Pupal exuviae as descriptors of the chironomid (Diptera: Nematocera) communities of large rivers

This study firstly compares the sample structure of the chironomid assemblage of a given station (a dyke field of the Lower Rhone River), obtained by two methods: net tows and emergence traps (at five different dates). Secondly, it compares the records obtained using a single method (the collection of the drifting pupal exuviae) at different times of day, in three different cases. Net tows provided lower densities than those recorded with emergence traps, but higher species richness values per total number of individuals. Highest abundance of Thienemanniella spp., Rheocricotopus atripes, Dicrotendipes nervosus, Paratanytarsus dissimilis and Cricotopus bicinctus, and in contrast relatively low abundance of Cladotanytarsus mancus, Microchironomus tener and Procladius choreus + sagitallis, occurred in net samples, these differences being correlated with the microhabitat occupied by each species. Drift samples collected over 48 h periods showed a great variability in the numbers of individuals and species. A single drift sample seems to be incapable of integrating the strong circadian variability of species emergence, especially in the liver channel. In this station, sample structure changed gradually over the course of the day with some species only occurring at night (e.g., Polypedilum aegyptium + quadriguttatum) and others only in the day (e.g., Nanocladius bicolor + rectinervis + distinctus).