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
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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Update: June 2012