Hydrochus aeruginosus, new species Figs. 1, 26, 29 Type material. Holotype (male): “ MEXICO: Sonora, Rio Cuchujaqui, 7 mi. SE Alamos, (26°57’N, 108°51’W), black lite, 25-x-1972, E. M. Fisher ” (MCZ). Paratypes (27): Same data as holotype (21 MCZ); Mexico: Jalisco, Agua Caliente, unnamed stream, (20°23’N, 105°26’W), 15.i.2005, leg. W. D. Shepard (WDS-A-1624) (5 SEMC); Nayarit, Acaponeta, 15.viii.1960, leg. E. S. Ross, D. C. Rentz, P. H. Arnaud Jr. (1 CAS). Differential diagnosis. Among Central American species, recognized by the combination of small size (ca. 2.50 mm), the smooth dorsum with relatively sparse pronotal punctation, the lack of a callus on the 5 th elytral interstria, the greenish-blue dorsal color, and the male genitalia (Fig. 26; described below). This species has a generalized Hydrochus habitus; reliable determinations will require dissection of males. The male genitalia are readily recognized by the paramere tips which arc over the aedeagus, the latter being widest near the distal end, an unusual condition; also the shape of the basal piece orifice is unusual. Description. Habitus as illustrated (Fig. 3). Size: holotype (length/width, mm): body (length to elytral apices) 2.50/0.92; head width 0.65; pronotum l/w 0.58/0.65; PA 0.62; PB 0.52; elytra 1.63/0.92. Habitus as illustrated (Fig. 26). Body size moderate (ca. 2.50 mm). Dorsum greenish blue, with strong to moderately strong iridescence, elytra without black spots. Punctation on elytra ca. 3–5x that of pronotum. Elytra interstriae ca. 0.5–1.5× strial puncture diameter. 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 slightly wider than long, widest slightly behind anterior margin, narrowed at base, sides arcuate, smooth; anterior margin slightly bisinuate; reliefs very sparsely punctate, depressions more densely punctate; depressions deep. Elytra with spaces between strial punctures narrow walls to ca. 1x their diameter; apices rather sharply conjointly rounded in dorsal view, in lateral view outer margin with very slight angulation. Ventral characters: Mentum coarsely moderately densely punctate, with large median fovea. Submentum with two large foveae. Legs dark brown, tibiofemoral joints slightly darker. Male genitalia (Fig. 26) general characters: paramere tips only moderately slender, without plsa, with pmsm, slightly narrower in lateral view than in dorsal/ventral views, in dorsal/ventral views with distal tips meeting, medial margins arcuate around distal end of aedeagus; pseudoparameres attaining ca. midlength of parameres, tapering distally; aedeagus in dorsal/ventral views widest subapically, tapering from widest point to base; in lateral view wider, especially basally; lateral margins of aedeagus sclerotized; basal piece shorter than parameres, parallel-sided or nearly so in dorsal/ventral views, orifice sclerotized and produced in semi-cup shape. Dorsal surface: pdmm wide at adtl, then very narrowly separated; adbl small; adtl at about distal 1/5. Ventral surface: pvmm much more widely separated than pdmm, approximating lateral margin of aedeagus; avtl at ca. midlength of aedeagus. Etymology. Named in reference to the greenish-blue dorsal color.

Published as part of Perkins, Philip D., 2021, Taxonomy of Central American water beetles in the genus Hydrochus Leach, 1817 (Coleoptera: Hydrochidae), pp. 459-503 in Zootaxa 4974 (3) on pages 462-465, DOI: 10.11646/zootaxa.4974.3.2, http://zenodo.org/record/4777839