Habitat: larvae of O. castor occur in headwater streams, where they were found in the sandy gravel substrate (Boonsoong and Chainthong, 2016).
Distribution: - Tak Province - Chanthaburi Province - Ratchaburi Province - Kanchanaburi Province
Work cited: - Hämäläinen, M. and Pinratana, A. 1999. Atlas of the Dragonflies of Thailand: Distribution Maps by Provinces. Brothers of St. Gabriel in Thailand, Bangkok. - Boonsoong, B. and Chainthong, D. 2016. Description of two final stadium Onychogomphus larvae from Thailand (Odonata: Gomphidae). Zootaxa 4066 (5): 561-570.
