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.
Robber Fly - Holcocephala calva
Holcocephala calva

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.

Robber Fly - Laphria thoracica
Laphria thoracica, a superb bumblebee mimic


  Laphria grossa with leaf beetle prey
Laphria thoracica

Laphria flavicollis

Laphria with weevil prey
Adult robber flies attack other flies, beetles, butterflies and moths, various bees, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders. Courtship behavior consists of the male glomming onto the female as if she were prey. Copulation is accomplished tail-to-tail, with the genitalia interlocked. The duo has no trouble flying around in this condition; the male generally towing the female backwards. (below)


Figure 4. Robber flies mating - Machimus sp.


Laphria with flower beetle prey


Dioctria hyalipennis


Laphria canis


Cerotainia albipilosa


Holcocephala fusca
 

Robber Fly - Leptogaster sp.
Robber Fly - Leptogaster sp. This is a very slender, small, secretive fly.

Robber Fly - H. abdominalis
Holcocephala abdominalis -- All flies in the genus are less than 10mm long
 

Promachus rufipes with housefly prey
Proboscis is uniquely adapted for stabbing and sucking

 

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