The latest record of Hexagenitidae (Insecta: Ephemeroptera) with elongated abdominal sternum IX from mid-Cretaceous Myanmar amber

A new species, Hexameropsis elongatus sp. nov., in the extinct family Hexagenitidae of Ephemeroptera, is described and figured based on two well-preserved male mayflies from the mid-Cretaceous Myanmar (Burmese) amber. The new species has the following diagnostic characters: fewer cross-veins and intercalary veins, R3b in contact with R2, the first curved loop-like triad wider than the other two triads in forewing, MA of hind wing branched at the quarter length near wing margin and abdominal sternum IX well-developed, more than three times as long as abdominal tergum X and tapered from the base of gonostyli to the apical. The well-developed abdominal sternum IX of H. elongatus sp. nov. is a unique feature in Ephemeroptera because abdominal sternum IX is usually covered by abdominal tergum X in almost all extant or fossil mayflies documented up to date. The new finding, the first record in Myanmar amber and the latest Hexagenitidae hitherto, provides important morphological characters to enhance our understanding of the evolution of Hexagenitidae. To clarify and compare the morphological characters of various genera and species of Hexagenitidae imagoes, a Key based on venational characters is provided.