Whilst not a particularly large insect the kite-tailed robber fly (Machimus artricapillus) is certainly a fearsome creature as far as other insects are concerned. It is quite capable of taking flies as big as some species of hoverfly which are actually larger than itself. They do so by lying in wait on plant leaves or tree branches and pounce when a suitable prey item come by. They have a lot of hairs which protects them against struggling prey.
The larvae are found in leaf litter and are omnivorous eating both rotting vegetation and taking live prey. The adults can be seen from May to August but are most numerous later in the summer when they often form large mating swarms. A locally common species in Dorset you can see them in places where the soil is sandy so heathland woodland is a good spot.
They can give a nasty bite if handled but do not bite humans unless provoked.