Robber Fly – Holcocephala abdominalis
Family Asilidae – Robber Flies
Live adult robber flies photographed in the wild at DuPage County Illinois. Size = 8mm
This is a small fly as robbers go. Very ethereal and difficult to follow, I have never been able to photograph with prey or witness any hunting.
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.
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.
Reference: Bugguide.net Holcocephala abdominalis
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Tree Encyclopedia / North American Insects & Spiders