Sacosternum Hansen, 1989

Fikáček, Martin & Short, Andrew E. Z., 2010, A revision of the Neotropical genus Sacosternum Hansen (Hydrophilidae: Sphaeridiinae: Megasternini), Zootaxa 2538, pp. 1-37 : 5-8

Genus Sacosternum Hansen, 1989

Sacosternum Hansen, 1989: 257 . — Type species: S. megalopus Hansen, 1989 (by monotypy).— Gender: neuter.

Differential diagnosis. Eyes moderately large to very large, separated by 1.3–4.4× of width of one eye (Figs. 1–6, 27–28); antennal club blunt at apex (Fig. 38); pronotal punctures uniform in size; each pronotal puncture bearing a seta; median portion of prosternum separated from lateral portion, forming an elevated plate extending over mesal parts of lateral portions; prosternal plate carinate medially; anteromedian excision of prosternum absent (Figs. 39, 43–45); antennal grooves present, moderately large, not reaching lateral portion of hypomeron (Fig. 39); elytron with 9 punctural series; lateral margin of elytron without denticles; mesepisternal grooves for reception of procoxae moderately large, not reaching mesocoxal cavities posteriorly ( Figs. 50–55 ); anterolateral ridge of metaventrite divergent from posterior margin of mesocoxal cavities laterally ( Figs. 50–55 ); median portions of anterolateral ridges bent posteriad, forming median longitudinal carina of variable length on metaventrite ( Figs. 50–55 ); abdominal ventrite 1 with longitudinal carina and additional longitudinal ridges (Figs. 31–33); posterior margin of abdominal ventrites 1–4 with small but distinct denticles (these rarely reduced on ventrite 4) (Figs. 31–35); median lobe of aedeagus with complex sclerotized and membranous apical structures (Figs. 8, 10, 12, 14, 16, 18, 20, 22, 24, 26); base of median lobe attached to base of parameres; male sternite 8 with anteromedian narrow projection (Fig. 37); median portion of male sternite 9 without median tongue-like projection (Fig. 36); body length 1.5–2.6 mm.

Recognition. Although rather variable in external morphological characters, the representatives of Sacosternum may be easily distinguished from remaining Neotropical genera of Megasternini by the combination of the following characters:

(1) anterolateral ridge of the metaventrite divergent from mesocoxal cavity, bent posteriad (or rarely anteriad) laterally ( Figs. 50–55 );

(2) median portion of prosternum shield-like, with median carina and without anteromedian emargination (Figs. 39, 41–43);

(3) mesepisternal grooves for reception of procoxae moderately large, not reaching mesocoxal cavities ( Figs. 50–55 );

(4) median carina on the metaventrite present at least in anterior third, often completely dividing metaventrite into two halves ( Figs. 51–55 );

(5) posterior margins of abdominal ventrites 1–3 with small denticles (Figs. 31–33).

FIGURES 1–6. Body habitus. 1–2: Sacosternum sp. B; 3–4: S. garciai ; 5–6: S. lebbinorum . 1, 3, 5: dorsal habitus; 2, 4, 6: lateral habitus.

Besides of these characters distinguishing all known Sacosternum species, the following characters are usually helpful during the routine identification:

(a) dorsal side usually reddish brown (Figs. 1, 3) (dark brown only in S. lebbinorum and S. sp. A; Fig. 5);

(b) eyes usually very large, separated by 1.3–2.8× of width of one eye (moderately large, separated by 4.4× width of one eye only in S. lebbinorum );

(c) head usually with a triangular interocular area defined by shallow impressions (Fig. 27; absent in Sacosternum sp. A and S. sp. B, sometimes weakly developed also in other species);

(d) lateral portion of pronotum usually with 1–2 longitudinal impressions on each side (Figs. 29–30; impressions absent Sacosternum sp. A and S. sp. B, sometimes weakly developed also in other species); (e) abdominal ventrites 1–4 (or 1–5 in S. megalopus ) usually with very distinct longitudinal sharp ridges entirely covering ventrites (Figs. 31.35; not developed only in Sacosternum sp. A and S. sp. B, in which only the ventrite 1 bears rather weakly developed ridges).

By the general body shape, presence of the arcuately bent anterolateral ridges of metaventrite and demarcated median portion of prosternum, Sacosternum species may be only confused with some species of the genus Oosternum and Australocyon within the Neotropical Region. Oosternum species do not bear the characters (5) and (b) to (e) mentioned above, Australocyon does not bear the characters (3) to (5) and (b) to (e).

Description. Body elongate oval, elytra gradually narrowing posteriad; body outline not interrupted between pronotum and elytra. Elytra unicolored.

Head: Anterior margin of clypeus with distinct rim, lateral portions of clypeus slightly deflexed. Interocular area not elevated above the remaining surface but bearing a triangular impressed area in many species (except Sacosternum sp. A and S. sp. B). Eyes moderately large to very large, separated 1.3–4.4× of width of one eye. Mentum narrowing anteriad. Gula rather wide, gular sutures divergent posteriad, posterior tentorial pits minute. Maxilla of male with sucking disc; maxillary palpomere 2 strongly widened apically, palpomere 4 spindle-like. Antenna with nine antennomeres, antennal club compact; antennomere 9 forming ca. anterior third of the club, indistinctly constricted subapically, blunt and not projecting at apex.

Prothorax: Pronotum arcuatelly narrowed anteriad, slightly emarginate at anterior margin, evenly convex or with a more or less distinct set of longitudinal impressions; not deflexed laterally. Lateral margin of pronotum not serrate, bearing distinct rim. Pronotal puntures uniform in size, similar on whole surface of pronotum; each puncture bearing decumbent seta. Transverse row of punctures on posterior margin of pronotum absent or hardly defined (in S. lebbinorum ). Prosternum highly carinate medially, median portion separated from lateral portions; median portion plate-like, slightly overlapping lateral portions; anterolateral excision of prosternal plate absent; ridges lying along anterior margin of pronotal plate absent. Pair of deep pits just lateral to the prosternal plate absent. Antennal grooves present, moderately large, not reaching lateral margin of hypomeron, rounded to subangular. Lateral glabrous portion of hypomeron narrow anteriorly, moderately widened posteriorly.

Mesothorax: Scutellar shield small, in shape of equilateral triangle. Elytron with nine series of punctures, not deflexed or explanate laterally. Distance between elytral series 7 and 8 equal to distance between other series. Elytra not costate, with weakly to moderately convex intervals and modeately to highly impressed elytral series. Elytral interval 2 as wide as intervals 1 and 3 posteriorly, as convex and as high as adjacent intervals, reaching elytral apex. Interval punctation with punctures bearing pale decumbent setae. Lateral margin of elytra finely serrate. Epipleuron moderately wide, slightly narrowing posteriad, reaching posterior portion of metathorax; pseudepipleuron reaching elytral apex. Grooves for reception of procoxae moderately large, not reaching to anterior margin of mesocoxal cavities. Preepisternal elevation variable in shape, triangular, arrow-head shaped or narrowly to very narrowly suboval; posterolateral bulges of preepisternal elevation well developed.

Metathorax: Metaventrite with subpentagonal elevated median portion; lateral portions of metaventrite dull, with fine microsculpture. Anterolateral ridges distinctly developed, divergent from posterior margin of mesocoxal cavities laterally, bent posteriad along lateral margin of metaventrite in most species (except Sacosternum sp. A and S. sp. B, in which they are bent anteriad); the ridges meeting and bent posteriad mesally, forming median carina of variable length: length of carina sexually dimorphic in some species, usually completely developed and nearly reaching posterior margin of metaventrite in females (except Sacosternum sp. B). Femoral lines absent or developed only laterally of anterolateral ridge and delimiting a triangular area in some species ( S. lebbinorum , S. delta , S. inconnivum ). Anepisternum very narrow, 8.5– 11.5× as long as wide. Hind wings present, all examined species macropterous (all dissected specimens were inspected for this character).

Legs short; procoxae with an elevated transverse ventral ridge; hind tibiae straight; tarsi slightly shorter than tibiae, bearing sparse yellowish pubescence ventrally.

Abdomen: Abdominal ventrite 1 carinate medially, bearing sublateral longitudinal ridges. Posterior margins of ventrites finely to very distinctly denticulate.

Male genitalia: Each paramere bearing 2 setae apically. Median lobe of aedeagus basally attached to parameres, ca. 1.5× longer than parameres, projecting slightly further than parameres distally; apical portion of median lobe with a complex structure consisting of an apical sclerite and a pair of lateral sclerites (the latter missing in some species) and a set of membranous structures. Apical sclerite bearing anterolateral crenulate rim, the rim continuous apically, its margins straight; lateral margins of median lobe lacking backward directed setae; apex of median lobe lacking setae. Sternite 8 with anteromedian narrow projection, the tergite subdivided into two halves. Sternite 9 simple, crescent-like, lacking median tongue-like projection.

Female genitalia: Not examined.

Figures

FIGURES 50 – 55. Meso- and metaventrite. 50: S. auribleps; 51: S. emissarium; 52: S. lebbinorum; 53: Sacosternum sp. B; 54: S. delta; 55: S. inconnivum.