Essex Biodiversity Project
Identification - The Hornet Robberfly (Asilus crabroniformis) is a very large and distinct predatory fly (Diptera - true flies) and is one of the biggest in Britain. They are very bristly, have sturdy legs, a deep groove between the eye and a 'beard' to protect the eyes. (Chinery 1986) It is a nationally notable species that is much declined in south-eastern England, and is currently only known in Essex from several cattle or horse grazed sites near the Thames. (Photo copyright Peter Harvey)
General ecology - It is found in unimproved or old grassland and heath, with a long continuity of grazing. The fly breeds on cow pats and other dung and the larvae (maggots) are thought to feed on dung beetle larvae. The adults are found from late July to early October and catch a variety of insects in mid-air and suck them dry. They are usually found resting on cow or horse pats and when disturbed they tend to fly a short distance to a nearby pat.
Surveying tips - We need your help to find colonies of this spectacular fly – have you seen it?
Although the Hornet Robberfly is large and distinctive, the adults spend much of their time sitting on cow or horse pats,where they are quite well camouflaged.
Adults are found mainly between late July and early September.They often only fly when disturbed and then only go short distances to another dung pat. They prey on other insects such as grasshoppers and flies, and never feed on flowers, so they are actually easy to miss unless you disturb them as you walk.
The Hornet Robberfly does not bite and is completely harmless to people and animals.
If you think you have seen this fly, please complete and return the survey card (a photograph would be a great help). Thank you.
Download the Hornet Robberfly survey information leaflet (PDF 343KB)
Download the Hornet Robberfly press release (PDF 8KB)