Ephemera vulgata
... that male Ephemera vulgata congregate in swarms in which each insect moves repeatedly up and down as part of a "nuptial dance"?
... that male <i id="mwBLE"><b id="mwBLI"><a href="./Ephemera_vulgata" id="mwBLM" rel="mw:WikiLink" title="Ephemera vulgata">Ephemera vulgata</a></b></i> <i id="mwBLQ">(pictured)</i> congregate in swarms in which each insect moves repeatedly up and down as part of a "nuptial dance"?