A study of Chromagrion Needham, 1903, Hesperagrion Calvert, 1902, and Zoniagrion Kennedy, 1917: three monotypic North American damselfly genera with uncertain generic relationships (Zygoptera: Coenagrionidae)

Comparative morphology identifies Chromagrion as the sister genus of Pyrrhosoma Charp. The genera Hesperagrion, Anisagrion Selys, Apanisagrion Kennedy and Calvertagrion St. Quentin probably form a monophyletic group as they share a bifid apical penis segment armed with a pair of sclerotized spine-like processes. A new interpretation of certain penis structures, and biogeographic considerations, suggest that Zoniagrion is probably a primitive genus, which occupies a basal position on the stem of the Acanthagrion-series, within the ischnurine Coenagrionidae.