Vertical stratification of adult aquatic insects: a single flight challenges the boundaries of a river ecosystem

The movement of organisms within river ecosystems has been understood mainly in terms of three dimensions: longitudinal, lateral, and vertical within the water column. However, little is known about the vertical extent of the aerosphere, which is utilized by adult aquatic insects. In this study, vertical adhesive traps were installed on a 59 m-high bridge spanning a mountain stream to investigate a community consisting of adult mayflies, stoneflies, and caddisflies that utilized the aerial zone directly above the river. The results showed that adult insects were concentrated within the zone up to 25 m above the water surface; however, a single female Stenopsyche marmorata was captured at a height of 55 m. This observation makes a significant contribution to knowledge regarding the flight altitude of adult Ephemeroptera, Plecoptera, and Trichoptera (EPTs) in river ecosystems. An individual of the insectivore bat species Myotis pruinosus was also captured at an altitude of 41 m, suggesting that this high-altitude zone may serve as an arena for predator–prey relationships. Our findings thus indicate that the functional boundaries of river ecosystems extend much higher than previously thought. Our paper aims to provide new perspectives for re-evaluating conservation strategies for riverine ecosystems, including in relation to the dispersal ecology of aquatic insects and the fragmentation of “aerial corridors” by artificial structures such as dams.