Tritonus Mulsant

Short, Andrew Edward Z., 2008, Revision of the Afrotropical genus Tritonus Mulsant (Coleoptera: Hydrophilidae), Zootaxa 1855, pp. 41-55 : 43-44

Tritonus Mulsant

Tritonus Mulsant, 1844: 378 . (Masculine)

Type Species. Hydrobius cribratus Mulsant (original designation)

Diagnosis. Body size large: 6.2–9.0 mm. Within the Laccobiini , Tritonus can be characterized by the following characters: Antennae 9-segmented. Abdomen with five ventrites; large glabrous region on ventrites 2– 5. Head, pronotum and elytra each with distinct systematic punctures. Posterolateral margins of the pronotum not spinose. Prosternum, procoxae, and mesoventrite all without spines. The genus contains the largest known members of the tribe.

Description. Form narrowly to very broadly oval. Head. Systematic punctures present on the lateral margins of the clypeus and frons; absent or undetectable on the labrum. Ground punctation fine to coarse; not uniform in size. Labrum small, often retracted beneath or not extending beyond the anterior margin of the clypeus such that it is not visible from above. Mentum quadrate ( Figs. 2 b, 3b), slightly depressed at lateral margins; often with fine to coarse microsculpture consisting of fine transverse microreticulation. Maxillary palps very short, always shorter than width of labrum; apical segment longer than penultimate ( Fig. 14 ). Antennae 9-segmented, including a 3-segmented pubescent club. Dorsal surface of the eyes not bulging, in the same plane as the surface of the head. Thorax. Pronotum with systematic punctures forming anterolateral and posterolateral series; each series forming an irregular arc or row; systematic punctures larger than ground punctation. Posterolateral corners of pronotum rounded, never angulate or spinose. Prosternum ( Figs. 7–8 ) with a low median carina which may be fine and without a strong ridge ( T. steineri ), or more commonly, strongly ridged ( T. complanatus , T. cribratus ). Mesoanepiplerual sutures straight, giving the mesoventrite a diamond-shaped appearance ( Fig. 6 ). Ten very fine rows of elytral serial punctures are distinct in T. complanatus , but not discernable (or absent) in the remaining species, although some rows of coarse punctures may represent serial punctures in addition to systematic punctures. Five rows of elytral systematic punctures present although frequently difficult to distinguish from enlarged serial punctures in some species (e.g. T. complanatus ). Elytra with sutural stria at least in posterior half ( Figs. 4–5 ). Interior of elytra with a small roughened region (=plectrum; Fig. 10 ) along the central margin. Procoxae ( Figs. 7–8 ) finely pubescent, without thickened setae or spines. Hind femora entirely glabrous ( Fig. 13 ). All tarsi with five segments. Meso- and metatarsi with two rows of ventral spicules and a dorsal row of fine setae. Abdomen. Abdomen with five exposed ventrites ( Fig. 11 ). Abdominal ventrites 2 to 5 each with median glabrous region, varying in width and length; ventrite 1 often with a small, posteromedial glabrous margin, but not as extensive as the region of the other ventrites. Anterior margin of ventrites 3 and 4 sometimes slightly to strongly crenulate. Ventrite 5 entire, without apical emargination. Laterosternite 3 with a distinct, ridged stridulatory file ( Fig. 9 ). Aedeagus of the simple tri-lobed type, with parameres and median lobe generally unmodified.

Discussion. Members of Tritonus are rather large water scavenger beetles, and represent the largest members of the tribe Laccobiini . Indeed, some species ( T. complanatus ) are larger than all but a few lineages of the family as a whole. Although rare in collections (less than 100 specimens are known to me, more than half representing just one of the seven species), this appears to be an artifact of collector bias rather than actual abundance. Most collectors who have examined hygropetric habitats in Madagascar have collected series of the genus, and frequently more than one species.

Within the Oocyclus -group, Tritonus appears to be most similar to the presently monotypic genus Scoliopsis d’Orchymont , which is known from Sri Lanka (and with unpublished records from southern India). Both these genera share broadly glabrous regions on abdominal ventrites, distinct sutural stria, and 9-segmented antennae; conversely, Beralitra d’Orchymont , Ophthalmocyclus Komarek and Oocyclus Sharp each possess entirely pubescent abdominal ventrites, lack sutural striae, and have 8-segmented antennae. However, unlike Tritonus , Scoliopsis also possesses spinose posterolateral margins of the pronotum and coarse spines on the prosternum and mesoventrite (features also found in some Oocyclus ).

Figures

FIGURES 2 – 3. SEMs of head, dorsal and ventral views; 2) T. complanatus; 3) T. riambavy. Scale bar = 1 mm.

FIGURES 13 – 14. Details of Tritonus spp.; 13) T. complanatus, metafemora, scale bar = 1 mm; 14) T. riambavy, maxillary palpus, scale bar = 0.20 mm.

FIGURES 6 – 8. SEMs of thoracic sterna of Tritonus species; 6) meso- and metaventrite of T. complanatus, scale bar = 1 mm; 7) prosternum of T. complanatus, 8) prosternum of T. riambavy. Scale bars = 0.5 mm.

FIGURES 4 – 5. SEMs of elytra; 4) T. complanatus; 5) T. riambavy. Scale bar = 1 mm.

FIGURES 9 – 10. SEMs of stridulatory apparatus in T. complanatus; 9) laterosternite 3; 10) interior of elytra showing plectrum. Scale bars = 0.5 mm.

FIGURES 11 – 12. Abdomen of Tritonus spp.; 11) T. riambavy; 12) T. crenulatus. Scale bars = 1 mm.