Ubristes Walker

Reemer, Menno & Stahls, Gunilla, 2013, Generic revision and species classification of the Microdontinae (Diptera, Syrphidae), ZooKeys 288, pp. 1-213 : 71-72

Ubristes Walker Figs 417-421

Ubristes Walker, 1852: 217. Type species: Ubristes flavitibia Walker, 1852: 217, by original designation.

Description.

Body length: 10-11 mm. Slender flies with long antennae and long, brush-like pilosity on hind tibiae. Mimics of Trigona -like stingless bees. Head wider than thorax. Face slightly convex, almost straight in lateral view; wider than eye. Lateral oral margins produced. Vertex flat. Occiput ventrally narrow, dorsally widened. Eye very sparsely and short pilose, appearing bare under low magnification. Eye margins in male converging at level of frons; mutual distance about three times width of antennal fossa. Antennal fossa about as high as wide. Antenna longer than distance between antennal fossa and anterior oral margin; basoflagellomere longer than scape. Postpronotum pilose. Anepisternum sulcate; pilose anteriorly and posteriorly, widely bare in between. Katepimeron convex; bare. Scutellum semicircular; without calcars. Wing: vein R4+5 with posterior appendix; vein M1 perpendicular to vein R4+5; cell r4+5 with postero-apical angle widely rounded; crossvein r-m located between basal 2/5 and 1/2 of cell dm. Hind tibia with long, brush-like pilosity. Abdomen elongate: parallel-sided or somewhat triangular. Tergite 2 with lateral tubercle at half of length. Tergites 3 and 4 fused. Sternites 1, 2 and 3 not separated by very wide membranes. Male genitalia: phallus furcate basally; epandrium with lateral ‘fenestrae’: well-defined, translucent, oval depressions; surstylus more or less oval.

Diagnosis.

Hind tibia with long, brush-like pilosity. Scutellum without calcars. Vein R4+5 with appendix. Tergite 2 with lateral tubercle at half of length.

Discussion.

Thusfar, Ubristes has been characterized by the brush-like pilosity of the hind tibia, giving the flies the appearance of stingless Trigona -like bees ( Cheng and Thompson 2008, Thompson et al. 1976). Based on this definition, 31 species were assigned to this group by Thompson et al. (1976), including the type species of Carreramyia , Hypselosyrphus and Stipomorpha . The latter two groups were considered as ‘subgroups’ of Ubristes by Cheng and Thompson (2008), because the characters previously used to define the groups (abdominal shape) were considered of little taxonomic value.

Closer examination of the morphology reveals several important differences between these taxa. The structure of the male genitalia of Ubristes is very different from those of the species here included in Carreramyia , Hypselosyrphus and Stipomorpha : the phallus is long and slender and furcate near its base, the base of the hypandrium is not bulged and there are well-defined, translucent, oval lateral depressions in the epandrium (here called ‘fenestrae’). In external morphology Ubristes is readily distinguished from the mentioned genera by e.g. the lateral tubercles on tergite 2. For other differences see the accounts of the other taxa. Considering the phylogenetic results of Reemer and Ståhls (in press) and the morphological differences between these taxa, Ubristes sensu Thompson et al. (1976) and Cheng and Thompson (2008) is here considered to be polyphyletic, with Carreramyia , Hypselosyrphus and Stipomorpha each as separate lineages. Besides the type species, two undescribed species are assigned to Ubristes .

Thompson et al. (1976) and Cheng and Thompson (2008) ranked Ubristes as a subgenus of Microdon . However, the species of Ubristes differ in several characters from the species of Microdon s.s., as defined in the present paper. Here, the view is taken that it is better to treat Ubristes as a genus instead of a subgenus, in order to make sure that Microdon comprises less heterogeneous groups with uncertain affinities.

Diversity and distribution.

Described species: 1. Descriptions of two additional species are in preparation by the first author. Central and South America.