Review of Thelastoma Leidy, 1849 (Nematoda: Oxyuridomorpha: Thelastomatidae) and description of T. variabile sp. nov. from the North American indigenous millipede Narceus americanus (Spirobolida: Spirobolidae).

Thelastoma variabile sp. nov. (Nematoda: Oxyuridomorpha: Thelastomatoidea) is described from the intestine of 82 specimens of the Narceus americanus/annularis complex (Diplopoda: Helminthomorpha: Spirobolidae) collected from April through October 2021 in Hochatown, McCurtain County, Oklahoma, U.S.A. A total of 343 T. variabile sp. nov. consisting of 315 females, 1 male and 27 juveniles were extracted primarily from the hindguts of the millipedes. Examination of the new species was conducted with the aid of brightfield, differential interference contrast (DIC), confocal, and scanning electron microscopies (SEM). The head of the female T. variabile sp. nov. is characterized by having a domelike lip region, large first annule, secretory-excretory pore about level with the base of the basal bulb, and a tail shape ranging from short and conical to long and filiform. The new species represents only the sixth Thelastoma spp. described from a North American diplopod. This new species was used as a template for review and evaluation of the characters used to differentiate the 74 currently known species of the genus. Several new characters are proposed for the separation of Thelastoma spp.