A description of a new species of horsehair worm, Acutogordius taiwanensissp. n., a redescription of Chordodes formosanus, and novel host records for the latter are provided. Acutogordius taiwanensissp. n. is morphologically similar to A. protectus with moderately flat areoles on its tail tips, but is distinguishable by small mid-body ornamentations. Despite the distinct differences in the post-cloacal crescents between 14 male samples, their conspecific status, along with that of nine female samples, was upheld by a phylogenetic comparison of partial cytochrome oxidase subunit I (COI) sequences. Chordodes formosanus is another common horsehair worm species in Taiwan, which was previously believed to specifically parasitize Hierodula mantids. However, in this study, five C. formosanus were observed emerging from an Acromantis mantid, and two long-horned grasshopper hosts (Leptoteratura sp. and Holochlora japonica). These five worms showed high degrees of similarity in COI sequences and morphology, but one of these individuals bore abnormal crowned areoles, which has never been observed in C. formosanus, and may be attributed to the incomplete development of this particular individual.
Keywords: Acutogordius taiwanensis; Chordodes formosanus; immature stage; new species; novel host.