The Lost Emerald of J. C. Fabricius: The identity of the dragonfly Libellula tomentosa Fabricius, 1775 (Odonata: Libellulidae)

The eighteenth-century nominal species Libellula tomentosa Fabricius, 1775 is a dragonfly that has been forgotten in time, with an unknown identity overlooked for 250 years. With a complex taxonomic history, the mystery surrounding its identity stems mainly due to the misplacement by H. A. Hagen and M. E. Selys in the polyphyletic Corduliidae s.l. (the emerald dragonflies) during the 19th century, resulting in numerous transfers to distinct genera within this family, as it has been placed in Cordulia Leach in Brewster, 1815, Somatochlora Kirby, 1890, Paracordulia Martin, 1907 and finally Epitheca Charpentier, 1840. In the present study, we carefully reviewed this species, examining and providing photos of its name-bearing type for the first time. The nomenclatural, taxonomic, and collection histories have all been investigated and documented here. We concluded it was a subjective senior synonym of two dragonflies of the skimmer’s family Libellulidae, the Caribbean Macrothemis pleurosticta (Burmeister, 1839) and Macrothemis celeno (Selys in Sagra, 1857), due to wing venation, body coloration, and caudal appendages morphology. Since both nomina, i.e., Libellula tomentosa and Libellula pleurosticta, represent until now enigmatic species that were rarely mentioned, while the name Libellula celeno is regularly applied to a well-established species in the genus Macrothemis Hagen, 1868 with an unequivocal identity for almost two centuries, we believe that the application of the Principle of Priority would be destructive to stability and universality. Therefore, L. tomentosa and L. pleurosticta are considered nomina oblita, not taking precedence over its nomen protectum L. celeno, by prevailing usage, whenever they are considered synonyms.