The Mayfly genus Paegniodes currently contains two species, Paegniodes cupulatus, from China, and Paegniodes dao, from Vietnam. Neither of these species is well known, and only Paegniodes cupulatus has been genetically sequenced, but they are considered to both be members of the same, unique, genus with some confidence, based upon unique characters of the imaginal and larval stages.
In a paper published in the journal ZooKeys on 10 May 2021, Boonsatien Boonsoong of the Animal Systematics and Ecology Speciality Research Unit at Kasetsart University, Chonlakran Auychinda of the Department of Biology and Health Science at Mahidol Wittayanusorn School, Michel Sartori of the Museum of Zoology at the Palais de Rumine, and the Department of Ecology and Evolution at the University of Lausanne, and Nuttakun Khanyom, also of the Animal Systematics and Ecology Speciality Research Unit at Kasetsart University, describe a third species of Paegniodes from Nan Province in Thailand.
The new species is named Paegniodes sapanensis, meaning 'from Sapan', in reference to the Sapan waterfall, a popular tourist attraction in Bo Kluea District of Nan Province, where the species was discovered.
The species is described from subimagos (a subimago is the penultimate stage in the development of a Mayfly, with the ability to fly but not reproduce) and larvae. A single female subimago is described, this having a body length of 12.8 mm, with cerci (paired appendages on the rear-most segments) 20.0 mm in length, a forewing measurement of 14.4 mm and a hindwing measurement of 2.6 mm. The head, thorax and abdomen are dorsally brown and ventrally light brown; the legs are yellow and the caudal filaments brown. The wings are semitransparent, with veins yellowish to brown, the sternum light brown, and a pair of dark dots is present on both the pronotum and mesonotum. A single male subimago described is coloured similarly to the female, but with a body length of only 8.5 mm, cerci of 17.5 mm, a forewing length of 10.9 mm, and a hindwing length of 1.5 mm. A described larval specimen is 16.2 mm in length, and dark brown in colour, with a whitish underside.
The species is known only from Nan Province in Thailand. The specimens were collected from tropical mountain streams which are slightly disturbed by tourist activities. The larvae of the new species were found in flowing areas and the littoral zone of the streams, underneath a mostly cobble substrate.
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