Lateral and longitudinal dispersal of aquatic insects in mountain streams, with notes about Trichoptera

Understanding the spatial patterns of energy flow from mountain streams via emerging aquatic insects remains limited due to a lack of empirical data. Upon emergence, adult flying insects disperse in both longitudinal (upstream/downstream) and lateral (into terrestrial habitats) directions. Here, we quantified the dispersal patterns of adult aquatic insects in both dimensions using a combination of sticky and Malaise traps. To assess longitudinal dispersal, we deployed sticky traps in transects along three streams, with Petri dish arrays designed to capture insects flying upstream, downstream, or laterally across the channel. Lateral dispersal was measured using the same trap design placed at increasing distances (up to 32–64 m) from the stream edge, complemented by Malaise traps at one site. Trichoptera exhibited the highest family-level richness among captured taxa, and the genus Micrasema showed a clear exponential decay in abundance with distance from the stream, consistent with lateral dispersal theory. Our findings contribute empirical evidence on the spatial extent of aquatic insect emergence and dispersal, informing future studies on stream–riparian energy flow across larger spatial and temporal scales.
Key words: Aquatic subsidy, caddisflies, Malaise trap, riparian microclimate, sticky trap