Abstract
1) Simulium (Gnus) nacojapi is the most predominant species in the northern suburbs of Kyoto-city and this species bites chiefly men and domestic animals violently. 2) The biting collections of S. (G.) nacojapi that attacked on human, cattle and goat were carried out in August and September, 1955, and the relations between the diurnal rhythm of biting activity of this species and the meteorological factors were observed. After the observations it was known that the biting activity of this black fly is influenced by illumination chiefly, while the air temperature and wind velocity were favorable usually all day long during the observations. 3) The optimum intensity of illumination seemed to lie between 1, 000 to 20, 000 lux in this species, but not a few flies were active even under the direct sunshine of midsummer (more over 100, 000 lux).
