Collection records and distributional updates of six rarely collected North American large river mayfly taxa (Insecta: Ephemeroptera)

Sampling the Lower Mississippi River (LMR) main channel and secondary channels during 2014–2015 and subsequent dietary analysis of river sturgeons (<i>Scaphirhynchus</i> spp.) captured in the Missouri River resulted in the collection of larvae of six mayfly species that specialise in large river habitats. Some of these mayflies are found in large numbers when encountered, while others are infrequently collected and always in low numbers. Two species, <i>Raptoheptagenia cruentata</i> (Walsh, 1863) and <i>Cercobrachys</i> sp. cf. <i>serpentis</i> Soldán, 1986 (Insecta: Ephemeroptera) are reported for Mississippi for the first time. <i>Pseudiron centralis</i> McDunnough, 1931 and <i>R. cruentata</i> are reported for Arkansas for the first time. Updated North American distributions are presented and include records available in major collections and published literature. The apparent restriction of these taxa to large river environments highlights the need for implementation of conservation and restoration efforts to ensure their persistence.