The relationship of the two families Bibionidae and Scatopsidae is discussed and for each, accounts are given of their structure, life histories and economic importance, followed by details of methods of collection and examination and revised check lists. Keys are presented to the two genera and 20 species of Bibionidae known from Britain. For the Scatopsidae, keys are given to the four subfamilies, three of which contain only one British genus, and to the three tribes of the fourth subfamily, the Scatopsinae, and to their included genera; altogether 15 genera and 37 species of this family are recognized as British.
In the Bibionidae, a new species Bibio edwardsi Freeman & Lane is described and B. hybridus Haliday is raised from varietal rank to be treated once again as a full species. In the Scatopsidae, a new species Rhexosa richardsi Freeman, is described, and a new genus Cookella Freeman proposed for Scatopse albitarsis Zetterstedt, formerly placed in Holoplagia. Ferneiella is described as a new genus by E.F. Cook, with Scatopse incompleta Verrall as type species.