New polymorphic species of the genus Menimus Sharp (Coleoptera: Tenebrionidae: Gnathidiini) from Borneo and Sumatra

New polymorphic species of the genus Menimus Sharp, 1876 (Diaperinae Latreille, 1802, Gnathidiini Gebien, 1921, Gnathidiina Gebien, 1921) from Borneo and Sumatra are described: M. burutsp. n., M. crockerisp. n., M. dasunsp. n., M. dayaksp. n., M. grimmisp. n., M. jacobsonisp. n., M. kadazansp. n., M. kinabalucussp. n., M. loeblisp. n., M. matangicussp. n., M. merklisp. n., M. punggulicussp. n., M. sabahicussp. n., M. sarawakicussp. n. A new synonym is proposed: Menimus seriepunctatus Gebien, 1927 (Menimus malayicus Schawaller, 2016 syn. n.). A lectotype is designated for Menimus seriepunctatus Gebien, 1927. Menimus burutsp. n. and Menimus sabahicussp. n. are the first known species with 9-segmented antennae, whereas Menimus matangicussp. n. completely lacks eyes. An identification key is compiled for all 16 species so far known from Borneo and Sumatra. The high morphological plasticity of the epigean species of Menimus should be re-examined in a more comprehensive study of all Gnathidiini, not only with morphological but also with molecular data.