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Robber Flies - Family Asilidae Taxonomic Heirarchy:
Kingdom Animalia / Phylum Arthropoda / Subphylum Hexapoda / Class
Insecta / Subclass Pterygota /
Infraclass Neoptera / Order Diptera / Suborder Brachycera / Infraorder Muscomorpha / Family Asilidae -- robber flies
Live adult robber flies photographed in the wild at Winfield, Illinois,
USA and other North American locations. |
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There are over 7,000 species of
robber flies world wide; nearly 1,000 in North America. All robber
flies have stout, spiny legs, a dense moustache of bristles on the face
(mystax), and 3 simple eyes (ocelli) in a characteristic depression
between their two large compound eyes. The mystax helps protect the head
and face when the fly encounters prey bent on defense. The antennae are
short, 3-segmented, sometimes with a bristle-like structure called an
arista.
The short, strong proboscis is used to stab and inject victims
with saliva containing neurotoxic and proteolytic enzymes which paralyze
and digest the insides; the fly then sucks the liquefied meal much like
we vacuum up an ice cream soda through a straw. Many species have long,
tapering abdomens, sometimes with a sword-like ovipositor. Others are
fat-bodied bumble bee mimics; the effect is quite convincing. Take a
close look at any insect that looks like a bumble bee if it's sitting on
a leaf - chances are, it's a robber fly. (Bumble bees as a rule do not
sit in one spot for more than a few seconds). |

Tolmerus sp. with moth prey |

Efferia aestuans |

Promachus sp. |

Promachus rufipes |
| Much has been made of the speed
and agility of these flies. Many books and sites cite them as fast and
agile flyers, taking insects on the wing. Others attribute to them
"still-hunting", that is, perching and attacking in mid-air; My National
Audubon Society Field Guide to Insects asserts they "pounce on resting
insects from above." Well, from my experience, I can say not all
robber flies are fast, and not all robber flies are agile. The big
robbers (Promachus) lumber and buzz in flight - it is very easy to
follow them, and I've often seen them pounce on honeybees busy at
flowers.
I've seen some smaller species (Tolmerus, Laphria canis) perch
and attack - many times missing even slow-flying moths. So much for
agility. Then there are the fast, agile ones - good luck seeing them do
anything but disappear. Whatever the species, robbers are fun to watch.
I consider myself lucky to see a capture: most often I find the fly
enjoying a meal, or flying off with prey firmly impaled on that terrible
beak. |

Laphria thoracica, a superb bumblebee mimic

Promachus rufipes with housefly prey |

Proboscis is uniquely adapted for
stabbing and sucking |
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