Sacosternum

Fikáček, Martin & Short, Andrew E. Z., 2010, A revision of the Neotropical genus Sacosternum Hansen (Hydrophilidae: Sphaeridiinae: Megasternini), Zootaxa 2538, pp. 1-37 : 4-5

Key to known species of Sacosternum

Characters used for the identification of species largely agree with those used for the phylogenetic analysis, therefore refer to the respective chapter for their more detailed explanation.

1 Body highly convex in lateral view (Fig. 2). Pronotum evenly convex, without sublateral impressions (Fig. 1). Preepisternal elevation of mesothorax triangular, lacking median longitudinal carina (Figs. 49, 53). Lateral portion of anterolateral ridge subparallel to posterior margin of mesocoxal cavity and bent anteriad laterally ( Fig. 53 ). Abdominal ventrites 2–5 without distinct longitudinal ridges (Fig. 31). Head without oblique impressions between eyes. ............................................................................................................................................................................. 5

- Body weakly convex in lateral view (Figs. 4, 6). Pronotum evenly convex or bearing 1 to 2 weak impressions on each side (Figs. 29, 30). Preepisternal elevation linear, narrowly suboval or arrow-head shaped with median longitudinal carina (Figs. 46–48), never in the form of a triangular plate. Lateral portion of anterolateral ridge bent posteriad laterally, therefore markedly divergent from the posterior margin of mesocoxal cavity ( Figs. 50–52, 54–55 ). At least abdominal ventrites 1–4 bearing distinct longitudinal ridges (Figs. 32–33). Head at least with weak oblique impressions between eyes (Fig. 27)........................................................................................................................................ 2

2 Lateral portion of metaventrite with a triangular area situated between anterolateral ridge and lateral margin of the metaventrite, delimited by an additional oblique ridge anteriorly ( Figs. 52, 54–55 ). Preepisternal elevation narrowly drop-like (2.5× longer than wide; Figs. 48, 52) or arrow-head-shaped with distinct median longitudinal carina (Figs. 47, 54–55). Prosternal plate with lateral projections divided by ridges (Fig. 44). Pronotum with very weakly developed longitudinal sublateral impression on each side (sometimes distinct only posteriorly) (Fig. 29), rarely evenly convex, lacking longitudinal impressions..................................................................................................................... 3

- Lateral portion metaventrite without distinct triangular area between anterolateral ridge and lateral margin of metaventrite ( Figs. 50–51 ). Preepisternal elevation in the shape of a very narrow plate (4.0–5.5× longer than wide; Figs. 46, 50–51). Prosternal plate without lateral projections divided by ridges (Fig. 43). Pronotum usually with two well developed longitudinal impressions on each side (Fig. 30) .................................................................................. 6

3 Preepisternal elevation narrowly drop-like, flat or slightly concave medially, without median longitudinal carina (Figs. 48, 52). Lateral portion of anterolateral ridge continually bent posteriad ( Fig. 52 ). Posterior margin of abdominal ventrite 4 with or without denticles. Apical sclerite of median lobe rather wide, triangular apically ( Figs. 24–26 ). ........................................................................................................................................................ S. lebbinorum sp. n.

- Preepisternal elevation arrow-head shaped with distinct median longitudinal carina (Figs. 47, 54–55). Lateral portion of anterolateral ridge convex to angulate before reaching lateral margin of metaventrite and therefore “zig-zag”- shaped ( Figs. 54–55 ). Posterior margin of abdominal ventrite 4 always with distinct denticles. Apical sclerite of median lobe narrow, with almost parallel lateral margins (Figs. 12, 20)..................................................................... 4

4 Anterolateral ridge of metaventrite indistinctly angulate ( Fig. 55 ). Median prosternal carina slightly elevate anteriorly in lateral view. Median lobe wide apically, with sclerotized median part triangularly widening postreriad; lateral sclerites present (Figs. 19–20). Abdominal ventrite 5 entire posteriorly in both sexes................ S. inconnivum sp. n.

- Anterolateral ridge of metaventrite distinctly angulate ( Fig. 54 ). Prosternal carina straight in lateral view. Median lobe rather narrow apically, only slightly widened subapically; lateral sclerites absent (Figs. 11–12). Abdominal ventrite 5 weakly emarginate posteriorly in females. .................................................................................... S. delta sp. n.

5 Pronotum with very distinct mesh-like microsculpture. Preepisternal elevation concave. Posterior margin of all abdominal ventrites entire, without denticles. ................................................................................. Sacosternum sp. A

- Pronotal interstices shiny, without distinct microsculpture. Preepisternal elevation flat, its median portion not impressed. Posterior margin of all abdominal ventrites denticulate ................................................ Sacosternum sp. B

6 Head with a small tuft of erect yellowish setae anteriorly of each eye, having the appearance of small “horns” (Fig. 28). Abdominal ventrite 5 with very distict longitudinal ridges (Fig. 32). Body length 2.1–2.2 mm ............................ ...................................................................................................................................................... S. megalopus Hansen

- Head without distinct tufts of setae anteriorly of eyes. Abdominal ventrite 5 without distinct longitudinal ridges (Fig. 33). Body length 1.5–2.1 mm ...................................................................................................................................... 7

7 Interstices on head and pronotum lacking microsculpture, shiny. Pronotal punctures rounded. Median ridge on metaventrite reaching its posterior margin in both sexes. Female abdominal ventrite 5 entire posteriorly. Body length 1.9–2.1 mm .......................................................................................................................................... S. epulum sp. n.

- Interstices of head and pronotum with more or less distinct microsculpture, interstices therefore opaque. Pronotal punctures rasp-like. Median ridge almost reaching posterior margin of metaventrite in both sexes or sexually dimorphic, reaching ca. anterior third of metaventrite in males and posterior margin of the metaventrite in females (compare Figs. 50 and 51 ). Female abdominal ventrite 5 entire or emarginate posteromedially (Fig. 33, 34). Body length 1.5–1.9 mm. The “ S. cruciphallus complex”................................................................................................................ 8

8 Apical sclerite of the median lobe long and very narrow; lateral sclerites present, very narrow, situated almost per-

pendicularly to the apical sclerite (Figs. 9–10). Body length 1.5–1.7 mm .................................. S. cruciphallus sp. n. - Apical sclerite of the median lobe wider; lateral sclerites absent or present, if developed than rather wide and short (Figs. 14, 18). Body length 1.5–1.9 mm ...................................................................................................................... 9

9 Lateral sclerites of median lobe of aedeagus absent, apical membranous portions of the median lobe only slightly wider than its width at midlength (Figs. 7–8). Median carina of the metaventrite long, nearly reaching its posterior margin in both sexes ( Fig. 50 ). Posterior margin of abdominal ventrite 5 emarginate in females (as Fig. 34).............. .......................................................................................................................................................... S. auribleps sp. n.

- Lateral sclerites of median lobe of aedeagus developed, apical membranous portion of median lobe much wider than its width at midlength (Figs. 13–14, 17–18). Median carina of the metaventrite shortened posteriorly in males ( Fig. 51 ) or fully developed in both sexes. Posterior margin of abdominal ventrite weakly emarginate or entire in females ................................................................................................................................................................................... 10

10 Abdominal ventrite 5 entire posteriorly in both sexes. Apical and lateral sclerites of median lobe very large, lateral sclerite semicircular (Figs. 13–14). Median carina of the metaventrite sexually dimorphic, distinct only in anterior third of the metaventrite in males ( Fig. 51 ) .................................................................................... S. emissarium sp. n.

- Apical and lateral sclerites of median lobe moderately large, lateral sclerite irregular in shape, bearing small lateral tooth (Figs. 17–18). Median carina of the metaventrite long, nearly reaching its posterior margin in both sexes......... .............................................................................................................................................................. S. garciai sp. n.

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.

FIGURES 23 – 26. Aedeagophores. 23 – 25: S. lebbinorum, holotype; 26: S. cf. lebbinorum from Brazil, Rondonia, SW of Ariquemes. 23: whole aedeagus, dorsal view; 24, 26: detail of the apical portion of the median lobe; 25: detail of apical sclerite after clearing of the aedeagus in KOH.