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Summary
Prosopistoma foliaceum is an Ephemeroptera of which only the nymph and the female subimago obtained by Vayssiere in 1880 are known. The nymph can be found sporadically, generally in rivers. One of our students picked up about twenty nymphs out of the Rhône in Lyon at the end of September 1951. In this river it reaches the most northerly point of its known range.
The nymph has six pairs of gills and not five as is commonly described. The sixth gill is a simple, rolled blade. The antennae consist of six joints. It has been possible to keep a nymph alive for seven months in still water. So, the tossing and the changing of the water are not necessary.
Some features of the gills, the shield, and the underlip tempt us to connect the nymph of Prosopistoma, so different from the typical nymph of the Ephemeroptera, with the nymphs of the genera Coenis and Boetisca.