Masculine.

Acidocerus aphodioides Klug, 1855: 649; by monotypy.

Small beetles, body length nearly 2.8 mm. Body shape elongate oval in dorsal view, moderately convex in lateral view, with dorsal outline nearly straight along anterior 2/3 of elytra (Fig. 17). Surface of head and pronotum granulate (Fig. 17C). Body pale/yellowish brown, with head slightly darker. Eyes with anterior margin straight in lateral view (not emarginate), in dorsal view slightly projecting from outline of head (Fig. 17C). Labrum not concealed by clypeus (Fig. 17C). Antennae with nine antennomeres, with strongly asymmetric cupule, with longer side acute. Maxillary palps elongate, with palpomere 4 nearly as long as palpomere 3 (d’Orchymont 1943f: 7, in key). Elytra without sutural striae, narrowly explanate laterally, serial punctures strongly marked, arranged in rows (Fig. 17A). Prosternum flat, rather sharply carinate medially, with angulate anteromedian projection. Posterior elevation of mesoventrite only weakly bulging. Metaventrite with hydrofuge pubescence. Metafemora without distinct tibial grooves, mostly pubescent, only glabrous at apex. Metatarsomeres 1–4 similar in length; metatarsomere 5 similar in length to metatarsomeres 1–4 combined. Fifth abdominal ventrite apically emarginate, with stout setae.

The long fifth metatarsomere (longer than metatarsomeres 1–4 combined) is unusual but not unique in the subfamily (Hansen 1991). The granulate surface of the head and body resembles that of Helobata, but besides their geographic origin, the exposed labrum of Acidocerus (as opposed to concealed in Helobata) allows its recognition. The small size and coarse punctation of the elytra of Acidocerus resemble some of the Old World Helochares (e.g., Fig. 36D–F) and some Agraphydrus (e.g., Agraphydrus hanseni, Fig. 19A), from which it can be differentiated by the medially sharply carinate prosternum (Hansen 1991).

Afrotropical: Mozambique; Fig. 4.

There is no natural history information available for the genus.

Immature stages are not known for the genus.

The taxon was originally described as related to Spercheus Kugelann, with maxillary palps similar to those of Hydraena Kugelann (Klug 1855), and even later afforded its own subfamily (see taxonomic history of the Acidocerinae section, above). d’Orchymont (1943f: 7) provided a list of diagnostic characters in a key, including the relative length of its tarsal segments, specifically that the fifth tarsomere is as long as tarsomeres 1–4 combined. Hansen (1991) redescribed the taxon based on syntypes. Hansen (1991: 149) further commented that he had seen other “typical” species of Helochares that also shared this feature and stated that “although Acidocerus may be somewhat reminiscent of a small Helochares… I prefer to maintain it as a distinct genus at the present stage”. The genus was not included in the molecular phylogeny in Short et al. (2021), and its assignment to the Helochares group is based primarily on its overall dorsal sculpturing, lack of a sutural stria, and Afrotropical distribution.

Only one described species. Hansen (1991) studied Klug’s syntypes housed at the Museum für Naturkunde der Humboldt-Universität in Berlin, Germany (ZMHB), which are the only known specimens for the genus. The diagnostic features listed above include information from d’Orchymont (1943f), Hansen (1991), and our own observations of photographs of the syntypes. Given that the specimens were mounted on cards when photographed, features of the ventral surface were not viewed by us. Characters of the ventral features (as well as the maxillary palps) as described above are based on d’Orchymont (1943f) and Hansen (1991), as the maxillary palps appeared to be missing by the time Hansen examined the syntypes. Until additional specimens are found, it is unlikely there will be a satisfactory resolution on deciding if Acidocerus is in fact a distinct genus or rather another variant of Helochares.

Acidocerus aphodioides (photographs of syntypes).

Klug 1855: 649: original description; d’Orchymont 1943f: 7: offers diagnostic features in a key; Hansen 1991: 149: redescription; Short and Fikáček 2013: 741: Acidocerus listed in subfamily Acidocerinae; Short et al. 2021: phylogenetic position and affinities discussed.