Limnotrechus gracilicornis Horváth, 1879: 109 (type locality: Kyushu, Japan).

Hydrometra jankowskii Jakovlev, 1889: 337 (type locality: Sidemi, Russia) (synonymized by Andersen, 1975: 22).

Gerris selma Kirkaldy, 1903b: 181 (type locality: Sikkim and Kurseong, N India) (synonymized by Distant, 1904: 178).

Gerris lepcha Distant, 1910 a: 140 (type locality: Sikkim, Shamdang, India) (synonymized by Andersen & Chen, 1993: 152).

Gerris (Gerriselloides) gracilicornis (Horváth); Andersen, 1975: 22; Andersen & Chen, 1993: 152, Figs. 2, 11, 12, 16 (synonymy).

Gerris (Macrogerris) gracilicornis (Horváth); Andersen, 1993: 2482–2483.

Material examined. VIETNAM: Lao Cai Prov.: 2 males, 1 female (brachypterous), 1 male, 5 females (macropterous), Sa Pa, Cat-Cat, FVBC, 1250m, coll. Frontier Vietnam, 8 Apr.–2 Jun. 1998 (MTMB); 1 male (apterous), Sa Pa, Cat-Cat, Fr. Sat.Camp FTO, 1890m, coll. Frontier Vietnam, 30 Apr. 1998 (MTMB); 3 males (brachypterous), Sa Pa, Hoang Lien NP, Nui Xe, small waterflow, coll. Tran A.D., 2 Jun. 2003, TAD0338 (ZRC); 1 female (brachypterous), 1 male (macropterous), Sa Pa, Nui Xe, small waterflow near the Forest Ranger Station, coll. Tran A.D., 2 Jun. 2003, TAD0339 (ZRC); 1 male, 1 female (brachypterous), 4 males, 4 females (macropterous), Sa Pa, small stream near Thac Bac and waterpools nearby, coll. Tran A.D., 3 Jun. 2003, TAD0344 (ZMHU); 4 males, 3 females (brachypterous), 10 males, 15 females (macropterous), Sa Pa, Nui Xe, waterpools near Suoi Vang, coll. Tran A.D., 3 Jun. 2003, TAD0346 (ZRC); 1 male, 1 female (brachypterous), 1 female (macropterous), Sa Pa, Hoang Lien N'Park, Nui Xe, Suoi Vang, coll. Tran A.D., 4 Jul. 2004, TAD0417 (ZRC); 1 female (brachypterous), 1 male 1 female (macropterous), Sa Pa, Hoang Lien N'Park, Nui Xe, a small water flow, coll. Tran A.D., 4 Jul. 2004, TAD0418 (ZRC); 5 males, 4 females (brachypterous), 1 male, 1 female (macropterous), Sa Pa, Nui Xe, water pools near Suoi Vang stream, coll. Tran A.D. & Dinh, T.N.H., 23 Apr. 2011, TAD1108 (ZMHU); 1 male (brachyterous), Sa Pa, Nui Xe, small tributary of Suoi Vang stream, coll. Tran A.D. & Dinh, T.N.H., 23 Apr. 2011, TAD1109 (ZMHU).

CHINA: 1 male, 2 females (macropterous), “Chekiang, Hangtoheou, A. Pichon, 1925” (MNHN); 3 females (macropterous), “ Nanking, China, Yettmar, 1931” “det. H.B. Hungerford ” (NHMW). Yunnan Prov.: 1 male, 1 female (brachypterous), 10 males, 17 females (macropterous), 100km W Kunming, Diaolin Nat.Res., coll. E. Jendek & O. Sausa, 25 May–2 Jun. 1993 (NHMW); 2 males, 4 females (macropterous), NW-Yunnan, Yulongxue Shan NP, 30km N Lijiaang, Beishui, 2900–3200m, leg. Schillhammer (17a), 7–11 Jul. 1994 (NHMW). Jiangxi Prov.: 1 male (macropterous), Jiangsi, W Jingang Shan-Ciping, coll. E. Jendek & O. Sausa, 2–14 Jun. 1994 (NHMW). Liaoning Prov.: 1 male, 1 female (macropterous), Yiwulushan, 5km NW Beizhen, 200m, coll. Jäch (32), 22 Aug. 1994 (NHMW); Jilin Prov.: 1 male, 1 female (macropterous), Baihe City, near Power Plant, coll. Jäch (26), 19 Aug. 1994 (NHMW); 2 females (macropterous), Baihe City, near Power Plant, coll. Jäch (29), 19 Aug. 1994 (NHMW); 1 female (macropterous), near Baihe City, 650m, Erdao Baihe, coll. Jäch (3), 15 Aug. 1994 (NHMW).

TAIWAN: 4 males, 2 females (macropterous), Kiaohsiung, Chishsieu, coll. L.J. Wang & J.C. Liu (52), 29 Mar. 1995 (NHMW); 3 males (macropterous), Ilan, Tsuifeng Lake, coll. L.J. Wang, 16 Aug. 1995 (NHMW); 1 male, 1 female (macropterous), Taipei, Gongliao Gongliao, coll. L.J. Wang, 21 Jan. 1995 (NHMW); 1 male, 1 female (mpt), Taipei, Sanhsia Tsouhe, coll. L.J. Wang, 18 Apr. 1995 (NHMW); 4 females (macropterous), Taipei, Waihsuangehi, coll. L.J. Wang, 19 Apr. 1995 (NHMW); 3 males, 2 females (macropterous), Taipei, Kungkuan, coll. L.J.Wang, 21 Apr. 1995 (NHMW); 6 males, 7 females (macropterous), Nantou county, Kao-Leng Dyi, 18km W of Wushe, 24°4.605’N 121°7.583’E, 2074m, from surface of waterbasin, coll. D. Anstine, Gy. Fábián & O. Merkl, 18–19 Apr. 2002 (MTMB).

KOREA: 1 male, 1 female (macropterous), De Sang-san, 10km NE Pyongyang, netting in water, coll. Dely & Draskovits, 1 Jul. 1977, No. 336 (NHMW).

Remarks. Gerris gracilicornis was previously recognised by Andersen & Chen (1993) as a species of the subgenus Gerriselloides. It was later transferred by Andersen (1993) to the subgenus Macrogerris Andersen, 1993. The latter subgenus was defined by several characteristics: protonal lobe rufous; hind margin of male sternum 7 without median emargination; antennae relatively long and slender; hind margin of female sternum 7 medially produced, covering bases of gonocoxae (for further details, see Andersen 1993). Gerris gracilicornis can be separated from other species of the subgenus Macrogerris by the following characteristics: connexiva of female produced, surpassing hind margin of sternum 7; sternum 8 of male with two impressions covered with silvery hairs; endosoma: ventral sclerite separated and small, distal part of dorsal sclerites recurved, shovel-shaped, and wider than basal part (see Andersen 1993).

Distribution. Vietnam: Lao Cai (first records for Vietnam!). Others: East Asia: from Far East of Russia, Korea, Japan to China and Taiwan.

Habitats. Gerris gracilicornis and G. latiabdominis have been found only in the montane area in northwestern Vietnam. They were found in schools on the surface of stagnant, shallow water pools with grassy banks. Although both species sometimes occur sympatrically, the latter species is very common there, while the former seems to be less so (personal observation).