The male, pupa and mature larva of Simulium (Asiosimulium) wanchaii Takaoka & Choochote, one of the four species of the small Oriental black fly subgenus Asiosimulium, are described for the first time based on samples collected from Thailand. The male S. (A.) wanchaii is characterized based on the enlarged hind basitarsus and the ventral plate which is much wider than long. The pupa and larva are characterized by the gill with 19 filaments and the deep postgenal cleft, respectively. Keys are provided to identify all the four species of the subgenus Asiosimulium for females, males, pupae and mature larvae.

Simulium chayamaritae Takaoka & Srisuka from Thailand belongs to the Simulium darjeelingense species-group of Simulium (Simulium) (Diptera: Simuliidae). The female of this species is described for the first time based on a female reared from a pupa collected from Chiang Mai, Thailand. It is characterized by the sensory vesicle elongate and the inner margins of the arms of the genital fork divergent, then convergent apically. It is similar to the female of S. eshimai Takaoka & Adler of the same speciesgroup from Vietnam. Taxonomic notes are given to separate it from two other species of the S. darjeelingense species-group from India and Malaysia, and 28 of 31 other species of the subgenus Gomphostilbia recorded from Thailand.