Gymnochthebius rhombus, Perkins, 2005

Perkins, Philip D., 2005, A revision of the water beetle genus Gymnochthebius Orchymont (Coleoptera: Hydraenidae) for Australia and Papua New Guinea, Zootaxa 1024 (1), pp. 1-161 : 1-161

Gymnochthebius rhombus new species

(Figures 60, 61, 74)

Type Material. Holotype (male): Australia: Northern Territory: Finke Gorge NP, Palm Valley , small temporary pools, rocky, some sandy base, algal growth, detritus, 24° 3' S, 132° 43' E, 14 March 1995, T. Weir. Deposited in the ANIC . Paratypes (2): Australia: Northern Territory: Finke Gorge NP (Boggy Hole), large permanent pool in river

bed, grasses and bulrushes at edge, algal growth, 24° 9' S, 132° 52' E, 15 March 1995, T. Weir (1 ANIC); Finke River xing, Stuart Highway, flowing river (temporary), sandy base, over­hanging vegetation, 24° 33' S, 133° 14' E, 4 March 1995, T. Weir (1 ANIC) .

Differential Diagnosis. Recognized by the strongly punctate dorsum, the elytral serial punctures deep and serially contiguous, the intervals appearing zig­zag due to large serial punctures, and the large, rhomboidal, subtabelliform metasternal glabrous area. Distinguished from the somewhat similar G. perpunctus by the smaller size (ca. 1.53 mm vs. 1.93 mm), the less strongly punctate pronotum, the microreticulate discal pronotal foveae with very discrete margins (not obscured by punctures), and the widened pronotal and elytral setae (Figs. 58, 60). The male genitalia of the two species also show a relationship, but G. rhombus differs in having the lobes narrower, more weakly rounded apically, and not barbed, the duct narrower compared with the width of the parameres, and the paramere setae shorter compared with the width of the notch (Fig. 59, 61).

Description. Size (length/width, mm) holotype: body (length to elytral apices) 1.53/ 0.69; head 0.28/0.42; pronotum 0.42/0.56; elytra 0.93/0.69. Form moderately ovate, moderately convex. Color dark brown, dorsal surface dull, strongly punctate, with short whitish setae.

Head with moderately dense whitish setae; frons densely coarsely punctate; interocular foveae deep; interocular tuberculi distinct; basomedial fovea not confluent with interocular foveae. Frontoclypeal suture deeply impressed, bisinuate. Clypeus midlength 0.5 apical width, moderately densely coarsely punctate, microreticulate laterally. Labroclypeal suture weakly bisinuate in dorsal view, evenly arcuate in anterior view. Labrum width:length as 15:6, anterior margin arcuate, without apicomedian tooth.

Pronotum lateral hyaline border well developed, origin at base of lateral depression, arcuate to posterior angles, very narrow around posterior margin; anterior margin of pronotum arcuate in midregion; each lateral depression with posterior angle produced to acute point, lateral margin weakly arcuate from anterior angle, very weakly emarginate in front of posterior angle, with short fringe of setae; lateral fossulae deep, microreticulate; pronotal disc moderately transversely convex, coarsely densely punctate, not microreticulate on reliefs, weakly microreticulate in foveae; each puncture with a recumbent whitish seta which contrasts with dark cuticle; median groove moderately narrow, deep, constricted slightly in midlength, extending nearly to margins; anterior foveae large, deep; posterior foveae oblique, 1/2 as long as and about twice as wide as median groove; posterolateral angles strongly punctate, without distinct impressions.

Elytra transversely convex on disc, with six rows of large, deep punctures between suture and humeri; rows 1–2 weakly striate­impressed and punctures serially contiguous, rows 3–6 with punctures slightly more widely spaced, not striate­impressed; each puncture with a distinctive decumbent short weakly squamose whitish seta; intervals slightly narrower than punctures; posterior declivity only moderately abrupt, summit slightly before midlength; striae 2 and 3 terminating into 4 at about apical 0.15; elytral explanate margin narrow, lacking prominent fringe setae.

Metasternal glabrous area slightly wider than long (length:width as 18:21), rhomboidal, subtabelliform, anteriorly attaining mesosternal intercoxal process as very short carina, moderately coarsely sparsely punctate posteriorly on disc, finely sparsely punctate anteriorly. Abdominal ventrites 1–3 and minutely basally 4 with hydrofuge pubescence.

Females have the explanate elytral margin and the labrum similar to males.

Aedeagus (Fig. 61): Length of main­piece 0.26 mm, length to tip of parameres 0.30 mm; lobes moderately long, apically weakly arcuate in ventral aspect, truncate in oblique aspect, not barbed; notches U­shaped, nearly parallel­sided, ca. 2x width of lobes, dorsal deeper than ventral; duct nearly straight, not expanded apically; apical paramere seta moderately elongate, ca. 2x length of next longest seta.

Etymology. Named in reference to the shape of the metasternal glabrous area.

Distribution. Currently known only from in and near the Finke Gorge National Park in southern Northern Territory (Fig. 74).