A new species of Simulium (Gomphostilbia) (Diptera, Simuliidae) from Thailand, with a key to identify females of 14 species of the species-group.

Simulium (Gomphostilbia) khelangense is described on the basis of females, collected by a sweeping net in Lampang, Phitsanulok and Chiang Mai Provinces, Thailand. This new species is placed in the subgroup of the species-group in the subgenus Gomphostilbia Enderlein by having the antenna with eight flagellomeres, pleural membrane bare, and female subcosta lacking hairs. It is similar to Pramual & Tangkawanit from Thailand in the same subgroup but is barely distinguished from the latter species by the head width relative to the greatest width of the frons and length of the labrum relative to the clypeus. A genetic analysis using the COI gene sequences similarly shows that is most closely related to , with a genetic distance of 1.23-2.81%. A revised key to identify females of 14 species of the species-group is provided.