Description of Gattollius gen. nov. (Ephemeroptera, Baetidae), documenting a new morphological type of accessory gill

A new genus, Gattollius Suttinun & Boonsoong gen. nov. (Ephemeroptera: Baetidae), is described from northern Thailand, together with a new species, Gattollius thailandicus Suttinun & Boonsoong sp. nov., based on all life stages. The nymph is unique within Baetidae in possessing accessory gills in 3 positions: a single gill on each side of the mentum, a single gill on each side between the coxa and prosternum, and a single gill at the inner proximal margin of the femur on all legs bearing a femoral patch. The adult is also distinctive in retaining accessory gills, including coxal gills in both subimago and imago, and femoral gills on each leg of the subimago. Persistence of accessory gills into the adult stage represents the first such record in Baetidae. This study further documents new type of femoral accessory gills in mayflies. These structures are not only inferred to function in respiration but also bearing femoral patch, and we propose the term "femoral villopore gills" for this specialized organ. The phylogenetic position of the genus is established, supporting its placement within the tribe Acentrellini, based on diagnostic characters observed across all life stages and may be closely related to the monotypic Asiobaetodes Gattolliat, 2012 from Borneo and the species Acentrella fimbriata Tungpairojwong, Phlai-ngam & Jacobus 2022 from Thailand.