Study site This study was conducted in a stream section of the Shiibadani River, a tributary of the Kuma River system located in southern Kumamoto Prefecture, Japan. This area lies in a temperate climatic zone and is covered by mixed forests composed of planted conifers (e.g., Cryptomeria japonica) and natural broadleaf trees (e.g., Quercus glauca), forming a mosaic of seminatural riparian woodland. Such temperate forest environments support a rich EPT community with synchronized adult emergence typically occurring during spring to autumn, making this period optimal for capturing vertical flight activity. The study site is located in a remote mountain valley with minimal artificial light and limited human disturbance, providing an ideal setting for aerial insect surveys with reduced confounding effects. Data sampling To capture adult EPTs flying at elevated flight heights, we utilized the existing Nohara Bridge (32°34′52.0″N, 130°83′58.7″E) that spans the Shiiba River (height from the railing to the water surface: 59 m). Twenty-five sticky traps(No.1–25) were suspended at equal intervals from the bridge railing to just above the water surface using ropes. The terminal of last rope was placed on gravel in the river. The sticky traps were created by spraying a colorless, odorless adhesive (Haetori nenchaku aerosol, Nisso Green Co., Ltd., Japan) on both sides of transparent acrylic plates (dimensions: 257 mm × 100 mm × 1 mm). This trap design, which made use of the structure of the bridge, enabled direct and continuous sampling of the aerial habitat above the river. The survey was conducted intensively over 95 h from August 1 to August 5, 2024, when EPTs were active. There was no rain and there were no persistent strong winds during this period. The captured EPTs were identified and classified into families then counted in the laboratory. Observations of environmental conditions To understand the vertical distribution of the microclimatic conditions, IBS-TH1 mini temperature and humidity loggers (Inkbird Tech. C. L., China) were installed on 7 of the traps (No. 1, 5, 10, 14, 18, 22, 24 from bridge side), and the air temperature and relative humidity were recorded at 30-minute intervals throughout the survey period.

This dataset contains a comprehensive set of ecological and meteorological data collected to investigate a vertical stratification of the microclimatic conditions and the distribution of flying adult aquatic insects above a mountainous stream. The study was conducted in a headwater stream located in the Kyushu region of Japan during the summer of 2024. The insect data and the microclimate data were collected simultaneously at multiple heights of 1m, 3m, 5m, 7m, 10m, 13m, 15m, 17m, 20m, 22m, 25m, 27m, 29m, 32m, 34m, 36m, 39m, 41m, 44m, 46m, 48m, 50m, 52m, 55m, 57m and 3m, 7m, 17m, 27m, 36m, 48m, 57m to create a detailed vertical profile, respectively. The dataset includes mean and standard diviation of air temperature and relative humidity, as well as the abundance and taxonomic composition of adult aquatic insects (e.g., Ephemeroptera, Plecoptera, Trichoptera) captured at each height.