Coelostoma (s. str.) aeneolum Régimbart, 1903

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 : 7-9

Coelostoma (s. str.) aeneolum Régimbart, 1903

Fig. 3 A–M

Coelostoma aeneolum Régimbart, 1903: 337 .

Coelostoma aeneolum – Zaitzev 1908: 403 (catalogue). — Knisch 1924: 110 (catalogue). — Orchymont 1928: 54 (catalogue).

Coelostoma (s. str.) aeneolum – Mouchamps 1958: 28 (assigned to Coelostoma s. str.). — Hansen 1999: 242 (catalogue).

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 (here designated)

INDIA • ♂; “ MAHÉ, COTE DE MALABAR ”; [11º42ʹ0.09ʺ N, 75º31ʹ59.96ʺ E]; [27 m a.s.l.]; “ Juillet 1901 ” [Jul. 1901]; “ M. Maindron leg.”; MNHN.

Paralectotype

INDIA • ♀ ; “ CALICUT [Kozhikode], COTE DE MALABAR ”; [11º15ʹ38.85ʺ N, 75º46ʹ49.48ʺE]; [15 m a.s.l.]; “ 20–25 Juin 1901 ” [20–25 Jun. 1901]; “ M. Maindron leg.”; MNHN .

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.

Other material

INDIA • 1 ♂; “ Maharashtra, Bombay ” [ Mumbai ]; [19º01ʹ3.96ʺ N, 72º51ʹ22.09ʺ E]; IRSNB .

Redescription

FORM AND COLOUR. Body length 4.6 mm, body width 2.6 mm (in both examined type specimens). Body oval in dorsal view, moderately convex in lateral view. Head black; pronotum and elytra uniformly 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 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.

MESOTHORAX. Elytral punctation dense and moderately coarse, consisting of punctures without transverse ridges. Series of punctures absent. Sutural stria weakly impressed, present in apical half. Mesoventral plate 1.2× 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 metaventral process narrowly projecting between mesocoxae; posterior process bifid. Wings welldeveloped (macropterous).

LEGS. Profemur with dense pubescence except in apical fifth; mesofemur and metafemur with sparsely arranged stout setae only.

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

AEDEAGUS ( Fig. 3 J–L). 0.4 mm long. Median lobe triangular, broad at base, tapering towards apex, bluntly rounded at apex; gonopore situated at apex, widely semicircular. Parameres longer than median lobe; narrow at base, rounded and slightly angulate at apex. Phallobase small, slightly wider than long.

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).

Figures

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.