Primocerus Giron & Short, 2019

Giron, Jennifer C. & Short, Andrew Edward Z., 2021, The Acidocerinae (Coleoptera, Hydrophilidae): taxonomy, classification, and catalog of species, ZooKeys 1045, pp. 1-236 : 1

Genus Primocerus Giron & Short, 2019 Figs 1R , 2 , 6 , 46 , 47

Primocerus Girón & Short, 2019: 133.

Gender.

Masculine.

Type species.

Primocerus neutrum Girón & Short, 2019: 147; by original designation.

Diagnosis.

Small to medium sized beetles, body length 2.4-4.9 mm. Body shape elongated oval in dorsal view; moderate to strongly convex in lateral view; dorsal outline uniformly convex or nearly straight and anteriorly inclined along anterior half (Fig. 46 ). Color brown, dark brown, reddish brown, or rather orange, usually uniform along body regions, but sometimes with slightly paler margins, pronotum or ventral surfaces and appendages; ground punctation shallow to moderately marked (Fig. 46 ). Shape of head trapezoid. Eyes small to moderate, seldom very small, not emarginated anteriorly, usually projected from outline of head. Clypeus trapezoid, with anterior margin broadly and roundly emarginate. Labrum fully exposed. Mentum rather flat and smooth, sometimes with lateral oblique ridges, and few crenulations; median anterior depression sometimes marked by a transverse carina (Fig. 46C, F, I ). Antennae with eight antennomeres; cupule slightly asymmetric, with rounded outline. Maxillary palps moderately stout, shorter to nearly as long as width of head; inner margin of maxillary palpomere 2 nearly straight, outer margin curved along apical 2/3; maxillary palpomeres 3 and 4 similar in length (Fig. 46C, F, I ). Prosternum flat to mesally only slightly produced (Fig. 46C, F, I ). Elytra with sutural striae; elytral punctures from shallow to sharply marked; ground punctures rather uniformly distributed; some species with serial punctures; outer margins of elytra slightly flared (Fig. 46A, D, G ). Posterior elevation of mesoventrite usually with curved transverse ridge, rather sharp and low, or bearing sharp, pyramidal (triangular) projection; anapleural sutures concave to forming obtuse angle, separated at anterior margin by distance 0.3-0.4 × width of anterior margin of mesepisternum (Fig. 46C, F, I ). Metaventrite with posteromesal glabrous patch nearly as wide as long (Fig. 46C, F, I ). Protibiae with spines of anterior row as thick, long semi-erect setae; apical spurs of protibiae moderately stout, reaching midlength of protarsomere 3. Metafemora with tibial grooves moderately developed; hydrofuge pubescence coverage ranging from sparse (nearly glabrous metafemora) to dense along basal 3/4 (Fig. 46C, F, I ). Tarsomeres 1-4 with long spiniform setae on ventral face; metatarsomere 2 nearly as long as 5 and as 3 and 4 combined. Fifth abdominal ventrite apically rounded, truncate, or slightly emarginate, usually with fringe of stout setae. Aedeagus trilobed (Fig. 47 ); basal piece as long or longer than parameres; median lobe triangular, nearly as wide at base as basal width of each paramere, with apical projection; gonopore absent.

Differential diagnosis.

At first sight, the smoother members of Primocerus (e.g., Fig. 46A-C ) can be mistaken for Chasmogenus (Fig. 24 ), 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; 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 shorter, nearly as long as the width of the head.

Punctate members of Primocerus (e.g., Fig. 46D-F ) may resemble some species of Tobochares ( Kohlenberg and Short 2017, Girón and Short 2021a); striate Primocerus (e.g., Fig. 46G-I ) may resemble Radicitus (Fig. 50 ; Short and García 2014). In those cases, Primocerus can be easily recognized by the presence of sutural striae. Some species of Primocerus may also superficially resemble certain New World cylomine genera, such as Andotypus Spangler ( Fikáček et al. 2014), from which it may be distinguished by the fully exposed labrum of Primocerus .

Distribution.

Neotropical: Brazil ( Pará), Guyana, Suriname, and Venezuela; Fig. 6 . We have seen additional specimens that slightly expand the range of the genus, but all still fall within the Guiana Shield region of South America.

Natural history.

The habitats occupied by members of Primocerus range from forested pools to seepages. One specimen was 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 ( Girón and Short 2019).

Larvae.

Immature stages are not known for Primocerus .

Taxonomic history.

Primocerus was only recently described.

Remarks.

With only nine known species in the genus, Primocerus is one of the most variable genera of New World acidocerines in terms of their external morphology. Additional recent study and collections have revealed that the species described as P. neutrum likely represents a species complex (Short pers. obs.).

Species examined.

Holotypes and paratypes of all known species were examined for this study.

Selected references.

Girón and Short 2019: original description of the genus and all its known species; Short et al. 2021: phylogenetic placement.

Figures

Figure 1. Variation across Acidocerinae, dorsal and lateral views A Colossochares ellipticus B Peltochares sp. C Peltochares conspicuus D Aulonochares tubulus E Helochares sp. F Helochares tristis G Novochares sp. H Helopeltarium ferrugineum I Batochares sp. J Helobata larvalis K Radicitus sp. L Nanosaphes tricolor M Agraphydrus cf. attenuatus N Tobochares luteomargo O Tobochares sulcatus P Quadriops similaris Q Crucisternum ouboteri R Primocerus neutrum S Agraphydrus coomani T Agraphydrus sp. U Globulosis flavus V Crephelochares nitescens.

Figure 2. Phylogeny of the Acidocerinae simplified from Short et al. (2021), indicating the distribution, preferred habitat, and currently described number of species for each genus. For habitat, filled black circles indicate that at least some species of the genus are commonly found in this habitat; light grey circles indicate the genus has been found in this habitat, but is rare or not typical for the group; white circles indicate no species have been recorded for the genus in this habitat.

Figure 6. Known distribution of genera of Acidocerinae: Novochares, Peltochares, Primocerus, Quadriops, Radicitus, Sindolus, Tobochares, and Troglochares.

Figure 24. Habitus of Chasmogenus spp. A-C C. ruidus: A dorsal habitus B lateral habitus C ventral habitus D-F C. cremnobates: D dorsal habitus E lateral habitus F ventral habitus G C. lineatus H C. amplius I C. itatiaia J C. fluminensis. G, H from Smith and Short 2020; I, J from Clarkson and Ferreira Jr 2014. Scale bars: 1 mm.

Figure 46. Habitus of Primocerus spp. A-C P. neutrum: A dorsal habitus B lateral habitus C ventral habitus D-F P. maipure: D dorsal habitus E lateral habitus F ventral habitus G-I P. semipubescens: G dorsal habitus H lateral habitus I ventral habitus. Scale bars: 1 mm.

Figure 47. Aedeagi of Primocerus spp. A P. neutrum B, C P. maipure: B dorsal view C lateral view D, E P. pijiguaense: D dorsal view E lateral view F P. gigas G P. petilus H P. striatolatus I P. cuspidis. Scale bars: 0.25 mm.

Figure 50. Habitus of Radicitus spp. A-C R. ayacucho: A dorsal habitus B lateral habitus C ventral habitus D-F R. granitum: D dorsal habitus E lateral habitus F ventral habitus. Scale bars: 1 mm.