Spolia mentawensia: dragonflies (Odonata
F. F. Laidlaw’s 1926 paper “Spolia Mentawiensia: Dragonflies (Odonata)” reports the results of a September–November 1924 collecting expedition to the Mentawi Islands (Siberut, Sipora, and the Pagi group) off the west coast of Sumatra, describing 34 species of dragonfly and naming one new species and two new subspecies. The paper’s central argument is that deep-water isolation has produced a distinct island fauna, characterised by the absence of certain widespread Oriental genera, the presence of endemic subspecies, and the inclusion of a few species of decidedly eastern or Papuan range.
Summary
Laidlaw’s account is part of a multi-author series (“Spolia Mentawiensia”) documenting the natural history of the Mentawi group, a chain of low, swampy islands lying 60–80 miles west of Sumatra and separated from it by the deep Mentawi Basin (500–1000 fathoms). The expedition party, which included N. Smedley (Raffles Museum, Singapore) and H. H. Karny (Zoological Museum, Buitenzorg), collected from sea-shore, lowland, swamp, cultivated areas, and accessible hills near the Government stations of Siberut and Sioban. Laidlaw notes that the islands are zoologically distinct from both the Sunda Shelf landmasses and from Nias, despite bathymetric charts suggesting possible shallow-water connections to Sumatra via the Batu Islands.
The systematic section lists 34 species (with a further 18 from Ris’s 1915 Simalur collection included for comparison), spanning both Anisoptera and Zygoptera. Laidlaw describes Drepanosticta krugeri sp. nov., a platystictid distinguished by the shape of the male anal appendages and a remarkable paired prothoracic process, and two new subspecies: Caconeura verticalis karnyi (differing from the Bornean typical form in narrower dorsal synthorax stripe and entirely black upper anal appendages) and Amphicnemis louisae smedleyi (differing from the Bornean race in reduced development of the prothoracic spine in both sexes). He also provides the first description of the male of Leptogomphus lansbergi, noting a horn-like vertex projection in the female that recalls the armature of the Neotropical genus Epigomphus.
The faunistic discussion identifies three patterns of interest: the apparent absence of the widely distributed Oriental genera Trithemis, Rhinocypha, and Coeliccia (the latter two also absent from Ceylon, suggesting they are late arrivals in Malaysia); the occurrence of subspecies peculiar to the West Sumatran island chain (e.g. Agrionoptera insignis chalcochiton, Brachydiplax chalybea simalura) that are replaced by distinct races on the Sumatran mainland; and the presence of “outlier” species of eastern or Papuan range such as Xiphiagrion cyanomelas and Rhyothemis regia, whose significance Laidlaw leaves unresolved.
Key Findings
- 34 species of Odonata recorded from the 1924 Mentawi collection, nearly matching the 35 species Ris listed from Simalur Island in 1915; Laidlaw estimates the list represents less than 80 per cent of the total fauna (p. 219).
- One new species described: Drepanosticta krugeri (Platystictidae), with male abdomen 39 + 0.75 mm and hind-wing 24 mm; characterised by a paired club-shaped prothoracic process and a pentagonal pterostigma (pp. 228–229).
- Two new subspecies named: Caconeura verticalis karnyi (hind-wing 17 mm, abdomen 27 mm) and Amphicnemis louisae smedleyi (11 ♂♂ 14 ♀♀ from Siberut alone) (pp. 230–232).
- Jagoria modigliani, previously regarded as a rarity, was collected in fair numbers (at least 8 specimens across Siberut and Sipora), suggesting it is locally abundant at certain seasons (p. 226).
- The genera Trithemis, Rhinocypha, and Coeliccia are entirely absent from two fairly large collections, a pattern Laidlaw interprets as evidence that these are late arrivals in the Malaysian region (p. 219).
- The female of Leptogomphus lansbergi bears a horn-like vertex projection behind each posterior ocellus, morphologically comparable to the armature of Epigomphus armatus from Costa Rica, supporting a probable distant alliance between Leptogomphus and the American genus (pp. 227–228).
Conclusion
Laidlaw concludes that the Mentawi Islands support a dragonfly fauna that is distinct from, though closely related to, that of the Sumatran mainland, shaped by deep-water isolation that has altered interspecific competition in favour of certain primitive forms and permitted the persistence of endemic subspecies. The absence of key Oriental genera and the presence of eastern outliers point to a complex biogeographical history that cannot yet be fully explained, but which marks the West Sumatran island chain as a zoologically significant unit in its own right.
Context
- The primary material derives from the 1924 expedition collections held at the Raffles Museum (Singapore) and the Zoological Museum, Buitenzorg (Java), with specimens distributed among collectors C. B. Kloss, N. Smedley, and H. H. Karny.
- The paper contributes to the early 20th-century programme of faunistic survey of the deep-water islands of the West Sumatran chain, building directly on Ris’s 1915 monograph of Simalur and de Selys’s 1898 description of Pseudophaea modigliani from “Mentawi,” and establishing the Mentawi group as a distinct biogeographical entity within Malaysian odonatology.