Taxonomy of water beetles in the genus Hydrochus Leach, 1817, from Bolivia, Brazil and Paraguay (Coleoptera: Hydrochidae) Perkins, Philip D. Zootaxa 2021 2021-06-28 4994 1 1 93 8Y4RN Perkins, 2021 Perkins 2021 [151,435,834,861] Insecta Hydrochidae Hydrochus Animalia Coleoptera 35 36 Arthropoda species triangularis sp. nov.   Figs. 3, 7, 57   Type Material.   Holotype(male): Brazil: Mato Grosso, “  Rio Caraguata, M. Grosso, Brazil, IX-53, 21°48’S, 52°27’W, 400M alt., Fritz Plaumannleg.” ( NMNH).  Paratypes(5): Bolivia: Beni Department,  7 kmSW Trinidad, 14°52’12’’S, 64°57’32’’W,  163m,  18.vii.1998, leg. K. B. Miller” (1 SEMC).  Brazil: Mato Grosso, Corumbaleg. F. Plaumann[no date] (1 NMNH).  Paraguay: Central Dept.,  2 kmN Luque,  23–24.vi.1969, leg. P. & P. Spangler(1 NMNH); Dep. Cordillera, San Bernadino,  23.vii.1998, leg. U. Drechsel(2 NMW).   Differential Diagnosis.Among species in the study area, recognized by the combination of moderate size (ca. 3.20 mm), the brown dorsum without black spots, the nearly parallel-sided body form, the pronotal depressions moderately deep, much more densely punctate than the low reliefs between depressions, and the male genitalia ( Fig. 7; described below). This species has a generalized  Hydrochushabitus; reliable determinations will require dissection of males. The paramere tips are shaped similarly to those of  H. distinctus( Fig. 6); however, the other aspects of the male genitalia, and especially the dorsal habitus, differ in the two species.   Description. Size: holotype(length/width, mm): body (length to elytral apices) 3.20/1.02; head width 0.75; pronotum l/w 0.69/0.70; PA 0.70; PB 0.59; elytra 2.06/1.02. Habitus as illustrated ( Fig. 7). Body size moderate (ca. 3.20 mm). Dorsum brown, with very slight to no iridescence, elytra without black spots. Legs testaceous, with tibiofemoral joints darker. Body form narrow (l/w ratio ca. 86/27). Punctation on elytra ca. 1–2x that of pronotum. Elytra interstriae ca. 1–1.5x strial puncture diameter. Interstria 9 thmore convex than others, overhanging 10 thinterstria. Usual area of callus on 5 thmoderately raised, moderately elongate, ca. 5 punctures on each side. Pronotum very slightly wider than long (ratio ca. 19/18), widest at anterior margin, narrowed at base, sides slightly arcuate or very weakly sinuate, minutely denticulate; anterior margin slightly bisinuate; depressions moderately deep, much more densely punctate than low reliefs between depressions. Elytra with spaces between strial punctures narrow walls to ca. 0.5x their diameter; apices rather sharply separately rounded in dorsal view, in lateral view outer margin with slight angulation. Ventral characters: Mentum punctate, with central depression; submentum with two foveae. Male genitalia ( Fig. 7) general characters: genitalia large, elongate, length ca. 5x greatest width, basal piece very slightly shorter than parameres (ratio ca. 39/41); paramere tips extending slightly beyond aedeagus tip, medial margins beyond aedeagus tip parallel, plsa large, rounded, lateral margins between plsa and apex slightly arcuate; in lateral view parameres slightly arcuate, wide basally, tapering to sharply pointed apically; aedeagus wide, agw at ca. proximal 1/3, narrowing from there to apex and to base; basal piece in dorsal/ventral views parallel-sided, width in dorsal/ventral views and lateral view ca. equal, in lateral view very slightly arcuate, orifice slightly asymmetrical, margin sclerotized. Dorsal surface: adtl at ca. distal 1/5; pdmm well separated in distal 1/5, remainder sinuate and overlapping alm; adbl very large, rounded laterally. Ventral surface: avtl not apparent; pvmm more widely separated than pdmm, overlapping alm except for ca. distal 1/5.   Etymology.Named in reference to the triangular paramere tips. 1969-06-23 1998-07-23 1969-06-23 NMNH, SEMC, NMW Rio Caraguata, M & Fritz Plaumann & K. B. Miller & Corumba & F. Plaumann & P. & P. Spangler & San Bernadino & U. Drechsel Brazil Rio Caraguata 163 -21.8 Central Dept. 1262 -52.45 Brazil 35 36 2 Cordillera holotype