Euryachen, Pires-Silva, 2023

Pires-Silva, Carlos M., 2023, Euryachen seuzei a new genus and species of Placusini (Coleoptera: Staphylinidae Aleocharinae) from Brazil, Zootaxa 5230 (4), pp. 467-477 : 468-469

Genus Euryachen gen. nov.

Type species Euryachen seuzei gen. et sp. nov., present designation.

Diagnosis: Head spherical, widest behind eyes; with a broad and distinctive neck occupying the entire basal head width. Posterior angles of head indistinct. Antennomeres VII–X distinctively transverse. Mandibles with dorsal molar area with seven transverse rows of large denticles. Maxillae with galea slightly longer than lacinia; lacinia with about nine large teeth apically. Labium short, 2-articulated. Ligula short and broadly rounded. Pronotum transverse; anterior margin straight with fore angles short and rounded; posterior margin broadly and weakly arcuate, hind angles straight. Tibial spur present, formula 2-1-1. Last tarsomeres of all legs longer than the previous together. Tarsal formula 4-4-5. Abdomen parallel-sided; segments III–VII with two pairs of paratergites each; tergite V of males bearing 7–9 spine-like thicker bristles near middle; tergite VII slightly broader at posterior region; on males with thicker and darker bristles on lateral margins.

Comparative notes: The genus Euryachen gen. nov., is most similar to Euvira Sharp in overall appearance, but is distinguishable in Placusini from any other genus by a combination of unique characters: distinct and broad neck, occupying the entire basal head width (unique in the tribe), distinct and short anterior angles of pronotum, and antennomeres VII–X transverse. Moreover, members of Placusini may also present secondary sexual characteristics. For instance, males of Placusa Erichson have modified tergite VIII, forming band of varied sizes spines on posterior margin, while in males of Euvira the tergite VIII is larger and prolonged posteriorly. Males of Euryachen have thicker spine-like bristles on tergite V, and also bear thick bristles on lateral margins of tergite VII. As far as is known, this kind of sexual dimorphism is unique in Placusini .

The pronotum in the tribe is in mostly transverse. In Euryachen , the pronotum is transverse, similar to that of Euvira , but is distinguishable by the short anterior angles, contrasting with the broadly rounded angles, and in some cases indistinct, of Euvira species. Further, members of Euvira have straight and distinctive posterior angles on the head, while it is indistinct in Euryachen due to the presence of the broad neck.

The tergite VIII in Euryachen is a transverse to subquadrate plate in both males and females, contrasting with the modified tergite VIII from males of Placusa (above mentioned); in species of Placukorna it is like a conspicuously curved horn. In Euvira , the tergite VIII is broader in males rather than females.

Etymology. The generic name is a junction of two Greek words, ευρύς (evrýs, wide) and the nominative singular noun αὐχήν (aukhḗn, neck). Thus, Euryachen means “wide neck” in allusion to the large neck distinctively in the tribe. Gender masculine.

Updated Key to the genera of Placusini

(modified from Santiago-Jiménez & Santiago-Navarro 2016)

1 Head with distinct neck................................................................................ 2

- Head with an indistinct neck............................................................................ 4

2 Neck broad, almost same width of head; head spherical with indistinct posterior angles; anterior angles of head shortly rounded; males with thicker spine-like bristles on the central region of tergite V.......................... Euryachen gen. nov.

- Neck narrower than head. Head quadrate with strong posterior angles; anterior angles of pronotum broadly rounded; tergite V of males without spine-like bristles....................................................................... 3

3 Antennomeres 4–10 distinctively transverse; spermatheca rounded, stem somewhat triangular ( Pace 2008, fig 92).................................................................................................. Kirtusa Pace

- Antennomeres 5–10 distinctively transverse; spermatheca not as above................................ Euvira Sharp

4 Head rounded or transversally sub-orbicular; tergite VIII transverse, not modified... Speiraphallusa Pace ( Pace 2013; fig 1)

- Head transverse; tergite VIII not transverse, modified, bearing spines or as a curved horn............................ 5

5 Male tergite VIII with a variable number of small to large spines ( Klimaszewski et al. 2001; figs 33–49)....................................................................................................... Placusa Erichson

- Male and female tergite VIII without spines, forming a conspicuously curved horn ( Santiago-Jiménez & Santiago-Navarro 2016; figs 2, 11)............................................................... Placukorna Santiago-Jiménez