Abstract
A new subgenus, Daviesellum, in the genus Simulium is established to accommodate two species from Thailand and Peninsular Malaysia. One of these species is Simulium pahangense, for which the male and larval silk-gland chromosomes are described for the first time. A new species, S. courtneyi, is illustrated and described from larvae, pupae and pharate males and females. The immature stages of both species inhabit torrential habitats, and larvae of both species contain the trichomycete fungus Harpella melusinae, representing the first record of this species from Thailand. Simulium (Daviesellum) is related perhaps most closely to the subgenera Gomphostilbia and Morops. Sulcicnephia unidens is synonymized with S. (D.) pahangense.
