<p>Hydraulic variables (water depth, current velocity, and average substrate size) have been demonstrated to control the spatial distributions of macroinvertebrate species in streams strongly. By contrast, responses of organisms to the edges of in-stream habitats remain poorly understood. Sediments often deposit as dispersed patches on a bedrock surface. In some bedrock reaches, the aquatic bug <i>Aphelocheirus vittatus</i> Matsumura occurs at high densities in such sediment patches and is rarely found on the bedrock surface; sediment patches are clearly defined microhabitats for this species. We investigated how the bug abundance responds to hydraulic variables on the microhabitat scale and whether it responds to the edges across sediment patches. The bug abundance showed unimodal responses to all hydraulic variables, and no sex-related differences in the hydraulic suitability on the microhabitat scale were observed. Female abundance was greater on the edges than in the interiors across the sediment patches, whereas male abundance did not differ according to the within-patch position. The positive response of female abundance to the edges may result from resource mapping in which food resources were concentrated along the edges, or convenient access to both food resources in a bedrock surface and substrate resources in sediment patches at the edges. We reveal that organismal abundance responds to the edges of in-stream microhabitats. We should explore the edge effects in various stream microhabitats as well as the hydraulic suitability of diverse invertebrate species.</p>