Nigidius, MacLeay, 1819, MacLeay, 1819

Paulsen, M. J., 2018, Revision of the obesus species group of the stag beetle genus Nigidius MacLeay (Coleoptera: Lucanidae: Lucaninae: Figulini), Insecta Mundi 2018 (624), pp. 1-13 : 2-5

obesus species group

Identification of species within the obesus group has been complicated by erroneous species hypotheses and synonymies. Members of this species group can be readily separated from other Southeast Asian Nigidius species by the presence of large, sub-oval lobes of the ocular canthus adjacent to the eyes ( Fig. 1 ). This is in contrast to other Southeast Asian groups that can be generalized as follows: b) the cornutus group, including N. cornutus MacLeay , N. sabahensis Okuda , and N. oblongus Van Roon with a shelf-like canthus that is truncate anteriorly; c) the distinctus group, including N. distinctus Parry and N. lewisi Boileau with a deeply emarginate canthus; and d) the laevicollis group, including numerous species such as N. laevicollis Westwood , N. elongatus Boileau , and N. formosanus Bates with a posteriorly expanding triangular canthus. In some species (such as N. kinabaluenis Ritsema , N. lichtensteinii Ritsema ) the canthus is weakly emarginate at the anterior third.

Within the obesus species group, species can be distinguished by the shape of the flagellum of the male genitalia, pronotal punctation, and the form of the anterolateral projection of the pronotum. Although useful for species diagnosis, the latter character is informative only for larger specimens because in all species the smaller individuals have a less well-developed projection. The form of the mandibles is conserved in these species, and partially sexually dimorphic. Large males can be sexed easily due to the broad lobe-like tooth near the base of the mandible. In females and small males, the tooth is much narrower and sub-acute.

The first species of the group was described from “ Penang, Malacca ” [West Malaysia], by Parry (1864) as N. obesus ( Fig. 2 ), and the species was subsequently illustrated by Westwood (1874). Since that time, most specimens from Malaysia and Indonesia in this group were identified as N. obesus ( Gestro 1881, Benesh 1960, Bomans 1991, Mizunuma and Nagai 1994). Bomans and Benoit (2007) gave an exceptionally wide distribution for N. obesus , encompassing the range of all species in the group, as well as Java and Sumba from which I have not seen specimens. Fujita (2010) limited N. obesus to specimens from Malaysia and Sumatra and treated the Borneo specimens as an undescribed species. However, specimens from Sumatra and Borneo possess distinct male genitalia from those of N. obesus , and specimens from these areas can themselves be separated into two distinct species. The true N. obesus is thus far known only from peninsular Malaysia and shares the same gross morphology of the male genitalia as the other mainland Asian species in the group, N. dawnae Gravely ( Fig. 3–4 ).

Specimens from Borneo and Sumatra share similarities in the overall male genitalic form but comprise two distinct species based on obvious differences in the pronotal punctation and the form of the anterolateral margin of the pronotum. In the Sumatran species, the punctation of the pronotum is more uniformly distributed and coarser. The pronota of the Borneo species (and both mainland Asian species) show clear subfoveate areas of coarse punctures juxtaposed with almost impunctate areas with only fine, shallow punctures, and the anterolateral marginal area is more strongly produced as well.

While initially considering both species to be undescribed, I researched existing names that might pertain to these two Indonesian species. The Borneo species in the obesus group has not been previously named and it is described and illustrated ( Fig. 5 ) below. The original description of N. helleri Boileau ( Fig. 6 ) from Sumatra mentioned its similarity to N. obesus and, critically, the presence of a very prominent lobe on the head that is rounded anteriorly, truncate posteriorly, and perpendicular to the body ( Boileau 1905). This describes the lobed canthus of the obesus group perfectly. I requested an image of the holotype of N. helleri from the MNHN ( Fig. 7 ), which confirmed that it is the Sumatran species of the obesus group. This is surprising because the name N. helleri has been traditionally used for a Javan or Sumatran species of the cornutus species group that is present in most collections ( Mizunuma and Nagai 1994; Okuda 2002; Fujita 2010).

The second mainland species of this group is from Myanmar, northern Thailand, and Laos and was first described as N. dawnae Gravely (1915) . Subsequently it has been identified incorrectly as N. larssoni de Lisle (de Lisle 1974; Fujita 2010) or N. obesus ( Mizunuma 2000) . Bomans (1993) synonymized N. larssoni de Lisle with N. obesus . Araya (2003) included images of the type material for all three of these available names. Fujita (2010) recalled N. larssoni from synonymy without mention of N. dawnae . Examination of the holotype from ZMUC confirmed that N. larssoni is a synonym of N. dawnae . Compared to N. obesus , the anterolateral pronotal projection is broader and more strongly produced in this species. When compared with similarly sized specimens, N. dawnae has the largest, broadest mandibles of any species in the obesus group.

Figures

Figure 1. Species groups of Southeast Asian Nigidius species based on ocular canthus shape. 1a) Nigidius obesus group. 1b) cornutus group. 1c) distinctus group. 1d) laevicollis group.

Figure 2. Nigidius obesus Parry, lectotype male, with labels (inset).

Figure 5. Nigidius gravelyi Paulsen, n. sp., holotype male, with labels (inset).

Figures 3–4. Nigidius dawnae Gravely. 3) Major male, dorsal habitus. 4) Lectotype male with labels.

Figures 6–7. Nigidius helleri Boileau. 6) Major male, dorsal habitus. 7) Holotype male with labels. Photo courtesy of Antoine Mantilleri, MNHN.