A rare non-sex mosaic abnormality represented by genitalia-like appendages on the ventral surface of abdominal segment 8 of a male black fly collected in Hokkaido, Japan, is reported. The appendages consist of a pair of style-like projections each arising from a coxite-like base, inverted-Y shaped ventral plate-like structure, and isolated round structure. This male was morphologically and molecularly identified as an abnormal form of S. (S.) iwatense (Shiraki), the only species in the Simulium (Simulium) ornatum species-group in Japan, although certain morphological characteristics of this male including the reduced number of uppereye (large) facets and elongate cerci are different from those of S. (S.) iwatense.