Lateral and vertical distribution of adult aquatic insects (Ephemeroptera, Plecoptera, and Trichoptera) in topographically constrained mountain stream

River systems in the Japanese archipelago feature steep gradients and narrow valleys, creating a tight spatial linkage between aquatic habitats and riparian forests’ vertical structure. However, how adult aquatic insects utilize the lateral and height (2D) space within these topographically constrained ecosystems remains largely unexplored. This study quantified the spatial distribution of adult Ephemeroptera, Plecoptera, and Trichoptera in a mountain area in Kumamoto, Japan, by considering both lateral distance from the water and vertical height above the ground. The abundance patterns of the dominant taxa were determined using sticky traps deployed at varying lateral distances (0–95.7 m) and vertical heights (0.04–9.89 m). Our results revealed most adult insects were laterally concentrated on the water surface and at the water edge, whereas they were not restricted to lower strata. Moreover, an increase in the abundance of Heptageniidae (Ephemeroptera) toward higher strata is demonstrated. However, Rhyacophilidae (Trichoptera) were strongly concentrated near the channel and at low heights. Perlodidae (Plecoptera) showed no significant responses to spatial variables. These findings demonstrate differences among taxa in their spatial distribution patterns ranging from above the water surface to within riparian forests, even in steep mountain areas. Consequently, biodiversity conservation in these environments requires maintaining habitat heterogeneity, including riparian vegetation–provided 2D structures, rather than focusing solely on lateral buffer zones.