Diagnosis. The forewings are rather triangular, though with a rounded distal margin. They are very dark violet grey, fasciated with black in the distal half. The reniform is paler, ochreous brown and rather anvil-shaped. The zone at the base of the forewing, bounded by the antemedial, is also suffused paler brown, particularly the antemedial and subbasal fasciae. The black postmedial is an uneven zigzag, with a longer, straight section anterior to the subbasal angle. The submarginal is more irregular and weaker. The hindwings are a more or less uniform paler, brownish grey.
Taxonomic note. The male abdomen has a number of unusual features that suggest that the external resemblance of tanaocrossa to the type species is superficial. The eighth segment has long, slender, well separated apodemes on the tergite that are absent from perumbrosa. In the genitalia, the uncus is very slender, without setae, rather than very deep and setose. The tegumen is very short, but the vinculum is long (or incorporates a paratergal sclerite) and extends well dorsal to the articulation with the valve costa. The valves are very broad, falcate at the costal apex, and with an interiorly directed spine at the saccular apex. The valve shape is unusual, reminiscent of species of the genus Athetis Hubner. The aedeagus vesica is typically herminiinae and much more coarsely scobinate than in perumbrosa.
Geographical range. Borneo.
Habitat preference. The species is only known for the holotype male, taken at about 1800m on G. Murud in Sarawak.
