The phylogeny of the insect infraorder Bibionomorpha (Diptera) is reconstructed based on the combined analysis of three nuclear (18S, 28S, CAD) and three mitochondrial (12S, 16S, COI) gene markers. All the analyses strongly support the monophyly of Bibionomorpha in both the narrow (sensu stricto) and the broader (sensu lato) concepts. The major lineages of Bibionomorpha sensu lato (Sciaroidea, Bibionoidea, Anisopodoidea, and Scatopsoidea) and most of the included families are supported as monophyletic groups. Axymyiidae was not found to be part of Bibionomorpha nor was it found to be its sister group. Bibionidae was paraphyletic with respect to Hesperinidae and Keroplatidae was paraphyletic with respect to Lygistorrhinidae. The included Sciaroidea incertae sedis (except Ohakunea Edwards) were found to belong to one clade, but the relationships within this group and its position within Sciaroidea require further study.

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The Coal Creek Member of the Kishenehn Formation in northwestern Montana, USA, is an emerging middle Eocene Lagerstätte. While fish, plant, mammal and molluscan fossils are present, the most numerous and well-preserved fossils are those of insects. In this study, we initiate an effort to enumerate, at the family level, the diversity of flies (Insecta: Diptera) at this locality. Seventeen specimens from 17 different families (15 families with Limoniinae and Cylindrotominae within Tipulidae s.l.), 15 new species and three new genera are described. These include Tipula fji sp. nov. (Tipulidae), Ellipteroides kishenehn sp. nov. (Limoniidae), Cyttaromyia lynnae sp. nov. (Cylindrotomidae), Sylvicola silibrarius sp. nov. (Anisopodidae), Efcookella nigra sp. nov. (first fossil known in the genus) (Scatopsidae), Bibiodes kishenehnensis sp. nov. (Bibionidae), Eosciarites hermes gen. et sp. nov. (Sciaridae), Rymosia hypnolithica sp. nov. (Mycetophilidae), Litoleptis araeostylus sp. nov. (Rhagionidae), Kishenehnoasilus bhl gen. et sp. nov. (Asilidae), Drapetis adelomedos sp. nov. (Hybotidae), Salishomyia eocenica gen. et sp. nov. (Dolichopodidae), Agathomyia eocenica sp. nov. (first known fossil in genus) (Platypezidae), Lonchoptera eocenica sp. nov. (Lonchopteridae) and Aenigmatias kishenehnensis sp. nov. (Phoridae). Two specimens in the families Psychodidae and Pipunculidae are described but not assigned to a genus. In addition, we revise several related fossil species housed at the NMNH. Asilopsis fusculus Cockerell, 1921, formerly described in Asilidae, is transferred to Cyttaromyia (Cylindrotomidae) as C. fuscula, Sciara florissantensis Cockerell, 1917 is assigned to Sciaroidea incertae sedis, and Sciara gurnetensis Cockerell, 1916, Sciara lacoei Cockerell, 1915 and Sciara protoberidis Cockerell, 1915, are assigned to Sciaridae incertae sedis. Given their diversity and high degree of preservation, continued characterization of the Coal Creek Member fossils may help elucidate the Eocene radiation of Diptera in North America.

Holarctic distributions in the genus Bibio, minus the Bibio clavipes-group, are discussed. The species Bibio brunnipes (Fabricius), Bibio rufipes (Zetterstedt) and Bibio siebkei Mik are found to have Holarctic distributions. Bibio fulvipes (Zetterstedt), Bibio fumipennis Walker, Bibio tenellus Hardy, and Bibio rufitibialis Hardy are synonymized with Bibio brunnipes. Bibio velorum McAtee and Bibio columbiaensis Hardy are synonymized with Bibio rufipes. Bibio labradorensis Johnson and Bibio monstri James are synonymized with Bibio siebkei. The relationship between the Palaearctic Bibio pomonae (Fabricius) and the Nearctic Bibio holtii McAtee and Bibio bryanti Johnson is discussed. Bibio strobli Duda is tentatively retained as a good species. Diagnoses are given for each species and illustrations of the male fore tibia, hypopygia and hind leg and the female terminalia and head are given for comparison. The ecological preferences and flight periods of each species are commented on.

The New World species of Penthetria Meigen are revised. Eight species are recognized: P. appendicula Hardy, P. arizonensis Fitzgerald n. sp., P. distincta Hardy, P. dolichopeza Fitzgerald n. sp., P. heteroptera (Say), P. mexicana (Hardy), P. neonigrita Fitzgerald n. sp., and P. yakima Fitzgerald n. sp.. Penthetria heros Say and P. nigerrima (Bellardi) are considered unrecognized (nomen dubium) and P. nigrita Perty is transferred to the genus Plecia Wiedemann, where it is a junior synonym of Plecia plagiata Wiedemann. The fossil species Penthetria dubia Geinitz is removed from family Bibionidae. Additionally, a summary of current knowledge about the genus, a generic description and diagnosis, and a catalog of the 36 (extant) world species are provided.