Key to species of the subgenus Sprentascaris parasitic in Neotropical freshwater fishes

1 Males with caudal alae and six pairs of postanal papillae.......................................................2

- Males without caudal alae and less than six pairs of postanal papillae.............................................3

2 Ventricular appendix/oesophageal length ratio in males and females is 1:0.16. Caudal alae well-developed. Vulva situated at 1.82–3.94 mm from anterior extremity, approximately at 1/3 of body length........................... R. (S.) mahnerti

- Ventricular appendix/oesophageal length ratio in males is 1:0.34 and in females 1:0.36. Caudal alae rudimentary. Vulva at short distance below oesophagus, at 0.40–1.06 mm from anterior extremity............................... R. (S.) lanfrediae

3 Lateral alae well developed (broad). In males, three pairs of postanal papillae......................................4

- Lateral alae absent or rudimentary. In males, five pairs of postanal papillae........................................5

4 Symmetrical and equal lips. In males, 22 pairs of preanal papillae. In females, mature eggs embryonated, thin-shelled................................................................................................ R. (S.) marano

- Dorsal lips smaller than ventrolateral ones. In males, 16 pairs of preanal papillae. In females, mature eggs embryonated with rough shell and membranous striations................................................. R. (S.) saltaensis sp. nov.

5 Narrow oesophagus. Rounded tail in both sexes. Posterior end of males with ventral cuticular ornamentation. Length of spic- ules 0.14 mm. Parasites of Pimelodidae fishes................................................... R. (S.) pimelodi

- Oesophagus markedly broad. Pointed tail tip in both sexes. Males with smooth posterior ventral surface. Length of spicules between 0.15–0.25 mm .................................................................... R. (S.) hypostomi