Any of about 4,000 species of predatory dipterans in the family Asilidae, found worldwide. Robber flies are the largest of all dipterans; some species are 3 in. (8 cm) long. Most have a dull-coloured, stout body resembling that of a bumblebee and a moustache of bristles between the large-faceted eyes. They use their long legs to capture insects in flight and hold them while eating; a fluid injected into the victim breaks down muscle tissue. A few species are serious pests of apiaries.