Coelostoma Brullé, 1835

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 : 4-7

Genus Coelostoma Brullé, 1835

Subgenera of Coelostoma

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).

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.

Key to Indian species of Coelostoma (s. str.)

1. Median lobe cylindrical, uniformly wide throughout; gonopore on its very apex ( Figs 2D , 6 J–K) .. ............................................................................................................................... C. lyratum sp. nov.

– Median lobe more or less triangular, wider at the base than at the apex; gonopore subapical or basal ( Fig. 2 A–C, E–F) .............................................................................................................................. 2 2. Apex of the median lobe narrowly spatulate, gonopore basal ( Figs 2F , 8 J–K). Small species ( Fig. 2F ) ............................................................................................... C. vividum Orchymont, 1936

– Apex of the median lobe not extremely narrow, gonopore subapical ( Fig. 2 A–C, E) ..................... 3

3. Median lobe very short and wide, gonopore very large ( Figs 2D , 7 J–K) ........................................... ................................................................................................................. C. vitalisi Orchymont, 1923

– Median lobe in form of narrow and elongate triangle ( Fig. 2 A–C) .................................................. 4

4. Median lobe much longer than its basal struts, lateral sides of the median lobe concave. Apex of paramere strongly asymmetrical, with apical tooth-like projection ( Figs 2C , 4 J–K) ......................... ........................................................................................................ C. fallaciosum Orchymont, 1936

– Median lobe only slightly longer than its struts, lateral margins of the median lobe straight. Apex of paramere more less symmetrical, without distinct tooth-like projection ( Fig. 2 A–B) ..................... 5

5. Aedeagus larger (0.7 mm), outer face of the paramere slightly concave in its apical third. Median lobe relatively wider at apex ( Figs 2B , 5 J–K) ............................................... C. nostocinum sp. nov.

– Aedeagus smaller (0.5 mm), outer face of the paramere continually arcuate, not concave subapically. Median lobe narrower at apex ( Figs 2A , 3 J–L) .................................. C. aeneolum Régimbart, 1903

Key to Indian Coelostoma (Holocoelostoma)

1. Median lobe of the same width from base to apical fifth, in apical fifth distinctly narrowed; lateral margins of the median lobe strongly sclerotized, median lobe spoon-like when observed slightly laterally ( Figs 2G , 9 J–K) .................................................................. C. bhutanicum Jayaswal, 1972

– Median lobe widest at the base, gradually narrowing ca to the midlength, apical half narrowly parallel-sided, ca of the same width; lateral margins of the median lobe weakly sclerotized, median lobe not spoon-like when observed slightly laterally ( Figs 2H , 10 K–M) .......................................... ................................................................................................................... C. stultum ( Walker, 1858)

Figures

Fig. 2. Overview of Coelostoma Brullé, 1935 s. str. and Holocoelostoma Mouchamps, 1958 of the Indian subcontinent (habitus on the top, aedeagus on the bottom). – A–F. Coelostoma s. str. A. C. aeneolum Régimbart, 1903. B. C. nostocinum sp. nov. C. C. fallaciosum Orchymont, 1936. D. C. lyratum sp. nov. E. C. vitalisi Orchymont, 1923. F. C. vividum Orchymont, 1936. – G–H. Holocoelostoma. G. C. bhutanicum Jayaswal, 1972. H. C. stultum (Walker, 1858). All habitus figures and all aedeagi are to scale. Scale bars: habitus = 1 mm; aedeagus = 0.25 mm.

Fig. 6. Coelostoma (s. str.) lyratum sp. nov., holotype, ♂ (NMPC). A. Dorsal habitus. B. Lateral habitus. C. Frontal view. 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: ventral (J) and dorsal (K).

Fig. 8. Coelostoma (s. str.) vividum Orchymont, 1936, specimens from Assam: Kohora (NMPC). A. Dorsal habitus. B. Lateral habitus. C. Frontal view. 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).

Fig. 7. Coelostoma (s. str.) vitalisi Orchymont, 1923, ♂ specimen from Assam: Kohora (NMPC). A. Dorsal habitus. B. Lateral habitus. C. Frontal view. 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: ventral (J) and dorsal (K).

Fig. 4. Coelostoma (s. str.) fallaciosum Orchymont, 1936, ♂ specimen from India: Fatepur Sikri (NMPC). A. Dorsal habitus. B. Lateral habitus. C. Head in dorsal view. 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: ventral (J) and dorsal (K).

Fig. 5. Coelostoma (s. str.) nostocinum sp. nov. A. Dorsal habitus. B. Lateral habitus. C. Frontal view. 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: ventral (J) and dorsal (K). Photographs based on: A–G. Paratype from Lonavala, Bhushi dam (NMPC). H–K. Holotype, ♂ (NMPC).

Fig. 3. Coelostoma (s. str.) aeneolum Régimbart, 1903, lectotype. A. Dorsal habitus. B. Lateral habitus. C. Frontal view. 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–L. Aedeagus. J–K. Lectotype, ♂ (MNHN) (J, ventral; K, dorsal). L. Non-type specimen from Mumbai, ♂ (IRSNB). M. Labels of the lectotype.

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.

Fig. 10. Coelostoma (Holocoelostoma) stultum (Walker, 1858). 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–J. Detail of abdominal apex. K–M. Aedeagus: dorsal (K) and ventral (L–M). N. Labels of the lectotype. Specimens: A–I, K–L. Non-type specimens from Uttar Pradesh, Fatehpur Sikri, ♂ (NMPC). J, N. Lectotype, ♀ (BMNH). M. Non-type specimen, ♂, from Sri Lanka: Colombo (BMNH).