Taxonomic insights into the diversity of Cloeon Leach, 1815 (Ephemeroptera, Baetidae) in Thailand

Four species of the genus <i>Cloeon</i> have previously been reported in Thailand: <i>C. bicolor</i> Kimmins, 1947, <i>C. bimaculatum</i> Eaton, 1885, <i>C. harveyi</i> (Kimmins, 1947) and <i>C. marginale</i> Hagen, 1858. However, since 1961, no systematic studies or investigations of this genus have been conducted in that country. This study reviews <i>Cloeon</i> Leach, 1815 in Thailand and investigates five species-<i>C. bengalense</i>, <i>C. bicolor</i>, <i>C. harveyi</i>, <i>C. rubellum</i>, and <i>C. viridulum</i>-based on morphological, molecular, and taxonomic analyses. Three species (<i>C. bengalense</i>, <i>C. rubellum</i>, and <i>C. viridulum</i>) are recorded for the first time in Thailand, and <i>C. rubellum</i> is described for the first time as a mature nymph and adult female. Additionally, the egg chorionic surface of all five species is described for the first time, and its use is demonstrated for species identification in Thailand. A key to the species of <i>Cloeon</i> in Thailand is provided based on the egg structure, mature nymph, and adult female and male.