Triacanthagyna septima is a relatively small darner growing to a total length of 59 to 66 mm; and its thorax (the midsection or tagma of the insect body) is pale green with a brownish wash on the front.
Triacanthagyna septima is a relatively small darner growing to a total length of 59 to 66 mm (2.3 to 2.6 in) with a wingspan of 68 to 86 mm (2.7 to 3.4 in). The thorax is pale green with a brownish wash on the front and the legs are pale. The thorax is the midsection (tagma) of the insect body. 
