urn:lsid:zoobank.org:act: A8C46E17-6023-4393-B527-9DDD7C82C216
Promecostethus Enderlein 1909: 535.
Nuncia: Hickman 1939: 160 (Promecostethus syn.jr. of Nuncia Loman, 1902)
Promecostethus: Kury et al. 2014: 4 (restored as valid).
Type species. Promecostethus unifalculatus Enderlein, 1909, by monotypy.
Diagnosis: Promecostethus differs from all other triaenonychid genera by the morphology of the male genitalia that exhibits a ventral plate without a cleft, forming an arc shape, a simple capsula externa forming a dorsal plate—bigger than the ventral plate—with a wide median cleft dividing the dorsal plate into two large halves diverging apically and with lateral plate absent. It differs further by a capsula interna with latero-ventrally micro-sculpturated fused conductors, located ventral to a wide and apically blunt stylus that is ventrally curved. Promecostethus is relatively similar to Nuncia obesa obesa (i.e. Nuncia Loman 1902, stricto sensu) in the morphology of the dorsal scutum and ocularium, but differs in the number of stout tubercles of coxa I (two in Promecostethus and four in N. obesa obesa), coxa II (two in Promecostethus and none in N. obesa obesa), and in the femur of the pedipalp which has dorsal tubercles and ventral proximal tubercles longer than N. obesa obesa. Also, in N. obesa obesa the ventral plate of the penis is cleft and larger than the dorsal plate and the stylus is remarkably long and apically pointed, in contrast with Promecostethus where the penis ventral plate is entire, smaller than the dorsal plate and the stylus slightly overpasses the conductors and is wide and apically blunt. Compared to South American “ Nuncia ” (i.e. the polyphyletic group of species not belonging to Nuncia, see Baker et al. 2020), the differences are more significant. The penis of Nuncia americana Roewer, 1961, and all Argentine and Chilean species of “ Nuncia ”, have the ventral plate cleft and several shapes of the conductors of the capsula interna: partially covering the ventral plate in N. americana, divided into two halves which are fused apically in N. chilensis (Soares, 1968), forming a bag in N. rostrata Maury, 1990, with a hood shape in N. spinulosa Maury, 1990 and tubular with apical setae in N.verrucosa Maury, 1990. The ocularium of N. chilensis and N. rostrata have an acute spine. Nuncia spinulosa has the dorsal scutum covered with stout tubercles and N. verrucosa is covered by wart shape tubercles. Promecostethus exhibits the closest similarities with the genera Calliuncus Roewer, 1931 (Australia), Neonuncia Roewer, 1915 (New Zealand), Nunciella Roewer, 1929 (Australia, New Zealand) and Nuncioides Hickman, 1958 (Australia), including the general body shape, the absence of stout tubercles on the dorsal scutum and the presence of a highly modified bifid setiferous tubercle placed proximo-ventrally on the pedipalp femur. Calliuncus differs from Promecostethus by the presence of an acute spine on the ocularium and by a penis with a ventral plate partly separated by a thin mediodistal cleft, stylus anteriorly (i.e. dorsal) curved, and lateral plates present. Promecostethus shares with Neonuncia, Nunciella and Nuncioides a strong proximal boss on the first segment of the chelicerae of the males. Neonuncia differs from Promecostethus by the absence of microsculpture in the capsula interna and the presence of a cleft in the ventral plate of the penis. Nunciella and Nuncioides differ from Promecostethus by the presence of a ventral plate with a cleft, except in Nunciella badia (Hickman, 1958) and Nunciella dentata (Hickman, 1958). Almost all species of Nunciella differ from Promecostethus by the presence of a lateral plate on the penis; however, there is no information about the genital morphology of Nunciella cheliplus Roewer, 1931, Nunciella granulata Roewer, 1931 or Nunciella parvula Roewer, 1931.
Distribution. Endemic to the Crozet Islands