Oligeriops chalybeus (Ferguson, 1926):176

Microdon is the nominotypic group, hence, remains some what a catch all for various unrelated species not placed in other genera. The species related to the type, mutabilis Linnaeus, are restricted to the North Temperate region and are characterized by long antenna, scutellum with apical calcars, vein R4+5 with an appendix, simple legs and abdomen. The European species have been revised (Doczkal & Schmid 1999, Schmid 2004), Nearctic (Thompson 1981b) and Japanese (Maruyama & Hironaga 2004, Hironaga & Maruyama 2004) (from Cheng and Thompson 2008).

Adapted from original description (Ferguson 1926).

A metallic-blue species with reddish legs.

MALE.

Head deep blue, forehead about one and a half times the width of an eye, rather sparsely punctate, deep blue around ocelli, greenish iridescent anteriorly, pilosity pale in front and on vertex, elsewhere dark; face convex, sides slightly divergent, rather sparsely punctate, deep blue, with pale pile; gena black with pale pile; eyes bare. Antennae black; scape short; pedicel very short; basoflagellomere long, thickened, especially at base, and somewhat curved, the upper surface distinctly concave, arista stout, rising near base, shorter than segment, a shallow groove on outer surface of basoflagellomere below arista; relative proportions of segments 4: 2: 16.

Scutum black on disc, blue at sides, the black portion slightly iridescent, finely and rather sparsely punctate with some fine transverse fasciae; pilosity long, dark, with paler pile intermingled, especially at sides. Scutellum deep blue, mainly pale pilose, apical margin rounded, not dentate. Pleurae blue with pale pile.

Abdomen deep blue, the first segment black, laevigate, the remainder rather closely and finely granulate, pubescence short, dark, appressed, longer pale hairs extending along the lateral margins of the whole abdomen, and forming a small patch on each side of base of second segment. Venter blue.

Legs light reddish-yellow, the coxae, trochanters and tarsi black, the latter with yellow pile beneath. Wings grey, slightly darker along veins, venation normal, M1 and dm-cu somewhat sinuous, appendix of R4+5 present. Calypters white; halteres yellow, brownish at base.

FEMALE.

Differs from male mainly in the entire absence of pale pubescence; the basoflagellomere is somewhat less thickened at base.

Microdon chalybeus Ferguson, 1926.

Ferguson, E.W. (1926) Revision of Australian Syrphidae (Diptera). Part i. Proceedings of the Linnean Society of New South Wales 51, 137-183, pl. 14. [1926.07.01]

Hull (1937) described a new genus for this species, Oligeriops, but he did not provide any diagnostic information, only a description. Hull (1949) used two characters, the "greatly reduced" size of the eyes, hence, the larger gena, vertex and occiput, and the shape of the basoflagellomere in his key to world genera of Microdontinae. As noted by Ferguson (1926: 171) in respect to another Australian species with an unusual basoflagellomere (alcicornis Ferguson, type of Cervicorniphora), "the antennae are most variable in the genus" Microdon. He later emphasized this variation by illustrating the antennae of most Australian Microdon species. From those illustrations one can see that at least the antennal structure of chalybeus is not unique, but common among Australian species (Cheng and Thompson 2008).

Body length: 8 mm (Ferguson 1926).