Following the key of Cheng and Thompson (2008), Ptilobactrum has abdomen short, conical, about twice as long as wide; head much wider than thorax; male basoflagellomere covered with long pile, greatly elongate, 4 or more times longer than scape; arista reduced, short, thick; scape long; scutellum unarmed; metatibia without long pile along dorsal edge, forming a distinct brush; and vein R4+5 with an appendix extending posteriorly into cell R4+5.
Ptilobactrum was erected by Bezzi (1915: 136) for a Microdon species which had a very broad head and the basoflagellomere was elongate and densely pilose in the male. Cheng and Thompson (2008) considered it as a synonym of Microdon (Microdon) without seeing the species.
Adapted from original description (Bezzi 1915).
A black species, with the second and third abdominal segments yellow, the third having on each side a broad round black macula.
Head black, punctate, moderately shining; frons near the eyes, and gena partly of a dark yellowish colour; the dark yellowish hair along the vertex is very long and erect, while on the middle of the frons at the sides the hair is short; pile on face pale yellowish, with silky reflexions; pile on basoflagellomere of a dark grey colour; rudiment of the arista reddish; proboscis pale yellow; head below clothed with long whitish pile.
Thorax and scutellum entirely black, punctate, with rather long but slanting pale yellow pile; front part of the pleuron with rather long, erect, whitish pile, posterior part bare; scutellum semicircular, with a faintly marked median longitudinal furrow. Metanotum shining black. Calypter white, with short white fringes; halteres pale yellowish. Legs bare, the femora shining, the tibiae whitish-dusted; coxae and metatrochanters black; femora and tibiae yellow, the latter with a blackish macula above at tip; tarsi broad and flat (especially the front pair), black, with yellowish pile; metabasitarsomere not thickened; claws black, pale at the base; pulvilli yellow. Wings hyaline, shining, with a faint pale yellowish tinge; veins yellow, more or less darkened outwards; vena spuria very faintly marked.
Abdomen: First segment of the abdomen black, with a narrow yellow posterior border on each side; second entirely yellow; third yellow, with a broad rounded black macula on each side, which reaches the fore border but not the sides; fourth black, with lateral borders narrowly yellow, and also bearing a triangular dark yellow median macula at the distal extremity; abdominal pile very short, whitish, but black along the middle line; venter black, yellowish pilose; genitalia black.
Ptilobactrum neavei Bezzi, 1915.
Bezzi, M. (1915) The Syrphidae of the Ethiopian Region based on material in the collection of the British Museum (Natural History), with descriptions of new genera and species. British Museum (Natural History), London. 146 pp.
unless otherwise stated