Paussus lanxangensis, Nagel, 2009

Nagel, Peter, 2024, The Paussus (Scaphipaussus) lanxangensis species group of Indomalayan ant nest beetles (Coleoptera, Carabidae, Paussinae, Paussini) with description of a new species, Zootaxa 5501 (3), pp. 447-457 : 454-455

The Paussus lanxangensis species group

Important morphological structures just as the ligula or the lacinia could not be studied because only a singleton ( P. agatae sp.n.) or very few specimens ( P. lanxangensis male holotype and one single female) were available for the present treatise. Destructive dissection was avoided. Other characters of the mouthparts (maxillary palps, labial palps) could not be used for all species in this study because the original descriptions of P. wakaharai ( Maruyama & Nakase 2023) and P. haucki ( Bednařik & Bocak 2023) did not provide this information.

The text and illustration of the original description of P. drumonti describes the maxillary palpomere II as “large, compressed” ( Maruyama 2014) but does not inform if its “mesal margin ... is strongly rounded or more or less straight” as in Scaphipaussus ( Robertson & Moore 2016, pp. 29, 32, 33). The importance of several characters became only known as a result of the recent revision of Paussus L. ( Robertson & Moore 2016). Publications that were issued earlier or contemporarily with the revision could not yet consider these criteria and their authors described the standard traits of the taxon instead.

I use the term “species group” for an informal, phenotypically denoted taxon. The term clade is reserved for a prospective proven monophylum (lineage).

Description of the group (diagnostic, phenotypical characters):

Small to very small species, BL 3.8–5.0 mm.

Head wider than long. Head approximately of same width as the pronotum, or little or considerably narrower than pronotum. Eyes small, temples large. Head with stigmatoid vestige of a short elongate opening at medial vertex.

Antennal club strongly inflated to oblong lens-shaped, posteriorly with roundish to elliptic cavity or with narrow longitudinal furrow. Basal margin uninterrupted, without central incision or semi-transparent impression.

Terminal labial palpomeres not distinctly spade-shaped.

Pronotum with deep transverse furrow with small dorsolateral trichomes. Anterior part of pronotum broader and much higher than posterior part. Posterior part of pronotum deeply excavated in the middle. Two prominent tubercles behind the transverse excavation at both sides of a longitudinal furrow.

Pygidium excavated, with posterior dorsal or latero-dorsal margins explanate, sometimes blade-like and upturned, and bent inwardly in some species. Pygidial disc even, smooth, and glabrous. Marginal pygidial trichome absent.

Tibiae slightly compressed, not edged, margins rounded. Tibiae widened towards apex, exterior apical angle not or little pronounced, apically rounded, rarely rectangular ( P. drumonti excepted, see key above).

Elytral lateral trichome (serially arranged setae) absent.

The group lacks falciform ridges or crests at the dorsal head. It also lacks elytral mirror markings (present at most species of the Paussus (Scaphipaussus) hystrix group ( Maruyama 2016; Bednařik & Bocak 2023) and the rounded elytral patches as well as the subapical trichome of antennomere 1 (scape) of the Paussus (Paussus) jousselinii group ( Nagel 2016; Robertson & Moore 2016).

Distribution: Southeast Asian mainland, and most probably an Indomalayan endemic.

The group is characterized by the distinctive characters of the Paussus series II (see above) and the following possible common traits (candidates for common derived character states):

- Head with stigmatoid vestige of short elongate opening at medial vertex.

- Pygidial disc smooth, glabrous and shining, positioned at the bottom of a hollow or large excavation.

- Maxillary palpomere II not subcylindrical but large, explanate, with mesal margin sinuate, faintly emarginate or almost straight, but not convex.

The group is classified as part of genus Paussus L., and as a member of the clade “ Paussus series II” in the latest revision ( Robertson & Moore 2016). Most probably, it forms part of the subgenus Scaphipaussus Fowler , see checklist below.

The recent revision ( Robertson & Moore 2016) listed two taxa near or in Sg. Scaphipaussus for which an exact placement could not be demonstrated with sufficient certainty: P. ghanensis Luna de Carvalho and P. nr. leechi Luna de Carvalho were both classified as “incertae sedis” of the “ Paussus II series”. I checked specimens of P. cochlearius Westwood ( Zimbabwe, CN) (part of the same species group as P. leechi ) and P. ghanensis ( Benin, CN) for characters that possibly indicate similarity beyond phenotypy with the P. lanxangensis species group, yet did not find any.

Equally small Paussus exist in other species groups and subgenera, such as the P. jousselinii group (Sg. Paussus L.) ( P. wittei Reichensperger, 1950 ), the P. cucullatus group (Sg. Hylotorus Dalman, 1823 ) ( P. minutulus Nagel & Rasool in Abdel-Dayem et al. 2019), or the P. hystrix group (Sg. Scaphipaussus ) ( P. kecil Maruyama, 2016 ). Therefore, the small size is not a good diagnostic character of the group.

The group could not yet convincingly be established as a clade, because not all or most species could sufficiently be studied with regard to some of the more relevant morphological characters, such as the ligula or the lacinia. The small number of species and specimens also prevented the application of a proper cladistic analysis.

It could, however, be demonstrated that the P. lanxangensis species group is a promising candidate for a new monophylum nested within the subgenus Scaphipaussus sensu Robertson and Moore (2016) . Future research will have to consider this aspect.