Genus Strongylus Müller, 1870

Synonyms: Sclerostomata Rudolphi, 1809; Sclerostoma (Rud.): Blainville, 1828; Sclerostomum Dujardin, 1845; Alfortia Railliet, 1923; Delafondia Railliet, 1923

General. Large Strongylinae. MC inflated, high, ring-shape, divided into inner and outer rings. Posterior edge of MC posterior to edge of BC. Amphids not markedly projected through MC surface. Tip and longer stalk of submedian papillae extend through MC. Tip of submedian papillae cone-shaped, round or oval, short. Stalk of submedian papillae longer than broad. Numbers of elements of ELC less numerous than ILC and markedly longer than ILC. Elements of ELC longer than broad, tip pointed; insertion point on tips of ILC. Elements of ILC longer than broad, tips pointed; insertion point on anterior edge of BC. Line formed by insertion of elements of ILC straight. Form of posterior edge of elements of ILC straight, unadorned. Support for ELC surrounds anterior edge of BC, helmut-like. Septum intracoronare origin on on BC. Medial insertion of septum intracoronare situated at junction of ELC and ILC. Walls of BC concave, thicker posteriorly, but without ring-like thickening. Buccal cavity oval, deeper than wide. Dorsal gutter elongate, more than 1/2 depth of BC. Buccal teeth present. Esophageal funnel shallow. Esophageal teeth not prominent. Anterior muscular portion of esophagus about ¼ to 1/3 of esophagus length. Excretory pore markedly anterior to, or at level of NR. Anterior deirids at level of NR.

Male: Dorsal ray with 6 branches. Ventral rays shorter than laterals. Dorsal lobe shorter then lateral lobes. Externodorsal rays origin on stem of dorsal rays. Gubernaculum small, straight, ventral groove. Genital cone short, conical. Spicule tips straight or slightly curved.

Female: Vulva more 2-3, or more, tail lengths from anus. Vagina shorter than sphincter. Ovejector vestibule vestibule T-shaped, infundibulum shorter than sphincter. Tail conical, short, less than 2x diameter at anus.

Type species: S. equinus Müller, 1784

Discussion

Looss (1900) recognized, in Sclerostomum (following Rudolphi, 1809; Diesing, 1851), 2 new species, S. edentatus and S. vulgaris, among specimens of S. equinus. Railliet and Henry (1909) reestablished Strongylus for these species. A fourth species, S. asini, was described from donkeys, Equus asinus, from East Africa by Boulenger (1920). The 4 species were grouped by Railliet (1923) in 3 subgenera (Strongylus (Strongylus) equinus, S. (Alfortia) edentatus and S. (Delafondia) vulgaris and S. (D.) asini), which were raised to generic level by Skrjabin (1933). More recently, most workers followed Lichtenfels (1975, 1980) in placing all 4 species in Strongylus, without recognizing Alfortia or Delafondia even at subgenus level. However, several experts on these nematodes have continued to recognize Alfortia and Delafondia either at the genus level (Popova, 1955; Dvojnos and Kharchenko, 1994; Zhang and Kung, 2002) or the subgenus level (Hartwich, 1994). The recognition of Alfortia and Delfondia has been based on differences in buccal teeth and dorsal rays of males. When included in the analysis, S. asini has been included in Delafondia with S. vulgaris because of the similar buccal tooth structure. This opinion was based on different life cycles, L3 morphology, pathology and convenience in namingdisease syndromes. However, Boulenger (1920) considered S. asini to be more similar to S. edentatus and S. equinus in characteristics of the female and male tails and in having an excretory pore at the level of the buccal capsule (near the nerve ring in S. vulgaris). Hung at al., 1996 found the ITS-2 sequence of S. asini to be more similar to those of S. edentatus (87.1%) and S. equinus (95.3%) than to that of S. vulgaris (73.9%). This result supports the retention of the 4 species within 1 genus. Thus, because of a lack of congruent morphological characters for subdividing Strongylus, we prefer to follow the simplest, most commonly used, taxonomy for these 4 similar species.