With 1,400 described species, Megaselia is one of the most species-rich genera in the animal kingdom, and at the same time one of the least studied. An important obstacle to taxonomic progress is the lack of knowledge concerning the phylogenetic structure within the genus. Classification of Megaselia at the level of subgenus is incomplete although Schmitz addressed several groups of species in a series of monographs published from 1956 to 1981. Another problem is the lack of molecular phylogenetic analyses to support morphology-based conclusions. As a contribution towards addressing these problems, we here circumscribe a previously unrecognized monophyletic lineage of Megaselia consisting of species similar to Megaselia lucifrons. We base this taxonomic decision on morphological study of an extensive phorid material from Sweden, complemented by molecular analyses of select exemplars using two markers (COI and 28S). We name the clade the lucifrons species group, and show that it contains three distinct species. Our results also demonstrate that Megaselia subnitida Lundbeck, 1920, previously treated as a synonym of Megaselia lucifrons Schmitz, 1918, is a separate species, and we remove it from synonymy. The third species in the group was previously unknown; we describe it here as Megaselia albalucifrons sp. n.

The genus Megaselia Rondani (Diptera: Phoridae) is one of the largest in the animal kingdom, with nearly 1700 described species and many remaining to be discovered. Work on this group is notoriously challenging due to the extreme species diversity, poor knowledge of higher‐level relationships and lack of molecular data. In this paper, we present the largest study to date of Megaselia relationships based on molecular data from one nuclear (28S rDNA) and three mitochondrial (ND1, COI and 16S) markers for 175 Nordic specimens representing 145 species of Megaselia, plus outgroups. Based on phylogenetic analyses of these data, we propose 22 informal Megaselia species groups, all of which match well‐supported terminal clades. Relationships among these groups, and between them and several isolated species, remain largely uncertain. Of the 22, 20 species groups fall into a moderately well‐supported monophyletic clade of ‘core Megaselia’. Two species groups, the spinigera and ruficornis groups, fall outside of core Megaselia, as does the single representative of Myriophora, a genus that is included in Megaselia by some specialists. Here, we explore the morphology of these molecular species groups to aid future studies, and we discuss the implications of our findings for the generic circumscription of Megaselia. Hopefully, our results can aid further characterization of subgroups within the enormous Megaselia radiation and among its closest relatives, thus facilitating future work on this challenging but fascinating group of small flies.