Diagnosis. This species is medium grey, marked more darkly in black, distinguished from fractalis Guenée by a more diffuse forewing medial band, and a submarginal that runs straight, whitish, with a blacker zone basad, from the forewing costa almost halfway to the dorsum before curving distad to the margin at its central angle. The hindwing is only slightly paler than the forewing. In the male genitalia the valves are much more elongate than in fractalis, with the curved section restricted to the distal half, and the uncus has a shorter dorsal spur and a longer section distal to this. The aedeagus vesica has a multidentate basal cornutus (almost saw-like) and much coarser scobination distally.
Taxonomic note.H. porphyrialis Pagenstecher (Moluccas, New Guinea) has similar forewing facies except the equivalent of the straight section of the forewing submarginal is distinctly dentate. The male genitalia of the two species are very similar except the distal, curved section of the valve is more expanded in porphyrialis.
Habitat preference. In a survey of logged and unlogged forest at around 170m near the Danum Valley Field Centre (S.J. Willott, unpublished data), a species identified as biasalis was recorded in frequency from the former, and from the understorey, but not the canopy, of the latter. As noted in the generic account, the species is also abundant in lowland softwood plantations.