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 casuarius , new species Figs. 1 , 45 , 57 , 70 Type Material. Holotype (male): “ BRAZIL : Amazonas ; Presidente Figueiredo (c. 57 km E) on AM-240, ‘airport stream’ ( -1.98826 , -59.51618 ), 80 m , leg. Short ; 18.vi.2019 , stream margins, BR18-0619-02B” ( INPA ). Differential Diagnosis. Among Brazilian species, recognized by the combination of small size (ca. 2.30 mm ), black dorsal color, the highly sculptured dorsum, with deep pronotal foveae, costate elytral interstriae, the 5 th interstria raised, but lacking usual callus, and the male genitalia ( Fig. 48 ; described below). Reliable differentiation from other similarly small sized black species with costate elytra should include dissection of males. Description . Size: holotype (length/width, mm): body (length to elytral apices) 2.30/0.95; head width 0.58; pronotum l/w 0.59/0.57; PA 0.57; PB 0.43; elytra 1.47/0.95. Habitus as illustrated ( Fig. 45 ). Dorsum black; very slight iridescence in depressions. 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. 3–5x that of pronotum. 5 th interstria raised, but lacking usual callus. Pronotum length and width ca. equal, widest at anterior margin, strongly narrowed at base, sides weakly sinuate, non-denticulate; anterior margin bisinuate; lateral and anterior margins with row of moderately long setae; depressions large, deep, much more densely punctate than almost impunctate rather sharp reliefs. Elytra with intervals 3, 5, 7, and 9 costate; costa of 7 th interval interrupted in midlength, other costae complete; punctures very large, spaces between strial punctures narrow walls, many punctures in off-set zig-zag pattern; apices acuminate, rather sharply separately rounded in dorsal view, in lateral view outer margin without angulation. Ventral characters: Mentum punctate, setose, with central depression; submentum very narrow, without foveae. Male genitalia ( Fig. 45 ) general characters: genitalia without parameres, basal piece elongate, arcuate in lateral view, straight in dorsal/ventral views; distal part enlarged and strongly sclerotized on dorsal surface, between distal enlarged part and apex with concavity on dorsal surface. Etymology . Named in reference to the cassowary-like shape of the distal end of the male genitalia.