Vertical stratification of flying adult aquatic insects (Ephemeroptera, Plecoptera, Trichoptera) above a river in Japan

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 utilised 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 utilised the aerial zone directly above the river. The results showed that a single female Stenopsyche marmorata Navás, 1920 was captured at a height of 55 m, however, adult insects were concentrated within the zone up to 25 m above the water surface. These observations make a significant contribution to knowledge regarding the flight altitude of adult Ephemeroptera, Plecoptera, and Trichoptera in river ecosystems. Our findings thus indicate that the functional boundaries of river ecosystems can extend to aerosphere. Our article 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.