Stenosternus costatus Karsch, 1881

Frolov, Andrey V. & Akhmetova, Lilia A., 2015, Rediscovery of the enigmatic Stenosternus costatus Karsch (Coleoptera: Scarabaeidae: Orphninae) from São Tomé Island, Zootaxa 4007 (3), pp. 440-444 : 440-442

Stenosternus costatus Karsch, 1881

Additional material examined. SÃO TOMÉ AND PRÍNCIPE: São Tomé Island: Vista Alegre, 200-300 m, 0°19′00″ N 6°40′ 59″E, X.1900, 3 males, 1 female, XI– X.1900, 3 males, 2 females, L. Fea leg. ( MCGI); Agua Ize, 400–700 m, XII.1900, L. Fea leg, 1 female ( MCGI); Parque National Obo, forest between Lagoa Amelia and Bom Sucesso, 0°16′48″ N 6°35′ 29″E, 5–14.V.2001, J.M. Ledford leg., 1 female ( CASC).

Description of female. Body ( Fig. 1 ) uniformly blackish brown to brick-red with bronze and green tint. Surface densely punctate, almost rugose. Head and most of pronotum punctate with oval, deep punctures separated by 0.5–0.2 puncture diameters, sometimes almost touching. Each puncture has shagreened microsculpture with 1 short seta (only slightly protruding above surface of pronotum). Intervals between punctures smooth. Clypeus emarginate anteriorly, with crenate margin, without tubercles. Genal and frontoclypeal sutures absent. Genae not protruding past eyes, indistinct. Frons feebly convex medially. Labrum feebly protruding past clypeus. Eyes relatively small, eye width 1.5 times smaller than distance between eye margin and gula (in ventral view). Antenna with 10 antennomeres, with 3-antennomere club. Mandibles similar to those of Aegidium (see Frolov 2013: fig. 11): rather long, with 2 distinct outer teeth (tooth 2 and 3) and a small angulate tubercle on the outer margin basad of the tooth II; this angulate tubercle apparently correspond to the tooth 1 in the other orphnines. Similar to the New World orphnines, mandible of Stenosternus has a long, bristled incisor comb reaching the mandibular tooth 3.

Pronotum with lateral margins broadly arcuate in dorsal view, elongate (1.27 times wider than long), with distinct longitudinal middle depression from base and almost reaching anterior margin. Lateral margin crenulate, base not bordered. Punctation of pronotum is similar to that of head.

Scutellum 1/24 length of elytra, narrow, angulate apically.

Elytra somewhat oblong, 1.2 times longer than combined width. Humeral umbones small but distinct. Elytra without striae but each elytron with low, longitudinal ridge from base to approximately 5/6 the length. Elytra densely punctate with characteristic semicircular punctures each bearing a short setae. Because of rugose punctation, lateral margin of elytra appear crenulate in dorsal view. Epipleuron with concavity receiving hind margin of metepisternum.

Wings vestigial, about 1/2 length of elytra.

Protibiae slender, almost parallel-sided, with 3 short outer teeth ( Fig 5 ). Protarsi absent. Procoxa with relatively deep longitudinal fossa on ventral side. Middle and hind legs similar in shape. Mesofemora and metafemora relatively slender, almost parallel-sided, with elongate punctures. Mesotibiae and metatibiae without ridges externally, rugosely punctate, with 2 apical spurs (outer spur about 1.5 times longer than inner one) and with modified spur-like basal tarsomere. Stridulatory field on metacoxae present, consists of fine parallel ridges.

Metepisternum narrow, almost parallel-sided, with rounded distal part slightly overlapping epipleuron. Orifice between mesocoxal cavities absent.

Abdominal sternites with irregular, somewhat V-shaped punctures. Abdominal sternite 8 about twice as long as sternite 7 medially, without concavity or tubercle; sternite 7 slightly longer than sternite 6 and 7. Apical margin of pygidium visible in ventral view. Plectrum trapezoidal, with minute seta medially near apical margin.

Sexual dimorphism. Females can be separated from males ( Fig. 2 ) in having a short, conical apical spur on protibia and three outer protibial teeth with the apical tooth directed almost parallel to the inner margin of protibia. In males, the spur is absent and the apical protibial tooth is directed somewhat medially. Elytra of females 1.1 times wider than pronotum, while in males they are about as wide as pronotum. Females have relatively longer abdomen with pygidium only feebly visible in ventral view ( Fig. 1 ) while in the males the abdominal sternites are shorter medially and the pygidium is more convex and well visible in ventral view ( Fig. 2 ).

Variability. Body length of the examined specimens varied from 15.0–17.0 mm in males and from 14.5–17.0 mm in females. Coloration varied from blackish brown to tile-red. The lighter, red specimens are teneral having complete pubescence of the body, sharp, unworn apices of mandibles, spurs, and outer protibial teeth.

Distribution and habitat. Based on the available material, S. costatus has been collected from three or four localities ( Fig. 6 ). Two of them, Vista Alegre and a forest between Lagoa Amelia and Bom Sucesso, could be traced with a map or coordinates from the label, respectively. Locality “Agua Ize, 400–700 m ” is unclear because Água Izé is located on the coast at sea level. We think that Fea might have collected this specimen about 10 km inland from Água Izé, in the foothills of the escarpment. Precise collecting locality of the holotype of S. costatus is also unknown, but it is possible that it was collected near Roça Rio d'Ouro ( Frolov, 2013).

Based on the body and legs shape, it was suggested that S. costatus might be a rotten wood dweller ( Frolov, 2013). There are now some data available to us that support this assumption. Leonardo Fea, in his letter to Raffaello Gestro, then director of MCGI, wrote about a “scarab beetle without tarsi living in rotten wood” and provided a sketch of the beetle leg. There is no doubt that this information refers to S. costatus although the Karsch’s work was apparently unknown to Fea. It was Gestro who identified Fea’s specimens.

Figures

FIGURES 1 – 2. Stenosternus costatus Karsch, habitus in dorsal view and abdomen in ventral view. 1 — female, 2 — male.

FIGURES 3 – 6. Stenosternus costatus Karsch. 3 – 4 — apex of protibia (3, 4 — male, 5 — female; 3 — holotype, 4 — specimen from “ Vista Alegre ” locality, 5 — specimen from “ Agua Ize ” locality). 6 — locality map; round dots represent reliable localities, square—possible collection locality of the holotype, triangle—possible collection locality of specimen with the “ Agua Ize, 400 – 700 m ” label.