Key to 5 species of genus Hemiarthrus (based on females)

1 Pleon with bifurcated end....................................................... H. abdominalis (Krøyer, 1840)

- Pleon ending in a single point........................................................................... 2

2 Pereomeres almost completely fused......................................... H. surculus Boyko & Williams, 2004

- Pereomeres separated.................................................................................. 3

3 Barbula with three pairs of projections......................................................... H. alphei n. sp.

- Barbula with fewer than three pairs of projections........................................................... 4

4 Lateral plates of pleon with distal, hook-like projection.............................. H. nemaocarcini Stebbing, 1914

- Lateral plates of pleon entire, without any projections................................... H. synalphei (Pearse, 1950)