Robber Fly is the commonly used name for a particular family of predatory flies. Individual species vary in size, but all are powerfully built, with strong, stout and bristly legs with which they seize their prey in flight. Not always easy to see, they wait motionless on favourite perches (grassy stems, scrub, saplings) ready to ambush passing insects in a short, fast flying attack.
They often return to the same perch with their prey. They have a short, stout proboscis which protects their sharp, piercing mouth parts, used to puncture the skin of their victims and inject saliva which not only paralyses, but liquefies their tissues, enabling the robber fly to suck out their insides. Their prey can be much larger than themselves and some will attack bumblebees. The largest UK species is the Hornet Robber Fly, which is over an inch long and has black and yellow markings, hence its name.
Robber flies frequent grassy, scrubby places, including gardens, where there is plenty of sun. In between their flying ambushes, they like to soak up the sun’s warmth, keeping their flight and leg muscles primed and ready for the attack.
One of the best places to see them is on Rodborough Common. Take the tracks that climb the south -western facing slopes above Tabernacle Walk and Little London. They often perch right beside the track on the heads of grasses and low saplings.
The larvae, or immature stages, of robber flies are worm-like predators. They live in soil, rotting stumps and logs, and similar moist organic material.
All photos by Stroud Nature.