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Biodiversity experiments have shown that plant diversity has largely positive effects on insect diversity and abundance. However, such relationships have rarely been studied in undisturbed and more complex ecosystems such as forests. Flies (Diptera) are among the most dominant taxa in temperate ecosystems, influencing many ecosystem processes. As it is unknown how Diptera respond to changes in forest biodiversity, we examined how community characteristics of Diptera respond to varying levels of tree and herb diversity and vegetation structure. The study was conducted in the Hainich National Park (Central Germany) on 84 plots along a gradient of tree (from two to nine species) and herb (from two to 28 species) diversity. We found that herb and canopy cover as well as spatial effects were the best predictors of Diptera community composition, consisting of 62 families, including 99 Empidoidea and 78 Phoridae species. Abundance of Empidoidea was positively influenced by herb diversity, indicating bottom-up control. A complex causal pathway influenced Dipteran species richness: species-rich forest stands, with low beech cover, had lower canopy cover, resulting in higher Dipteran species richness. In addition, Diptera benefited from a more dense and diverse herb community. Individual species responded differentially to herb layer diversity, indicating that effects of plant diversity on higher trophic levels depend on species identity. We conclude that tree and herb canopy cover as well as herb diversity predominately shape Dipteran communities in temperate deciduous forests, which is in contrast to expectations from grassland studies exhibiting much closer relationships between plant and insect diversity. [ABSTRACT FROM AUTHOR]