A review of Elocomosta Hansen with a description of a new species with reduced eyes from China (Coleoptera, Hydrophilidae, Sphaeridiinae)

Abstract A new species of the genus Elocomosta Hansen, 1989 (Coleoptera: Hydrophilidae: Sphaeridiinae: Coelostomatini), Elocomosta lilizheni sp. n., is described from Guangxi Province, China. It is compared in detail with the only other known species of the genus, Elocomosta nigra Hansen, 1989 from Borneo, and the genus is diagnosed from the remaining coelostomatine genera. The new species is unusual among Hydrophilidae by having extremely reduced eyes.


Introduction
The tribe Coelostomatini is one of the largest groups of hydrophilid beetles, represented by 17 genera containing more than 200 described species Fikáček 2011, 2013). Only two of these genera are species-rich: the aquatic genus Coelostoma Brullé, 1835 with slightly more than 100 described species (e.g., Fikáček 2011, Jia et al. 2014), and the terrestrial genus Dactylosternum Wollaston, 1854 with ca. 70 described species (Short and Fikáček 2011). The remaining coelostomatine genera contain far fewer species, usually characterized by peculiarities of ventral morphology. In some cases, these peculiarities are likely related to the specific biology as in the case of Lachnodacnum Orchymont, 1937 inhabiting bromeliads in Brazil (Clarkson et al. 2014) or Coeloctenus Balfour-Browne, 1939 inhabiting the tidal zone of Lake Victoria (Balfour-Browne 1939). In some genera (e.g., Phaenonotum Sharp, 1882 andPhaenostoma Orchymont, 1937), new species continue to be discovered by recent faunal inventories (Gustafson andShort 2010, Deler-Hernández et al. 2013), indicating that they are not as species-poor as they currently may appear.
Elocomosta Hansen, 1989 was erected for a single aberrant species of Coelostomatini from the Malaysian province of Sarawak on the island of Borneo, at that time placed in the tribe Sphaeridiini. The genus was transferred to Coelostomatini by Hansen (1991) and remained virtually unknown, with only few series of specimens collected after the description, all from Borneo and all belonging to the type species. It was hence a big surprise when an aberrant coelostomatine species with reduced eyes collected in the Guangxi province of south China was identified as Elocomosta using the key by Hansen (1991). It encouraged a detailed comparison of the Chinese species with the type species of Elocomosta, which is summarized here.

Material and methods
Two specimens of Elocomosta nigra and 12 specimens of the new species have been examined for this study. The holotype and several of the paratypes of the new species were dissected. After eight hours in 10% KOH at room temperature, male genitalia were mounted in glycerol on a piece of transparent plastic pinned below each specimen. Morphological characters and aedeagi were examined with the use of Leica S8APO compound microscope. The external morphology was examined using the Hitachi S-3700N environmental electron microscope at the Department of Paleontology, National Museum in Prague. Habitus photographs were taken using Canon D-550 digital camera with attached Canon MP-E65mm f/2.8 1-5 × macro lens, and subsequently adapted in Adobe Photoshop CS2. Aedeagus photographs were taken with an Axioskop 40, and then stacked using Auto-Montage software. Morphological terminology largely follows Clarkson et al. (2014), the higher-level taxonomic nomenclature follows Hansen (1999) and Fikáček (2011, 2013).
Examined specimens are deposited in the following collections: NMPC National Museum, Prague, Czech Republic; SHNU Shanghai Normal University, China; SYSU Sun Yat-sen University, Guangzhou, China.

Elocomosta Hansen, 1989
Elocomosta Hansen, 1989: 254. Type species. Elocomosta nigra Hansen, 1989 (by original designation). Diagnosis. The genus may be easily diagnosed from other coelostomatine genera by the combination of the following characters: antenna with thin, loosely segmented club, maxillary palpomere 2 without apparent distal widening, elytron with ten series of punctures (usually not very apparent among interval punctures) but without sutural stria, prosternum rather long in front of procoxae, mesoventrite with subpentagonal to circular plate with marginal bead, without anepisternal sutures and anteromedian pit, metaventrite very short but with long and wide metaventral process abutting posterior margin of mesoventral plate, abdominal ventrite 1 without median carina and abdominal ventrite 5 without apical emargination and/or group of stout setae.
Elocomosta is easy to distinguish from other Oriental coelostomatine genera by the morphology of the mesoventral plate, which is more or less flat and with a bead along the whole margin. In this it differs from Coelostoma Brullé, Coelofletium Orchymont, and Dactylosternum Wollaston, in which the mesoventral plate is keel-or roof-like and generally of the arrow-head-shape morphology. Rhachiostethus Hansen has the mesoventral plate flat, but it is tightly fused to the elevated metaventral keel and lacks the marginal bead. Dactylostethus Orchymont bears the mesoventral plate which is very similar to that of Elocomosta in the shape and in the presence of the marginal bead, but can be easily distinguished from Elocomosta by an extremely short prosternum, elytra without any traces of puncture series, abdominal ventrite 1 with median carina, more compact antennal club and mesoventrite with the anteromedian pit-like depression.
Prothorax. Pronotum transverse, deeply excised on anterior margin, strongly widened posterially; anterior and lateral margin with complete marginal bead, posterior margin without marginal bead; posterolateral corners rectangular to acute, posterior margin nearly straight to slightly concave; sublateral portions of pronotum with minute but distinctly developed trichobothria. Prosternum in front of procoxae rather wide, medially flat, without longitudinal ridge, only with a weak transverse impression along anterior margin; prosternal process hidden below procoxae, hence posterior margin of exposed portion of prosternum widely triangular; anterior margin angulate medially, with fine marginal bead, posterior margin finely beaded; concealed portion of prosternal process slightly widened posterior of procoxae. Procoxal cavities contiguous medially, open posteriorly, anterolateral aperture of procoxae very narrowly open to completely closed. Hypomeron with wide lateral bare portion, divided by a very fine line from median pubescent portion; anteromesal portion with a rather indistinct "antennal groove" defined by a weak ridge.
Pterothorax. Scutellar shield rather small, in shape of equilateral triangle. Elytron with ten longitudinal series of punctures but without sutural stria; scutellar stria not developed. Trichobothria minute but present on alternate elytral intervals; epipleuron wide and horizontal throughout, with wide external bare portion. Punctures of elytral intervals with a characteristic structure of several concentric ridges. Mesothorax with strongly elevate mesoventral plate of elongate subpentagonal to circular shape, margins of the plate with distinct wide marginal bead; posterior margin of the plate widely abutting metaventral process; anepisternal sutures reduced, not visible, anteromedian pit absent. Mesanepimeron rather narrow but long, completely closing mesocoxal cavity laterally. Metaventrite transverse, behind mesocoxae very short, shorter than the length of mesoventrite; median portion with wide and long metaventral process, ca. as long as metaventrite between meso-and metacoxae. Posterior margin of mesocoxae with a postcoxal ridge which does not continue to metaventral process. Median portion of metaventrite slightly elevated, with sparse pubescence similar to lateral portions of metaventrite, without surface microsculpture. Posterior half of metaventrite with fine longitudinal median carina. Metanepisternum narrow, ca. of the same width throughout, sparsely pubescent, with wide and long posterolateral process contacting abdomen. Apterous species.
Mesocoxae rather widely divided from each other by metaventral process, transverse; metacoxae transverse, contiguous medially; meso-and metafemora rather wide basally, with sharply defined tibial grooves in distal half, ventral surface without hydrophobic pubescence, only with sparsely arranged setae; meso-and metatibiae slightly bent outwards, slightly widened distally, with short but stout spines distally and along lateral and mesal faces; meso-and metatarsomere 1 the longest, ca. 1.5-2.0 × as long as tarsomere 2; ventral face with dense pubescence, dorsal face with few isolated long setae.
Abdomen with 5 ventrites, all ventrites without median carina, abdominal apex without emargination or series/group of enlarged setae. All ventrites with dense hydrofuge pubescence.
Biology. All known specimens were collected from terrestrial habitats. Holotype and nine paratypes were collected from mixed leaf litter in beech forest, and two specimens from dead wood with polypores in logging field.

Elocomosta nigra
Biology. Terrestrial species, part of type series was collected from rainforest leaf litter (Hansen 1989). No biology data are available on locality labels in the specimens examined by us.

Discussion
The new species of Elocomosta is very unusual in Hydrophilidae due to its extremely reduced eyes. The examination using the scanning electron microscope revealed that the eye is not reduced completely, as it may appear from an observation under a stereomicroscope, but that the eye is extremely reduced in size, with only few ommatidia recognizable. Interestingly, it seems that not only the eye itself is reduced in size, but that the whole lateral portion of the head is reduced, which is best seen when the relative widths of the heads are compared between E. nigra and E. lilizheni sp. n. (compare Figs 3 and 6). This is unique among known Hydrophilidae, as in all known cases of eye reduction, the shape of the head remained unaffected or nearly so. The reduction of eye size is commonly observed in the Sphaeridiinae, probably in relationship with the cryptic leaf-litter life style of the majority of the members of the subfamily. Reduced eye size was observed for example in some species of the megasternine genera Motonerus Hansen and Oosternum Sharp Short 2006, Fikáček andHebauer 2009) and seems to be frequently correlated with the reduction of the hind wings: in both aforementioned genera reduced eyes were observed in micropterous or apterous species. In case of Elocomosta, both E. nigra with well-developed eyes and E. lilizheni sp. n. with extremely reduced eyes are apterous. Moreover, the extent of eye reduction observed in E. lilizheni is not comparable to any other known member of the Sphaeridiinae, which may indicate not only flightlessness but also some other reasons which may be responsible for the eye reduction; a specialized lifestyle would be a possible candidate. For example, the loss of eyes was recently discovered in larvae of the myrmecophilous genus Sphaerocetum Fikáček which live inside of the nest of the Crematogaster + Camponotus ants, likely in its pupal and larval chambers (Fikáček et al. 2015). However, the collecting circumstances of E. lilizheni sp. n. do not conclusively indicate any highly specialized lifestyle nor an association with ants.