1. Recently documented insect declines have caused major concerns and an increased interest in studies using long-term population-monitoring data.
2. Samples from a 12.2-m suction trap were used to examine trends in phenology and abundance of Diptera over four decades.
3. The timing of peak flight has advanced by an average of 17 days, from 23 July in 1974 to 6 July in 2014.
4. The abundance of flies has decreased by 37% over the studied period (from April to September), and peak abundance has decreased by 48%. The flight period has started earlier in recent years, and in 2014, the number of flies was higher in spring until the 31st of May than in 1974. Possible causes and impacts of these changes are discussed.