Thysanarthria Orchymnont 1926

Thysanarthria Orchymont, 1926 Thysanarthria Orchymont, 1926a:195. Type species: Hydrobius atriceps Régimbart, 1903 by original designation. = Chaetarthriomorphus Chiesa, 1967: 276. Type species: C. sulcatus Chiesa, 1967 by monotypy. Synonymized by HANSEN (1991: 126). Diagnosis. The genus can be recognized from other co-occurring genera of the Hydrophilidae based on the following combination of characters: body small to medium sized (1.4–2.2 mm); head black, pronotum and elytra yellowish to pale brown in most species, uniformly brown in the remaining ones (Figs 1 A–D); head with large exposed well sclerotized labrum (e.g., Fig. 1B); antenna with 9 antennomeres, scape very long, pedicel bulbose, antennomeres 3–5 very small, cupule and three-segment antennal club pubescent (Fig. 3D); maxillary palpomere 4 basally with row of many peg-like setae (Fig. 3E); mentum projecting anteromedially, with row of setae along anterior margin (Fig. 3A); gular sutures contiguous (Fig. 3C); mesoventrite distinctly divided from anepisterna by sutures, sutures widely separated on anterior margin of mesothorax (Fig. 3B); mesoventrite flat except of small semicircular elevation posteromesally (Fig. 3B); metaventrite short, sparsely pubescent only mesally and anterolaterally (Fig. 3B); elytra with 10 sharply impressed striae (Figs 1 A–D); whole dorsal surface covered by sparsely arranged setae which are trifid basally with a long median projection (Figs 1C, 2 a–b); profemora pubescent in basal half (Fig. 3C); mesofemora pubescent anterobasally (Fig. 3B); metafemora bare except on anterobasal margins (Fig. 3B); tarsi rather short and stout, metatarsus with all tarsomeres c. equal in length (Fig. 3F); abdomen with 5 ventrites, basal two bearing shallow cavity covered by long setae arising from base of ventrite 1, holding whitish gelatinous substance (Figs 3 G–H); ventrite 1–2 with median carina; male abdominal sternite 8 with narrow median projection (Fig. 2H); male sternite 9 V-shaped medially (Fig. 2J); aedeagus with long tubular phallobase, base of median lobe not reaching deeply into phallobase (Figs 4–9). Differential diagnosis. The base of abdomen with series of long setae covering a gelatinous substance and antenna with bulbous pedicel distinguish Thysanarthria from all other non-chaetarthriine genera. Within Chaetarthriini, the well sclerotized and widely exposed labrum differentiates it from Hemisphaera Pandellé, 1876 (which is also smaller and has more depressed body: see FIKÁČEK et al. 2012, JIA et al. 2013) which can co-occur with Thysanarthria, and from the Neotropical genus Guyanobius Spangler, 1986 (see GUSTAFSON & SHORT 2010). Thysanarthria can be distinguished from all three groups of Chaetarthria defined above by the elytra with 10 sharply impressed striae (Figs 1 A–D). Most species of the genus are easy to recognize in the samples by their small body size and pale coloration of pronotum and elytra constrasting with the black head (this coloration is not present only in the Near East T. persica sp. nov. and T. wadicola sp. nov.). Characters important for species-level identification. All known species of Thysanarthria are very similar to each other in most external characters, and their tiny size makes the observation of many characters very difficult. The only external characters are (1) the presence/absence of the microsculpture on the head and labrum, pronotum and elytra, which can be either strongly mesh-like, weak and granulate, or totally absent; and (2) the body shape which can be wider (Fig. 1A) or more elongate (Figs 1B, D), but this is hard to compare in specimens which are not mounted in extended position on labels. Body coloration differs between species, with pronotum and elytra either uniformly yellowish (Figs 1A, C) or partly darkened (e.g. pronotum in Fig. 1D) or uniformly dark brown (Fig. 1B). However, examination of longer series of some species (T. championi and T. siamensis) revealed that the coloration can vary within a species, and hence is not always reliable for identification. The same is true for the dorsal body microsculpture which seems to vary in intensity, at least in T. brittoni (see under that species). The body size also differs between species, and the presence of specimens of different size in the same series may indicate the presence of multiple species. However, in species in which more specimens were available, the body size was revealed to vary to some extent as well, and the body size can be hence used as an additional character only. Therefore, examination of the male genitalia is necessary for reliable identification in all cases. Ideally, the genitalia should be examined under a medium magnification of the compound microscope, and attention should be paid also to the membranous structures on the apical part of the median lobe (including short, paired, subapical projections which are present only in some species, e.g. Figs 5 D–E, I–J, 6 D–E, I–J). The proportions of parameres may be uneasy to observe as they are partly affected by the position of the aedeagus, and the genitalia should be carefully observed under slightly different angles in case of doubts. The ratio of paramere length to phallobase length should be evaluated in lateral view, due to the strongly bent phallobase in many species. The examination of the material used for this study shows that especially the form of the median lobe is constant and diagnostic, whereas the shape of parameres may slightly vary. The apex of the median lobe is membranous in many species, even though usually rather constant in shape, and it seems that at least in some species it can include parts which are normally inverted and hence not easy to observe, and may sometimes get fully everted after the treatment in KOH (which however may distort other parts of the aedeagus; see e.g. 4J and 7E which show fully everted apical membranous parts). Since the observation of this part is difficult, we did not consider it for species diagnosis. As male genitalia are the only reliable character for species identification, below we provide detailed illustrations of the genitalia with which new specimens to be identified should be compared. Once the candidate species is found based on genital morphology, the external characters mentioned above (body size and coloration, presence/ absence of the microsculpture) should be compared with the (re)descriptions provided below. No identification key is hence provided. Species groups. The limited number of characters makes it difficult to group the species into supposedly monophyletic species groups. Based on the genital morphology, the African Thysanarthria atriceps is very similar to the Arabian T. brittoni, and these two species seem to form a group of closely related species. The presence of subapical membranous lobes on the median lobe in T. brincki, T. bifida, T. cardamona, T. madurensis, and T. trifida (Figs 5 D–E, I–J; 6 D–E, I–J; 8 N–O) may also point to a close relationships. Thysanarthria brincki and T. cardamona may form a clade within this group characterized by lateroapical spine on the paramere (Figs 5 A–J). Function of the abdominal gelatinous substance. All members of the tribe Chaetarthriini including Thysanarthria bear a series of long setae on the base of the abdomen which cover a shallow depression in ventrites 1–2 filled in by whitish gelatinous substance (Figs 3 G–H). When submerged, Thysanarthria floats with dorsal body surface facing up, i.e. in the position usual for most other Hydrophilidae. The gelatinous substance hence does not serve to increase the buoyancy of the beetle as might be the case in the non-related genus Amphiops Erichson, 1843 which bears similar-looking gelatinous substance at the base of abdomen and is swimming in an upside-down position when submerged (Fikáček & Angus, pers. observ.). Moreover, the gelatinous substance does not interfere with the ventral air bubble of the submerged beetle: the bubble covers the whole ventral side of the beetle including the whole abdomen. Therefore, it seems that the substance cannot be functional in submerged beetle but may be an adaptation to its usual environment on the wet sand outside water. The gelatinous substance is sticky in alive specimens. Its function remains unknown.
Published as part of Fikáček, Martin & Liu, Hsing-Che, 2019, A review of Thysanarthria with description of seven new species and comments on its relationship to Chaetarthria (Hydrophilidae: Chaetarthriini), pp. 229-252 in Acta Entomologica Musei Nationalis Pragae 59 (1) on pages 234-236, DOI: 10.2478/aemnp-2019-0020, http://zenodo.org/record/4488918
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