Key to species groups of Otostigmus (Otostigmus)

1 Only tergite 21 marginate ................................................................................................................. spinicaudus group

- Marginate at least from tergite 17 ................................................................................................................................. 2

2 Three antennomeres glabrous dorsally ........................................................................................................................ 3

- 2–2.5 antennomeres glabrous dorsally.......................................................................................................................... 5

3 Ultimate leg prefemur with two rows of ventrolateral spines ............................................................ aculeatus group

- Ultimate leg prefemur with one row of ventrolateral spines ...................................................................................... 4

4 Tarsal spurs minute or absent..................................................................................................................... nudus group

- Most legs with tarsal spurs........................................................................................................................ politus group

5 With sharp keels at least from tergite 7..................................................................................................... scaber group

- Tergites lacking keels or with them at most moderately developed ............................................................................ 6

6 With 24 or more antennomeres .......................................................................................................... longicornis group

- With 17–22 antennomeres ........................................................................................................................................... 7

7 Posterior sternites tuberculate.......................................................................................................... tuberculatus group

- Posterior sternites without tubercules .......................................................................................................................... 8

8 Coxopleuron of segment 21 with dorsal spine..................................................................................... rugulosus group

- Coxopleuron of segment 21 without dorsal spine .............................................................................. orientalis group