Primocerusneutrum sp. nov.

Small to medium sized beetles (2.4–4.9 mm), elongate oval in dorsal view, moderate to strongly convex in lateral view (e.g., Figs 11F, 12B), brown, dark brown, reddish brown, or rather orange. Antennae with eight antennomeres (e.g., Fig. 10G). Maxillary palps short to moderately long (e.g., shorter to nearly as long as the width of the head; e.g., Figs 10H, 12H). Elytra with sutural striae; elytral punctures from shallow to sharply marked (e.g., Figs 11E, 12E); serial punctures, ground punctures and systematic punctures similar in size and degree of impression throughout elytra; all punctures seemingly arranged in rows, sometimes evidently so. Prosternum flat to mesally only slightly produced. Posterior elevation of mesoventrite with a curved transverse ridge, rather sharp and low (Fig. 14A), except in P.cuspidis which bears a sharp, pyramidal (triangular) projection. Posteromesal glabrous patch on metaventrite nearly as wide as long. Pubescence on anterior surface of metafemora ranging from sparse to densely covering basal three fourths of the femur (e.g., Figs 12C, 10G, 11G). Fifth abdominal ventrite apically rounded, truncate or slightly emarginate, usually with stout setae (e.g., Fig. 14C). Basal piece of aedeagus as long as or longer than parameres; median lobe triangular, nearly as wide at base as basal width of one paramere, with apical projection (Fig. 14D–L); gonopore absent (Fig. 14D–L).

At first sight, the dorsally smoother members of Primocerus (Figs 10, 11) can be mistaken for Chasmogenus, given that both genera exhibit sutural striae. The presence of a transverse curved ridge (sometimes very low) on the posterior elevation of the mesoventrite distinguishes Primocerus from Chasmogenus, in which the mesoventrite is either flat, broadly elevated or with a longitudinal elevation (e.g., figs 2, 4 in Clarkson and Ferreira 2014). In addition, the maxillary palps of most Chasmogenus species are nearly 1.5 × longer than the maximum width of the head, whereas in Primocerus the maxillary palps are nearly as long as the width of the head.

Punctate members of Primocerus (in particular P.maipure and P.pijiguaense, Fig. 12) may resemble some species of Tobochares (e.g. T.canthus, T.pallidus; Kohenberg and Short 2017); striate Primocerus may resemble a very small Radicitus (see Short and García 2014). In those cases, Primocerus can be easily recognized by the presence of sutural striae. Some Primocerus may also superficially resemble certain New World cylomine genera, such as Andotypus (see Fikáček et al. 2014), from which it may be distinguished by the fully exposed labrum of Primocerus.

In addition, the presence of sutural striae and the relative size of the basal piece of the aedeagus resemble some species of Enochrus (Enochrinae) in that the basal piece is as long as or longer than the median lobe + parameres (e.g. see figs 11 and 14 in Fernández 2006). The maxillary palps curved inwards in Primocerus (as opposed to zig-zag-like as in Enochrus) allows for its recognition.

The aedeagus of Primocerus is so far unique among the Acidocerinae in the lack of a well-developed gonopore, and the presence of a lightly sclerotized projection beyond the apex of the median lobe.

Small to medium sized beetles, total body length 2.4–4.9 mm; body elongate oval, moderate to strongly convex in lateral view (e.g., Figs 11F, 12B); orange brown (Fig. 11A–D), reddish brown (Fig. 10 A–D), to dark brown in coloration (e.g., Fig. 13), usually uniform along body regions, sometimes slightly paler along margins, pronotum, ventral surfaces, and appendages, particularly maxillary palps and tarsi. Head. Frons and clypeus with either shallow (e.g., Fig. 10D) or sharply marked (e.g., Fig. 12D) ground punctures, irregularly and rather densely distributed over the surface, accompanied by scattered seta-bearing systematic punctures, particularly noticeable along anterior and inner margins of eyes, and lateral areas of clypeus; surface between punctures smooth and shiny. Clypeus roughly trapezoid, with posterior margin wider than anterior margin; anterior corners roundly angulated, anterior margin widely roundly emarginate; membranous preclypeal area not visible (visible in Chasmogenus; e.g., fig. 28 in Clarkson and Ferreira 2014); surface mesally moderately convex, laterally flattened to slightly concave (Figs 12D, H). Eyes subquadrate in dorsal view, usually protruding from outer outline of head. Labrum wide, fully exposed, collinear to perpendicular to clypeus, and usually around 0.3 times as long as clypeus (e.g., Fig. 10D); dorsal surface flat to convex, with scattered fine punctures; anterior margin markedly roundly bent inwards, mesally emarginate, with tiny denticles along emargination, and setae on lateral areas of anterior margin. Temporae densely covered by very short and fine setae (hydrofuge pubescence). Mentum parallel sided, often with lateral margins densely fringed by short setae; surface rather flat, smooth, and glabrous, sometimes with lateral oblique longitudinal ridges, and few crenulations; anterior margin with wide, deep, concave median impression, sometimes marked by a transverse carina. Submentum sunken, concave, and pubescent at base, glabrous, shiny, flat and ascending at apex; ocular ridge of variable development. Maxilla (e.g., Fig. 10G) with ventral surface of cardo and stipes smooth, shiny, and glabrous; outer dorsal margin of palpifer with a row of stiff, decumbent, spiniform setae; limit between cardo and stipes oblique; maxillary palps curved inward, brown to orange or yellow, longer than antennae, short to moderately long (e.g., shorter to nearly as long as the width of the head; e.g., Figs 10H, 12H); maxillary palpomere 1 gradually broadening towards apex, with inner margin straight and outer margin apically convex; apex of palpomere 3 bearing sensilla; palpomeres 1 and 3 similar in length, palpomere 2 only slightly shorter. Mandibles with apex bifid (observed in P.gigas, P.pijiguaense, P.striatolatus and P.petilus; e.g., Fig. 12H). Labial palps yellowish to brown, usually nearly as long as mentum, dorsoventrally flattened; palpomere 2 with outer margin convex apicad of midpoint, sometimes with setae near apex; palpomere 3 digitiform to somewhat kidney-shaped, with one or two long subapical setae on outer margin. Antennae (e.g., Fig. 10G) with eight antennomeres, slightly paler than general coloration of head; antennomere 1 anteriorly projected near base, at most reaching midpoint of ventral surface of eye, reaching to surpassing cardo-stipes joint, nearly 2.0 × longer than antennomere 2; antennomere 2 nearly as long as antennomeres 3–4 combined; antennomere 5 forming a well differentiated, symmetric cupule; antennomeres 6–8 slightly flattened, forming a loosely articulated, pubescent club (antennomere 7 shortest, 8 longest); apex of antennomere 8 with longer setae than general pubescence of club. Thorax. Pronotum widest at base, narrowed anteriorly, surface evenly convex; anterior and posterior corners widely rounded, sometimes posterior corners almost forming a sharp straight angle (e.g., Fig. 10G); anterior and posterior margins nearly straight; ground punctation either shallow or sharp, uniformly dense, with surface between punctures smooth and shiny; seta-bearing systematic punctures forming paired anterolateral semicircles. Scutellar shield of moderate size, triangular, nearly as long as wide, with punctation as in pronotum. Prosternum nearly as long as 0.7 × the length of a procoxa; anterior margin of prosternum mesally projected as a wide triangle, apically either acute or rounded (except in P.ocellatus); surface of prosternum flat to only weakly broadly convex, covered by scattered, fine, rather long setae; intercoxal process projected from posterior margin of procoxal cavities, rectangular in outline, mesally longitudinally carinate. Mesoventrite (Fig. 14A) not fused to mesepisterna, with anterior margin nearly 0.3 × as wide as anterior margin of mesepisternum; anterior rib of mesoventrite bearing paired medial teardrop-shaped, pearlescent maculae; posterior elevation of mesoventrite with a transverse curved ridge, rather sharp and low, reduced in P.maipure, P.pijiguaense and P.ocellatus, with a sharp, pyramidal (triangular) spine-like projection in P.cuspidis (Fig. 11C); surface of mesoventrite reticulated for the most part, covered by scattered, fine and rather long setae, with anteromedial depression, and posterolateral smooth and glabrous areas; mesepisternum obliquely widely concave, with reticulated surface; mesepimeron trapezoid, with reticulate and pubescent surface. Mesofurca (examined in P.neutrum) with short arms, 0.75 × length of mesocoxae; apical half of arms free, explanate at apex, somewhat square. Metaventrite mesally widely elevated, rather wide throughout and flat posteromesally; surface densely pubescent, except for posteromesal nearly as wide as long glabrous patch, and soemtimes postero-lateral areas (Fig. 10G; except in P.ocellatus, Fig. 10D). Metepisterna 3–4 × longer than wide, narrowing only at posterior end. Metepimeron clearly visible, triangular. Metafurca (examined in P.neutrum, Fig. 14B) 1.3 × wider than long, with furcal arms slightly shorter than stalk; stalk triangular (wider near the crux, gradually narrowing ventrally), with paired longitudinal keels extending along basal third of posterior face, fusing together towards crux, with a well-developed median keel on anterior face extending to anterior margin of dorsal sheets; outer margins of stalk gradually diverging from base towards basal third of furcal arms; furcal arms somewhat parallelogram-shaped, with apex (hemiductus) only slightly explanate, with apex pointing obliquely; anterior tendons inserted basad of mid length of dorsal edge of furcal arms; well-developed dorsal sheaths, narrower than widest point of lateral sheaths. Elytra. Surface even (without elevations or depressions), with sutural striae; ground punctures and systematic punctures either shallow or sharply marked (e.g., Figs 11E, 12E), similar in size and degree of impression throughout elytra, seemingly arranged in rows; serial punctures, when present (e.g., Fig. 13A, D, G), larger and deeper than ground punctures, and clearly arranged in longitudinal rows (striae); serial punctures only very slightly impressed into grooves along posterior half of elytra in striate species (e.g., P.petilus, P.striatolatus, and P.semipubescens; see Fig. 13); seta-bearing systematic punctures rather scarce; elytral outer margins flared, usually along entire length. Epipleura usually well developed, surface either flat or oblique, with sparse setae and irregular sculpture, anteriorly wide, gradually narrowing posteriorly, extending up to midlength of first abdominal ventrite; inner margin of epipleura only slightly indented at anterior outer corner of metepisternum; pseudepipleura usually well developed and perpendicularly positioned, ranging in width from nearly as wide as anterior portion of epipleura, to half as wide, extending up to basal half of abdomen along outer margin of elytra. Hind wings well developed. Legs. Pubescence on anterior surface of metafemora ranging from scarce and limited to anterior margin (e.g., Fig. 12C), to densely covering most surface up to apical fifth (e.g., Fig. 11G); glabrous area of metafemur with shiny and sometimes slightly reticulated surface; all femora antero-posteriorly flattened; metafemora usually with sharply marked tibial grooves. Tibiae slender, rather cylindrical; longitudinal rows of well-developed spines along pro-, meso- and metatibiae, composed of rather sharp and stout spines, slightly sparser along metatibiae; protibiae with a median longitudinal row of rather long and thick setae along anterior surface; protibial apical spurs large, extending beyond apex of protarsomere 2, sometimes reaching apex of protarsomere 3. All tarsi with five tarsomeres, bearing long apical hair-like setae on dorsal face, and spine-like or hair-like setae on ventral face of tarsomeres 2–4, sometimes also tarsomere 5; pro- and mesotarsomeres 1–4 similar in size and shape; pro- and mesotarsomere 5 approximately as long as 3–4 combined; metatarsomere 2 similar in length to metatarsomere 5; claws rather large, curved; well-developed empodium, bearing a pair of long, curved apical setae. Abdomen. Abdomen with five ventrites, rather flat to medially convex; all ventrites with uniform, dense, fine pubescence; posterior margin of fifth ventrite either rounded, truncate, or slightly emarginate, usually fringed with spine-like setae (Fig. 14C). Aedeagus (Fig. 14D–L) with basal piece as long or longer than parameres; median lobe triangular, with base nearly as wide as base of a paramere, with well-developed lateral basal apodemes; apex of median lobe variable, with a membranous to lightly sclerotized apical projection; gonopore not differentiated; parameres nearly as long as median lobe, with outer margins usually straight along basal 3/4, with setae at apex.

The immature stages are unknown.

Named from the Latin primus, meaning first, with the ending -cerus, in reference to the belonging of the genus to the Acidocerinae. To be treated as masculine.

Broadly distributed across the Guiana Shield region of South America, including Brazil (Pará), Guyana, Suriname and southern Venezuela (Amazonas, Bolívar) (Fig. 15).

The habitats occupied by members of Primocerus range from forested pools to seepages (Fig. 16), in an elevational range from 80 to 1950 m. Only one specimen has been collected with a flight intercept trap. Specimens of Primocerus are relatively rare, given that so far have only been found in low numbers of specimens per collecting event.