Sexual mosaicism in the black fly Simulium (Gomphostilbia) hongthaii (Diptera: Simuliidae) in Vietnam.

Sexual mosaicism was found in an adult black fly reared from a pupa in Vietnam. It is considered an antero-posterior type of gynandromorph, showing female phenotype for the head, thorax and wing; male phenotype for the abdomen including the genitalia; and mixed phenotype for the legs. The fly is identified as Simulium (Gomphostilbia) hongthaii in the S. asakoae species-group by having the sensory vesicle medium-long (0.28-0.30 times as long as the third palpal segment), mandible without teeth on its outer margin, and ventral plate transverse, with its lateral margins emarginated basally when viewed ventrally, and trapezoidal, with its ventral margin nearly straight when viewed posteriorly. This specimen represents the first record of a black fly with sexual mosaicism in Vietnam.