Drilini are soft-bodied predatory click beetles with incompletely metamorphosed females. Approximately 150 described species are distributed in the Afrotropical, Palaearctic and Oriental realms, with the highest diversity known from sub-Saharan Africa. In this study, we describe Namibdrilus albertalleni gen. et sp. nov. from Namibia which brings the total number of genera in Drilini to 16. The discovery of this unique taxon sheds new light on the diversity and evolution of the enigmatic paedomorphic beetle lineage and is interesting for several reasons. This new species is the only known representative of Drilini that has unidentate mandibles and lacks a hook on the dorsal part of the aedeagal median lobe, two of the few characters heretofore used for the unambiguous identification of members of this group. Furthermore, based on its morphology it belongs to a group of genera (Drilus clade) which heretofore contained only taxa from the Palaearctic Realm. We provide an updated diagnosis of the tribe Drilini, as well as an updated diagnosis and an identification key for the genera of the Drilus clade based on adult males. Further, we explain how to easily recognize adult Drilini from similar-looking soft-bodied elateroids like Elateridae: Omalisinae, Rhagophthalmidae, and Lampyridae: Ototretinae.

Afrotropical Realm, Agrypninae, click beetle, distribution, Elateroidea, identification key, male genitalia, new genus, new species, paedomorphosis, taxonomy

The tribe Drilini (Elateridae, Agrypninae) contains soft-bodied click beetles with flight capable males, incompletely metamorphosed larviform females, and larvae which feed on land snails (Crowson 1972; Baalbergen et al. 2014; Kundrata et al. 2015a; Kundrata and Bocak 2011, 2019). Although the center of diversity of this group lies in the Afrotropical Realm, Drilini are also well represented in the Palaearctic fauna, and several species are also known from the Oriental Realm (Kundrata and Bocak 2019). This group was earlier treated as a separate family, Drilidae, and contained various taxa which are currently placed in several other families within Elateriformia (e.g., Wittmer 1944; Crowson 1972; Geisthardt 2009; Kazantsev 2010). Kundrata and Bocak (2011) placed the group in Elateridae as tribe Drilini in Agrypninae based on a molecular phylogenetic approach. They also transferred genera Euanoma Reitter, 1889 and Pseudeuanoma Pic, 1901 to Omalisidae (currently a subfamily of Elateridae), and redefined Drilini to contain only Drilus Olivier, 1790, Malacogaster Bassi, 1834, Selasia Laporte, 1838, and tentatively also Paradrilus Kiesenwetter, 1866. The latter genus was soon after transferred to Omalisidae by Kundrata et al. (2015b) based on results of a molecular phylogenetic analysis. This concept of Drilini with only three genera lasted only until Kundrata and Bocak (2017) investigated the diversity of Drilini in western Africa and described five new genera from there. Kundrata and Bocak (2019), in the most comprehensive phylogeny of Drilini yet published, defined five main clades, and established six new genera, of which five were from tropical Africa and one from Pakistan. Kovalev et al. (2019) then described a new monotypic genus from Iran.

Here, we report the discovery of a morphologically unique and extremely interesting Drilini specimen from Namibia, which represents a new genus and species. Surprisingly, it is morphologically similar to geographically distant Palaearctic genera. The unique morphology of its mandibles and male genitalia compelled us to modify the diagnosis of the tribe Drilini.

The genitalia were dissected after a short treatment in hot 10% KOH. Images of habitus and main diagnostic characters were photographed using a digital camera Canon EOS M6 Mark II attached to a stereoscopic microscope Olympus SZX12. Stacks of photographs were combined with the software Helicon Focus Pro (version 7.6.4, Kharkiv, Ukraine), applying the ‘depth map’ or ‘weighted average’ rendering methods. We did not clean the surface of the holotype in order not to damage the unique specimen. The measurements were taken with a scale bar in an eyepiece. Body length was measured from the fore margin of the head to the apex of elytra (since abdomen is highly flexible in soft-bodied elateroids), body/elytra width at humeri, head width including eyes, minimum interocular distance in the frontal part of the cranium, maximum eye diameter in lateral view, pronotal length at midline, pronotal width at the widest part, scutellar shield length at midline, scutellar shield width at the widest part, aedeagus length medially from base to the apex of the median lobe, and aedeagus width at the widest part. We follow the morphological terminology and the classification of Drilini by Kundrata and Bocak (2019). Label data are cited verbatim. The holotype of the here-described new species is deposited in the National Museum, Prague, Czech Republic (NMPC). The Drilini specimens used for comparison of the here-described new species with its congeners are deposited in the following collections: The Natural History Museum, London, United Kingdom (BMNH), Koninklijk Museum voor Midden-Afrika, Tervuren, Belgium (RMCA), Museum National d’Histoire Naturelle, Paris, France (MNHN), Natural History Museum, Budapest, Hungary (HNHM), Museo Nacional de Ciencias Naturales, Madrid, Spain (MNCN), Naturkundemuseum Erfurt, Germany (NKME), Naturhistorisches Museum, Vienna, Austria (NHMW), Naturalis Biodiversity Center, Leiden, The Netherlands (RMNH), Naturhistorisches Museum, Basel, Switzerland (NHMB), Hessisches Landesmuseum, Darmstadt, Germany (HLMD), Oxford University Museum of Natural History, Oxford, United Kingdom (OUMNH), Museo Civico di Storia Naturale, Genova, Italy (MSNG), Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany (MFNB), Senckenberg Deutsches Entomologisches Institut, Müncheberg, Germany (SDEI), Lund Museum of Zoology, Lund University, Sweden (MZLU), Natural History Museum, Copenhagen, Denmark (NHMD), and the collections of Albert Allen, Idaho, USA (PCAA) and Robin Kundrata, Olomouc, Czech Republic (PCRK). The updated diagnosis of the tribe Drilini, as well as an updated diagnosis and an identification key for the genera of the Drilus clade, are partly based on Kundrata and Bocak (2019).

The here-described new genus of Drilini is extremely interesting from the morphological as well as from the evolutionary point of view. Its discovery has changed our view regarding the diagnosis of the tribe Drilini as well as our understanding of the morphology and distribution of one of the main clades of Drilini. Therefore, both the diagnostic characters used for the recognition of Drilini and the unique morphology of the new genus are worthy of a more detailed discussion.

Within the family Elateridae, Drilini are easily recognizable due to their various modifications connected with the soft-bodiedness and the loss of clicking mechanism. Compared to the typical well-sclerotized and clicking Elateridae, Drilini males have e.g., much softer body, reduced prosternum usually with a strongly reduced or missing prosternal process, reduced mesoventrite without a well-developed mesoventral cavity and often with a reduced mesoventral process, and the abdomen with seven or eight visible sternites (compared to usually five in hard-bodied elaterids; Kundrata and Bocak 2019). The females are larviform, lack elytra, and do not look like any other elaterid adult female (e.g., Hoffmannova and Kundrata 2022).

However, it is not always easy for some to distinguish adult Drilini from similar-looking soft-bodied elateroids, mainly Elateridae: Omalisinae, Rhagophthalmidae, and Lampyridae: Ototretinae, many of which were historically classified in the broadly defined Drilidae (Wittmer 1944). From all three above-mentioned groups, Drilini males were always easily distinguishable based on their bidentate and more robust mandibles, setae at the bases of pretarsal claws (the latter is the character typical for Agrypninae), and a distinct hook on the dorsal part of the aedeagal median lobe. All known Omalisinae, Rhagophthalmidae, and Ototretinae have simple, usually narrow and sickle-shaped mandibles; they lack setae on the pretarsal claws and lack a hook on the aedeagal median lobe (Janisova and Bocakova 2013; Bocek et al. 2018; Kundrata et al. 2022). Furthermore, Omalisinae have antennomeres 2 and 3 short and subequal in size (only antennomere 2 is short in Drilini), and the phallobase with posterolateral processes (without those processes in Drilini) (e.g., Kundrata et al. 2015b, Packova et al. 2024), and Rhagophthalmidae have always 12 antennomeres (compared to 11 in Drilini) and numerous species have deeply emarginate eyes (always simple in Drilini) (Kawashima et al. 2010; Kundrata et al. 2022; Packova and Kundrata 2023). Regarding the females, all these groups have them paedomorphic, retaining many larval characters in their adulthood. Females of many genera and species of Drilini, Omalisinae, Rhagophthalmidae, and Ototretinae are actually unknown. Nevertheless, based on available information, we can recognize females of Drilini by having only the head and legs completely metamorphosed and the rest of the body being larviform (e.g., Kundrata et al. 2015a; for females of Omalisinae see e.g., Geisthardt 1977 and Bocek et al. 2018, for females of Rhagophthalmidae see Kawashima et al. 2010 and Kundrata et al. 2022, and for females of Ototretinae see Kawashima 1999 and Yiu and Jeng 2018).

From three main characters which were heretofore used for the unambiguous identification of Drilini males (i.e., bidentate mandibles, setae at the bases of pretarsal claws, and a distinct hook on the dorsal part of the aedeagal median lobe), Namibdrilus gen. nov. has only the setae on the claws. Namibdrilus gen. nov. has unidentate mandibles (Fig. 2A–D) that are still more robust than the sickle-shaped mandibles of omalisines, rhagophthalmids and ototretines. However, all other Drilini have bidentate mandibles although some Drilus spp. and Malacogaster spp. have the inner tooth minute (Kundrata et al. 2015a, Hoffmannova and Kundrata 2022). All Drilini species for which male genitalia was known, representing all described genera (confirmed by several hundreds of dissections by the first author and his students), had an aedeagal median lobe with a distinct hook (see e.g., Geisthardt 2007; Kundrata and Bocak 2017, 2019; Kovalev et al. 2019). However, Namibdrilus gen. nov. is surprisingly the first representative of Drilini with the median lobe of the aedeagus simple, without any hook (Fig. 4E–G). Consequently, the diagnosis of Drilini needed to be modified accordingly.

Based on the results of molecular phylogeny supported by the morphological characters, Drilini were divided into five major informal groups, i.e., clades A, W, S, M and D (letters represent the first letters of the respective genus name which is typical for the given clade; Kundrata and Bocak 2019). Four of those clades (A, W, S, M) contain solely or predominantly Afrotropical taxa (with several species of the clade S extending to the Oriental Realm) while the clade D contained exclusively Palaearctic species. One would expect that Namibdrilus gen. nov. from southern Africa belongs to one of the African clades; however, surprisingly it falls morphologically into the clade containing Palaearctic species. All known African Drilini differ from Namibdrilus gen. nov. in having a short frontoclypeal region and an aedeagal median lobe with a dorsal hook. Furthermore, the representatives of two basal-most clades A (Austroselasia Kundrata & Bocak, 2019) and W (Habeshaselasia Kundrata & Bocak, 2019, Latoselasia Kundrata & Bocak, 2017, Mashaselasia Kundrata & Bocak, 2019, Wittmerselasia Kundrata & Bocak, 2017) differ from Namibdrilus gen. nov. in many important diagnostic characters including the pronotum with posterior margin medially rectangularly emarginate (arcuately emarginate in Namibdrilus gen. nov.), the prosternal process present, plate-like (absent in Namibdrilus gen. nov.), the mesoventral process well developed (absent in Namibdrilus gen. nov.), the elytral surface relatively smooth and with apparent elytral striae and interstriae or at least their remnants (elytral surface rather rough and without any striae in Namibdrilus gen. nov.), seven abdominal ventrites with the first four connate (eight free ventrites in Namibdrilus gen. nov.), and the intercoxal process present on the first abdominal ventrite (absent in Namibdrilus gen. nov.). Members of clade S (Illubaboria Kundrata & Bocak, 2019; Selasia) usually have an apparent prosternal process, well-developed mesoventral process, seven abdominal ventrites, and at least a slightly developed intercoxal process on the first abdominal ventrite. The representatives of clade M (Flabelloselasia Kundrata & Bocak, 2017, Kupeselasia Kundrata & Bocak, 2017, Lolosia Kundrata & Bocak, 2017, Microselasia Kundrata & Bocak, 2017) are usually tiny forest species from the western and central Africa which differ from Namibdrilus gen. nov. in having a smooth glabrous pronotal disc often with distinct sublateral carinae (pronotum with relatively rough surface and no sublateral carinae in Namibdrilus gen. nov.) and the prosternum with a prosternal process forming a narrow plate (absent in Namibdrilus gen. nov.).

On the other hand, Namibdrilus gen. nov. shares many diagnostic characters with the Palaearctic genera included in clade D (Drilorhinus, Drilus, Malacodrilus, Malacogaster), including the frontoclypeal region produced forwards between antennae, the pronotum not distinctly transverse and without sublateral carinae, the posterior margin of pronotum shallowly and arcuately emarginate, the prosternum without a well-developed prosternal process, the scutellar shield apically subtruncate, the mesoventrite v-shaped and without a well-developed mesoventral process, the elytral apices not conjointly rounded and instead being divergent at internal margins, the elytral surface rough and without distinct striae, and the abdomen with eight free visible sternites. Based on the above-listed characters, we place this genus within clade D (Drilus clade) as the only Afrotropical member in this otherwise Palaearctic group. Namibdrilus gen. nov. differs from all other genera in the group by having much larger eyes, with their frontal separation 1.15 times eye diameter (eyes in the Palaearctic genera are small to medium-sized, with their frontal separation 1.60–3.00 times eye diameter; Fig. 2A–D; e.g., Kundrata et al. 2015a, Hoffmannova and Kundrata 2022), the frontoclypeal region strongly produced forwards, carinate and anteriorly widely rounded (if the frontoclypeal region in the Palaearctic genera is strongly produced then it is anteriorly narrowed and clearly emarginate; Fig. 2A–D; Packova et al. 2021), unidentate mandibles (always bidentate in the Palaearctic genera), relatively narrower prosternum, and the aedeagal median lobe without any subapical hook (Fig. 4E–G). Based on its divergent morphology and distribution in tropical Africa, we hypothesize that Namibdrilus gen. nov. may be a sister group to all remaining genera in clade D. Of course, without a strong phylogenetic hypothesis we cannot exclude the possibility that Namibdrilus gen. nov. either represents a more developed paedomorphic phenotype in one of the previously identified African clades (less likely) or it forms a separate clade on its own. All these hypotheses should be tested using a molecular approach in future research.

One of the most striking characters of Namibdrilus gen. nov. is the frontoclypeal region which is strongly produced forwards and is visible even from the ventral view of the head (Fig. 2D). Although Drilini have variously shaped frontoclypeal region from e.g., a very short and wide in Selasia spp. to a narrow and high in Microselasia spp., major modifications are known in genera of the Drilus clade. Malacogaster ruficollis Dodero, 1925 from Libya is the only species of genus Malacogaster having the frontoclypeal region produced forwards and covering labrum; however, it is very wide, sloping downwards and broadly rounded apically (Hoffmannova and Kundrata 2022). On the other hand, the monotypic Drilorhinus from Iran (Fig. 5A) and several Mediterranean Drilus spp. have the frontoclypeal region strongly produced forwards and apically narrowed, with apex medially distinctly emarginate (Kovalev et al. 2019; Packova et al. 2021). None of these conditions is similar to the strongly produced frontoclypeal region of Namibdrilus gen. nov., which is oriented straight forwards and apically somewhat subtruncate and widely rounded (Fig. 2A–D).

In summary, the discovery of Namibdrilus gen. nov. is extremely important for our understanding of the diversity and evolution of Drilini and represents one of the most interesting findings regarding Drilini in recent decades. Recent increased research on Drilini has already led to a better understanding of their systematic placement (Kundrata and Bocak 2011), their phylogenetic relationships and diversity, including descriptions of several new genera (Kundrata et al. 2015a, Kundrata and Bocak 2017, 2019), or the discovery of the first Drilini from South East Asia (Kundrata and Sormova 2018). Based on morphology, Namibdrilus gen. nov. represents a connection between tropical Africa, the center of diversity of Drilini, with a solely Palaearctic group of genera with the highest degree of paedomorphosis-related morphological modifications (although this remains to be further tested using a molecular approach). Furthermore, its unique aedeagal morphology stresses the importance of male genitalia for the systematics of Drilini.

We thank Albert Allen (Idaho, USA), who allowed us to study the unique specimen of the here-described species, and who kindly donated it to NMPC. We also thank all curators from the institutions listed in Material and methods and other colleagues for allowing us to study Drilini specimens in their care, and Simone Policena Rosa (Brazil) and two anonymous reviewers for their helpful comments and suggestions. Our special thanks go to Jiří Halada, Pavel Kučera, František Černý and Walter Grosser (all Czech Republic) for their effort to search for more specimens of the here-described species.