Taxonomy of water beetles in the genus Hydrochus Leach, 1817, from Bolivia, Brazil and Paraguay (Coleoptera: Hydrochidae) Author Perkins, Philip D. text Zootaxa 2021 2021-06-28 4994 1 1 93 journal article 10.11646/zootaxa.4994.1.1 1175-5326 5043261 107FCA64-345F-40A4-99D3-5C1441EEAD93 Hydrochus constrictus , new species Figs. 3 , 47 , 55 , 70 Type Material. Holotype (male): “ BRAZIL , Amapá , Oiapoque (ca. 1 km E), 18.vii.2018 , stream margin ( 3.85039 -51.81683 ), 17 m ; leg. Short, BR 18-0718-03B” ( INPA ) . Paratypes (8): Same data as holotype (6 INPA , SEMC ) ; Amapá , Oiapoque (c. 5.5 km NE), balneário, 3.87281 -51.78939 , 10m , 18m . vii.2018 , Margin of larger stream, leg. Short, BR 18-0718-01C (1 SEMC ) ; Amapá , Oiapoque , behind house in town, 3.83424 -51.83372 , 13, 18.vii.2018 , Margins of small stream, leg. Short, BR 18-0718-04A (1 SEMC ) . Differential Diagnosis. Among Brazilian species, recognized by the combination of small size (ca. 2.62 mm ), the black to bronze dorsal color, the pronotum with moderately deep impressions and dense, rather uniform punctuation, the elytral interstriae narrow ridges, often forming zig-zag pattern, the pronotal shape, and the male genitalia ( Fig. 47 ; described below). Differentiation from other similarly small sized species with similarly shaped pronotum will require dissection of males. Description . Size: holotype (length/width, mm): body (length to elytral apices) 2.62/0.95; head width 0.62; pronotum l/w 0.62/0.64; PA 0.64; PB 0.52; elytra 1.63/0.95. Habitus as illustrated ( Fig. 47 ). Dorsum grey to dark brown or black, with very slight iridescence, elytra with black spots. Legs testaceous to brown. Dorsal punctuation dense. Punctation on elytra ca. 2x that of pronotum. Elytra interstriae narrow ridges, often forming zig-zag pattern, to ca. 0.5x strial puncture diameter, or less. Interstria 5 th higher than others, except 9 th . Interstria 9 th more convex than others, overhanging 10 th interstria. Usual area of callus on 5 th very weakly raised, if at all. Pronotum length less than width (ratio ca. 32/37), densely coarsely punctate, of quite uniform puncture size; widest at anterior margin, narrowed at base, sides weakly sinuate, smooth; anterior margin bisinuate; depressions shallow. Elytra with spaces between strial punctures narrow walls to ca. 0.5x their diameter; apices slightly produced, separately rounded in dorsal view, in lateral view outer margin without angulation. Ventral characters: Mentum punctate, with central depression; submentum without foveae. Male genitalia ( Fig. 47 ) general characters: genitalia without parameres; basal piece elongate, nearly straight in both dorsal/ventral and lateral views, basal 3/5 thickened, markedly arcuate in lateral view, distal ½ thin, apex bifid, distal piece issuing from between bifurcations; distal piece moderately short, straight in lateral view, with terminal minute tapering process, distal piece in dorsal/ventral views gradually increasing in width until subapical, where markedly narrowed in width, then widened to apex, tapering process issuing from distal margin, at ca. middle of apex. Etymology . Named in reference to the subapical constriction of the distal piece of the male genitalia.