Insects are a key component of natural biodiversity, providing food for birds, mammals and other invertebrates, and pollinating many natural and cultivated plants. Despite this importance, there is a real lack of long-term data across Britain on the general abundance of most insect orders. An important source of information on aerial insect populations comes from the national network of 12.2 m high suction traps of the Rothamsted Insect Survey, which started operation with the first site in 1965. All aphids have been identified and counted from these samples and, of the 12 longest-running sites, seven show no trends, three increases and two overall declines in total aphid numbers. Recently a small sub-set of stored samples from these traps has been analysed for biomass and added to the aphid biomass to estimate total aerial biomass across all insect orders for four sites in southern Britain. At three of these sites there are no long-term trends, but the total biomass has declined severely at one site, Hereford. A more detailed analysis of the Hereford samples has been done, which indicated that the major component of the biomass decline is in the larger insects, particularly the Diptera (flies). Such declines may have been more widespread before the present sampling system was initiated, and may have had important conservation consequences, particularly for insectivorous birds and mammals.