Microdon (Omegasyrphus) coarctatus Loew
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.
Lenght: 7.8-9.0 (body); 6.7 mm (wing) (Loew 1864).
Nearctic species found from Nebraska to Maryland, southwards to Mexico and Florida.