The phylogeny and classification of the tribe Halictini, with special reference to the Halictus genus-group (Hymenoptera: Halictidae)
Authors: Pesenko Yu.A. 
Published: September 2021

The monophyly of the cosmopolitan tribe Halictini, including over 2300 currently recognized species, is supported by at least a single manifested synapomorphy shared by all members of the tribe: metasomal tergum VII of the male is modified; this forms a transverse ridge giving a false apex beneath which the tergum is strongly reflexed to the morphological posterior margin. On the basis of the present phylogenetic analysis, the tribe Halictini is subdivided into five subtribes: Halictina (comprised of 7 genera: Echthralictus, Glossodialictus, Halictus, Homalictus, Patellapis, Seladonia, and Thrincohalictus), Sphecodina (4 genera: Eupetersia, Microsphecodes, Ptilocleptis, and Sphecodes), Thrinchostomina (2 genera: Thrinchostoma and Parathrincostoma), Caenohalictina (9 genera: Agapostemon, Caenohalictus, Dinagapostemon, Habralictus, Mexalictus, Paragapostemon, Pseudagapostemon, Rhinetula, and Ruizantheda), and Gastrohalictina (one large and diverse genus: Lasioglossum s. l.). The subtribe Halictina is a paraphyletic group; the remaining four tribes are strictly monophyletic (holophyletic). The monophyly of the Halictus genus-group, comprising the genera Halictus and Seladonia, is supported by two distinct synapomorphies of the male genitalia: (1) dorsal gonostylus simple (not double), flattened, broad, narrowed proximally, and provided with a clump of very coarse bristles on the inner surface; (2) ventral gonostylus sclerotized, relatively thin and long, directed backward. Only generalized members of the group possess both the character states above. In derived members, the clump of bristles and the ventral gonostylus are often lost independently. The sister group of the Halictus genus-group is the genus Thrincohalictus. This is supported by the following synapomorphy found among the tribe Halictini only in Halictus, Seladonia, and Thrincohalictus: the ventral gonobasal rim in the male genitalia is forming a right posterolateral angle with a short projection directed laterally. Another feature characterizing these three genera is the presence (except in the parasitic subgenus Paraseladonia) of posterior bands of tomentum or dense and much plumose appressed hairs on the metasomal terga. However, this character is shared also with Patellapis subgenera Patellapis and Lomatalictus. The following subgeneric classification of the genera Halictus and Seladonia is suggested. The genus Halictus includes 12 subgenera: Acalcaripes (2 species), Argalictus (8), Halictus s. str. (4), Hexataenites (11), Lampralictus (1), Monilapis (29), Nealictus (2), Odontalictus (2), Platyhalictus (14), Protohalictus (13), Ramalictus (2), and Tytthalictus (4). The genus Seladonia comprises 6 subgenera, including 2 new ones: Mucoreohalictus subg. n. (15), Pachyceble (22), Paraseladonia (1), Placidohalictus subg. n. (5), Seladonia s. str. (36), and Vestitohalictus (16). The subgenera of Halictus and Seladonia are keyed. The phylogenetic tree of the subgenera of Halictus and Seladonia is reconstructed with use of 46 morphological characters of adults. All other genera of the tribe Halictini were taken as outgroup. The genus Halictus is ascertained as a strictly monophyletic group based upon a single postulated synapomorphy: dorsal gonostylus with a triangular hair patch as a distal appendage on inner side. All the subgenera of this genus appear as strictly monophyletic groups, with the exception of Monilapis, which is a paraphyletic one in relation to Acalcaripes. The monophyly of the genus Seladonia is supported by three synapomorphies, including a novelty (unique synapomorphy): male dorsal gonostylus with a deep cleft. All the subgenera of this genus are strictly monophyletic with the exception of Placidohalictus, which is a paraphyletic one in relation to Vestitohalictus and Mucoreohalictus. A synonymical catalogue of species and species-group names in the genera Halictus and Seladonia, including 442 names, is provided as an appendix.