Forest Preserve District of Cook County, Illinois
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Robber Flies
Nature Bulletin No. 428-A   October 9, 1971
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Not long ago, we noticed a very large vicious-appearing fly crouched on the ground. Its streamlined body, and the way it crouched there, reminded us of one of those jet fighter planes that guard our great cities. As we approached, it darted away with an angry buzz. That was a Robber Fly. It has been called a bearded bandit, a bloodthirsty pirate, an assassin -- and it is, in the insect world -- but the robber fly is a friend of mankind.

The fly family is one of the four largest in the insect kingdom and, so far, over 80,000 species have been described. They differ from most other insects in that the adults have but a single pair of wings. Butterflies, dragonflies and other "flies" which have two pairs of wings are wrongly named.

The family has a bad reputation. It includes most of the insects -- such as the house fly, mosquitoes, and the tsetse fly -- that transmit human diseases. It includes the stable flies, horse flies, deer flies, gnats such as the "blackflies", and the midges which Indians called "No-see-ums", that bite and suck the blood of man and domesticated or wild animals. Bot flies and warble flies are serious parasites on livestock. The Hessian fly has made it impossible to successfully grow wheat in many regions. The Mediterranean fruit fly at one time threatened to wipe out the citrus industry in Florida and, in the Rio Grande valley of Texas, the Mexican fruit fly causes great damage to apples, cherries and blueberries.

On the other hand, the fly family includes many kinds which are extremely useful as scavengers -- house flies, blow flies, flesh flies and vinegar flies are some of them -- and others valuable as parasites or as predators upon noxious insects. Among the predators are the Robber Flies, of which there are several hundred kinds in North America. They do not attack man. Their victims are other flying insects, captured on the wing, or small spiders. Many kinds will kill and feed on almost any insect; some species will attack only certain types such as butterflies and moths, or slow-flying beetles, or mosquitoes.

Robber flies are active sun-loving insects that prefer dry open areas: fields, pastures, sandy places, openings or roads in woodlands, and bushy country. Most of them are slender and streamlined, with a long tapering abdomen, but one species is as big, bulky and hairy as a queen bumblebee. Some are an inch and a half or more in length; others are smaller and one kind is less than a half-inch long. The head is very large with two great eyes, each containing several thousand separate lenses, that provide remarkable vision enabling the robber fly to spot a small insect at a considerable distance.

Underneath its bushy beard, the robber fly has a stout beak enclosing a dagger-like shaft used to stab the victim in the head or thorax and inject a fluid which kills it. This fluid, apparently, soon causes the victim's "insides" to become liquid and the robber fly, on some favorite perch, then proceeds to suck it dry, leaving nothing but an empty shell.

The larvae or maggots live in soil or rotting wood, preying upon other larvae and preyed upon by their own enemies. A man who has studied robber flies for 40 years reports that the adults are eaten by ground spiders which attack them at night when they are sleeping. "Dog eat dog" is the rule in nature.

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