A comparison between local emergence patterns of Perla grandis and Perla marginata (Plecoptera, Perlidae)

In this study we investigated the emergence patterns of two Systellognatha stoneflies: Perla marginata and P. grandis (Plecoptera: Perlidae) in a first order stream of the Northern Apennines (NW Italy). Exuviae of both species were collected in a selected area of the Curone creek by hand-picking, measuring the distance between river edge and substrate typology. Exuviae were found on different substrate types near the shoreline: stones in the stream bank were mainly used by both species, and gravel beach was the least-utilised substrate. No evident differences were found between the species in the typology and colour of emergence substrates. Interestingly, we found that P. grandis exuviae were significantly located at greater horizontal and vertical distances from the water edge than P. marginata ones. We discuss these findings on the basis of ecological and behavioural considerations.