This checklist lists 98 species and subspecies of oligochaetes (Annelida) that have been reported so far as members of the natural fauna of Taiwan and its adjacent Lanyu (Botel Tobago), Gueishan (Turtle) and Penghu (Pescadore) Islands. Terrestrial oligochaetes have been studied for over a century since 1898, particularly intensively in the past ten years. They are represented by 72 species of which 42 (58.3%) species and subspecies are endemic to Taiwan. They belong to the families Moniligastridae, Glossoscolecidae, Lumbricidae, Octochaetidae, Ocnerodrilidae, and Megascolecidae. Megascolecidae is the most dominant family with 63 species, comprising 87.5% of the terrestrial Taiwanese earthworms reported so far. Aquatic oligochaetes have been investigated for about a decade since 1995, and are represented by 26 species. Of them, 25 are in the family Naididae (including Tubificidae) and Eiseniella tetraedra in the family Lumbricidae.

This paper describes a new species of athecate earthworm belonging to the illotus species-group of the genus Amynthas Kinberg from Taiwan. It is Amynthas hohuanmontis sp. nov. from Mt Hohuan at an elevation of 3000 m in the Central Mountain Range. The athecate earthworm Amynthas pusillus (Ohfuchi) from Ishigaki Island, Riukiu is found to be synonymous with Amynthas assacceus (Chen) from Hainan Island, China. Accordingly, the A. illotus species-group currently consists of A. hohuanmonti s sp. nov. from Taiwan, A. illotus (Gates) from Yunnan and Riukiu, A. assacceus (Chen) of Hainan and Riukiu, A. sheni (Chen) from Hong Kong and A. oyuensis (Ohfuchi) from Japan. Based on characters of particularly the porophore structure, the ® ve species are divided into two species-subgroups: the sheni species-subgroup of A. sheni and A. hohuanmonti s, and the assacceus species-subgroup of A. illotus, A. assacceus and A. oyuensis. The two subgroups might have diOE erentiated from two independent ancestors. The ® ve species show an arc-shaped distribution in the peripheral regions of the East Asian continent (Yunnan, Hainan, Hong Kong, Taiwan, Riukiu and Japan), suggesting them to be survivors from the Pleistocene ice ages.

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This paper describes four new species of earthworms, Amynthas shinanmontis sp. nov., Amynthas chilanensis sp. nov., Amynthas bilineatus sp. nov., and Amynthas cruxus sp. nov. from Taiwan. Amynthas shinanmontis is sexthecate and parthenogenetic with individuals with various levels of degeneration of spermathecae and male reproductive organs. It occurs at elevations of 1700-2700 m in the Central Mountain Range in southern Taiwan. Amynthas bilineatus and A. chilanensis are athecate and also parthenogenetic. They occur, respectively, at elevations of 1000 m in the central region and 1325 m in the northeastern region of Taiwan. Amynthas cruxus is octothecate, amphimictic, and occurs at elevations of 900-1700 m in southern Taiwan. This paper also discusses confusion and dilemma caused by uncertain, speculative synonyms and parthenogenetic morphs in species delimitation in oligochaete taxonomy, and recommends the use of absolute synonyms and phylogenetic species concept.