Nepenthes dubia certainly is one of the least well known species of the genus. It is only recorded from Gunung Talakmau in West Sumatra though there is a lot of confusion with a hybrid between N. inermis and N. talangensis which is found on Gunung Talang. This hybrid resembles it a little. Note the lid of the pitcher which points backwards and the flat peristome. Interestingly N. dubia grows exclusively as an epiphyte.
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