Hypselosyrphus is based on an Ubristes species in which the 4th abdominal segment is short and triangular. As with Stipomorpha, this character is considered only of species group value (Cheng and Thompson 2008).
Ubristes (Hypselosyrphus) species have postmetacoxal bridge complete; abdomen triangular, broad basally, strongly narrowed apically, short, almost equilateral in shape; metatibia with long pile along dorsal edge, forming a distinct brush of pile; basoflagellomere not furcate, usually not greatly elongate; metabasitarsomere not enlarged; and vein R4+5 with an appendix extending posteriorly into cell R4+5.
Microdon (Hypselosyrphus) trigonus (Hull, 1937).
Hull, F.M. (1937) New species of exotic syrphid flies. Psyche 44, 12-32, pl. 2.
Adapted from original description (Hull 1937).
Small flies, related to Microdon with the weak and flattened abdomen somewhat longer than broad. Face round convex, with the vertex produced into a curved jutting knob. Scutellum sulcate, tumid, angularly directed upward. Pro- and metabasitarsi swollen. Metatibiae convex, with a swollen ornament suggesting a load of pollen, heavily pilose. Metafemora not greatly thickened. Vein R4+5 iwith an appendix into cell R4+5. Veins M1 and dm-cu nearly straight, bulging at their basal corners and making angles of approximately seventy-five to eighty degrees with the veins they join.
MALE.
Head: Front enormously swollen as a shining tubercle, the ocelli on top. Front excavated, flat, leaving a considerable concavity in profile. Face in profile convex, evenly rounded. Antennae situated at upper third. Scape and basoflagellomere subequal, the latter pointed, pedicel quite short. Arista slender, as long as basoflagellomere. Color of antennas dark brown. Face, front and vertex shining dark mahogany red or brown, almost black.
Thorax very dark brown, almost black, a row of white pile across at suture, and on base of scutellum, remainder densely erect short black pilose. Pleurae shining dark brown, sparsely pilose. Halteres and calypters dark blackish. Scutellum dark blackish, prominent, directed upward at an angle of forty-five degrees, deeply sulcate medially, the sides swollen, rounded, without spines.
Abdomen short oval, flat, vitreous, dark reddish brown on first, second and narrowly on base of third segment. Remainder bright orange. Legs dark shining purplish or reddish black, tarsi except metabasitarsomere light yellowish brown. Probasitarsi swollen anteriorly. Metabasitarsi considerably swollen. Metatibiae greatly swollen and dorsally arcuate, a curious crease running around it, below the part of greatest swelling. Tibiae heavily long black pilose. Wings brownish especially basally, with a faint pale yellow macula beginning past the stigma. Apical section of fourth longitudinal vein (vein M) straight, not angular, no appendix present (vein M2 absent), not even from the angle of the fourth longitudinal vein.
This remarkable species suggests Microdon panamensis Curran, which lacks the interesting structural characters of this form. It is a handsome and peculiar species, that like Ubristes resembles Trigonid bees, but differs from that genus in the development of the vertex and scutellum, etc.
Hull (1937) designated trigonus as the type-species for his new genus, Hypselosyrphus. Thompson et al. (1976) considered Ubristes as subgenus of Microdon and made a new combination for this species, Microdon trigonus.
unless otherwise stated