Bembidion bukejsi

Schmidt, Joachim & Michalik, Peter, 2017, The ground beetle genus Bembidion Latreille in Baltic amber: Review of preserved specimens and first 3 D reconstruction of endophallic structures using X-ray microscopy (Coleoptera, Carabidae, Bembidiini), ZooKeys 662, pp. 101-126 : 104-106

Bembidion bukejsi http://zoobank.org/661D694B-E65E-4D6F-A0D6-A26C0BC3D50EFigs 1-2, 3-5, 6-8, 9-13, 14-17, 26, 27

Holotype.

Male in Baltic amber; size of amber piece approximately 8.9 × 4.7 × 2.5 mm (Fig. 2), with collection label data “012”, in Andris Bukejs collection, maintained at Institute of Life Sciences and Technologies, Daugavpils University (Daugavpils, Latvia). This amber piece was found elsewhere in the area of the Curonian Spit.

Preservation status.

The amber piece and its Bembidion fossil are in a comparatively very good conservation state. Most parts of the piece are clear, and the beetle body is well visible using light microscopy (Figs 1, 3-5). The mouth part is partly covered by milky coating and thus mandibles and maxillae cannot be investigated by light microscopy. The body is partly covered by minute particles (granules or tiny bubbles) of unknown origin. The exoskeleton of the specimen is partly slightly shrunken and thus dissociated from the inclusion wall (Figs 6-8). Four apical tarsomeres of the left proleg are lost. The aedeagus is well preserved and could be reconstructed together with its endophallic folding structures using micro-CT (Figs 12-17, 26).

Syninclusions.

One stellate hair and numerous dirt particles.

Description.

Body length: 3.5 mm.

Colour: The whole body surface appears blackish, very shiny, with metallic lustre; variation in colouration of the different parts of the beetle body is not recognizable.

Microsculpture: Surface of head including labrum with deeply engraved isodiametric sculpticells, pronotum with less deeply engraved slightly transverse meshes, elytral intervals with very finely engraved transverse meshes which are much smaller than on head and pronotum and which are not visible below magnification of × 100.

Head: Moderately large and transverse; length 0.75 mm, width 0.80 mm. Mandibles moderately slender. Labrum with apical margin slightly concave, dorsally with three pairs of setae near apical margin. Clypeus with one pair of setae in normal position. Shape and setation of maxillary palpi as typical for Bembidion , with apical segment subulate, approx. 2/5 of length of penultimate segment; penultimate segment markedly broadened towards apex. Antennae rather short, with pedicellus approx. 1.5 times longer than broad, and with two antennomeres extending beyond the pronotal base. Mentum and submentum distinct, mentum with medio-apical tooth simple, shortly rounded at tip, and with one pair of setae; pits absent. Eyes large, hemispherical, protruded; tempora very small, approx. 1/20 of eyes diameter, not visible in dorsal view. Disk moderately convex, smooth apart from the prominent microsculpture. Frontal furrows very shallow, very short, absent on disk. Supraorbital furrows flat, without punctures; two supraorbital setae present and in normal position for Bembidion .

Prothorax: Pronotum moderately large, length 0.76 mm, width 1.04 mm, transverse (width/length = 1.37), 1.3 times broader than head, subcordate, broadest slightly before middle, with sides faintly concave in posterior third. Laterobasal angles large, almost rectangular, not protruded laterally. Basal margin 1.1 times broader than apical margin. Disk moderately convex, smooth. Anterior margin finely convex in middle, lateroapical angles distinctly protruded, rounded. Posterior margin not beaded, slightly convex in middle, slightly sinusoidal towards laterobasal angles, latter very faintly shifted anteriad with respect to posterior margin of pronotum. Median longitudinal impression deep in middle, deepest before posterior transverse impression, but absent near pronotal apex and base; anterior transverse impression very shallow, smooth; posterior transverse impression moderately deep, smooth; laterobasal foveae large and rounded, moderately deep, smooth. Lateral gutter narrow throughout, smooth. Laterobasal carina long and straight, approx. 1/3 of length of pronotum. Both lateral and laterobasal setae present, with the lateral seta located slightly before middle of pronotum. Proepisternum glabrous, smooth.

Pterothorax: Elytra moderately convex on disc, in dorsal view narrow ovate, length 2.05 mm, width 1.37 mm, length/width = 1.50, widest near anterior third, distinctly wider than pronotum (width of elytra/width of pronotum = 1.32). Surface and lateral border glabrous and smooth apart from the primary elytral setation. Shoulders moderately broad with humeral margin angulate: The lateral bead forms an almost right angle with the abbreviated basal bead; the latter extends to the tip of the 4th elytral stria. Crista clavicularis absent. Sides with preapical sinuation indistinct; subapical plica present. Parascutellar stria moderately long, parascutellar seta present. All striae complete, deeply impressed, impunctate, with intervals convex; apical stria (= common prolongation of the 5th, 6th, and 7th striae) deeply impressed from level of the apical cross of 5th and 6th stria towards apex; recurrent stria lacking. Ninth interval moderately broadened from level of humeral umbilicate series towards apex. Each elytron with two discal setae in third interval, with relevant pores located close to, but separated from, third stria. Preapical seta located in the deepened apical portion of the seventh stria; the fine apical seta located at apical margin. Umbilicate series consist of eight setae: four humeral setae, with distance between first and second as well as second and third setae slightly larger than that between third and fourth setae; the fourth seta is located distinctly basad of the level of the anterior discal seta; both the subapical setae are markedly advanced and located at the beginning of the apical elytral third; two apical setae of the umbilicate series situated anterior of the junction of the eighth stria and lateral gutter. Metepisternum long, glabrous and smooth, with outer margin 1.6 times longer than anterior margin. Metasternal process without borders, moderately convex in middle. Hindwings fully developed.

Abdomen: Abdominal sternites V–VII each with one (male) pair of setae near apical margin; surfaces smooth, without hairs or micropunctures.

Legs: Relatively short, unmodified, femora moderately robust, protibiae straight and moderately dilated towards apex. First protarsomere markedly dilated, second protarsomere moderately dilated with apicolateral projection on inner margin.

Male genitalia (Figs 12-17, 26): Shape and size of median lobe as well as general structures of endophallus similar to species of the Bembidion subgenera Bracteon Bedel, 1879, and Odontium LeConte, 1848 (see Maddison 1993: 257): Median lobe moderately large, moderately slender, in lateral view moderately bent, with terminal lamella distinct, short tongue-shaped, slightly bent ventrad. Details of the parameres could not be recovered. Endophallus (in inverted positon) with a large ostidial flag with its internal tip distinctly bent back to the dorsal side of median lobe; dorsal field of ostium distinct; dorsal plate very large with left side more than right side. Below the dorsal plate an extended folding structure is developed (central fold system) which covers the central sclerite and the brush sclerite complexes dorsally and, which proceed in the flagellum distally; latter rather short, almost straight, not extending to the dorsal tip of the ostidial flag. Central and brush sclerite complexes both moderately large and situated side by side in middle portion of median lobe, overlaying each other in lateral view; central sclerite with left lobe lenticular and with right lobe large, situated more basad; brush sclerite apico-ventrally markedly prolonged with complicate folding structures.

Derivatio nominis.

The species epithet is a dedication to the collector Andris Bukejs, which kindly allowed us to investigate this unique specimen.

Relationships and recognition.

The angulate humeral margin together with the position of the elytral discal setae separated from third stria and the shape of the endophallic structures provide important evidence for probable relationships of the fossil species. An angulate humeral margin is also developed in species of the Bembidion sensu lato lineages Andrewesa Netolitzky, 1931, Hoquedela Müller-Motzfeld, 1988, Peryphophila Netolitzky, 1939, Phyla Motschulsky, 1844, Plataphodes Ganglbauer, 1891, as well as in the Hydrium and Odontium complexes sensu Maddison (2012). Pekinium Csiki, 1901, is another taxon described based on a species with angulate humeral margin, however, the type specimen seems lost and the state of the taxon remain questionable ( Toledano and Sciaky 1998) and will thus not further considered here.

Based on a comprehensive phylogenetic analysis of Bembidion and related ground beetles, Maddison (2012) has shown that the character state 'angulate humeral margin’ evolved probably five times independently within Bembidion sensu lato. In Phyla , the abbreviated basal bead extends to the tip of the 5th elytral stria and is therefore distinctly shorter than in B. bukejsi sp. n.. The same holds true for Peryphophila , which was not included in the analysis of Maddison (2012). Species of both taxa can be additionally distinguished from B. bukejsi sp. n. in the elytral discal setae, which are situated in the third stria, and by its very different endophallic morphology. In Phyla , an additional endophallic sclerite is developed basad of the central sclerite complex (probably homolog to the “N-sclerite” of the Notaphus series sensu Maddison 2012, Fig. 19), the central sclerite is irregularly shaped, and the dorsal field is lacking. In Peryphophila , the central sclerite is markedly small and the aedeagal median lobe is much more slender compared to B. bukejsi sp. n. (Fig. 22). In the Plataphus complex sensu Maddison (2012), including Plataphodes , the central sclerite is only hardly sclerotized or sometimes lacking and thus, the endophallic structures are very differently developed (Fig. 20) compared to the situation in B. bukejsi sp. n.. Moreover, also in species of the Plataphus complex, the elytral discal setae are situated in the third stria. In the High Asian taxon Hoquedela , the elytral discal setae are separated from the third stria similarly to the organization in B. bukejsi sp. n.. However, Hoquedela differs from B. bukejsi sp. n. by the shape of the aedeagal median lobe (Fig. 18), which is markedly robust with a swollen middle portion and very stout apex, by an extensively sclerotized folding structure of endophallus particularly near median lobe ostium, and by modified mesotibia which are suggestively sinuate due to slightly convex interior surface in middle of tibia and slightly inwardly bent tibial apex.

The external shape of the aedeagal median lobe and sclerotization patterns of the endophallic structures present in B. bukejsi sp. n. resemble those of species of Andrewesa (Fig. 23), the Hydrium complex and Odontium series (Figs 24, 25, Maddison 1993: Figs 112-128), as well as in Liocosmius Casey, 1918 ( Maddison and Cooper 2014: Fig. 12), Melomalus Casey, 1918 (Fig. 21), Trechonepha Casey, 1918, Trichoplataphus Netolitzky, 1914 ( Toledano and Schmidt 2010: Figs 33-42), and several species of the Ocydromus series sensu Maddison (2012). The common male genital characters of most species of these lineages are: (i) median lobe rather slender, slightly bent throughout, with (ii) terminal lamella short tongue-like, not bent backwards; (iii) endophallic central sclerite and brush sclerite complexes both moderately large and (in inverted position) situated side by side in middle portion of median lobe, overlaying each other in lateral view; (iv) central sclerite with left lobe lenticular and (v) with right lobe large and situated more basad; (vi) brush sclerite apico-ventrally markedly prolonged; (vii) dorsal plate largely developed and (viii) without additional sclerites near basal opening of median lobe. The multi-gene analyses presented by Maddison (2012) support a monophyly of all these groups together with the Plataphus complex (in the following called the " Odontium - Plataphus - Ocydromus (OPO) clade). Within this highly diverse clade, a natural group ' Ocydromus series plus Plataphus complex plus Trichoplataphus ' is recovered, however, the relationships of the other main lineages remain unresolved by the molecular data. In three species groups of the OPO clade, the character states 'angulate humeral margin’ and 'elytral discal setae are separated from the third stria’ resemble the organization observed in the fossil B. bukejsi sp. n., Andrewesa , the Hydrium and the Odontium complexes.

Molecular data suggest that Andrewesa is the sister group of the Hydrium complex, while the Odontium complex is the sister group of the Hydriomicrus complex; both the latter taxa together form the Odontium series ( Maddison 2012). The Hydriomicrus complex includes species with rounded humeral margin and elytral discal setae situated in the third stria. Close relationships of the Hydrium complex plus Andrewesa with the Odontium series, are not supported by the molecular data. Thus, it remains unclear whether the character states 'angulate humeral margin’ and 'elytral discal setae separated from third stria’ were evolved in the OPO clade only once or even twice; in latter case, an independent evolution of these features in the Hydrium complex plus Andrewesa , and in the Odontium complex of the Odontium series has to be assumed.

We could not find additional derived characters justifying a further assignment of the fossil species B. bukejsi sp. n. with one of these three lineages of the OPO clade. Similarities with representatives of extant species groups are considered to be symplesiomorphies, e.g., the shape of the pronotum and the development of the elytral striation. In B. bukejsi sp. n., the pronotal basolateral foveae are large and rounded as in Andrewesa . In many species of the Hydrium complex and in all species of the Odontium series, the basolateral foveae are linear impressed, and the area between foveae and laterobasal carina is markedly wide and convex. The latter represent apomorphic states. The elytral striae are deeply impressed throughout in B. bukejsi sp. n., and likewise in Hydriomicrus Casey, 1918, Hirmoplataphus Lindroth, 1963, and Pseudoperyphus of the Odontium series. In all the remaining lineages of the Odontium series as well as in Andrewesa and the Hydrium complex, the external elytral striae are more or less distinctly more shallowly impressed before the apex. Latter is considered an apomorphic character state.

Due to the impunctate elytral stria B. bukejsi sp. n. differs strikingly from all species of Andrewesa , the Hydrium complex and the Odontium series. However, impunc tate or indistinctly punctate elytral striae are also present in other lineages of the clade, e.g., Melomalus , the Ocydromus series, and the Plataphus complex. This character shows also continuous variation in other Bembidiina clades and is therefore not informative with regard to its phylogenetic implications.

Due to the peculiar combination of the character states observed in B. bukejsi sp. n., we conclude that this fossil species represents an extinct lineage of the OPO clade, probably related to either Andrewesa plus the Hydrium complex or the Odontium series, or to both of these sub-clades. Given the present state of knowledge, the assignment to one of the known subgeneric taxa is impossible and therefore we decided to describe a new subgenus for this fossil representative of Bembidion (see below).

Remarks on biogeography and ecology.

The above mentioned OPO clade represents one of the most species rich clades of Bembidion sensu lato ( Maddison 2012) with hundreds of recent species occurring mainly in the Holarctic region and, in much smaller number, also in the Oriental region and Africa. Species of the OPO clade are adapted to very different climates (tropical: e.g., Microserrullula , to arctic: e.g., some species of Plataphus ), and prefer very different habitats although most of them are ripicolous or paludicolous. The occurrence of B. bukejsi sp. n. in the Baltic amber forest as the first fossil representative of the OPO clade is therefore in accord with the expected distribution of that clade during the Early Cenozoic.