This is the southernmost member of the genus. The description is based on the reared Argentine material from Buenos Aires. Unlike in T. australis (Hag ), the only congener of which the immature stage has been described, in T. risi there are dorsal hooks on segments 4-8 only. This requires a slight modification of the generic key provided by J.G. NEEDHAM & M.J. WESTFALL, 1955, Dragonflies of North America, Univ. California Press. A modified key is appended.