A revision of the Neotropical genus Sacosternum Hansen (Hydrophilidae: Sphaeridiinae: Megasternini)

Abstract

The previously monotypic genus Sacosternum Hansen, 1989 is revised and redefined. Eleven species are recognized, of which eight are described as new: Sacosternum auribleps sp. n. (Brazil), S. cruciphallus sp. n. (Panama), S. delta sp. n. (Brazil, Paraguay); S. emissarium sp. n. (Costa Rica), S. epulum sp. n. (Brazil), S. garciai sp. n. (Costa Rica, Panama, Venezuela); S. inconnivum sp. n. (Costa Rica, Panama), and S. lebbinorum sp. n. (French Guiana, Peru, Brazil). Sacosternum megalopus Hansen, 1989 is newly recorded from Costa Rica. Two species, referred to as Sacosternum sp. A (Peru) and S. sp. B (Costa Rica) are left undescribed pending on the collecting of males. Additional unidentified specimens are recorded from Mexico (Veracruz State), Guatemala, Costa Rica, Panama, and Venezuela. A key to known species of the genus is provided, important characters are illustrated. A reconstruction of the phylogenetic relationships of Sacosternum species is performed, based on 51 adult morphological characters. The genus Sacosternum is decisively resolved as monophyletic based on six unique synapomorphies. Three species (S. epulum sp. n., S. lebbinorum sp. n. and an unidentified species of the “S. cruciphallus complex”) were collected in association with ecitonine army ants of the genera Eciton and Labidius. The possible preference of the Sacosternum species for the organic-rich leaf litter below the bivouacs of these ants is hypothesized from their label data and possible morphological adaptations are discussed.