Review of Coelostoma of the Indian subcontinent (Coleoptera: Hydrophilidae) Part 1: Coelostoma s. str. and Holocoelostoma

Species of the genus Coelostoma Brullé, 1935 belonging to the subgenera Coelostoma s. str. and Holocoelostoma Mouchamps, 1958 from the Indian subcontinent are revised. Six species of Coelostoma s. str. and two species of Holocoelostoma are recognized, of which two are described as new: C. (Coelostoma) lyratum sp. nov. (India: Maharashtra) and C. (Coelostoma) nostocinum sp. nov. (India: Maharashtra, Goa, Karnataka, Kerala). Coelostoma (Coelostoma) fallaciosum Orchymont, 1936 and C. (Coelostoma) vividum Orchymont, 1936 were recorded for the fi rst time from India and Bangladesh, respectively. Lectotypes are designated for C. aeneolum Régimbart, 1903 and Hydrobius stultus Walker, 1858. The previously confusing situation with Coelostoma (Holocoelostoma) stultum (Walker, 1858) and C. (Holocoelostoma) bhutanicum Jayaswal, 1972 is clarifi ed based on new material of both species from India. Coelostoma sulcatum Pu, 1963 from China is removed from the synonymy of C. stultum and considered as a likely synonym of C. bhutanicum, a status which needs to be confi rmed by a detailed study of type specimens. All species are (re)described and illustrated. Diagnosis of the subgenera of Coelostoma are modifi ed in order to accommodate the species of the Indian subcontinent, resulting in narrowing down the concept of Coelostoma s. str. and widening the concept of Lachnocoelostoma Mouchamps, 1958.


Introduction
Hydrophilidae Latreille, 1802 is a major group of water beetles with about 3000 described species classified in 6 subfamilies and 12 tribes (Short & Fikáček 2013). This group is ancestrally aquatic, but one of its lineages colonized terrestrial habitats ca 175 million years ago (Bloom et al. 2014). This lineage gave rise to a diverse fauna currently classified in the subfamilies Sphaeridiinae Latreille, 1802 and Cylominae Zaitzev, 1908(Toussaint & Short 2018. A few lineages of these secondarily terrestrial groups returned to water. This is the case with Cylomissus Broun, 1903and Anticura Spangler, 1979(Minoshima et al. 2015 and larvae of the otherwise flower inhabiting Rygmodus White, 1846 (Minoshima et al. 2018) in Cylominae, a few lineages of the genus Cercyon Leach, 1817 (Arriaga-Varela, unpubl. data) and the whole genus Coelostoma Brullé, 1835 in Sphaeridiinae.
The coelostomatine genus Coelostoma is mainly distributed in the Afrotropical and Oriental regions, with a few species also present in the Palearctic and Australian regions (Hansen 1999). About 120 species have been described in this genus, with the highest known diversity occurring in China (Jia et al. 2014(Jia et al. , 2017(Jia et al. , 2019. All known species with available biological data are aquatic, occurring in habitats ranging from shallow ponds to horizontal and vertical wet cliffs (Jia et al. 2019).
The genus is characterised by broadly oval and dorsally convex body, dark brown to black colour, loosely arranged antennal club, prosternum bulging medially and dentiform anteriorly, mesoventrite with an elevated arrowhead-shaped structure in the middle, metaventrite with strongly elevated median portion, anteriorly projecting between mesocoxae and abutting mesoventral elevation, the first metatarsomere longer than second tarsomere, elytra with sutural stria and usually without serial punctures (with few exceptions), and the first abdominal ventrite not carinate medially (with a few exceptions; Hansen 1991;Jia et al. 2014).
The Asian fauna of Coelostoma is rather species-rich, but the lack of modern revisions prevents reliable identification. However, the fauna was recently revised in China (Jia et al. 2014(Jia et al. , 2017(Jia et al. , 2019, Taiwan (Liu et al. 2020) and Japan (Hayashi 2008). These studies provided good quality illustrations of genital characters crucial for the species identification and recognized many previously undescribed species. Other parts of tropical and subtropical Asia remain unrevised, including the Indian subcontinent, a region characterized by high species diversity and endemism (Myers et al. 2000;Mittermeier et al. 2005). So far, sixteen species of Coelostoma have been recorded from the Indian subcontinent (Walker 1858;Orchymont 1923aOrchymont , 1923bOrchymont , 1928Orchymont , 1936Mouchamps 1958;Jayaswal 1972;Hebauer 2000Hebauer , 2002, but a detailed taxonomic revision is wanting, and the status of many of these species remains unclear (Linnean shortfall; Hortal et al. 2015). Thus, the aim of this revision is to overcome Linnean and Wallacean shortfalls (as defined by Hortal et al. 2015) by providing basic information about the diversity and distribution of species and to provide taxonomic keys, descriptions and illustrations allowing an easy identification of Coelostoma species.
For practical purposes, we divided the revision into two parts. Here we present the revision of two subgenera: Coelostoma s. str. Brullé, 1835 and Holocoelostoma Mouchamps, 1958. The revision of the more diverse subgenus Lachnocoelostoma Mouchamps, 1958 from India will be presented later. Here, we describe two new species from India, provide new distribution records of all known species from the Indian subcontinent and illustrate the characters of all of them.
Mountains in the north, because of which the region experiences tropical monsoon. Additionally, the subcontinent is lined by shoreline on western, eastern and southern sides. Diverse ecological conditions ranging from dry deserts, evergreen forests to temperate and alpine environments exist in the region. Furthermore, the Western and Eastern Ghats form the edges of the geologically stable Peninsular Plateau of the subcontinent, which was one of the parts of the ancient Gondwana. All these physical features contribute to gradients of interacting climatic conditions of precipitation, altitude and temperature on the subcontinent (Mani 1974).

Specimen examination
The genitalia were dissected from water-relaxed specimens and examined on temporary glycerine slides or in permanent mounts in Euparal, in both cases without a cover glass. Photographs of glycerine mounted genitalia were taken with a Canon D1100 digital camera attached to an Olympus BX41 compound microscope. Photographs of external morphology were taken using a Canon EOS 550D digital camera with an attached Canon MP-E65 mm f/2.8 1-5 × macro lens. We standardized European Journal of Taxonomy 690: 1-32 (2020) the photographic documentation provided for each species to include principal characters important for the reliable identification of all Indian Coelostoma (including the subgenus Lachnocoelostoma not treated here). For this reason, the plates even include characters which seem invariable (e.g., dorsal punctation) but are crucial for diagnosing the species treated here from some Indian species of Lachnocoelostoma. Original unedited photographs are available in the Zenodo archive under https://doi.org/10.5281/zenodo.3949349. For details of the methods used, see Sheth et al. (2018) and Fikáček & Liu (2019). Geographical coordinates were obtained from GoogleMaps and GoogleEarth © in case they are not listed on the labels. Incorrect coordinates provided on labels were rectified, based on locality data interpreted from the label and additional data (e.g., altitude) was added in some cases; all these data that we added are placed in brackets. Maps were constructed in QGIS (ver. 2.18.5, https://qgis.org/downloads/).

Review of published data
We have reviewed the literature referring to the occurrence of Coelostoma species on the Indian subcontinent. These publications are listed in the synonymy section, and faunistic data from corresponding publications are summarized under Published records. Publications with records outside of the Indian subcontinent are not listed in these sections.  Mouchamps (1958) defined four subgenera of Coelostoma on the basis of the combination of three characters: (1) the pubescence on the mesofemora (with or without dense pubescence), (2) morphology of the abdominal apex (with or without stout setae) and (3) the morphology of the tarsal claws (simple or bifid, the latter in the subgenus Hammacoelostoma Mouchamps, 1958, not occurring in the Indian subcontinent). These characters have since then been successfully used to place all described species in subgenera (e.g., Hansen 1999). Characters (1) and (2), relevant for recognizing the subgenera occurring in India, were illustrated by Jia et al. (2014).

Depositories
The study of the Indian Coelostoma revealed that the morphological diversity within the genus is much higher than expected, especially in the subgenus Lachnocoelostoma, with some species not fitting the combination of characters used by Mouchamps (1958). To deal with this problem, we are adapting the definition of the subgenera of Coelostoma in the way specified in Table 1, using five characters. The re-examination of C. (Hammacoelostoma) afflatum Knisch, 1922 revealed that the additional strongly sclerotized projections between the tarsal claws are in fact modified empodial setae, and not basal projections of the claws as suggested by Mouchamps (1958). Our concept narrows down the definition of Coelostoma s. str. and significantly widens the concept of Lachnocoelostoma. Reasons for that will be demonstrated in detail in the review of Indian Lachnocoelostoma (Sheth et al., in prep.). The atypical species of Lachnocoelostoma (i.e., those having largely bare mesofemora or abdominal apex without stout setae) always differ from the subgenera treated in this paper in the presence of the abdominal median carina. The carina is absent in all Coelostoma s. str. and in all Holocoelostoma, including all species treated in this paper.

Differential diagnosis
Coelostoma aeneolum resembles C. nostocinum sp. nov. and C. fallaciosum in having a narrowly triangular median lobe and the subapical gonopore. It can be distinguished from C. fallaciosum by parameres reaching much further than median lobe (in contrast to parameres only slightly longer than median lobe in C. fallaciosum), apex of the parameres more less symmetrically rounded (strongly asymmetrical and projecting into mesal 'toothʼ in C. fallaciosum), the median lobe triangular with straight sides (sides clearly concave in C. fallaciosum) and much smaller aedeagus (ca 0.5 mm compared to 1.0 mm in C. fallaciosum). Coelostoma aeneolum is very similar to C. nostocinum sp. nov.; see under the latter species for differential characters and discussion.

Material examined
Lectotype ( The species was described based on an unspecified number of specimens deposited in MNHN. Both above specimens were found in coll. Régimbart in MNHN and their locality data agrees with the original description. We designate the male specimen here as a lectotype, to fix the identity of the species. brown with slightly paler margins; ventral surface uniformly dark brown. Tarsi pale brown. Mouthparts and antennae yellowish, antennal club brown.
Head. Dorsal punctation dense, consisting of simple punctures without associated ridges; trichobothria present; surface between punctures smooth. Anterior margin of clypeus arcuate. Eyes large, interocular distance ca 3.3 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Lateral margin with very indistinct sculpture; anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation sparser and finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, carinate mesally, anterior portion of carina raised, producing tooth-like process seen laterally.
legs. Profemur with dense pubescence except in apical fifth; mesofemur and metafemur with sparsely arranged stout setae only.

Remarks
The species is only known from three historical specimens and the types had not been re-examined since the original description. We are here clarifying the identity of the species based on the examination of the types (see the redescription above) and an additional specimen. The species seems very rare, probably because of its distribution in lowland coastal areas from Maharashtra to Kerala, which are nowadays affected by high human population and high pollution. Additional research is needed to reveal whether the species is still present.

Biology
Unknown.

Distribution
Only known from coastal areas in northern Kerala (Mahé) and in Maharashtra (Mumbai).

Differential diagnosis
Among Indian Coelostoma without pubescent mesofemora, C. fallaciosum may be recognized by its relatively large body size (resembling that of C. stultum and C. bhutanicum), simple abdominal apex without emargination or stout setae (with emargination and stout setae in both latter species) and with a typical aedeagus with subapical gonopore and strongly asymmetrical apex of the paramere. In the morphology of male genitalia it also resembles C. aeneolum; see under that species for diagnosis. Published records NEPAL: Bheri Prov., Nepalgunj, 28º02ʹ59ʺ N 81º36ʹ56ʺ E (Hebauer 2002); Annapurna Prov., Pokhara, W of Phewa river and lake (Hebauer 2006).

Redescription
Form and colour. Body length 5.6-6.4 mm (holotype 5.7 mm), body width 3.0-3.6 mm (holotype 3.3 mm). Body oval in dorsal view, moderately convex in lateral view. Head black, pronotum and elytra dark brown to black with slightly paler margins; ventral surface uniformly dark brown, abdomen with pale spot at sides of each ventrite; appendages paler distally; mouthparts and antennal club brown.
Head. Dorsal punctation moderately dense, consisting of simple punctures without associated ridges; surface between punctures smooth; few trichobothria present. Anterior margin of clypeus arcuate. Eyes large, interocular distance ca 4 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus, dark brown dorsally, sinuate on anterior margin. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin almost straight, posterolateral corners rounded. Anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation moderately dense, slightly coarser than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, weakly carinate mesally, anterior portion of carina not raised.
mesotHorax. Elytral punctation moderately dense and coarse, similar to that on pronotum, consisting of punctures without transverse ridges. Series of punctures absent. Sutural stria weakly impressed, present in apical two thirds. Mesoventral plate 1.1 × as long as wide, arrowhead-shaped, bluntly pointed anteriorly, posteriorly widely attached to metaventrite.
legs. Profemur with dense pubescence except in apical fifth; mesofemur and metafemur with sparsely arranged short setae only.

Biology
Aquatic species, in Japan and Taiwan reported to inhabit shallow, well-vegetated pools (Liu et al. 2020;Nakajima et al. 2020). The biology of the specimens from the Indian subcontinent is unknown; all examined specimens were collected at light.

Differential diagnosis
Coelostoma nostocinum sp. nov. is characterized by smaller body size, by which it especially resembles C. vividum. Its male genitalia with the triangular median lobe easily distinguish it from all species except C. fallaciosum and C. aeneolum. The species may be distinguished from C. fallaciosum by its much shorter median lobe (compared to parameres) with straight lateral margins (concave in C. fallaciosum) and wider apex, and by the more or less symmetrically pointed apex of the paramere (strongly asymmetrical in C. fallaciosum). Coelostoma nostocinum sp. nov. is very similar to C. aeneolum, but may be distinguished from it by its (1) smaller body size (3.5-4.5 mm, compared to 4.6 mm in C. aeneolum), (2) larger aedeagus (0.7 mm, compared to 0.4 mm in C. aeneolum), (3) relatively longer apodemes of the median lobe (longer than half of the length of the apical triangular part of the median lobe, compared to much shorter than half the length in C. aeneolum) and (4) paramere distinctly concave on outer margin subapically and pointed apically (compared to evenly arcuate on whole outer margin and more less rounded apically in C. aeneolum).

Etymology
The species name refers to the finding of the holotype of this species in association with Nostoc Vaucher ex Bornet & Flahault (see Biology). Paratypes INDIA -Goa • 11 specs; same collection data as for holotype; NMPC • 2 specs; same collection data as for holotype; BMNH • 1 spec; same collection data as for holotype; UASB 01923074 • 1 spec; same collection data as for holotype; ZSI • 3 specs; "30 km S of Margao, Palolem env.; 15º00.47ʹ N,

Description
Form and colour. Body length 3.3-4.5 mm (3.8 mm in holotype), body width 2.2-2.5 mm (2.4 mm in holotype). Body oval in dorsal view, moderately convex in lateral view. Head black, dark brown clypeus; pronotum and elytra uniformly dark brown to black; ventral surface pale to dark brown. Femora and tarsi yellowish brown, tibia dark reddish brown, tarsi pale brown. Mouth parts and antennae yellowish, antennal club brown.
Head. Dorsal punctation dense, consisting of simple punctures without associated ridges; trichobothria present; surface between punctures smooth. Anterior margin of clypeus non-arcuate. Eyes large, interocular distance ca 4.0 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere markedly wide.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum nearly straight on anterior margin, gently carinate mesally.
mesotHorax. Elytral punctation dense and moderately coarse, consisting of punctures without transverse ridges. Weakly developed series of impressed punctures present along suture and laterally. Sutural stria well impressed, present in apical half, extends beyond middle; lateral elytral margins with sculpture. Mesoventral plate as long as wide, arrowhead-shaped, bluntly pointed anteriorly, posteriorly widely attached to metaventrite.
legs. Profemur with dense pubescence except in apical fifth; mesofemur and metafemur with sparsely arranged short setae only.

Variation
Specimens from Maharashtra are slightly smaller than those from more southern areas. The aedeagus varies slightly in the shape of the parameres, the apical part of which is slightly wider in the specimens from Kerala; in all other aspects these specimens agree with those from Goa and hence we consider them conspecific.

Remarks
Coelostoma nostocinum sp. nov. and C. aeneolum are very similar in all characters including the morphology of the male genitalia, and both species seem to have very similar (and overlapping) distribution ranges. We were hence working with the hypothesis that they may be conspecific for some time, with the observed variation in body size and aedeagus morphology being an intraspecific variation. The examination of all available material, however, indicates that this is not the case, and that we really have two distinct morphotypes without intermediate characters: the species with larger body and smaller aedeagus with more or less rounded apices of parameres (C. aeneolum) and the smaller species with larger aedeagus with narrower and apically pointed paramere (C. nostocinum sp. nov.). Based on the material examined, both morphotypes are constant in the characters listed in the differential diagnosis across the distribution range (i.e., from Maharashtra to Kerala in both). For these reasons, we are treating them as separate species, with the smaller species described here as C. nostocinum sp. nov.

Biology
The specimens from Goa were collected under 'ballsʼ of Nostoc blue-green algae growing on wet sandy places on rock cliffs at the sea coast. Specimens from Maharashtra were collected at light.

Differential diagnosis
The species is easily recognized based on its aedeagus which resembles those of C. bhutanicum and C. stultum in its apically situated gonopore and parallel-sided median lobe. In contrast to both latter species, the outer margin of the parameres in strongly bisinuate in C. lyratum sp. nov. (in contrast to a nearly continuously arcuate outer margin in C. bhutanicum and C. stultum) and the abdominal apex is simple, without emargination and stout setae.

Etymology
The species name refers to the lyriform shape of the aedeagus; adjective.  Paratypes INDIA • Maharashtra • 3 specs; same collection data as for holotype; NMPC • 2 specs; same collection data as for holotype; BMNH • 1 spec.; same collection data as for holotype; SMNS • 1 spec.; same collection data as for holotype; ZSI • 2 specs; same collection data as for holotype; NCBS-BL018-019 • 2 specs; same collection data as for holotype; UASB 01923068-69.

Description
Form and colour. Body length 4.2-4.7 mm (holotype 4.5 mm), body width 2.7-3.1 mm (holotype 3.0 mm). Body narrowly oval in dorsal view, moderately convex in lateral view. Head black, reddish brown near clypeus and anterior to eyes; pronotum and elytra uniformly dark brown to black with slightly paler margins, ventral surface blackish brown, legs reddish brown. Mouthparts and antennae pale brown, antennal club brown.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation denser and finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, weakly carinate mesally.
legs. Profemur with dense pubescence except in apical fifth; mesofemur and metafemur with very sparsely arranged short setae only.
abdomen. All ventrites densely pubescent. First ventrite without carina. Posterior margin of last ventrite entire, without stout spines mesally. aedeagus (Fig. 6J-K). 0.84 mm long. Median lobe of nearly the same width throughout; apex roundly cut off; gonopore situated at apex, widely semicircular. Parameres slightly longer than median lobe; heavily tri-sinuate on outer margin; apex acute at inner margin, rounded laterally. Phallobase small, slightly wider than long.

Biology
Unknown. The labels of the type specimens indicate that they were found at the riverside.

Distribution
Known from the type locality in southern Maharashtra.

Differential diagnosis
Coelostoma vitalisi is easy to recognize based on its aedeagus, with a very wide median lobe with a very large subapical gonopore. Thanks to the unique genital morphology, it cannot be confused with any other species of the genus.

Description
Form and colour. Body length 4.0-4.9 mm, body width 2.5-2.9 mm. Body oval in dorsal view, moderately convex in lateral view. Head black; pronotum and elytra uniformly black to dark brown with slightly paler margins; ventral surface uniformly dark brown. Tarsi pale brown. Mouthparts and antennae yellowish, antennal club brown.
Head. Dorsal punctation dense, consisting of simple punctures, few punctures at posterior-most portion with associated ridges; trichobothria present; surface between punctures smooth. Anterior margin of clypeus gently arcuate. Eyes large, interocular distance ca 4.3 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus, sinuate on anterior margin, brown. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Lateral margin with very indistinct sculpture; anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation similar to that on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, very weakly carinate mesally, anterior portion raised, producing tooth-like process seen in lateral view.
mesotHorax. Elytral punctation dense and moderately coarse, similar to that on pronotum, consisting of punctures without transverse ridges. Series of punctures absent. Sutural stria impressed, present in apical half. Mesoventral plate 1.1 × as long as wide, arrowhead-shaped, bluntly pointed anteriorly, posteriorly widely attached to metaventrite.
metatHorax. Metaventrite raised medially, posterior third and anterior portion of median elevation bare, remaining median surface with sparse regular setae; lateral portions densely pubescent. Anterior legs. Profemur with dense pubescence except in apical fifth; mesofemur with sparsely arranged stout setae only, metafemur with sparse pubescence.

Biology
Aquatic species, Nakajima et al. (2020) reported it from shallow wetlands in lowland to hilly areas. Most specimens available in the collections were collected at light.

Differential diagnosis
Coelostoma vividum is easy to recognize due to its small body size and the aedeagus with the apically strongly narrowed median lobe.

Description
Form and colour. Body length 3.4-4.1 mm, body width 2.1-2.3 mm. Body oval in dorsal view, moderately convex in lateral view. Head black; pronotum and elytra uniformly black to dark brown with slightly paler margins; ventral surface uniformly dark brown. Tarsi pale brown. Mouthparts and antennae yellowish, antennal club brown.
Head. Dorsal punctation dense, consisting of coarse simple punctures, a few punctures with associated ridges in posterior-most region; trichobothria present; surface between punctures smooth. Anterior margin of clypeus gently arcuate. Eyes large, interocular distance ca 4.0 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus, pale brown in coloration. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad. ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Lateral margin with very indistinct sculpture; anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation sparser and finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, carinate mesally, anterior portion of carina raised, producing tooth-like process seen in lateral view.
legs. Profemur with dense pubescence except in apical fifth; mesofemur with sparsely arranged stout setae only; metafemur with sparse pubescence.

Biology
Unknown. Published specimens were all collected at light.

Differential diagnosis
Coelostoma bhutanicum is easy to recognize from most other Coelostoma species by the apical gonopore and widely parallel-sided median lobe; in these characters it resembles only C. stultum and C. lyratum sp. nov. From C. lyratum sp. nov. it differs by the evenly arcuate outer face of the paramere and by the abdominal apex bearing a shallow emargination armed with stout setae. In both these characters it agrees with C. stultum, from which it is only recognizable by an examination of the male genitalia: the aedeagus of C. bhutanicum is more sclerotized and seemingly more 'robustʼ than that of C. stultum when examined quickly. The main difference is in the form of the median lobe, which has strongly sclerotized lateral margins in C. bhutanicum. This causes the median lobe to be of nearly the same width from base to apical fifth, from which it abruptly narrows (in contrast, the median lobe is narrowing from base to ca midlength and is of more or less the same width in the apical half in C. stultum). The more sclerotized margins in C. bhutanicum also affect the 3D form of the median lobe, which is distinctly spoon-like (best seen when the aedeagus is slightly rotated when examined).

Material examined
Type material Holotype not examined. [Types are deposited in the Zoological Survey in Kolkata but were not studied.] Published records BHUTAN: Ganga lakha (Jayaswal 1972). INDIA: Uttarakhand: Nainital (Jayaswal 1972). Records published from Nepal by Hebauer (2002) need confirmation, as our examination of a few specimens identified by F. Hebauer revealed that he often confused C. bhutanicum with C. stultum.

Redescription
Form and colour. Body length 4.7-5.8 mm, body width 2.8-3.5 mm. Body widely oval in dorsal view, moderately convex in lateral view. Head black; pronotum and elytra uniformly dark brown to black with slightly paler margins; ventral surface uniformly reddish brown. Tarsi pale brown. Mouthparts and antennae yellowish brown, antennal club brown.
Head. Dorsal punctation dense, consisting of simple punctures without associated ridges; trichobothria present; surface between punctures smooth. Anterior margin of clypeus arcuate. Eyes large, interocular distance ca 2.8 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation slightly finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum straight on anterior margin, weakly elevated mesally, anterior portion of carina not very elevated.
mesotHorax. Elytral punctation dense and moderately coarse, consisting of punctures without transverse ridges. Series of impressed punctures absent. Sutural stria weakly impressed, present in apical half. Mesoventral plate as long as wide, arrowhead-shaped, bluntly pointed anteriorly, posteriorly widely attached to metaventrite.  Jayaswal (1972). M. Aedeagus illustration from the description of C. sulcatum in Pu (1963). N-O. Apex of the aedeagus of the holotype of C. sulcatum. P. Aedeagus of the paratype of C. sulcatum from Yunnan: Jingdong. Q. Labels of the holotype of C. sulcatum. metatHorax. Metaventrite raised medially, posterior third and anterior of median elevation bare, lateral portions pubescent. Anterior metaventral process narrowly projecting between mesocoxae; posterior process bifid. Wings well-developed (macropterous).
legs. Profemur with dense pubescence except in apical fifth; mesofemur with sparsely arranged stout setae only; metafemur with very sparse pubescence.

Variation
The apical part of the median lobe is less narrowed in specimens from Taiwan (see Liu et al., 2020), but otherwise the aedeagus morphology seems very constant across the range in this species.

Identity of C. bhutanicum and synonymies
The identity of this species was long unclear, and many taxonomists considered it as a possible synonym of C. stultum, although this opinion was never published because of the inaccessibility of the types. The comparison of the aedeagus of the specimens from northern India examined here with the drawing by Jayaswal (1972) leaves little doubt that both refer to the same species: Jayaswalʼs (1972) drawing (Fig. 9L) shows the typical shape of the median lobe having the same width along the whole length except for the apical fifth. The slightly wider form illustrated on Jayaswalʼs (1972) drawing is likely caused by the fact that the genitalia were slide-mounted. Jia et al. (2014) illustrated the male genitalia of two specimens identified as C. stultum from China (from Guangdong and Xizang), which both clearly show the strongly sclerotized lateral margins of the median lobe and the shape of the median lobe corresponding to those in C. bhutanicum. This indicates that (1) Jia et al. (2014) mixed the two species, and C. bhutanicum clearly occurs in China, and (2) the status of C. sulcatum Pu, 1963 as a synonym of C. stultum proposed by Jia et al. (2014) is incorrect. Coelostoma sulcatum was described from specimens from Yunnan, Xishuanbanna (type locality) and Jindong, and the illustration by Pu (1963) shows the typical form of the median lobe present in C. bhutanicum (Fig. 9M). We hence suspect that C. sulcatum Pu, 1963 andC. bhutanicum Jayaswal, 1972 may refer to the same species. Martin Fikáček examined the types of C. sulcatum (the holotype from Xishunagbanna and a paratype from Jindong) briefly during his visit to Beijing in 2010 and took photographs of the holotype, with the abdominal apex showing a slightly protruding aedeagus (Fig. 9N-O), and of the completely dissected aedeagus of the paratype (Fig. 9P). The aedeagus of the paratype shows the median lobe with clearly sclerotized lateral margins, but it is rather narrow and not abruptly narrowing at the apex as it is typical for C. bhutanicum; this may be because the aedeagus is slightly deformed by dehydration. The apex of the aedeagus of the holotype shows the typical shape of the parameres present in both C. bhutanicum and C. stultum but absent from the illustration by Pu (1963) and also clearly shows the narrowing apex of the median lobe, further indicating that the shape of the median lobe of the paratype may be a deformation. The dissection of the holotype, remounting the aedeagus of the paratype or the examination of fresh material from Xishuanbanna would be necessary to understand the identity of C. sulcatum correctly and to confirm its synonymy with C. bhutanicum, which we are hypothesizing here. This synonymy would affect the nomenclature, since C. sulcatum would become a valid name and C. bhutanicum its junior synonym. However, we refrain from proposing the formal synonymy until the identity of C. sulcatum can be completely clarified, in order to prevent even more confusion about the species concepts within the subgenus Holocoelostoma.

Biology
Aquatic species; in Taiwan, the specimens were collected at the sides of ponds (among plant roots) and of running water (under stones and in mud) (Liu et al. 2020). The specimens examined by us were likely collected at light.

Distribution
Due to the confusion of this species with C. stultum, the distribution of C. bhutanicum requires more study. The occurrence is so far confirmed in Bhutan, Nepal and northern India (Jayaswal 1972, this paper), in China (based on photographs provided by Jia et al. 2014), in Taiwan (Liu et al. 2020) and Japan (Watanabe & Minoshima 2020). (Walker, 1858) Fig

Differential diagnosis
Large species, easy to recognize from all other Coelostoma species except C. bhutanicum by the combination of the widely parallel-sided median lobe with apical gonopore and emarginate abdominal apex with stout setae. From C. bhutanicum it may be only recognized by the morphology of the median lobe of the aedeagus, which is wide basally, gradually narrowing to ca midlength and more less of the same width in apical half. In contrast to C. bhutanicum, C. stultum does not have strongly sclerotized lateral margins of the median lobe and the median lobe is not spoon-like in form.

Published records
All published records of C. stultum from the Indian subcontinent need re-examination, as they very likely include misidentified C. bhutanicum.

Description
Form and colour. Body length 3.8-5.2 mm, body width 2.6-3.3 mm. Body oval in dorsal view, moderately convex in lateral view. Head black; pronotum and elytra uniformly dark brown; ventral surface uniformly reddish brown. Tarsi pale brown. Mouthparts and antennae yellowish brown, antennal club brown.
Head. Dorsal punctation dense, consisting of simple punctures without associated ridges; trichobothria present; surface between punctures smooth. Anterior margin of clypeus arcuate. Eyes large, interocular distance ca 3.3 × the width of one eye in dorsal view; eye emarginate anteriorly. Labrum moderately sclerotized, largely exposed anterior of clypeus. Antenna with 9 antennomeres, club loosely segmented. Second maxillary palpomere moderately broad.
ProtHorax. Pronotum bisinuate anteriorly, anterolateral corners obtuse; posterior margin moderately bisinuate, posterolateral corners rectangular. Anterior and lateral margins with distinct bead not extending to posterior margin. Pronotal punctation sparser and finer than on head, consisting of simple punctures without associated ridges; surface between punctures smooth. Prosternum slightly projecting mesally on anterior margin, carinate mesally, anterior portion of carina very weakly elevated.
legs. Profemur with dense pubescence except in apical fifth; mesofemur with sparsely arranged stout setae only; metafemur with very sparse pubescence.

Variation
The form of the median lobe differs slightly between the examined specimens from India and Sri Lanka and those from Taiwan and Japan (see Liu et al. 2020), but in all cases the median lobe is widest at the base and narrowed ca at mid-length. Additional studies are needed to understand whether this variation may represent a geographic variability of the species.

Lectotype designation
On the request to loan the type specimens from the Walker collection at BMNH, we received two specimens, both considered as syntypes. Both are females, but each of them belongs to a different species: the specimen labelled as cotype belongs to the subgenus Holocoelostoma based on the emarginated abdominal apex and mesofemora lacking dense pubescence; it agrees externally in all details with a male specimen from Colombo, the genitalia of which are illustrated on Fig. 10M. The second specimen, labelled as type, has an emarginate abdominal apex and densely pubescent mesofemora, and hence belongs to the subgenus Lachnocoelostoma. To fix the current concept of C. stultum and of Holocoelostoma (of which C. stultum is the type species), we are therefore designating the first specimen as the lectotype.

Biology
Aquatic species; in Japan and Taiwan it inhabits vegetation-rich places with muddy bottoms, typically ponds, rice fields and river sides (Liu et al. 2020;Nakajima et al. 2020).

Distribution
Coelostoma stultum is reported as a very widespread species, extending from the Arabian Peninsula (Fikáček et al. 2010) to Japan (Hayashi 2008) and New Guinea (Hebauer 2001) (see Hansen 1999 for a summary), also recorded from the Mascarene Islands and Madagascar (Hebauer 2006). This distribution Fig. 11. Known distribution of the Coelostoma s. str. and Holocoelostoma species in the Indian subcontinent.
is, however, surely partly based on misidentified specimens of C. bhutanicum, and needs revision. Based on the material examined for this study and published illustrations of male genitalia, we may confirm the species to occur in the following countries at present: United Arab Emirates (Fikáček et al. 2010), India (this paper), Srí Lanka (Walker 1858, this paper), Taiwan (Liu et al. 2020) and Japan (Hayashi 2008).

Discussion
The species diversity of the subgenera Coelostoma (s. str.) and Holocoelostoma in the Indian subcontinent is rather low (6 and 2 species, respectively), with 5 species being moreover very widespread across the whole Oriental Region (Fig. 11). In this respect the fauna of the Indian subcontinent corresponds, e.g., to the fauna of China, where 5 species of Coelostoma s. str. and two species of Holocoelostoma occur, all of them being widespread species (Jia et al. 2014(Jia et al. , 2017this paper). In contrast to the Chinese fauna, three species of the Indian subcontinent seem to be more local and likely endemic to the western coast of India (C. aeneolum, C. nostocinum sp. nov. and C. lyratum sp. nov.). The relatively low species diversity of Coelostoma (s. str.) and Holocoelostoma in the Oriental Region seems to contrast with the available data on the subgenus Lachnocoelostoma, which contains a high number of species, many of which are local endemics (e.g., Jia et al. 2014Jia et al. , 2017Jia et al. , 2019Liu et al. 2020). Habitat preferences of Coelostoma (s. str.) and Holocoelostoma species may be partly responsible for this pattern: all species of these two subgenera for which habitat data are available inhabit lowland standing waters. The only known exception is the finding of the above-described C. nostocinum sp. nov. in growths of Nostoc blue-green algae not associated with a nearby aquatic environment. In contrast, available data on the species of Lachnocoelostoma indicate a much wider spectrum of habitats, including the sides of running waters and wet rocks (Jia et al. 2014;Liu et al. 2020). Previous studies on aquatic beetles indicated that species of standing (lentic) waters tend to have larger ranges than species of running (lotic) waters (e.g., Ribera & Vogler 2000). Available data would indicate a similar pattern for Oriental Coelostoma, although further studies are needed to test this assumption.