Holotype (male): “Venezuela: Zulia State/ 09°50.490'N, 72°49.310'W; 270m/ Perija National Park, Tukuko,/ Rio Manantial: margins and pools/ 16.vii.2008/ leg. A. Short; AS-08-027”, “[barcode]/ SEMC0929744/ KUNHM-ENT”, “HOLOTYPE/ CHASMOGENUS/ castaneus sp. n./ des. Smith & Short” (MIZA). Paratypes (3): Venezuela: Zulia: Same data as holotype (3 exs., SEMC, including DNA voucher SLE1779).

From other Andean species with a triangular clypeal emargination, C. castaneus can be differentiated by its larger size (> 4.5 mm) and its distinct chestnut dorsal coloration (Fig. 3G–I). The overall body form with parallel sides is similar to C. lineatus, but that species is much smaller and entirely dark brown to black in coloration. In addition, the apex of the parameres are not sinuated.

Size and color. Total body length 4.0 mm. Body form elongate oval with slightly subparallel lateral margins. Dorsum of head bicolored, frons dark brown to mottled orange-brown (Fig. 8A), anterior margin of labrum and clypeus slightly paler in color. Pronotum mottled dark orange and dark brown, paler on lateral anterior margin (Fig. 3G). Elytra orange-brown, lateral margins very dark brown (Fig. 3G). Venter of head dark brown to dark red-brown. Maxillary palps and tarsi exhibit proximodistal gradation from light brown to light tannish yellow. Venter dark red-brown (Fig. 3I). Head. Ground punctation on head fine. Clypeus with anteromedial emargination which exposes a wide rounded to subtriangular gap between labrum and clypeus (Fig. 8A). Mentum strongly depressed in anterior half with rounded anteromedial notch. Maxillary palps long, longer than width of head immediately posterior to eyes. Thorax. Ground punctation on pronotum moderately coarse. Prosternum tectiform. Mesoventrite with broad elevation posteromedially, forming a posteromedial longitudinal carina. Metafemora densely pubescent in basal six-sevenths (Fig. 3I). Aedeagus. Aedeagus (Fig. 12A) with median lobe very wide at base, wider than the width of one paramere, tapering abruptly in apical quarter to form a narrow triangular projection which is even with to slightly extending beyond the apex of the parameres. Sclerite of the median lobe not expanded. Gonopore situated ca. half of one gonopore width below the apex of the median lobe. Parameres symmetrical, with outer margins slightly curved along entire length, appearing weakly convex; apex weakly angled inward. Basal piece long, slightly shorter than the length of the parameres.

The species name is derived from the Latin castaneus, meaning “of the color of chestnuts”, a reference to the distinctive reddish brown dorsal coloration of this species.

Only known from the type locality in northwestern Venezuela (Fig. 17).

All specimens of C. castaneus were collected in marginal pools by a stream (Fig. 19A, B).