A new ant-mimetic species of Metopiellus (Staphylinidae: Pselaphinae) from the Northern Colombian Amazon

The genus Metopiellus (Staphylinidae: Pselaphinae) is confirmed in Colombia. A new ant-mimetic species of Metopiellus, Metopiellus guanano sp. nov., is described from the northern Amazon. Major diagnostic characters and ecological data are given. A new symbiotic association (rove beetle and fungus-growing ants) is hypothesized for the genus Apterostigma (Formicidae: Myrmicinae) and recorded for the first time for the country.

HL head length (from anterior margin of antennal tubercle of the head to posterior margin of the head disc). HW head width (maximum). NW neck width (minimum). PL pronotum length (maximum). PW pronotum width (maximum, without the spines).

Results
Thorax (Figs. 1B,D-F): pronotum trapezoidal in dorsal view (PL: 0.35; PW: 0.44) widest anteriorly, stair shaped in profile. Two rounded protuberances on the medial region of anterior half, acuminated with two spines. Two smaller spines produced laterally on each side of the two rounded protuberances of the medial region of the anterior half of the pronotum. Posterior half well below height of anterior half, demarcated by a deep sulcus which connects with two deep lateral antebasal foveae. Pronotum coarse, covered in thick curved setae. Pronotum anterior margin slightly convex and basal margin straight. Prosternum with lateral procoxal fovea. Mesoventrite with prepectal fovea and lateral mesosternal fovea. Metaventrite with lateral mesocoxal foveae, lateral metasternal fovea and median metasternal fovea. Region of metaventrite in articulation with metacoxae form triangular protuberance, inwardly convex. "Waist" between pronotum and elytra strongly produced, with dark coarsely reticulated integument.
Legs (Figs. 1A,D): legs long and robust. Femora thickened at apical half. Tibiae slightly curved and slightly shorter than femora, all tibiae thickened at apex. Protibiae in its internal face carinate and without microsetae on the posterior and mesial region, carinae lined with thick curved setae. Tarsi 3-segmented, first tarsomeres very short, last 2 tarsomeres longer, tarsomere 2 longer than segment 3; all tarsi with single claw and thick accessory seta. Pro Coxae conical and prominent, mesocoxae globularconical, less prominent than procoxae, metacoxae transverse, region that articulates with metatrochanter conical. Procoxae, mesocoxae and metacoxae contiguous.
Female with characters of head, pronotum, and elytra as are described for males. Abdominal sternum VIII with posterior margin rounded and without small prolongation (Fig. 3A,B).
Habitat and ecological notes. The specimens were collected through Winkler sampling in primary forest in the northern Colombian Amazon. The sampled localities correspond to areas with a relative humidity of 84% and an average temperature of 28° C, in both locations the vegetation was characteristic of a humid tropical forest. The processes that determine the diversity and floristic composition of the forests are not well known (Cano & Stevenson 2008). The sampling in Cucúra was carried out at no more than 20 m from a body of water, the area had been slightly intervened by eliminating plants from the understory and the terrain was humid due to recent rains, it is an area of primary forest, the predominant vegetation is arboreal with little presence of small plants. The sampling in Villa Fátima was carried out in a submontane primary forest, the collection area was mostly pristine, the predominant vegetation is arboreal, little understory vegetation was observed, this may be due to the superficiality of the first granite layer of the hill (Tepui) (Gröger 2000).
Apterostigma pilosum complex samples were abundant in the same sampling method in the holotype and paratype localities. It is important to highlight this new species' morphological similarity and possible obligate synoekete behavior with regards to the Apterostigma pilosum complex collected samples.
Etymology. This ant-mimetic rove beetle new species is named after the indigenous communities located at the type localities. The Guanano people inhabit the Vaupés River region of Colombia, from the Santa Cruz area below Mitú to Ibacaba in the lower Vaupés on the border of Brazil (Stenzel 2007).
Distribution. Metopiellus guanano sp. nov. is known from the holotype and paratype localities, Counties of Mitu and Villa Fatima, Department of Vaupes, Colombia (Fig. 4).

Comments. The new species belongs to
Metopiellus, based on the third antennal segment being much shorter than the second (Fig. 1C); the posterior coxae contiguous or nearly so; and the mesial face of the protibia being carinate and open at its base and apex (Fig. 1D). However, the new species appears to be highly automorphic, presenting sides of the spinose protrusion on the pronotum as well as a medial protrusion on the dorsum of the pronotum.   Newton et al. (2005) recorded Metopiellus for the country, then Sissa and Navarrete (2016) documented the genus in a study of the composition and structure of rove beetles in the department of Boyacá. Nevertheless, none of these works named or described the species reported, so they were considered dubious records. Here we provide the first confirmed record of the Metopiasini tribe, with the first official record of the genus Metopiellus Raffray (1908) for the country.
There are few reports of associations between beetles and Fungus-growing ants. Navarrete-Heredia (2001) reports ten Pselaphinae species associated with Atta and Acromyrmex, of which Attapsenius (Attapseniini) is an obligate guest of Atta sexdens fungus galleries (Park 1942). On the ant-camptosome associations' synthesis, Agrain et al. 2015, report the genus Atta, specifically the species Atta texana (Buckley 1860) and Atta mexicana (Smith 1858), as ant-host of Cryptocephalinae beetles. These authors also noted that fungus-growing ants nests could provide food to their beetle guests. Shepard (2020) records Cercyon sp. (Coleoptera: Hydrophilidae: Cercyon Leach), associated with Atta colombica Guerin-Meneville 1844 in Crique Aimara in French Guiana. Finally, the present report constitutes the first record of mimetics from a Staphylinidae in the genus Apterostigma and the first record of this association (leaf-cutting ants and rove beetle symbiosis) in Colombia.
The examined specimens suggest Metopiellus guanano sp. nov. as a true myrmecomorph. According to McIver & Stonedahl (1993), myrmecomorphic insects display morphological modifications that enhance mimicry. Pselaphinae beetles possess very specialized myrmecomorphic body plans and are even called "Ant-loving" beetles. Metopiellus guanano sp. nov demonstrates many modifications that help to mimic the Apterostigma sp. to an impressive degree. Specifically, the thick pilosity covering its body, resembling that of the Apterostigma pilosum complex, the reddish-brown color, and the conspicuous mesosomal constriction, which is typical of Apterostigma ants. (Lattke 1997). Personal observations, and the overlapping presence of the two species in consecutive sampling trips, provide possible evidence for a myrmecophilous relationship, but further study is required to verify this association.
This new report provides further evidence for the myrmecophilous status of the genus Metopiellus. The only previous report of myrmecophily for the genus was that of M. aglenus Reitter 1885 in its original description, where it was reported in association with Hylomyrma reitteri (Mayr 1887). The species M. silvaticus Bruch (1933) was also collected in an unconfirmed association with ants, But this association has yet to be confirmed (Asenjo et al. 2017). In the Neotropics, the only Pselaphinae species with a known myrmecophilous relationship is Jubogaster towai (Coleoptera: Staphylinidae: Pselaphinae: Trogastrini), which establishes a relationship with Pheidole xanthogaster Wilson (Parker and Maruyama 2013). In addition, these authors noted that the largest Pselaphinae species are myrmecophiles and termitophiles. Nevertheless, this new report expands the myrmecophile size range to include small species such as M. guanano sp. nov.  (Bruch)