Forty-four species known from North America are described and figured, and a key is erected for their identification. Most of the species described by earlier American workers (Eisen, Moore, Smith, Verrill) were de-discovered, most of the cosmopolitan species have now been reported, and twelve new species are described. The species may be divided into four geographical groups: - Cosmopolitan, Eastern (with at least related species in Europe), Western (related to Asiatic forms), and Pan- American species. Twenty species are unknown outside North America. Some further new species are now known to occur in the Great Lakes, but these will be described in a separate publication