rectangular; phallus projecting little beyond apex of hypandrium, furcate near apex. Considering these characters in combination with the results of Reemer and Ståhls (in press), it is deemed not appropriate to include this taxon in Microdon .

Diversity and distribution.

Described species: 5. Australia (incl. Tasmania).