Dimeraspis (Megodon), Keiser

Reemer, Menno & Stahls, Gunilla, 2013, Generic revision and species classification of the Microdontinae (Diptera, Syrphidae), ZooKeys 288, pp. 1-213 : 43-44

Subgenus Megodon Keiser Figs 187-195

Megodon Keiser, 1971: 252. Type species: Megodon stuckenbergi Keiser, 1971: 253, by original designation.

Description.

Body length: 8-13 mm. Broadly built flies with oval abdomen and long antennae. Head about as wide as thorax. Face convex in profile; narrower than an eye. Lateral oral margins slightly produced. Vertex flat. Occiput narrow and parallel-sided over entire length. Eye bare. Eye margins in male converging at level of frons, with mutual distance about equal to width of antennal fossa. Antennal fossa about as wide as high. Antenna longer than distance between antennal fossa and anterior oral margin; basoflagellomere shorter than to as long as scape; bare. Postpronotum pilose. Scutellum trapezoid; with strongly developed calcars. Anepisternum weakly sulcate; pilose anterodorsally and along posterior margin, widely bare in between. Anepimeron entirely pilose. Katepimeron convex; smooth; 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 angular to weakly rounded, with or without appendix; crossvein r-m located around basal 1/6 of cell dm. Abdomen oval, around 1.5 times as long as wide. Tergites 3 and 4 fused. Sternite 1 pilose or bare. Male genitalia: phallus furcate near base, with processes equally long and projecting well beyond apex of hypandrium; epandrium with ventrolateral ridge; surstylus unfurcate, elongate, curved dorsad.

Diagnosis.

Vein R4+5 with posterior appendix. Postpronotum pilose. Abdomen oval. Anepisternum widely bare medially. Propleuron bare. Postero-apical corner of cell r4+5 more or less rectangular. Antenna longer than distance between antennal fossa and anterior oral margin. Occiput narrow and parallel-sided over entire length (not widened dorsally). First tarsomere of hind leg dorsally with wide longitudinal groove.

Discussion.

Keiser (1971) erected this genus to include a species with very large, cone-shaped scutellar calcars. Cheng and Thompson (2008) did not study this species and refrained from commenting on the status of the group. The first author was able to study the holotype of Megodon stuckenbergi Keiser, 1971, as well as some additional material.

Megodon is very similar in external morphology to Microdon . Their genitalia also share important characters, like the deeply furcate phallus, the long aedeagal processes and the presence of a ventrolateral ridge on the epandrium. There are also differences, most notably the entirely narrow and parallel-sided occiput, and the dorsal, longitudinal groove on the first tarsomere of the hind leg. The shared characters are here considered more important than the differences. Because of these considerations, combined with the phylogenetic results of Reemer and Ståhls (in press), Megodon is here treated as a subgenus of Microdon .

Microdon planitarsus Keiser, 1971 is here also assigned to Megodon , because it agrees with the diagnostic characters as described above, and its male genitalia are very similar to those of Microdon stuckenbergi (Figs 193, 195). In Microdon planitarsis , the scutellar calcars are not as large and cone-shaped as in Microdon stuckenbergi . This indicates that the size and shape of these calcars should not be regarded as group-defining.

Diversity and distribution.

Described species: 2. Madagascar. One undescribed species from Madagascar is known to the first author.