The Collaborative International Dictionary of English v.0.48:

Hornet \Hor"net\, n. [AS. hyrnet; akin to OHG. hornaz, hornuz,
   G. horniss; perh. akin to E. horn, and named from the sound
   it makes as if blowing the horn; but more prob. akin to D.
   horzel, Lith. szirszone, L. crabo.] (Zool.)
   A large, strong wasp. The European species (Vespa crabro)
   is of a dark brown and yellow color. It is very pugnacious,
   and its sting is very severe. Its nest is constructed of a
   paperlike material, and the layers of comb are hung together
   by columns. The American white-faced hornet (Vespa
   maculata) is larger and has similar habits.
   [1913 Webster]

   Hornet fly (Zool.), any dipterous insect of the genus
      Asilus, and allied genera, of which there are numerous
      species. They are large and fierce flies which capture
      bees and other insects, often larger than themselves, and
      suck their blood. Called also hawk fly, robber fly.

   To stir up a hornet's nest, to provoke the attack of a
      swarm of spiteful enemies or spirited critics. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Robber \Rob"ber\, n.
   One who robs; in law, one who feloniously takes goods or
   money from the person of another by violence or by putting
   him in fear.
   [1913 Webster]

         Some roving robber calling to his fellows. --Milton.
   [1913 Webster]

   Syn: Thief; depredator; despoiler; plunderer; pillager;
        rifler; brigang; freebooter; pirate. See Thief.
        [1913 Webster]

   Robber crab. (Zool.)
   (a) A purse crab.
   (b) Any hermit crab.

   Robber fly. (Zool.) Same as Hornet fly, under Hornet.


   Robber gull (Zool.), a jager gull.
      [1913 Webster]

WordNet (r) 3.0 (2006):

robber fly
    n 1: swift predatory fly having a strong body like a bee with
         the proboscis hardened for sucking juices of other insects
         captured on the wing [syn: robber fly, bee killer]