Abstract
The wingbeat frequencies of two black fly species, Simulium bidentatum and S. iwatense were measured after emergence. The mean frequencies of both sexes of S. bidentatum were about 200Hz within 24hr after emergence and rose to 245-250Hz after then. On the other hand, the mean frequencies of male and female of S. iwatense were about 200 and 180Hz within 24hr after emergence, respectively. The ratios of the mean frequencies of male to female in the same age were 0.99 : 1 to 1.15 : 1 in S. bidentatum and 1.09 : 1 in S. iwatense.
