Robber fly (July 19, 2013)

The insect of the week is the Robber Fly, also known as the assassin fly.

Guest post by Sydney Glass, Integrated Pest Management Intern.


The robber fly is in the order of true flies, Diptera, in the family Asilidae, and the name of the robber fly actually is in reference to its aggressive feeding behavior. The flies are rather large, being anywhere from about ½ inch up to a little over 1 inch. The robber fly is grey-black with a large hairy body and a long abdomen similar to a dragonfly or damselfly. The robber fly has very long and powerful legs with which it catches its prey with in mid-flight. The fly also has an interesting eye structure with a hollow spot between the eyes, and the robber fly is called “bearded” because of the several long hairs around the mouth.


Robber flies are found on all continents but Antarctica. However, the preferred habitat is one that is open, sunny, and mostly dry. The robber fly feeds on beetles, moths and butterflies, grasshoppers, wasps and bees, and other flies.


The mouth of the robber fly is a very stout straw for piercing-sucking. The robber fly grabs its prey, stabs the insect with its mouth, “injects” enzymes into the insect causing paralysis and digestion of the insides of the insect, and the robber fly carries the prey off to drink him up. Robber flies are beneficial insects but if handled roughly they will bite you, very painful but not harmful.

There is a good little page from Texas A&M Extension on the Robber Fly.

Robber fly lunching on a southern cabbageworm moth. Photo Credit: Patrick Porter.

Robber fly eating a honeybee. Photo Credit: Patrick Porter.