Coelostoma (Holocoelostoma) bhutanicum Jayaswal, 1972

Sheth, Sayali D., Ghate, Hemant V. & Fikáček, Martin, 2020, Review of Coelostoma of the Indian subcontinent (Coleoptera: Hydrophilidae) Part 1: Coelostoma s. str. and Holocoelostoma, European Journal of Taxonomy 690, pp. 1-32 : 21-25

Coelostoma (Holocoelostoma) bhutanicum Jayaswal, 1972

Fig. 9 A–K

Coelostoma sulcata Pu, 1963: 77 , possible synonym [needs confirmation, see comments below]. Coelostoma (Holocoelostoma) bhutanicum Jayaswal, 1972: 409 .

Coelostoma (Holocoelostoma) bhutanicum – Hebauer 2002: 28 (faunistics).

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.]

Other material

NEPAL • 1 ♂; Bagmati, Sindhupalchok, Jarayetar-Dubhachaur ; [28º1ʹ45.43ʺ N, 85º20ʹ43.88ʺ E]; 800– 1600 m a.s.l.; 2 Jun. 1989; F. Hebauer determined as C. stultum ; M. Brancucci leg.; NMPC .

INDIA – Rajastan [Rajasthan] • 1 ♂, 1 ♀; Alwar district, 30 km N of Dausa, Gola-ka-bas village; 27º05ʹ31ʺ N, 76º18ʹ47ʺ E; 359 m a.s.l.; 24–28 Mar. 2004; P. Šípek and L. Šejnohová leg.; NMPC . – Meghalaya • 1 ♂, 1 ♀; W Garo Hills, Balphakram NP ; [25º29ʹ58.61ʺ N, 90º18ʹ57.16ʺ E]; 300–500 m a.s.l.; 22–27 May 1996; Jendek and Šauša leg.; NHMW • 1 ♂; W Garo Hills, Bagmara ; 25º11.5ʹ N, 90º26.5ʹ E; ca 100 m a.s.l.; 19–21 May 1996; Jendek and Šauša leg.; NHMW . – Kerala • 1 ♂; Cardamon Hills, 50 km NW of Pathanamhitta, Pambaiyar River ; 9º25ʹ N, 77º05ʹ E; 300 m a.s.l.; 5–9 May 1994; Z. Kejval leg.; NHMW .

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.

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.

ABDOMEN. All ventrites densely pubescent. First ventrite without carina. Posterior margin of last ventrite emarginate, with stout spines mesally.

AEDEAGUS ( Fig. 9 J–K). 1.0– 1.1 mm long. Median lobe nearly parallel-sided, not widened basally, slightly narrowing in apical fifth; gonopore totally apical, widely oval in shape. Parameres longer than median lobe; apex rectangular at inner margin, not projecting inwards, rounded laterally. Phallobase small, wider than long.

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 and C. 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. 9 N–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).

Figures

Fig. 9. Coelostoma (Holocoelostoma) bhutanicum Jayaswal, 1972 (A–L) and C. (H.) sulcatum Pu, 1963 (M–Q). A–K. C. bhutanicum, non-type specimen from India: Rajasthan, Alwar (NMPC). A. Dorsal habitus. B. Lateral habitus. C. Head. D. Thorax in ventral view. E. Dorsal punctation of head. F. Dorsal punctation of pronotum. G. Dorsal punctation of elytron. H. Abdominal ventrites. I. Detail of abdominal apex. J–K. Aedeagus: dorsal (J) and ventral (K). L. Illustration of the aedeagus of the paratype of C. bhutanicum by 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.