Robber Fly – Efferia aestuans

Robber Fly – Efferia aestuans
Family Asilidae – Robber Flies / Efferia group – Nerax aestuans
Live adult female robber fly photographed in the wild at Winfield, Illinois, USA.

Robber Fly - Efferia aestuans
Flies Index | Flies Main | Tachinidae | Syrphidae | Bombyliidae | Asilidae

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.
Robber Fly - Efferia aestuans

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.

Robber Fly with Millimeter Scale

Live female adult fly photographed at Winfield, DuPage County Illinois, July 13, 2005. Size: 24mm. This fly must have been ailing – it did not want to fly for some reason, although it could and ultimately did, away. I placed it on various surfaces including my finger and a white sheet of paper. This robber is covered with white dust from the limestone path it was sitting on when I found it. The sword-like ovipositor (egg-laying device) at the tip of the abdomen is indicative of female gender.

Robber Habitat: Pastures, open fields, forest clearings. / Range: North America east of the Rocky Mountains / Food: Adults suck body juices from small flying insects, larvae prey on insect larvae, especially beetles. Adults take their prey on the wing, in mid-air, much like dragonflies and damselflies. Life Cycle: Female lays eggs in holes in the soil. Larvae tunnel downward in search of prey, pupate in soil close to the surface.

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.

1.  Fritz Geller-Grimm, "Asilidae"
Flies of North America – Order Diptera. Flies are prevalent in virtually all habitats, with over 16,000 species in North America. Flies can be distinguished from all other insects in that they only have one pair of normal wings. The other pair has evolved into small ball-like structures called halteres. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids.
Syrphidae | Flies Index | Tachinidae | Bee Flies | Robber Flies