Derosphaerus metallescens (Fairmaire, 1893)

Cheong, Loong Fah, 2025, On nomenclature and taxonomy of some Carabidae and Tenebrionidae from Singapore, with descriptions of new species of the genera Uloma Dejean, 1821 and Promethis Pascoe, 1869, Raffles Bulletin of Zoology 73, pp. 233-242 : 235-236

Derosphaerus metallescens (Fairmaire, 1893) , status restored

( Figs. 1A–B , 2A–C )

Encyalesthus metallescens Fairmaire, 1893: 27 . Type locality: Sumatra , Singapore.

Derosphaerus viriditinctus : Schawaller, 2011: 295

Material examined. Derosphaerus metallescens : 1 ♀ ( HT): Singapore / TYPE [red label printed] / Encyalesthus metallescens Fairm. 1893 . Singapore / MUSEUM PARIS, Collection Léon Fairmaire ) [ MNHN] ; 1 ♀ ( PT): Encyalesthus metallescens n. sp., Sumatra / [ NHMB, collection Frey] ; 1 ♂: Sumatra , Medan / Encyalesthus metallescens F. / [ NHMB, collection Frey] .

Derosphaerus viriditinctus : 2 ♀♀: Borneo / cum typo comparatum / Encyalesthus viriditinctus Fairmaire / P. ARDOIN DET. 1973. / Museum Paris Coll. P. Ardoin 1978 / [ MNHN] ; 1 ♀: Borneo / Plesiotype! No. 291 [red label printed] / Encyalesthus viriditinctus Frm. / [ NHMB, collection Frey] .

Discussion. Fairmaire (1882) described Encyalesthus (= Derosphaerus ) viriditinctus , based on a single specimen from Palembang, Sumatra. Sex was not mentioned but it is female. Gebien (1914) provided additional descriptive notes on E. viriditinctus , based on one female specimen from Borneo in his possession (labelled as “plesiotype”). Specifically, he quantified the width of the frons (not reaching the transverse diameter of an eye), noted as important diagnostic features the long legs (elongate meso- and metatarsi) and the presence of a distinct anal impression which finely inflects the apical edge of the last ventrite. Unfortunately, Fairmaire’s holotype in MNHN could not be located. Instead, I have examined Gebien’s specimen in the Frey’s collection, labelled as “plesiotype” ( Fig. 1A ), and two specimens of D. viriditinctus from Ardoin’s collection in MNHN, labelled as “cum typo comparatum” and identified as females ( Fig. 1B ).

Fairmaire (1893a) described Encyalesthus metallescens based on two specimens from Singapore (HT, MNHN) and Sumatra (PT, NHMB), without mentioning the sex. However, it seems clear that the specimens are both female as Fairmaire did not note the distinctive dilated metatibia of the male E. metallescens in his description. Gebien (1914) did not describe E. metallescens directly but remarked that E. hauschildi can be distinguished from E. metallescens by “… ungefurchtes Prosternum, einfache Hintertibien ...”; which means that E. metallescens has prosternum furrowed and metatibiae dilated. While Gebien did not specifically say that he had a male specimen of E. metallescens , the tibial dilation is clearly a male feature. I follow Gebien’s concept of the male as it matches very well with the female, other than sexual differences that are typical for this genus. Gebien did not give any details of the male specimen on which he based his description. I found in Frey’s collection only one male specimen from Medan, Sumatra, and assume that that was the specimen from Gebien. Figs. 2A and 2B are respectively images of the holotype and paratype (both female) of Derosphaerus metallescens , whereas Fig. 2C is an image of a male D. metallescens featured in Schawaller (2011) (in which it has been synonymised to D. viriditinctus and named as such).

Kaszab (1987) provided a checklist of the species in Derosphaerus but D. metallescens is missing in the species list. Schawaller (2011) regarded Derosphaerus metallescens as a junior synonym of Derosphaerus viriditinctus . I note that Schawaller (2011) could not reexamine the types of both taxa, so this synonymy would be open to doubt with some justification. I propose to resurrect the former as a good species based on examination of the type materials and by returning to the descriptions of Fairmaire and Gebien.

The view that D. metallescens and D. viriditinctus are unlikely to be synonymous is supported by the following reasons: For D. metallescens and D. viriditinctus to be synonymous, one has to disregard quite significant differences in the female holotypes of these two species. These differences include the more obvious ones like the frons width (difference in IE/TD between the two species more than two times), leg length (specifically meso- and metatarsi), and presence/absence of anal impression in the last ventrite. Subtler differences include longitudinal impression on pronotum, shape of pronotum, and convexity of elytral interval. For instance, compared to D. viriditinctus , D. metallescens typically has a bulkier pronotum relative to elytra (e.g., see PB/EW in Table 1). Admittedly, there is quite a bit of pronotal shape variation across the wide distribution range of D. metallescens . As a result of examining the types mentioned above, it is believed that the derived characters possessed by these two species are not common and that they are not homologous.

One also has to consider what was regarded by Gebien as the male D. metallescens ; this seems to be a very reasonable male choice for the female holotype, based chiefly on the pronotum shape, its longitudinal impression, IE/TD, etc. If, however, D. metallescens and D. viriditinctus were to be considered synonymous, this would result in the following rather atypical sexual dimorphism for a coleopteran: the frons of the male (IE/TD: 1.5 in the male D. metallescens ) is much broader than that in the female (IE/TD: 0.6–0.7 in the female D. viriditinctus ).

Table 1 summarises some of the salient measurements that reflect the difference between the two species D. viriditinctus and D. metallescens .

Remarks. Additional materials of Derosphaerus metallescens were also studied in MNHN and NHMB. There is considerable variation in the body size, pronotum shape, degree of shine on dorsum. While the body length shows a considerable variation of almost up to 30%, most of the pronotum and elytra measurements conform to those in Table 1. However, there are a small number of specimens in which the pronotum is less bulky (PB/EW as low as 0.60) and also more transverse (PW/PL as high as 1.22). I am not sure if the whole series is conspecific or not, although it is difficult to separate them into different morphospecies.

Figures

Fig. 1. A–B, Derosphaerus viriditinctus (Fairmaire, 1882). A, a female specimen labelled as “plesiotype” from NHMB; B, a female specimen labelled as “cum typo comparatum” from MNHN.

Fig. 2. A–C, Derosphaerus metallescens (Fairmaire, 1893). A, holotype, female, from MNHN; B, paratype, female, from NHMB; C, photograph of a male from Schawaller (2011)’s paper (in which it has been synonymised to D. viriditinctus and named as such), Photo copyright: W. Schawaller.