Updated dichotomous key to species of Amphimonhystrella (based on Gagarin & Gusakov 2023)

1. Gubernaculum absent, outer labial setae 8 μm long................................................. A. megastoma

- Gubernaculum present, outer labial setae shorter than 5 μm long................................................ 2

2. Gubernaculum without dorsal apophysis, amphidial fovea adjacent to the base of buccal cavity............ A. paramonovi

- Gubernaculum with dorsal apophysis..................................................................... 3

3. One testis, tail drop-like, enlarged strongly in terminal portion........................................ A. bullacauda

- Paired testes, tail not drop-like, enlarged in terminal portion................................................... 4

4. Buccal cavity divided into two sections, anterior one wide and short, posterior one long and narrow.................... 5

- Buccal cavity single, whole............................................................................. 6

5. Spicules 37–41 μm long, proximal end strongly curved................................................ A. tropica

- Spicules 12 μm long, proximal end not curved...................................................... A. boucheri

6. Body longer than 650 μm, outer labial setae longer than 3 μm.................................................. 7

- Body shorter than 650 μm, outer labial setae shorter than 3 μm................................................. 8

7. Buccal cavity large, 12.5–13 μm deep, gubernacular apophyses straight.................................... A. major

- Buccal cavity small, 5 μm deep, gubernacular apophyses curved ventrally........................... A. sinica sp. nov.

8. Buccal cavity cup-shaped, spicules with tapered tip, tail with a long filiform distal part..... A. parva Gagarin & Thanh, 2009

- Buccal cavity conical, spicules with round tip, tail with a short cylindrical distal part.............. A. unita Lorenzen, 1977