It has been stated by Malloch and other authors that the superficial differences between the various species of Simulium in the adult stage are often so minute as to be discernible only after a very careful study of the genus. Previous studies had led the writer to believe that the characters of the pupal respiratory filaments would prove to be of constant specific value, but further investigation has caused this view to be modified with regard to the actual branching, though the structure of the chitinous wall and the general appearance still seem to be constant specific characters.