Holotype. Thailand • Chai Nat Province, Mueang Chai Nat District, Ban Kluai Subdistrict; 15°10.65'N, 100°08.70'E; 17 Mar. 2018; ASME members leg.; Lotus pond; MUMNH-HIR008-28; dissected (BL 111.70 mm, BW 15.50 mm; CL 12.45 mm, CW 12.85 mm). Paratype. Thailand • 2 specimens; same collection data as for holotype; MUMNH-HIR008-01; dissected (BL 129.15 mm, BW 13.35 mm; CL 11.90 mm, CW 11.95 mm), MUMNH-HIR008-02; dissected (BL 117.25 mm, BW 13.05 mm; CL 11.70 mm, CW 12.30 mm).

Thailand • 25 specimens; same collection data as for holotype; MUMNH-HIR008-03 to HIR008-27 • 1 specimen; Mukdahan Province, Wan Yai District, Wan Yai Subdistrict, Chanot Stream; 16°43.76'N, 104°43.77'E; 1 Apr. 2018; ASME members leg.; MUMNH-HIR001-01 • 1 specimen; Nakhon Phanom Province, Tha Uthen District, Songkhram River; 10 Apr. 2018; E. Jeratthitikul leg.; MUMNH-HIR004-01 • 8 specimens; Phrae Province, Sung Men District, Ban Kwang Subdistrict; 18°04.26'N, 100°11.26'E; 13 May. 2018; Local peoples leg.; MUMNH-HIR003-01 to HIR003-08 • 3 specimens; Buriram Province, Krasang District, Nong Teng Subdistrict, Chi River; 14°52.51'N, 103°22.72'E; 18 Sep. 2018; E. Jeratthitikul and C. Sutcharit leg.; MUMNH-HIR009-01 to HIR009-03 • 4 specimens; Ubon Ratchathani Province, Khemmarat District, Huai Na Muang stream; 16°01.75'N, 105°15.91'E; 12 May. 2018; E. Jeratthitikul leg.; MUMNH-HIR010-01 to HIR010-04.

In life, dorsal surface dark brown to green. Lateral spots on annuli b2 and b5, black, squared, not prominent, reduced to small spots on anterior somites. Ventral surface green to dark greenish or dark olive. Male gonopore in somite XI b5/b6. Female gonopore in somite XII b5/b6. Gonopores separated by 5 annuli. Atrium of male reproductive organ moderate-sized, bulbous, rather round. Penis sheath short. Ejaculatory ducts ventrally inserted into middle of atrium. Common oviducts open into the female bursa near the insertion point of vaginal caecum to female bursa.

Body firm, muscular, with constant width posteriorly. In life, dorsal surface dark brown to green in background color. Dorsal median line black, distinct, somewhat narrow on annuli b1 and b6 of each mid body somite, and very narrow or broken between somites V and VI, VI and VII, and VII and VIII. Two longitudinal, inconspicuous, and broken stripes on each side of median stripe, each stripe present in b2–b5 and absent in b1 and b6 of each mid body somite. Lateral spots on annuli b2 and b5, black, squared, not prominent, reduced to small spots on some anterior somites. Ventral surface green to dark greenish or dark olive, with narrow marginal pale-yellow stripe. Color faded in preservative, except the dorsal median line and other dorsal markers.

Number of annuli 103. Somites I–III uniannulate. Somites IV and V biannulate, (a1+a2)=a3. Somite VI dorsally triannulate, a1=a2<a3; ventrally biannulate, (a1+a2)=a3. Somite VII triannulate, a1=a2<a3. Somite VIII quadrannulate, a1>a2=b5>b6. Somites IX–XXIII quinquannulate, b1=b2=a2=b5=b6. Somite XXIV quadrannulate, b1=b2=a2=a3. Somite XXV being last complete annulus ventrally; triannulate, a1=a2=a3. Somite XXVI biannulate, (a1+a2)>a3. Somite XXVII biannulate, (a1+a2)>a3. Anus on somite XXVII a3. Clitellum between X b5 and XIII a2.

Eyespots 5 pairs, arranged dorsally in parabolic arch: first pair anterior on somite II, remaining eyespots situated laterally; second pair on somite III; third pair on somite IV (a1+a2); fourth pair on somite V (a1+a2); and fifth pair on somite VI a2. Sensillae on annulus a2 of complete somites; 3 bilateral rows dorsally, one proximal to midline, 2 paired distal to midline; and 2 bilateral rows ventrally. Nephridiopores 17 pairs, positioned on ventral surface, situated bilaterally at posterior margin of VIII a1, and at posterior margin of b2 of somites IX to XXIV.

Oral sucker, dorsal lip prominent, with several furrows. One median longitudinal furrow on ventral surface deep, extending from margin of lip to pharynx. Two deep furrows on each side, and one or two shallow furrows between deep furrows. Three jaws in oral cavity, 1 dorsal and 2 ventrolateral. Small salivary papillae circular, placed along denticular ridge. Large salivary papillae circular to ovate; irregularly placed laterally on each jaw; minimum of 30 visible on one side of right ventro-lateral jaw. Teeth on median jaw 105–125 per jaw. Pharynx muscular and tubular reaching to somite IX b2/a2. Crop reaching to somite XIX b5. Crop caeca10 pairs: Intestine tubular, acaecate, extending from somite XIX b6 to somite XXIII. Rectum thin-walled, tubular, extending from somite XXIII to somite XXVII. CL 11.9 mm, 13.0 mm wide, round, with eight rays of sensillae on dorsal surface radiating out from center.

Male gonopore in somite XI b5/b6. Female gonopore in somite XII b5/b6. Gonopores separated by 5 annuli. Nerve cord passing along the left side of reproductive system. Male reproductive system in somites XI and XII. Atrium moderate-sized, bulbous, rather round. Penis sheath short, recurved anteriorly to atrium. Epididymes round, paired, lateral to atrium. Ejaculatory bulbs present, muscular, and thick. Ejaculatory ducts short, ventrally inserted into middle of atrium. Testisacs 11 pairs. Female reproductive system in somites XII and XIII.

Ovaries paired, small, droplet-shaped. Oviduct paired, narrow, short, join to form common oviduct. Common oviduct moderate in length, slightly bent distally, then descending to vagina. Albumin gland at point of intersection of two oviducts and common oviduct. Common oviduct opens into female bursa near the insertion point of vaginal caecum to female bursa. Vaginal caecum elongated, two times as long as wide at widest point. Vaginal caecum anteroventral end connected to female bursa.

The specific name “thailandica” refers to the country in which specimens of the new species were collected.

This species is known from several river basins in Thailand, including the Chao Phraya and the Middle Mekong river basins (Fig. 4B).