Hydrochus obliquus, new species Figs. 1, 10 Type Material. Holotype (male): “ PERU: Dept. Huanuco, Rio Llullapichis, Panguana, 260 m a.s.l. 9°37’S, 74°56’W, 18.11–16.12.09, leg. Wachtel ” (NMW). Differential Diagnosis. Among Peruvian species, recognized by the combination of small size (ca. 2.14 mm), the black dorsum, the densely, deeply punctate pronotum with deep depressions, the elytra with sutural and intervals 3, 5, 7, and 9 costate, 5 th and 9 th more strongly raised than others, the spaces between strial punctures narrow walls to ca. 0.5x their diameter, producing zigzag pattern of intervals, and the male genitalia (Fig. 10; described below). Description. Habitus as illustrated (Fig. 10). Size: holotype (length/width, mm): body (length to elytral apices) 2.14/0.78; head width 0.52; pronotum l/w 0.52/0.50; PA 0.52; PB 0.41; elytra 1.33/0.78. Body size small (ca. 2.14 mm). Dorsum black, with slight to moderately strong iridescence, elytra without black spots. Legs brown, lateral margin of tibiae with row of spines. Head with frons medially slightly tumid, with longitudinal punctate groove on each side of tumidity. Punctation on elytra ca. 2–5x that of pronotum. Elytra interstriae narrow walls to ca. 0.5x strial puncture diameter. Interstria 9 th more convex than others, overhanging 10 th interstria. Usual area of callus on 5 th not raised. Pronotum densely deeply punctate, slightly longer than wide, widest at anterior margin, narrowed at base, sides arcuate or very weakly sinuate, smooth; anterior margin slightly bisinuate; depressions deep, slightly more densely punctate than reliefs between depressions. Elytra with sutural and intervals 3, 5, 7, and 9 costate, 5 th and 9 th more strongly raised than others; spaces between strial punctures narrow walls to ca. 0.5x their diameter, producing zigzag pattern of intervals; apices rather sharply conjointly rounded in dorsal view, in lateral view outer margin without angulation. Ventral characters: Mentum moderately coarsely and moderately densely punctate, with small shallow median fovea. Submentum narrow, without foveae. Legs brown, lateral margin of each tibia with row of prominent spines. Male genitalia (Fig. 10) general characters: genitalia without parameres; basal piece elongate, arcuate in lateral view, extreme distal end bifurcate; distal piece emerging from bifurcation, long, straight, tapering apically, lacking processes. Etymology. Named in reference to the zigzag pattern of the elytral intervals. Remarks. Makhan (1998, 2001) proposed several new genera in the Hydrochidae. Hansen (1999), Short & Hebauer (2006), and Short & Fikáček (2011) have considered these to be invalid, and have placed them in synonymy with Hydrochus. Based on the male genitalia, H. obliquus n. sp. is related to species placed in one of Makhan’s “genera.” The taxonomy of the very diverse genus Hydrochus would benefit from a comprehensive molecular phylogenetic study. Until such time, the following statement from Hansen (1999) seems to hold true: “Hence, there seems to be no justification for considering [Makhan’s “genera”] as distinct genera … there is certainly no phylogenetic support for generic rank (this would almost inevitably leave Hydrochus as paraphyletic).”

Published as part of Perkins, Philip D., 2020, Taxonomy of Peruvian water beetles in the genus Hydrochus Leach, 1817 (Coleoptera: Hydrochidae), pp. 61-89 in Zootaxa 4868 (1) on pages 74-77, DOI: 10.11646/zootaxa.4868.1.4, http://zenodo.org/record/4417281