"To clarify the distribution of three net-spining caddisfly (Insect, Trichoptera) species, Stenopsyche marmorata，S. sauteri，( Stenopsychidae) and Macrostemum radiatum (Hydropsycidae), their larvae and pupae were collected in 2004-2016 at 197 sites in the Yahagi River system, central Honshu, Japan. S. marmorata was widespread in the river system. S. sauteri was restricted in some large rivers but less abundant in the mainstream. M. radiatum was abundant in the reach from the Yahagi Daini Dam to theTenjin Bridge along the mainstream of the Yahagi River. To clarify the life history of S. marmorata and M. radiatum, their larvae，pupae, and adults were collected 18 times from August 2014 to December 2015 at the Heisei Kinen Bridge in the Yahagi River. Results of the collection indicate that S. marmorata is bivoltine whereas M. radiatum is univoltine. The results also indicate that large larvae (fourth and fifth instar) and pupae of both the two species can be simultaneously collected only in the Season from November to April of the following year. It suggests that the season is suitable for the field research to know the dominant species of net-spinners in the Yahagi River. Then, net- spinners were collected in April, November and Decemberof 2015 at 86 sites in the riffles of the mainstream of the Yahagi River, where both S. marmorata and M. radiatum are distributed. The results show that either S. marmorata or M. radiatum was dominant in the net-spinners at most of the 86 sites，and that distinct differences were often observed in the abundance of S. marmorata and M. radiatum even between adjacent sites. If M. radiatum is hypothesized to be the climax species and S. marmorata to be the pre-climax species in the succession of benthic macroinvertebrates in the Yahagi River, dominance of M. radiatum suggests the long-term stability of riverbed after the disturbance caused by flood, and dominanc of S. marmorata suggests that the stability continued in a relatively short time."