Key for identification of Biapertura species of the world

1. Thoracic limb I with IDL seta 1 thin, straight, not claw–like, about half length of seta 2................................................................................................... Biapertura lepida (Birge, 1892) (USA).

- Thoracic limb I with IDL seta 1 thick, strongly curved, claw–like, of similar length with seta 2........................ 2

2. Posteroventral angle of valves with denticles....................................................................................... Biapertura sibirica (Sinev, Karabanov & Kotov, 2020) (North Europe, North and Central Asia).

- Posteroventral angle of valves without denticles............................................................. 3

3. Postabdomen of female evenly narrowing distally in postanal portion............................................ 4

- Postabdomen of female with parallel margins in basal half of postanal portion..................................... 5

4. Spine on basal segment of antennal exopodite as long or longer than middle segment............................................................................................ Biapertura elliptica (Sinev, 1997) (East Australia).

- Spine on basal segment of antennal exopodite much shorter than middle segment............................................................................................... Biapertura martensi (Sinev, 2007) (South Africa).

5. PP in adult females is over 2.5 IP.................................. Biapertura kendallensis (Henry, 1919) (Australia)

- PP in adult females about not exceeding 2 IP................................................................6.

6. Postabdomen of adult male with right, clearly defined distoventral angle..................................................................................................... Biapertura affinis (Leydig, 1860) (Eurasia, Africa).

- Postabdomen of adult male with rounded distoventral angle..... Biapertura ossiani (Sinev, 1998) (North and South America, South Korea), two subspecies............................................................................ 7

7. Postabdomen of adult male without any marginal denticles............. Biapertura ossiani ossiani (Sinev, 1998) (Brazil) .

- Postabdomen of adult male with 1–3 marginal denticles at postanal angle...................................................................................... Biapertura ossiani herricki Sinev, 2013 (USA, Canada, South Korea).