The adult hornet robber fly has a body length of 19-28mm and a wing length of 15-18mm. The life expectancy of Asilus crabroniformis is 2-3 years.
The adult females lays her eggs in, on, around or underneath drying friable soil mixed with livestock dung or just the surface-dry dung of cows, horses and rabbits.
The larvae are associated with surface-dry dung, with only cattle and rabbit dung being chosen (Pinchen et al 1998).The eggs are laid in or beneath this dung . The larvae are thought to be free-living within the soil although the current research is limited
They take approximately 2 years to develop and are thought to feed on dung beetle larvae and other dung and soil-bourne invertebrates. There does appear to be an association with dung beetle larvae, with among others, a report of having found one within of a cluster of larval cells of Geotropes beetles.
The larval stage can last as long as 2-3 years. Pupation has been observed with the larvae forming their own burrow with pupation occurring within smooth lined cells, with adults emerging approximately 1 month later.
The adults emerge from mid-June with the peak being July and adults being seen to late October. Adults fly from late June to Late October when temperatures are about 16oC.