Robber flies grow a bristly moustache to shield their faces from struggling prey
Every one of the 7,000-plus robber fly species is a predator. They perch at lookout points, launch at passing insects, grab them in midair and tackle dragonflies, grasshoppers and stinging wasps. A thick moustache of stiff bristles on the face, called the mystax, may help guard the head while their victims fight back.
Robber flies, also known as assassin flies, form the family Asilidae within the true flies. Latreille established the family in 1802, and Alpheus Packard coined the English name in 1869 from the German for predatory flies. They are found worldwide. Adults range from 3 millimetres to more than 5 centimetres long, usually elongated with tapering abdomens, though genera such as Laphria are plump bumblebee mimics. Many wear warning colours imitating wasps and bees.
Their heads carry large compound eyes with three simple eyes set in a distinctive hollow between them, a feature that marks the family. Strong, spiny legs seize prey, and narrow wings allow fast flight. Most hunt in the hottest hours in open sunshine, relying on eyesight to spot targets, and shelter in dense vegetation at night. Handled carelessly, they can give a person an intensely painful bite.
The kill is chemical. The fly drives its short, hardened mouthparts into a weak point such as an eye, the neck or a joint between body segments. Its saliva carries neurotoxins that paralyse quickly and enzymes that break down internal tissue, which the fly then drinks. Some species pierce even the tough shells of beetles, apparently undeterred by the defensive chemicals many prey carry.
Females lay whitish eggs on low plants or in crevices, often coating the masses in chalky protection. Larvae live in soil or rotting wood and pass through four stages before pupating, a cycle that takes one to three years. Their diet is debated: in one species studied in 1983, first-stage larvae did not eat insects, the second fed on beetle larva secretions, and the last two hunted outright. With so many species all living as predators, the family helps keep insect communities in balance.
Source: Asilidae
