Gyrinus sericeolimbatus Régimbart, 1883 (Figs. 1A, 3A–B) Material examined. INDONESIA: South Sulawesi Province: Gowa Distr., 6 km E Malino, Gn. Bawakareng Area, border of gardens and mixed forest nr Lembanna camp, 5°15.4'S 119°54.5'E, 1520 m a.s.l., 11–13.II.2015, leg. J. Hájek & J. Sumpich, 1 female (GTGC). SOLOMON ISLANDS: Guadalcanal: ca. 3.5 km SE of Barana vill. 9°29.8'S 159°59.5'E, 190 m a.s.l., 24.XI–14.XII.2013, leg. J. Hájek, 1 female (GTGC). Diagnosis. Male: Aedeagus (after Svensson 1991; Mazzoldi 2010) with median lobe slightly shorter than parameres, strongly attenuate in apical 1/2, apex bifid, lateral margin of parameres weakly curved, apex rounded (Fig. 3A). Female (Fig. 1A): Size: 4.5 mm. Body form oval, strongly convex in lateral view; pronotum and elytral margins darkly colored, pronotum with median transverse depression, dorsal surface fairly evenly covered with microreticulation consisting of polyhedric meshes; pronotum and elytra laterally with bronzy-metallic sheen; striae I–XI evident with distinct, widely spaced punctures, all elytral striae non-sulcate; all intervals similarly convex; elytral apex truncate, without border, epipleural angle indistinct; metanepisternal ostiole small. Gonocoxae as in Fig. 3B. Differential diagnosis. The species is easily distinguished from all other Gyrinus known from Southeast Asia by its microreticulate dorsal surface and the lateral sides of pronotum and elytra having a distinct bronzy-metallic sheen. Distribution. This species is known from the Malay Peninsula, throughout Indonesia, Luzon of the Philippines, New Guinea and the Bismarck Archipelago (Ochs 1927, 1930, 1955; Brinck 1976; Svensson 1991; Freitag et al. 2016), and possibly also from Sri Lanka (Svensson 1991). Habitat. Gyrinus sericeolimbatus occurs in a wide variety of habitats from large lakes to streams at both low and high elevation (Svensson 1991). Discussion. The first report of a species of Gyrinus in Thailand was of G. sericeolimbatus (Ochs 1927, 1930) found in a stream at Khao Luang National Park, Nakhon Si Thammarat Province at an elevation of about 609 m a.s.l. For this study, a large number of aquatic insects were collected intensively at the Nakhon Si Thammarat Mountain Range, but G. sericeolimbatus was not found. However, we did not collect within Khao Luang National Park where G. sericeolimbatus has been reported. Therefore, this species may occur at only a few localities within the Nakhon Si Thammarat Mountain Range. More generally, G. sericeolimbatus exhibits substantial variation in both habitat and morphology across its wide geographic distribution with a couple of closely related species in New Guinea (i.e., G. toxopeusi Ochs 1955 and G. brincki Svennson 1991), and one subspecies (G. s. wegneri Ochs, 1959) in Timor. These taxa were distinguished from G. s. sericeolimbatus based on variation in dorsal microreticulation, extent of the brassy lateral sheen of the elytra, dorsal convexity, and width of the male protarsus (Ochs 1955, 1959; Svensson 1991). The specimens of G. sericeolimbatus that we examined had a fairly complete covering of microreticulation consisting of polyhedric meshes, which would qualify as a four on the reticulation index established by Svensson 1991. Sampling for phylogenetic analysis of this highly variable and widely distributed species complex across its range would be beneficial for both phylogeographic and species delimitation purposes.

Published as part of Suksai, Benjamart, Gustafson, Grey T., Sites, Robert W. & Sangpradub, Narumon, 2021, The Gyrinidae (Coleoptera) fauna of Thailand: Key to tribes and genera, with new records and keys to species of Dineutini and Gyrinini, pp. 97-117 in Zootaxa 5071 (1) on page 99, DOI: 10.11646/zootaxa.5071.1.5, http://zenodo.org/record/5723327