Macrothrix flabelligera Smirvov 1992

11. Macrothrix cf. flabelligera Smirvov, 1992 Localities and specimens found: 32 − 2♀, 2♀ ov, 1♀ em, 7j; 42 − 1♀ ov; 46 − 1♀; 50 − 14♀, 9j; 51 − 12♀, 1♀ ov, 7j. Distribution and ecology. Currently, M. flabelligera is known from Australia and the Neotropical and Oriental regions (Smirnov 1992; Güntzel et al. 2003; Kotov et al. 2013a). Among the Oriental countries, the M. flabelligera - like populations have been recorded in Thailand, Cambodia and Vietnam (Sanoamuang 1998; Maiphae et al. 2005; Tanaka & Ohtaka 2010; Korovchinsky 2013; Van Damme et al. 2013; Gusakov et al. 2014; Choedchim & Maiphae 2023). However, at the moment, the presence of this species in Southeast Asia is considered controversial (see Remarks for details). The lifestyle and ecology of M. flabelligera are poorly covered in the literature. Obviously, as in most other macrothricids, its life is mainly associated with the surface of bottom sediments and aquatic vegetation (Kotov 2006; Korovchinsky et al. 2021b). The species has been found in different types of water bodies, from lakes and reservoirs to rivers and swamps (Güntzel et al. 2003; Tanaka & Ohtaka 2010; Van Damme et al. 2013; Choedchim & Maiphae 2023). In Vietnam, we detected the M. flabelligera -like specimens in a man-made pond and two natural lakes, on the bottom and among the roots of floating plants (see Appendix 1). In the rhizosphere, they were more numerous. Remarks. The individuals of M. flabelligera that we found in Vietnam correspond to Smirnov’s (1992) description according to the general diagnostic characteristics. For the first time this macrothricid was noted in the fauna of the country in the work of Gusakov et al. (2014). At the same time, a number of researchers consider the presence of this species in Southeast Asia to be highly questionable (Kotov et al. 2013b; Van Damme & Sinev 2013; A.Y. Sinev’s personal communication). They suggest that populations from Southeast Asia identified as M. flabelligera may actually represent a morphotype of another widespread relative in the region, M. triserialis Brady, 1866, which has additional spine-like setules on the segments of antenna II, similar to the typical form of M. flabelligera. Thus, there is a clear need for a thorough comparative surveys of the Asian “ flabelligera – triserialis ” forms. Unfortunately, there are no such studies yet. So far, we are presenting the specimens we have discovered as in this checklist.