Masculine.

Sindolus optatus 1882: 72; by subsequent designation (Hansen 1991: 292).

Small to medium sized beetles, body length 2.5–5.0 mm. Body shape oval in dorsal view, moderately to strongly convex in lateral view (Fig. 51); dorsal outline usually evenly curved. Dorsal surfaces even and smooth, yellowish, orange brown to brown and rather uniform in coloration; ground punctation fine and extremely shallow (Fig. 51A). Shape of head trapezoid. Eyes not emarginate, moderate to relatively large in size, subquadrate, separated by nearly 5 × width of eye, only slightly projected from outline of head. Clypeus trapezoid, with anterior margin broadly and slightly emarginate. Labrum fully exposed, convex, and anteriorly emarginate. Mentum rather flat, with few shallow transverse crenulations on anterior region; median anterior depression relatively shallow, sometimes marked by transverse carina (Fig. 51C). Submentum smooth to very shallowly sculptured. Antennae with nine antennomeres, with strongly asymmetric and round cupule; antennomere 9 nearly 3 × longer than antennomere 8. Maxillary palps slender, 1.2–1.5 × longer than maximum width of head; inner margin of maxillary palpomere 2 usually evenly weakly curved, outer margin curved along apical third; palpomere 4 nearly 0.8 × as long as palpomere 3 (Fig. 51C). Pronotum evenly convex, usually with systematic punctures forming distinct anterolateral semicircles. Elytra without sutural striae, with margins only slightly flared; serial punctures absent; scarce systematic punctures, bearing moderately long setae (Fig. 51A). Surface of prosternum somewhat longitudinally elevated, sometimes with low and blunt longitudinal carina; anterior margin acutely to roundly projected anteriorly. Posterior elevation of mesoventrite with sharp and strongly elevated (laminar) longitudinal carina, with the ventral edge of the carina usually straight and parallel to the body (Fig. 51C); anapleural sutures concave, separated at anterior margin by distance nearly half width of anterior margin of mesepisternum. Metaventrite densely and uniformly covered by hydrofuge pubescence (Fig. 51C). Protibiae with anterior row of spines reduced (short appressed spines) to extremely reduced (tiny denticles); apical spurs of protibiae moderate, broad and reaching apex of protarsomere 2. Metafemora with tibial grooves sharply marked, and hydrofuge pubescence covering at least basal four fifths of anterior surface (Fig. 51C). Metatarsomere 2 slightly shorter or similar in length to metatarsomere 5, metatarsomere 2 similar in length to metatarsomeres 3 and 4 combined; ventral surface of all tarsomeres with long setiform setae on ventral surface (tarsomeres 1 and 2 with small stout spines). Abdomen with five pubescent ventrites. Fifth abdominal ventrite emarginate at apex; emargination fringed by stout setae. Aedeagus divided (Fig. 49E, F), somewhat pear-shaped, with basal piece nearly 0.3 × length of parameres; parameres slender, narrowing apically, with outer margins at least slightly sinuated, usually apically rounded; median lobe divided into dorsal and ventral plates; dorsal plate of median lobe medially bifurcate, with narrow, slender and apically rounded lobes; ventral lobe of median lobe varying in width and length, usually very lightly sclerotized; gonopore well-developed, usually positioned at midlength of aedeagus.

Sindolus is the only known genus of acidocerines that bears a sharp and strongly elevated (laminar) longitudinal carina.

Neotropical: Argentina, Bolivia, Brazil (Amazonas, Mato Grosso do Sul, Rio de Janeiro, Rio Grande do Sul), Colombia [in doubt; d’Orchymont, 1943d: 56], Costa Rica, French Guiana [in doubt; d’Orchymont, 1943d: 56], Guatemala, Lesser Antilles (Antigua), Mexico, Nicaragua, Paraguay, Uruguay; Fig. 6.

Sindolus mundus Sharp and S. optatus Sharp have been collected in stagnant waters at low elevations in dry areas; both species have been collected at mercury vapor lights in a drying lowland marsh where S. optatus Sharp was extremely abundant (Short 2005). Fernández and Kehr studied the annual life cycle (1994) and the spatial and temporal distribution (1995) of a population of S. femoratus in Argentina.

Immature stages are known for Sindolus talarum (Fernández) (as Helochares (Sindolus) talarum); egg case, first, second and third instar larvae and pupae were all described and illustrated by Fernández (1983). From each egg case between 25 and 40 larvae emerged; some larvae perforated and entered the aerenchyma of Spirodella intermedia (Araceae) and spent some time in there, apparently breathing the air stored in the plant tissues (Fernández 1983). In Argentina (Buenos Aires Province) first instar larvae start appearing in September, become abundant in October, and in November and the first two months of the summer all larval stages are abundant; at the end of March third instar larvae are the most common. Fernández (2004) also described the egg case and third instar larva of Sindolus femoratus (Brullé) (as Helochares (Sindolus) femoratus).

Originally described as a genus by Sharp (1882) to accommodate two species from Central America; downgraded to subgenus of Helochares by d’Orchymont (1919c); Hansen (1991): designates type species.

There are eight species of Sindolus described. The genus is among the most easily recognized acidocerines in the New World.

Sindolus femoratus (Brullé), S. mundus Sharp, S. optatus Sharp. One of the available specimens of S. mundus had been previously compared wit the holotype by A. Shohrt.

Sharp 1882: original description of the genus and two species; Fernández 1981: description of two new species; Fernández 1983: description of immature stages for Sindolus talarum (Fernández); Fernández 2004: description of immature stages for Sindolus femoratus (Brullé); Short et al. 2021: phylogenetic placement.