Acrostilicus Hubbard, 1896

Brunke, Adam J. & Schnepp, Kyle E., 2021, Taxonomic Changes in Nearctic Paederinae, New Records and a Redescription of the Enigmatic Genus Acrostilicus Hubbard (Coleoptera: Staphylinidae), The Coleopterists Bulletin 75 (4), pp. 883-894 : 885-888

Acrostilicus Hubbard, 1896

( Figs. 1A–F , 2A–F )

Acrostilicus Hubbard 1896: 299 . Blackwelder 1939 (in key); Newton et al. 2000 (in key, based on Blackwelder 1939).

Diagnosis. Acrostilicus is recognized among the Nearctic Stilicina based on a combination of: labrum with single, small and blunt median tooth (sometimes slightly recessed from margin), without paired apical teeth ( Fig. 1B ); head with dense umbilicate punctures lacking microsculpture between them ( Fig. 1F ).

Redescription. Dorsal surface of head covered with coarse and dense umbilicate punctures, most narrowly separated, those on frons slightly more distantly separated, disc without microsculpture between punctures ( Fig. 1F ); neck narrow, approximately 1/6 of maximum head width (including eyes); head slightly produced and convergent anteriad of eyes; labrum consisting of two lobes, with single small and blunt median tooth, lacking medial paired teeth ( Fig. 1B ), ventrally with long marginal row of stout setae to each side of median groove ( Fig. 1C ); mandibles without prostheca ( Fig. 2A ); galea setose, with about 6 setae along outer margin, with numerous additional setae elsewhere ( Fig. 1E ); apical maxillary palpomere aciculate but not minute, penultimate segment elongate and dilated to subapex, much longer than second palpomere ( Figs. 1B, D ); apical labial palpomere aciculate ( Fig. 1E ); gular sutures fused from about anterior third to base of head ( Fig. 1D ). Pronotum with pair of coarse, stiff bristles near anterior and posterior angles. Prosternum with median longitudinal carina nearly complete, from prosternal process to nearly reach- ing anterior margin ( Fig. 2B ); proepimeral fold absent, lateral marginal bead present as disconnect- ed lateral fragment, not connected to tergosternal suture. Procoxal cavities open behind; legs long and slender; inner face of protibia with short ctenidia in basal concavity, without ctenidium between basal concavity and apex ( Fig. 2C ); metatarsi long, with article I about twice as long as V. Abdominal segments III–VII with paratergites. Male with apical margin of abdominal sternite VIII bearing distinct wide, moderately deep triangular emargination, with fragment of transverse basal line ( Fig. 2F ); sternite VII unmodified; sternite IX slightly, broadly emarginate ( Fig. 2F ). Aedeagus with dorsal process of ventral process absent, ventral process shorter than dorsal and lateral plates of internal sac ( Figs. 2D, E ); with lateral plates of internal sac attached to sides of median lobe, entirely lacking sensilla ( Fig. 2E ); aedeagus with dorsal plate large and relatively weakly sclerotized compared to much smaller and more strongly sclerotized lateral plates ( Fig. 2E ); median lobe with internal pair of lateral struts ( Fig. 2D ).

Comments. Hubbard (1896) apparently intended to describe Acrostilicus in much greater detail in a later paper but only managed to compare his new genus to Stilicopsis Sachse (subtribe Stilicopsina ), which he said was identical in habitus and differed only in the labral dentition. However, the two genera differ in a number of aspects, including habitus. Acrostilicus does not have the head strongly produced anteriad and subparallel or divergent immediately in front of the eyes, a putative synapomorphy here ob- served ( Fig. 3A ) in the members of the clade consisting of Stilicopsina , Echiasterina, and Astenina , which was recently recovered by Żyła et al. (2021). The genus also lacks the characteristically expanded prosternum of Stilicopsina and a number of other features of Stilicopsis . Acrostilicus was keyed near members of the subtribe Stilicina in Blackwelder (1939) and was included there tentatively by Newton et al. (2000) pending re-evaluation of its status. Acrostilicus is here definitively placed in Lathrobiini : Stilicina based on the following character states: apical maxillary palpomere slender, aciculate but not minute ( Figs. 1B, D ); head slightly produced in front of eyes but this section weakly convergent anteriad ( Fig. 1F ); head without long sensory seta inserted into deep impression near inner eye margin (contra Scopaeina ); neck extremely narrow, less than 1/5 of the head width ( Fig. 1D ); gular sutures fused ( Fig. 1D ); tarsomeres not lobed; metatibia with well-developed apical ctenidium only on outer face, inner face without ctenidium (contra Lathrobiina ).

Currently, there are five valid Nearctic genera of Stilicina ( Newton 2019): Acrostilicus , Eustilicus Sharp , Megastilicus Casey , Pachystilicus Casey , and Rugilus Leach.According to the phylogenetic results of Frania (1986), Eustilicus is a separate lineage from Rugilus and may be closely related to some members of Medonina ( Deroderus Sharp , Stilocharis Sharp ). However, the monophyly of Medonina is doubtful and preliminary molecular evidence sug- gests that Stilicina , including Eustilicus and Rugilus , do form a clade, albeit with the medonine genus Thinocharis Kraatz ( Żyła et al. 2021) ; Deroderus and Stilocharis were not included in this analysis. Pachystilicus is not clearly distinguished from Rugilus , especially when the world diversity is considered ( Frania 1986; confirmed here). The emarginate head of Pachystilicus , often used to distinguish it from Rugilus (e.g., Newton et al. 2000), is also found in some Oriental Rugilus . Megastilicus is morphologically distinct from all other Stilicina , based on the unique head with rapidly converging temples and characteristic vestiture of scattered stiff and dark setae. However, it is probably closely related to or derived from within Rugilus based on shared derived characters ( Frania 1986). Therefore, Nearctic Stilicina appear to form two distinct lineages: Eustilicus (and potential relatives in Medonina ) and the remaining Rugilus -like genera.

Acrostilicus was the only Nearctic genus of Stilicina not examined by Frania (1986) and it has yet to be compared to its relatives. It is unique within Nearctic Stilicina for its labrum, which lacks paired teeth on the apical margin but does bear a single median tooth, sometimes situated slightly before the margin ( Fig. 1B ). This character state may be plesiomorphic within Stilicina as it is found in both Rugilus lineage genera ( Stiliderus Motschulsky ) and Eustilicus lineage genera ( Eustilicus , Deroderus ), but not in Rugilus ( Frania 1986) . Otherwise, Acrostilicus shares a number of morphological character states with Rugilus (not found in Eustilicus ) based on table 2 in Frania (1986): presence of umbilicate punctures, most separated by less than 0.25 their width (except center of disc and frons) ( Fig. 1F ); underside of labrum with long row of setae along anterior margin ( Fig. 1C ); mandible with prostheca absent ( Fig. 2A ); galea with more than 4 setae ( Fig. 1E ); inner face of protibia with ctenidia limited to basal concavity, not extending to apex ( Fig. 2C ); aedeagus with lateral plates of internal sac attached to sides of median lobe, without sensilla ( Fig. 2E ); aedeagus with dorsal plate large and somewhat sclerotized ( Figs. 2D, E ); median lobe with internal pair of lateral struts ( Fig. 2D ). Within Rugilus , Acrostilicus shares a few traits with subgenus Eurystilicus Fagel as tentatively diagnosed by Assing (2012) such as the strong pair of setae on the anterior and posterior angles of the pronotum, and the relatively small ventral process of the aedeagus. However, Acrostilicus differs from Rugilus (Eurystilicus) in several aspects including the coarser punctation, unmodified sternite VII, and the pronotum with an impunctate median line. Also, the general form of the aedeagus in Acrostilicus differs from those of most Rugilus species, including those of both Rugilus (s. str.) and Eurystilicus (e.g., Assing 2012), by the dorsal and lateral plates of the internal sac distinctly larger than and extending beyond the ventral process.

We maintain Acrostilicus as a valid genus, pending phylogenetic analyses of Stilicina , which are already underway and include the three endemic Nearctic genera (D. Żyła, personal communication). The larva of Acrostilicus remains undescribed.

Figures

Fig. 1. Acrostilicus hospes. A) Dorsal habitus, B) Labrum in dorsal view, showing median tooth (arrow), C) Labrum in ventral view, showing marginal row of setae (arrow), D) Head in ventral view, E) Head in ventral view, anterior close-up, F) Anterior disc of head, dorsal view, showing umbilicate punctures. G = galea; LP = labial palpus. Scale bars = 1 mm (A), 0.2 mm (B–F).

Fig. 2. Acrostilicus hospes. A) Left mandible, ventral view, B) Prosternum, C) Protibia, showing ctenidia, D) Aedeagus, ventral view, E) Aedeagus, lateral view, F) Male sternite VIII (upper) and male sternite IX (lower), setae and male tergite X omitted for clarity. VP = ventral process; LP = lateral plates of the internal sac; DP = dorsal plate of the internal sac; LS = lateral struts. Scale bars = 0.2 mm.

Fig. 3. Paederinae. A) Stamnoderus monstrosus, dorsal head, B) Pseudolathra oregonensis, protibia, ventral protrusion absent, C) Lobrathium grande, protibia, ventral protrusion present, D) Homaeotarsus sellatus, dorsal habitus, E) H. cribratus, dorsal habitus, F) H. serpentinus, dorsal habitus. Scale bars = 0.2 mm (A–C), 1 mm (D–F).