Adult females and males reared from pupae of Simulium sp. from Tak Province and those of S. chiangdaoense Takaoka & Srisuka from four localities in Chiang Mai and Lampang Provinces, northern Thailand, were molecularly and morphologically compared. Simulium sp. is morphologically almost indistinguishable from S. chiangdaoense except the hair tuft at the base of the radius of the adult female and male, which is composed of yellow and dark hairs (in place of yellow hairs only). Molecular analysis using COI gene sequences shows that S. sp. formed a distinct clade and was separated from S. chiangdaoense by a genetic distance of 1.56-2.44%. Based on the results of morphological and molecular analyzes, S. sp. is described as a new species, S. mokroense, from females, males, pupae and mature larvae. It is also found that S. chiangdaoense is genetically diverse, with five lineages, and is morphologically variable in the number of male upper-eye (large) facets, which are in 13 to 17 vertical columns and 14 to 17 horizontal rows on each side. This is a first case, in which the difference in the color of the wing tuft hairs is a clue leading to the discovery of a new cryptic species close to S. chiangdaoense.
Keywords: Biodiversity; Black fly; Molecular analysis; Taxonomy.
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