Frolov, A. V. & Akhmetova, L. A. 2015. Rediscovery of the enigmatic Stenosternus costatus Karsch (Coleoptera: Scarabaeidae: Orphninae) from São Tomé island. Zootaxa, 4007, 440–444.

DOI: 10.11646/zootaxa.4007.3.12

ZooBank: urn:lsid:zoobank.org:pub:2E9A27BE-1A65-4729-B3B2-D26127A85606

Rediscovery of the enigmatic Stenosternus costatus Karsch (Coleoptera: Scarabaeidae: Orphninae) from São Tomé Island

Frolov, A.V., L.A. Akhmetova

Laboratory of Insect Systematics, Zoological Institute, Russian Academy of Sciences, Universitetskaya nab., 1, St.-Petersburg, 199034 Russia. Email: frolov@scarabaeoidea.com

Universidade Federal de Mato Grosso, Instituto de Biociências, Departamento de Biologia e Zoologia, Av. Fernando Corrêa da Costa, 2367, Boa Esperança, 78060-900 Cuiabá, MT, Brazil.

 

Stenosternus Karsch, 1881 is one of the least-known genera of the Orphninae scarab beetles. It was described from a single specimen of the odd-looking S. costatus Karsch, 1881, from the island of São Tomé, Gulf of Guinea, West Africa. Karsch (1881) originally placed Stenosternus in the “Copridae” but later moved it to the Orphninae (Karsch, 1887). Since then, no more findings of S. costatus have been reported. The genus was redescribed by Frolov (2013) from the holotype of S. costatus and provisionally placed in the tribe Aegidiini Paulian based on a number of putative synapomorphies (Frolov, 2012). However, the material limited to only one male specimen did not allow the author to clarify which of its unique characters might be malformations or be sex dependent.

We discovered additional specimens of both sexes of S. costatus housed in the Museo Civico di Genova, Genova, Italy (MCGI) and California Academy of Sciences, San Francisco, California, United States of America (CASC). In this note we describe a female of S. costatus and discuss the species distribution and the taxonomic position of the genus.

Material used in this work is housed in the above-mentioned institutions. Photographs were taken with a Leica MZ9.5 stereo-microscope and a Leica DFC290 digital camera from dry specimens. Partially focused serial images were combined in Helicon Focus software (Helicon Soft Ltd.) to produce completely focused images. Photographs were not altered except for digital enhancing with Adobe Photoshop (Adobe Inc.): levels and tone correction, background elimination.

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.

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

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).

 Figures 3–6. Stenosternus costatus Karsch. 3–4 — apex of protibia   6 — locality map
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.

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.

Morphological traits and taxonomic position of Stenosternus

Suprageneric classification of the Orphninae was proposed by Paulian (1984) who erected Aegidiini for the four then known New World orphninae genera and Orphnini for the genera from the Old World. Stenosternus was not apparently studied by Paulian but was implicitly placed in the Orphnini. Frolov (2012) provisionally placed it in the Aegidiini based on the overall similarity with Aegidium and a number of putative synapomorphies: metepisternum widened posteriorly (forming posterior metepisternal lock for closed elytra), a keel separating basal and anterolateral parts of propleura, shape of the mandibles, and protibia with a medial tooth.

The data obtained from the examination of the newly available material of S. costatus do not provide additional arguments for the close relations of Stenosternus to the Neotropical taxa. Essentially, only the first two characters can still be considered putative synapomorphies of these lineages. On the other hand, Stenosternus lacks the following characters found in all New World taxa including the recently described Onorius Frolov et Vaz-de-Mello from the Andean cloud forest (Frolov & Vaz-de-Mello, 2015): a hole connecting mesocoxal cavities and a tube-shaped phallobase.

Also, despite a relatively large body size, S. costatus lacks sexual dimorphism characters that are present in most other Orphninae. Neither the New World type (lateral pronotal ridges and a medial pronotal tubercle) nor the Old World type (lateral pronotal ridges and a medial clypeal tubercle or horn) armature was found in a reasonable series of males.

The protibia of the female is slender as in the male but differs in the following characters. It has a well-developed apical spur similar to all female orphnines but rather short. There are three outer teeth: two basal are directed laterad and the apical tooth (outer tooth 1) is directed apicad, parallel to the lateral margin of the protibia (Fig 5). In males, there are also three protibial teeth, two outer and one inner (Figs. 3–4). It was suggested (Frolov, 2013) that the inner tooth could be homologous to the tooth-like medial process found in the males of some New World genera. However, examination of the female shows that it should rather be interpreted as the outer tooth 1.

Males of all Orphninae lack protibial spurs. This reduction is also found in many other scarab beetles of different lineages. However, in the Orphninae there is an apparent tendency that the lacking spur is substituted by either a few, normally three, robuster setae (in most Old World taxa), or a tooth-like process directed mediad (in all New World taxa except Paraegidium). Stenosternus represents the third type of this character with the outer tibial tooth 1 displaced medially.

All examined specimens of S. costatus lack protarsi and have the middle and hind tarsi reduced to only basal tarsomeres similar to the spurs in length and shape. Therefore these characters are not malformations of the holotype. They are also sex independent and represent autopomorphies of the genus.

All examined specimens of S. costatus have a similar shape of the metepisternum, which is slightly widened posteriorly and slightly overlaps the epipleuron; the epipleuron has a small but distinct concavity. This structure is a posterior metepisternal lock reduced in S. costatus apparently due to the brachyptery (Frolov, 2013). It is likely that in the flying ancestor of Stenosternus, the posterior part of the metepisternon was angulate, similar to those of the New World orphnine genera. However, the same shape of the metepisternon is also found in the dung-beetle genus Phanaeus MacLeay (Edmonds, 1972) and therefore does not necessarily suggests close relationships of Stenosternus and Aegidiini (sensu Paulian).

The stridulatory apparatus of Stenosternus is distinctive in the shape of the plectrum, which is almost rectangular, as opposed to being more or less triangular to round apically in other taxa. The large stridulatory file consists of almost parallel ridges similar to those of the Old World orphnines, while in all the New World genera the stridulatory ridges are distinctly curved posteriad (based on our personal observations).

Data available now suggest that Stenosternus represent a lineage distinct from both the other recent Old World and the New World orphnines. A more comprehensive phylogenetic analysis might necessitate separation of Stenosternus in a distinct taxon of tribal or subtribal rank.

Acknowledgments

We are thankful to Roberto Poggi (MCGI) for the possibility to examine a series of S. costatus housed in his museum and for translating L. Fea’s letter. AF is thankful to David Kavanaugh and Igor Sokolov for assistance during his visit to CASC and the Ernst Mayr Foundation for providing funding for this visit. This work was partly supported by the Russian state research project 01201351189 and Russian Foundation for Basic Research (grant 13-04-01002-a).

References cited

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