A new genus of coprophagous water scavenger beetle from Africa (Coleoptera, Hydrophilidae, Sphaeridiinae, Megasternini) with a discussion on the Cercyon subgenus Acycreon

A new genus of coprophagous beetle, Evanesternum gen. n. (Hydrophilidae: Sphaeridiinae: Megasternini), is described in order to accommodate Cercyon (Acycreon) pulsatus d’Orchymont, 1937 from the Republic of South Africa and the Democratic Republic of Congo. A detailed description is provided along with habitus photographs, line drawings and SEM micrographs of relevant diagnostic characters. The new genus possesses the tribal synapomorphies of Megasternini but bears several unique morphological characters which are discussed in detail. The morphology of the remaining three species classified in the subgenus Acycreon d’Orchymont, 1942 (i.e. C. punctiger Knisch, 1921, C. collarti d’Orchymont, 1942 and C. apiciflavus Hebauer, 2002), is illustrated in order to provide evidence that Acycreon is an assemblage of morphologically dissimilar and likely not related species. An identification key to the Megasternini genera and subgenera known from the Republic of South Africa is presented.


Introduction
Water scavenger beetles (Hydrophilidae) are mainly known as species associated with a wide variety of aquatic habitats; the majority of the species (ca.65%) inhabit aquatic and semi-aquatic environments (Short andFikáček 2011, Bloom et al. 2014).Aquatic species form the vast majority of the subfamilies Hydrophilinae, Chaetarthriinae, Enochrinae and Acidocerinae (Short and Fikáček 2013).However, more than a third of known hydrophilid beetles, mostly belonging to the subfamilies Cylominae and Sphaeridiinae, have colonised terrestrial habitats, typically those with large amounts of decaying organic matter (e.g.tropical forest leaf litter and rotten plant debris).Several groups are also associated with vertebrate dung (usually excrement from large herbivorous mammals) and many taxa are exclusively coprophagous.Dung provides an abundant and rich source of nutrients to a wide range of arthropods, with beetles being amongst them, along with Diptera, the most conspicuous and diverse.Hydrophilids, together with scarabs, are amongst the most important coprophagous beetles (Holter 2004).
The African continent is well known for its abundance of large mammal species, which in turn serve as a source of dung to be exploited.The diversity and abundance of large mammals (especially herbivores) likely promoted the diversification of beetle groups like scarab dung beetles and terrestrial hydrophilids of the subfamily Sphaeridiinae, which are both abundant and diverse in Africa (Davis and Scholtz 2001;Hebauer 2006).The tribe Megasternini is the most diverse group of hydrophilid beetles associated with terrestrial environments, comprising moreover the vast majority of obligatory coprophagous hydrophilid species.Seventeen genera are known to occur in Africa, of which Cercyon Leach, 1817, Pachysternum Motschulsky, 1863 and Cryptopleurum Mulsant, 1844 are especially species-rich.Nine small genera are endemic for the Afrotropical region: Cercillum Knisch, 1921 (Central and Southern Africa), Cyrtonion Hansen, 1989 (Central Africa), Delimetrium Hansen, 1999 (Republic of South Africa), Parastromus Balfour-Browne, 1948 (Central and Southern Africa), Pelocyon Balfour-Browne, 1950 (Central andSouthern Africa), Pseucyon d'Orchymont, 1948 (Ethiopia), Quadristernum Balfour-Browne, 1950 (Rwanda-Burundi), Acaryon Hebauer, 2003 (Madagascar) and Colerus Hansen, 1999 (Madagascar).All of these are rare in collections and poorly known.They have rarely been mentioned after their description (Balfour-Browne 1948, 1950;Knisch 1921;d'Orchymont 1948) or have been described very recently (Hansen 1999b, Hebauer 2003) and few of them (Colerus, Quadristernum) are known only from one or a few specimens.
During the recent field work in the Cape region of the Republic of South Africa, the authors discovered a morphologically aberrant tiny representative of the Megasternini which represents an undescribed genus.The review of previously known South African species revealed that the species is already described, but misclassified as part of the subgenus Acycreon d'Orchymont, 1942 of the genus Cercyon.In this paper, the generic assignment of this species is re-evaluated, a new genus described for it, the morphology of the remaining species assigned at the moment to Cercyon (Acycreon) is reviewed and the taxonomic composition of this subgenus is discussed.

Materials and methods
This study is based on the specimens deposited in the following entomological collections:

MNRJ
Museu Nacional, UFRJ, Rio de Janeiro, Brazil (B.During the field work, about 15 kg of relatively fresh horse and cow dung was collected in a thick 60 litre plastic bag.The bag was closed and an air buffer was left above the excrement.The bottom of the bag was perforated with 1 cm holes using a knife.This bag was enclosed in another intact bag and hung above the ground in a shaded area.The beetles accumulated overnight in the second bag and were collected in 96% ethanol, without needing to check the excrement by hand.
Part of the specimens examined was dissected, with genitalia embedded in a drop of ethanol-soluble Euparal resin on a small piece of glass glued to cardboard attached below the respective specimen.
Habitus photographs were taken using a Canon D-550 digital camera with attached Canon MP-E65 mm f/2.8 1-5 macro lens.Pictures of genitalia were taken using a Canon D1100 digital camera attached to an Olympus BX41 compound microscope; pictures of different focus were combined in Helicon Focus software.Scanning electron micrographs were taken using Hitachi S-3700N environmental electron microscope at the Department of Paleontology, National Museum in Prague.Pictures used for plates were adapted in Adobe Photoshop CS6.All original pictures including additional views, not presented in this paper, are published and freely available on Flickr (https://www.flickr.com/photos/142655814@N07/sets/72157681650620964) and submitted to Zenodo repository (https://zenodo.org/) under https://doi.org/10.5281/zenodo.806765.
Mesothorax.Mesoventrite completely fused with an episternum; anterior collar of mesothorax narrow.Median portion of mesoventrite elevated as a mesoventral plate slightly overlapping anterior margin of metaventrite; plate well defined posteriorly as a broad, semi-elliptical tablet abruptly vanished in anterior half leaving only a narrow median ridge.Grooves for reception of procoxae well defined by a conspicuous carina, short, transverse (Fig. 4g).Mesepimeron very narrow, widening laterad.Mesocoxal cavities moderately narrowly separated.Scutellar shield small, semi-elliptical, 1.7× as long as wide.Elytra (Fig. 1a, b) weakly convex, narrowly explanate laterally, each elytron bearing 10 series, series 1-9 consisting of setiferous punctures as large as interval punctures but surrounded by a foveolate depression; punctures in series situated in longitudinal impressed sulci; series 1-4 and 9 reaching apex, series 5 and 8 enclosing series 6-7 subapically, series 10 reduced both anteriorly and posteriorly; epipleuron horizontal, weakly gradually narrowing posteriad, accentuating about half the length of metaventrite, vanishing shortly after the posterior margin of the metaventrite, bearing sparse short setae (Fig. 3).
Metathorax.Metaventrite (Fig. 4h) with anterior rim narrow, widening on anterolateral corners; mesal elevate area flat and very wide, almost reaching lateral margins; anterior and posterolateral corners distinctly rugose, with short setae.Femoral lines and anterolateral ridges absent.Metanepisternum ca.5× as long as wide, with anterior oblique ridge, metepimeron with minute ventral portion.Metafurca well developed.Metathoracic wings well developed, with transverse vein r4 arising from basal portion of radial cell, RP rather long, reaching ca.halfway to wing base, basal cubito-anal cell small, closed, wedge cell absent, transverse vein mp-cua joining to MP 3+4 +CuA 1+2 ; anal lobe not defined.
Etymology.The generic name is derived from evanescere (Latin, "to vanish") and sternum (Greek, "chest") which refers to the anteriorly vanishing mesoventral plate.The gender is neutrum.
Metathorax.Metaventrite (Fig. 4h) with raised area very wide, almost reaching lateral margins, about 1.5× as wide as long, rather roughly punctate, punctures transverse, resembling those on dorsal surfaces, with very small fine setae, surface finely squamose.
Abdomen with five ventrites.Ventrite 1 with median longitudinal carina present, slightly narrowing posteriad, briefly projecting posteriorly in both sexes (Fig. 4i); ventrite 5 with rounded apex in both sexes, with a group of longer setae on apex.
Genitalia.Median projection of sternite 9 (Fig. 1f ) subtruncate apically, without subapical setae, median portion narrowing posteriorly, with posterior end distinctly acute, lateral struts joined at mid length of the median projection.Phallobase (Fig. 1c) slightly longer than parameres, asymmetrically narrowing basally, base slightly curved.Parameres weakly narrowing apically, subsinuate and briefly widened near apex.Median lobe (Fig. 1d) narrow, parallel-sided throughout, apex acuminate, with small parallel apical projection, gonopore moderately large, situated subapically.Biology.Recently collected specimens from Western Cape were extracted from cow and horse dung in a small farm close to a river (Fig. 5b).Other hydrophilids from the same dung collected in abundance were Sphaeridium caffrum Castelnau, S. abbreviatum Boheman, Pachysternum capense (Mulsant) and three small Cercyon species (one belonging to the Cercyon nigriceps group and other two unidentified species).Other specimens examined were collected in unspecified type of dung while specimens from the Democratic Republic of Congo were collected from elephant excrement.

Key to the Megasternini genera and subgenera known from the Republic of South Africa
The key includes the genera recorded from the Republic of South Africa by Hansen (1999a) and Hebauer (2006).Due to unclear limits of Cercyon and Parastromus, the subgenera of Cercyon recorded from RSA are also included into the key to allow the identification of as many species as possible.Re-description.2.0-2.5 mm long, 1.8-1.9×as long as wide, 2.9-3.0× as long as high.Integument shining (Fig. 6a).Colouration reddish-brown with pale palpi.Eyes without thickened ridge at posterior margin (Fig. 7a.).Mentum subtrapezoid, with anterior mar-  gin almost straight-lined (Fig. 7b).Pronotum with lateral margins moderately impressed (Fig. 7e).Pronotum and elytra deeply punctate without conspicuous microsculpture (Fig. 7c-d).Mesoventral elevation fusiform, short, not reaching anterior margin, procoxal rests not defined by transversal carinae (Fig. 7f).Raised portion of metaventrite about as long as wide, distinctly deeply punctate (Fig. 7h).Median lobe of aedeagus with parameres moderately broad, phallobase about 1.3× as long as parameres (Fig. 6b).Sternite 9 with median projection strongly narrowing anteriorly, lateral struts almost reaching base (Fig. 6c).
Distribution.Only known from the Democratic Republic of Congo.Redescription.1.8 mm long, 1.4× as long as wide, 2.4× as long as high.Integument shining (Fig. 9a).Colouration of head, pronotum, ventral surfaces and legs dark reddish-brown with pale palpi and antennae, elytra black with apex testaceous.Eyes without thickened ridge at posterior margin.Mentum subtrapezoid, with anterior margin straight-lined (Fig. 9b).Pronotum with lateral margins narrowly impressed (Fig. 9e).Pronotum and elytra deeply punctate without conspicuous microsculpture (Figs 9d-e).Mesoventral elevation fusiform, short, not reaching anterior margin, procoxal rests defined by oblique carinae, with a deep setose pore on each side of the mesoventral elevation (Fig. 9g).Raised portion of metaventrite about as long as wide, distinctly deeply punctate (Fig. 9h).

Cercyon (Acycreon) apiciflavus
Distribution.Only known from the type locality in Nepal.Comments.The species differs from the remaining two Acycreon species in the rather globular and widely rounded body.In these aspects, as well as in the morphology of the ventral parts of thorax, it is very similar to several undescribed species from the Chinese provinces, Yunnan and Sichuan (S.Ryndevich, in prep.).The deep, rounded setose pores on the mesoventrite on the side of the central elevation have not been recorded in any other Cercyon species and, along with the other morphological features of this species, suggest its distant relation to the type species of Acycreon subgenus.
Evanesternum pulsatum comb.n. differs from C. punctiger (type species of Acycreon) and the other two species in the following characters: (1) male 9 th sternite with lateral struts attached ca. at midlength (Fig. 1f ) (basally or sub-basally in C. punctiger and C. collarti (Fig. 6c, f ), not known for C. apiciflavus); (2) rugose-reticulate dorsal integument of head and pronotum (Fig. 4d-e) (without any microsculpture in Acycreon (Figs 7c-d, 8c-d); (3) medial raised part of metaventrite nearly reaching lateral margin (Fig. 4h) (present only mesally and not reaching laterally in Acycreon (Figs 7h, 8g, 9h); (4) thickened ridge running along posterior ventral margin of eye (Fig. 4a) (without such ridge in Acycreon (Figs 7a, 8a); (5) procoxal rests of mesoventrite defined by sharp transverse ridges (Fig. 4g) (in Acycreon without such ridges (Figs 7g, 8f ) or with oblique ridges (Fig. 9g)); (6) pronotum deeply impressed along lateral margin (Fig. 4e) (at most weakly impressed in Acycreon; Evanesternum pulsatum comb.n. was mentioned by d' Orchymont (1942) as having the lateral portion of the pronotum similar to that of C. punctiger, but the margin is much deeper and broader in Evanesternum (compare Fig. 3h with Figs 7e and 9e).The above-mentioned morphological differences between Evanesternum pulsatum comb.n. and the members of Acycreon are substantial and it is believed that E. pulsatum is not congeneric with them.Evanesternum gen.n. may, at first sight, resemble the small-bodied African megasternine genera Delimetrium, Pelocyon and Pseucyon or the smaller-sized species of the genus Cercyon (e.g. C. minax Balfour-Browne).The simple prosternum (i.e.not elevated medially) easily distinguishes it from all these genera except Cercyon.Evanesternum would be keyed out asCercyon in the key to genera by Hansen (1991), which remains to date the most comprehensive resource for identification of Megasternini.The new genus has all the diagnostic characters of Megasternini: antenna with compact club, maxilla of male with sucking disc on galea; prosternum with antennal grooves, first abdominal ventrite carinate medially.However, it can be distinguished from Cercyon (as well as other megasternine genera) by the following combination of characters: (1) the mesoventral plate well-defined only posteriorly, not properly demarcated anteriorly (always well-demarcated as a complete plate in Cercyon); (2) mesal bare portion of metaventrite extending far laterally, covering the majority of the metaventral surface (always confined to median portion of metaventrite in Cercyon); (3) pronotum with deep groove along the lateral margin (absent in Cercyon); and (4) the morphology of male sternite 9 with very short lateral struts attaching at mid-length of the medial sclerite (lateral struts long and attaching basally in Cercyon and all genera of the Cercyon-group of genera).The morphology of the male terminalia and surrounding structures (sternite 9, articulation of the median lobe and parameres) seems to be phylogenetically informative in Megasternini, corresponding to the clades recognised by molecular phylogenetic analyses (Short and Fikáček 2013, Arriaga-Varela unpubl. data).Hence, the very unusual morphology of male sternite 9 of Evanesternum may indicate its rather isolated position in the Cercyongroup of the Megasternini.Preliminary analyses of molecular data (Arriaga-Varela, in prep.)seem to support this hypothesis.
Despite the mesal portion of prosternum being flat in Evanesternum, it bears a very faint demarcation of the mesal part with respect to the lateral parts by irregular diagonal sulci.These sulci 'define' the mesal portion which can be also inferred from the differences in the sculpture on the posterior margin and from the long setae present on the anterior margin.The prosternum as found in Evanesternum may possibly represent an intermediate condition between the flat prosternum (as present, for example, in Cercyon) and the mesally demarcated and elevated one (as present in genera Cryptopleurum, Cyrtonion, Delimetrium, Pachysternum, Pelocyon and Pseucyon in Africa) or a highly reduced version of a mesally demarcated prosternum.
Of the valid subgenera, Cercyon s. str.contains the largest number of species (slightly over 200) which are morphologically very diverse, indicating that the subgenus is likely to be an artificial assemblage of species rather than a monophyletic group.The remaining ten subgenera were created to accommodate some morphologically aberrant Cercyon species.They contain many less species and since they were mostly defined by some unique characters of the meso-or metaventrite, they more likely represent monophyletic groups.Still, the delimitation of some of them is unstable, resulting in frequent changes in subgeneric assignments of some species.This confusion concerns especially the subgenera Clinocercyon d'Orchymont, 1942 defined by the oblique, rath-er than horizontal epipleura, which contains a very diverse assemblage of species from the Old World, with some species repeatedly moved in and in and out (e.g.Ryndevich 2007) and Conocercyon Hebauer, 2003 defined by a shape of the postcoxal ridge of the metaventrite (originally described for a few species from Madagascar and Seychelles, with two eastern Palaearctic species assigned to it later: Hebauer 2003, Ryndevich 2007, Hoshina 2008).Three subgenera, Himalcercyon Hebauer, 2002, Oedocercyon d'Orchymont, 1942and Prostercyon Smetana, 1978, are monotypic (containing only one species).The results of this review of the subgenus Acycreon corresponds to the situation observed in these subgenera, i.e. it seems that the subgenus is defined by a single (and moreover weakly defined) character which picks up superficially similar but likely not closely related species.It is however surprising that this problem also concerns Acycreon, i.e. one of the smallest subgenera of Cercyon and clearly indicates that even the small subgenera need to be carefully tested for monophyly in future molecular phylogenetic studies.
After the transfer of C. pulsatus to Evanesternum, Acycreon contains three species, C. (A.) punctiger, C. (A.) collarti and C. (A.) apiciflavus.The mesoventrite of all of them forms a well-defined fusiform plate with a well-marked acute anterior tip, the plate being however rather short, ca.half as long as the length of the mesoventrite.The similarity of all three species hence concerns the relative length of the mesoventral plate, rather than the absence of the plate-like elevation mentioned by previous authors.In all other aspects, the Acycreon species are not very similar to each other in terms of external and genital morphology (compare Figs 6a-f, 7a-h, 8a-g) which indicates that they are likely not closely related and potentially not forming a monophyletic group.The distribution of the species (Oriental region versus tropical Africa) is also congruent with this view.Hence, even after excluding C. pulsatus, Acycreon consists of quite dissimilar species and may still be an artificial rather than natural assemblage.The concept of Acycreon needs to follow the morphology of its type species (C.punctiger) and can be only adapted after the phylogenetic position of that species has been resolved.

Figure 1 .
Figure 1.Evanesternum pulsatum (d'Orchymont) a dorsal habitus b lateral habitus c tegmen of aedeagus d median lobe of aedeagus e detail of apex of median lobe f 9 th sternite.

Figure 4 .
Figure 4. Evanesternum pulsatum (d'Orchymont) a ventral view of head b labium c prosternum d detail of head surface e detail of pronotal surface and lateral margin f detail of elytral surface g mesoventral plate h metaventrite i ventral view of abdomen.

Figure 5 .
Figure 5. Evanesternum pulsatum (d'Orchymont) a distribution map b habitat collecting locality: Summerset Getaway Farm, Western Cape, Republic of South Africa c beetles collected from dung in Summerset Getaway Farm, Western Cape, Republic of South Africa.Red arrow is indicating an Evanesternum pulsatum (d'Orchymont) specimen.

Figure 7 .
Figure 7. Cercyon (Acycreon) punctiger Knisch a ventral view of eye b labium c detail of elytral surface d detail of head surface e detail of pronotal surface and lateral margin f prosternum g mesoventral plate h metaventrite.

Figure 8 .
Figure 8. Cercyon (Acycreon) collarti d'Orchymont a ventral view of eye b labium c detail of elytral surface d detail of head surface e prosternum f mesoventral plate g metaventrite.

Figure 9 .
Figure 9. Cercyon (Acycreon) apiciflavus Hebauer a dorsal habitus b labium c detail of elytral surface d detail of head surface e detail of pronotal surface and lateral margin f prosternum g mesoventral plate (arrow pointing to the setose pore) h metaventrite.