Four new species of black flies (Diptera: Simuliidae) from Sumatra, Indonesia.

Four new species of black flies are described based on the females, males and pupae from Sumatra, Indonesia, and are assigned to three species-groups in two subgenera of the genus Simulium Latreille s. l.: Simulium (Gomphostilbia) kandisense sp. nov. in the S. asakoae species-group, S. (Simulium) kotsuboyui sp. nov. and S. (S.) solokense sp. nov. in the S. iridescens species-group, and S. (S.) bengkuruense sp. nov. in the S. striatum species-group. The two new species in the S. iridescens species-group and one new species in the S. striatum species-group were formerly regarded as S. (S.) iridescens De Meijere and S. (S.) argyrocinctum De Meijere, respectively, in Sumatra, although these two species were originally described from Java and later recorded from Sumatra. The number of male upper-eye (large) facets, the presence or absence of tubercles on the pupal head and thorax, and the arrangement and relative thickness of pupal gill filaments were useful for separating these three new species from the two known species. This study increased the number of species of black flies in Sumatra from 27 to 31. However, records of some old species including S. (S.) iridescens and S. (S.) argyrocinctum should be reconfirmed for their distribution in Sumatra by future surveys.