Key to all known species of Leidynema Schwenk in Travassos, 1929

1. Males with several protuberances in posterior region............................. L. portentosae Van Waerebeke, 1978

- Males without protuberances in the posterior region.......................................................... 2

2(1). Spicule=63µm long in male............................. .. L. socialis (Leidy, 1850) Adamson & Van Waerebeke, 1992

- Spicule less than 63µm long in male....................................................................... 3

3(2). Lateral alae present in females only.......................................................................4

- Lateral alae present in both sexes.........................................................................6

4(3). Lateral alae in female ends in backwardly pointed projection............................. L. schwencki Farooqui, 1967

- Lateral alae in female do not end in backwardly pointed projection.............................................. 5

5(4). Lateral alae in female extending along whole length of body without spinous process; cuticular bosses present.......................................................................................... L. delatorrei Chitwood, 1932

- Lateral alae in female start from mid-body and continuous up to tip of tail with spinous process; cuticular bosses absent......................................................................... L. stylopygi Biswas & Chakravarty, 1963

6(3). Female oesophagus 1/5th –1/6th of body length; males with five pairs of caudal papillae....... .. L. periplaneti Farooqui, 1967

- Female oesophagus 1/8th of body length; males with 3–5 pairs of caudal papillae................................................................................................. .. L. appendiculatum (Leidy, 1850) Chitwood, 1932