PUPAL EXUVIAE AS DESCRIPTORS OF THE CHIRONOMID (DIPTERA, NEMATOCERA) COMMUNITIES OF LARGE RIVERS

This study firstly compares the sample structure of the chironomid ass emblage of a given station (a dyke field of the Lower Rhone River), ob tained by two methods: net tows and emergence traps (at five different dates). Secondly, it compares the records obtained using a single met hod (the collection of the drifting pupal exuviae) at different times of day, in three different cases. Net tows provided lower densities th an those recorded with emergence traps, but higher species richness va lues per total number of individuals. Highest abundance of Thienemanni ella spp., Rheocricotopus atripes, Dicrotendipes nervosus, Paratanytar sus dissimilis and Cricotopus bicinctus, and in contrast relatively lo w abundance of Cladotanytarsus mancus, Microchironomus tener and Procl adius choreus + sagitallis, occurred in net samples, these differences being correlated with the microhabitat occupied by each species. Drif t samples collected over 48 h periods showed a great variability in th e numbers of individuals and species. A single drift sample seems to b e incapable of integrating the strong circadian variability of species emergence, especially in the liver channel. In this station, sample s tructure changed gradually ver 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).