Introduction of the Exocelina casuarina-group, with a key to its representatives and descriptions of 19 new species from New Guinea (Coleoptera, Dytiscidae, Copelatinae)

Nineteen new species of Exocelina Broun, 1886 from New Guinea are described herein: E. adelbertensis sp. n., E. ambua sp. n., E. bewani sp. n., E. cyclops sp. n., E. ibalimi sp. n., E. keki sp. n., E. kumulensis sp. n., E. mendiensis sp. n., E. menyamya sp. n., E. okapa sp. n., E. piusi sp. n., E. pseudofume sp. n., E. pseudopusilla sp. n., E. pusilla sp. n., E. sima sp. n., E. simbaiensis sp. n., E. simbaijimi sp. n., E. sumokedi sp. n., and E. yoginofi sp. n. All of them, together with five already described species, have been united into the newly defined casuarina-group, a polyphyletic complex of related species with similar shape of the median lobe and paramere setation. An identification key to all known species of the group is provided, and important diagnostic characters (habitus, color, male protarsomeres 4–5, median lobes, and parameres) are illustrated. Data on the distribution of the species are given, showing that most of the species occur in the central, mountain part of Papua New Guinea