Distribution patterns and habitat characterization of Elmidae and Hydraenidae (Insecta : Coleoptera) in the Weidlingbach near Vienna, Austria

A total of 870 adult and 831 larval Elmidae and Hydraenidae were caught fro m September 1995 to September 1996 at the Weidlingbach, a fourth order trib utary of the Danube near Vienna, Austria, using a Hess sampler at 14 sampli ng stations from source to mouth. Elmis maugetii (LATREILLE) and Riolus sub violaceus (MULLER) accounted for 66.3% of the total, whereas Hydraena graci lis (GERMAR) was the most abundant Hydraenidae species. Based on head width, instars 1-6 were collected in E. maugetii, instars 2-6 in R. subviolaceus and instars 3-6 in Limnius volckmari (PANZER) and Riolu s cupreus (MULLER); from the remaining species, only instars 4-6 were sampl ed. E. maugetii was most abundant on coarse, moss-covered substrates (median = 22.2 mm) exposed to high water velocity (median = 69.4 cm/s; range 6.0-117. 6 cm/s); the latter was also true for R, subviolaceus although it favoured smaller sediment grain sizes (median = 10.7 mm). Sites exposed to only mode rate flow and with abundant filiform algae were preferred by Esolus paralle lepipedus (MULLER) and L. volckmari, whereas the Hydraenidae species, Esolu s angustatus (MULLER), R. cupreus and Oulimnius tuberculatus (MULLER) were collected mostly at sites with moderate current speed and abundant moss-cov ered pebbles of various size. Species richness and population density increased from source to mouth. At the spring sampling site Elmidae and Hydraenidae were completely lacking.