Abstract
The Asian species of the genus Pachysternum Motschulsky, 1863 are revised and redescribed. Three new species are described: Pachysternum kubani sp. nov. (Laos, China: Sichuan), P. rugosum sp. nov. (China: Gansu, Shaanxi) and P. sandacanum sp. nov. (Malaysia: Sabah, Sarawak).
Three new synonyms are established: Megasternum gibbulum Motschulsky, 1866 and Pachysternum sibiricum Kuwert, 1890 are junior synonyms of Pachysternum haemorrhoum Motschulsky, 1866; Pachysternum nigritum Jia, Wu & Pu, 1998 is a junior synonym of P. stevensi Orchymont, 1926. Pachysternum keralense Hebauer, 2002 is transferred to the pilocnemoides group of the genus Australocyon Hansen, 1990.
Megasternum japonicum Shatrovskiy, 1989 is resurrected from the synonymy with M. gibbulum and considered as a valid name. Lectotypes are designated for the following species: Pachysternum apicatum Motschulsky, 1863, P. curvatum Orchymont, 1925, P. haemorrhoum Motschulsky, 1866, P. nigrovittatum Motschulsky, 1863, P. sibiricum Kuwert, 1890, Megasternum gibbulum Motschulsky, 1866 and M. distinctum Sharp, 1873.
All Pachysternum species are diagnosed, relevant morphological characters are illustrated, and an updated identification key is provided. Pachysternum apicatum is interpreted as a polymorphic species exhibiting high geography-based variation and possibly consisting of some sibling taxa; diagnosing of these taxa requires more material and additional (ideally molecular) characters and is therefore left unresolved at the present time.