Behningiidae and Potamanthidae (Insecta, Ephemeroptera) in Thailand Author Kwanboon, Sedtawut https://orcid.org/0000-0002-3858-9522 Animal Systematics and Ecology Speciality Research Unit (ASESRU), Department of Zoology, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand Author Sartori, Michel https://orcid.org/0000-0003-3397-3397 Museum of Zoology, Palais de Rumine, Place Riponne 6, CH- 1005 Lausanne, Switzerland & University of Lausanne (UNIL), Department of Ecology and Evolution, CH- 1015 Lausanne, Switzerland Author Boonsoong, Boonsatien https://orcid.org/0000-0002-8166-0021 Animal Systematics and Ecology Speciality Research Unit (ASESRU), Department of Zoology, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand fscibtb@ku.ac.th text ZooKeys 2021 2021-10-29 1067 57 82 http://dx.doi.org/10.3897/zookeys.1067.72779 journal article http://dx.doi.org/10.3897/zookeys.1067.72779 1313-2970-1067-57 8DB9C8FE9DF7438D92BA4B40E81F5DE1 921145FDEC60583E831A6266908B4BCD Rhoenanthus (Potamanthindus) magnificus Ulmer, 1920 Figures 10B, D , 11B , 12 , 14 , 15 Materials examined. 5 larvae , Thailand , Chiang Mai province , Chiang Dao , Mae Na , 19°19'13.08"N , 98°53'25.98"E , 742 m , 11.III.2016 , B. Boonsoong leg. (ZMKU) ; 8 larvae , Loel province , Nam Thob ranger station, 17°15'36.5"N , 101°34'52.9"E , 338 m , 20.III.2016 , B. Boonsoong leg. (ZMKU) ; 1 larva , Nan province , Bo Kluea district , Sapan waterfall, 19°11'25.8"N , 101°11'56.3"E , 800 m , 28.XI.2020 , B. Boonsoong leg ; 2 larvae , Nan province , Bo Kluea district , Lamer resort, 19°09'08.8"N , 101°09'17.0"E , 28.XI.2020 , S. Kwanboon leg ; 3 larvae , Nan province , Bo Kluea district , Mae Nam Wa stream, 19°16'22.6" N 101°10'48.2" E , 848 m , 26.XI.2019 , B. Boonsoong leg ; 7 larvae , Chiang Rai province , Mueang district , Mae Kon stream, 19°51'46.1"N , 99°39'04.7"E , 534 m , 6.III.2021 , S. Kwanboon leg ; 2 larvae , Chiang Rai province , Mueang district , Mae Kon stream, Pong Phrabat waterfall, 20°00'41.8"N , 99°48'15.1"E , 470 m , 7.III.2021 , S. Kwanboon leg. Diagnosis. The larvae of Rhoenanthus magnificus (Fig. 12 ) can be distinguished from those of other Rhoenanthus (Potamanthindus) species based on the following characteristics: i) large body size (18-21 mm), ii) mandibular tusks arched inward about 33-34° (angle measurement as shown in Fig. 10D ), iii) length of the mandibular tusks ca 1.4 x length of head, and iv) length of the foretibiae ca 1.5 x length of the forefemora and about 2.9 x length of the foretarsi (Fig. 11B ) ( Nguyen and Bae 2004 ). Distribution. Chiang Mai, Chiang Rai, Loei, and Nan provinces. Remark. The larva of R. magnificus was originally described by Nguyen and Bae (2004) from material collected in northern and central Vietnam. The species is known from southern China and Vietnam. In the present study, we found this species in streams of several provinces (Fig. 14 ).