Omegasyrphus was established for some small Microdon species with a distinctive abdominal shape: 2nd segment is widened, flattened, flared, with its lateral margin subcircular, thickened and rounded; and the rest of abdomen (3rd–5th segments) narrowed and cylindrical (Hull 1949: 309).
Omegasyrphus species have first two abdominal segments with three distinct depressions, two lateral ones and an anterior medial one; abdomen more or less parallel-sided; scutellum with apical calcar; vein R4+5 with an appendix extending posteriorly into cell R4+5; vein M1 rounded, not angulate; face convex, without mystax; metatibia without long pile along dorsal edge, forming a distinct brush.
From original description (Loew 1864) in Latin.
♂ et ♀. Gracilis, chalybeoviridis, modice nitens, oculis subnudis, fronte et facie angustis, ocellis a vertice remotis, antennis brevibus nigris, segmenti abdominalis marginibus tumidis. — Long. corp. 3 1/2 — 4 lin., Long. al. 3 lin.
Gracillimus. chalybeo-viridis, modiee nitens, albido-pubescens. Frons aequalis, angusfa, foeminae panlo latior quam maris, ocellis a vertice remotis. Antennae nigrae, breves, articulo tertio praecedentibus simul sumtis paulo longiore. Thoracis dorsum aeneo-viride, sublilissime punctulalum et transversim aciculatum, vittis quatuor obscure cupreis. Scutellum ex viridi nigrum, apice emarginato et utrinque denticulo nigro instrucfo. Abdomen maris cylindricum, obtusum, foeminae acutum; segmentum secundem paulo latius quam sequentia, disco depresso, marginibus lateralibus quasi tumidis. Alae cinereo-hyalinae, venarum transversalium limbis nigris, angulis posticis cellularum posterioris primae et discoidalis rotundatis, brevissime appendiculatis. — (District Columbia; Osten-Sacken.)
New description:
MALE.
Head: Face concave, without tubercle, black, white pilose; gena linear, not visible from profile; frons black, white pilose; dichoptic, eye bare; vertical triangle black, pale pilose; antenna black, basoflagellomere and scape subequal, more than 2 times the pedicel.
Thorax: Scutum black, slightly punctuate, with pale appressed pile; postpronotum pilose;
scutellum semicircular, with two apical calcars, pale pilose; subscutellar fringe absent. Pleuron black to dark brown; anepisternum, anterior anepimeron, and katepisternum pale pilose; metasternum bare; calypter brownish; halter yellow; spiracular fringes brown. Wing: Wing membrane almost hyaline, dark brown vitta from stigma to vein bm-cu and veins M1 and dm-cu also darker, as well as the appendix of vein R4+5; microtrichose, except cells CuP, R and R1 bare basally, cell BM bare posteriorly. Vein R4+5 with an appendix; vein M2 present; vein M1 recessive. Alula broad, bare with some microtrichia apically. Legs: Entirely dark brown, joints between femora and tibia a bit lighter.
Abdomen: more or less parallel-sided after tergum 2; first two abdominal segments with three distinct depressions, two lateral ones and an anterior medial one. Tergum 2 flat, broader than the remaining terga, with oval lateral margins. Dorsum mainly black or dark brown, becoming lighter laterally and ventrally, punctuate, with appressed pale pile, very concave.
Microdon (Omegasyrphus) coarctatus Loew, 1864.
Loew, H. (1864) Diptera Americae septentrionalis indigena. Centuria quinta. Berl. Ent. Z. 8: 49-104. [1864.02.??]
unless otherwise stated