Spolia mentawiensia: Dermaptera. Introd. C.B. Kloss
Alfredo Borelli, with an introduction by C. Boden Kloss, published this taxonomic account of the Dermaptera (earwigs) of the Mentawai Islands in 1926, reporting on material collected during a September–November 1924 expedition to Siberut and Sipora. The paper documents 22 species—two of them new to science—and uses the assemblage to argue that the Mentawai Group, though small and low-lying, constitutes a zoologically distinct entity separated from Sumatra by deep-water basins rather than the shallow Sunda shelf.
Summary
Kloss’s introduction situates the Mentawai Group (Siberut, Sipora, North and South Pagi) approximately 90–110 km west of Sumatra, noting that the islands are connected to each other by a sea-bottom of less than 200 metres but are separated from the mainland by the Mentawai Basin, which reaches depths of 1,000–2,000 metres. He argues that bathymetric charts showing a continuous ridge linking the group to Sumatra via the Batu Islands are probably incorrect, because the faunas differ greatly and the Mentawai mammalian fauna, while richer than that of the deep-water islands Simalur and Engano, is far more peculiar and differentiated than that of Nias (p. 385–386). The collecting conditions were difficult: the islands are mostly swamp with hills rarely exceeding 500 metres, paths are scarce, and all members of the fifteen-person party except Kloss contracted malaria (p. 385).
Borelli’s systematic section reports 22 species, of which two are described as new: Diplatys karnyi (a large Pygidicranid from Siberut, 11.5 mm, distinguished from D. jacobsoni by size and the shape of the penultimate ventral segment) and Nesogaster gonopygius (a small Labiid from Siberut, 4.5 mm, allied to N. mounseyi from the Philippines but differing in pygidal shape and antennal coloration) (pp. 387–391). Of the 19 previously known species, Tagalina semperi Dohrn had not previously been recorded from Sumatra or Java, and several others (Parapsalis laevis, Gonolabidura piligera, Chaetospania lanceolata) were known from Sumatra and the Philippines but not yet from Java (p. 387). Notably, none of the seven species previously reported from the Mentawais by Modigliani (held in Genoa) were recovered by the 1924 expedition, suggesting either a change in local conditions or differences in collecting methodology (p. 387).
The paper is one of several contributions under the collective title “Spolia Mentawiensia,” which also included reports on flora (Ridley), birds (Chasen & Kloss), reptiles and amphibians (Smith), and Zoraptera (Karny), all published in 1926 (p. 386).
Key Findings
- The expedition collected 22 Dermaptera species from Siberut and Sipora, including two new species: Diplatys karnyi (♂, 11.5 mm body length) and Nesogaster gonopygius (♂, 4.5 mm body length) (pp. 387–391).
- Tagalina semperi Dohrn, previously known only from other regions, was recorded for the first time from Sumatra (Sipora and Siberut), with adults, nymphs, and larvae collected (p. 389).
- Chelisoches semirufus Borelli, described from the same expedition, was represented by multiple specimens from both Sipora and Siberut and constituted the only known examples of the species at the time of publication (p. 391).
- The Mentawai Basin between the islands and Sumatra reaches depths of 1,000–2,000 metres, and the group is surrounded by water 200–1,000 metres deep, placing it well beyond the shallow Sunda shelf (less than 75 metres) on which most Malaysian landmasses stand (p. 386).
- None of the seven Dermaptera species previously reported from the Mentawais by Dr. Elia Modigliani (collections in Genoa) were recovered by the 1924 party, a discrepancy Borelli notes without resolving (p. 387).
- Hamaxas feae Borm. was collected from Sipora (1 ♂, 3 ♀); the male showed two internal teeth on the forceps branches rather than the single tooth of the type specimen, a variation also recorded from the Philippines, Borneo, and Sumatra (p. 391).
Conclusion
Borelli’s account confirms that the Mentawai Islands, despite their modest size and low relief, harbour a Dermaptera fauna that is both richer and more distinctive than that of comparable deep-water islands in the West Sumatran chain, reinforcing Kloss’s biogeographical argument that the group should be treated as a zoologically discrete unit rather than a simple extension of the Sumatran mainland.
Context
- The primary material derives from the Raffles Museum (Singapore) and the Zoological Museum at Buitenzorg (Java), collected during the 1924 expedition; Borelli examined the specimens at the Museo Zoologico in Turin.
- The paper contributes to the broader “Spolia Mentawiensia” series, a multi-author, multi-journal synthesis of the island group’s natural history published in 1926, and represents one of the earliest systematic faunal surveys of the Mentawais.