(Geocosmius) Maddison 2023, new subgenus
Creators: name: Maddison, David R.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia

description: Geocosmius Maddison, new subgenus
Type species Bembidion nahuala Erwin, 1982
Derivation of name. The first portion of the name, “Geo”, consists of the first three letters of George E. Ball’s given name, and is included to honor George’s long-term commitment to discovering and documenting the biodiversity of México, and his role in collecting most known specimens of this subgenus. In addition, “Geo” is a Greek prefix referring to the Earth, and evokes the interesting relative geographic distributions of Geocosmius and its sister group, Liocosmius. The last portion of the name, “cosmius”, was chosen to provide a linguistic connection to its sister group.
Habitus. Beetles of the subgenus Geocosmius have the general appearance of typical members of non-riparian cloud forest Bembidion; that is, they are small, dark, unspotted, and convex; most specimens lack hind wings and have rounded shoulders. A similar form has evolved multiple times within Bembidion in similar habitats, as in the subgenus Ecuadion Moret & Toledano from Central and South America (Moret and Toledano, 2002), Nesocidium Sharp from Hawaii (Liebherr, 2008), subgenus Hypsipezum Alluaud from Africa, and some members of the Ocydromus complex in Central America (e.g., B. chiriqui Erwin) (Maddison et al., 2019b).
Morphological Diagnosis. Small to medium (2.6–3.5 mm), shiny Bembidion, without colored markings on the elytra. Frontal furrows single, broad, shallow, well-marked (Bembidion franiae), or broad, shallow, and disrupted by surface rugosity (other species), not convergent or extended onto clypeus. Eyes of normal size for Bembidion (B. franiae) or slightly reduced (other species). Mentum with anterior lateral region complete, triangular; mentum tooth triangular.
Posterior angle of pronotum with a posterolateral carina, although it is interrupted posteriorly in B. franiae and some specimens of other species. Posterior margin of pronotum not strongly sinuate laterally; basal transverse furrow weak, disrupted by shallow punctures, and in many specimens interrupted by the medial furrow extending posteriorly to the hind margin of the pronotum; hind angles right or slightly acute.
Elytron with lateral bead ending at humerus, not prolonged onto base, although in B. franiae a short carina extends from the end of the bead toward the center of the elytral disc at an angle, giving the impression of an angulate shoulder bead. Striae vary in depth, from anteriorly deep in B. franiae to much shallower in most other species, with lateral striae increasingly evanescent; all striae absent or nearly so in the posterior quarter of the elytra, except for the first stria; two discal setae in third stria. Elytra slightly iridescent in B. franiae, because of the transverse microsculpture, and slightly dull in B. elescarabajo, because of the nearly isodiametric sculpticells; elytral microsculpture absent from specimens of the nahuala group. Mesoventral process without subapical setae. Metaventral process unmargined.
Apex of last visible abdominal sternite with two setae in males, four setae in females. Apices of each paramere normally with three setae, but some individuals have two or four setae on the left paramere, and one has four setae on the right paramere.
Within the geographic range of Geocosmius live several other species of Bembidion with small, convex adults without spots: B. purulha Erwin (a member of the Ocydromus complex), as well as B. (Cyclolopha) cyclodes Bates and B. (Cyclolopha) championi Bates. Bembidion purulha can be distinguished by the elytral striae, which abruptly transition at about the halfway point from large, distinct punctures anteriorly to fully effaced striae posteriorly, as well as a fully margined metaventral process. The two Cyclolopha species can be distinguished by the much broader pronotum with very rounded hind angles, with the posterior lateral seta notably anterior to the posterior margin of the pronotum at the midline.
All known specimens of B. franiae and B. elescarabajo lack hind wings. Bembidion nahuala and B. “ Chiapas ” are wing dimorphic, with most specimens lacking hind wings, and with more rounded elytra and notably rounded shoulders (Fig. 1A). A minority of specimens of both species are fully winged, with more parallel-sided elytra and less rounded shoulders, giving these specimens a rather different appearance (Fig. 1B).
Habitat. Based upon the available label data, species in this group live in cloud forests (Guatemala, Chiapas, Oaxaca, Guerrero, and Puebla), oak-pine forests (Guatemala, México, Oaxaca), pine forests (Veracruz), pine-alder forests (Veracruz), wet oak forests (Puebla), and oak-alder forests (Guerrero).
Geographic distribution. This subgenus is known from southern México and Guatemala (Fig. 2).
Composition. The known species of subgenus Geocosmius are Bembidion franiae Erwin, Bembidion elescarabajo Maddison, sp. nov., and then a group of similar forms (the nahuala group) including Bembidion nahuala Erwin and B. “ Chiapas ”. I have examined 5 specimens of B. franiae, 13 of B. elescarabajo, and 311 specimens of the nahuala group (UASM, CUIC, CMNH, NMNH, UNAM, CTVR, and OSAC).
Members of the nahuala group are very similar to one another. With the exception of a form with deeper elytral stria from Volcán de Colima in Jalisco, and the differences associated with wing dimorphism and gender, this widespread complex shows no obvious patterns of external morphological variation. Bembidion nahuala Erwin and B. “ Chiapas ” appear to be different species based upon DNA sequences (Fig. 4) and male genitalia (Fig. 5). The two forms differ by 4 bases in 28S, and by 27 bases or 4.1% in COI; these nucleotide differences in COI imply two amino acid differences between the species.
description: Published as part of Maddison, David R., 2023, A New Subgenus of Bembidion Latreille from México and Guatemala, with Descriptions of Two New Species (Coleoptera: Carabidae), pp. 433-450 in Proceedings of the California Academy of Sciences 67 (19) on pages 441-443, DOI: 10.5281/zenodo.11067242