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.
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.
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.
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.
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.