We analysed an extensive data set of Diptera (953 species) and Coleoptera (699 species) collected by trunk-window traps and eclectors in the forest reserve Sihlwald (Canton Zurich. Switzerland) in 1996 and 1997. The samples contained 186 species of Diptera new to the Swiss fauna and approximately 20 species of the same group new to science. Temporal variability among the samples was larger than spatial variability. Despite the unusually large collecting effort, several methods applied for estimating total species richness revealed undersampling of > 30%. We therefore underline the necessity for using standardized sampling techniques which make different studies comparable and reduce sampling effort without considerable loss of information.