balancer

from WordNet (r) 3.0 (2006)

balancer
    n 1: an acrobat who balances himself in difficult positions
    2: either of the rudimentary hind wings of dipterous insects;
       used for maintaining equilibrium during flight [syn:
       {halter}, {haltere}, {balancer}]

from The Collaborative International Dictionary of English v.0.48

Balancer \Bal"an*cer\, n.
   1. One who balances, or uses a balance.
      [1913 Webster]

   2. (Zool.) In Diptera, the rudimentary posterior wing.
      [1913 Webster]

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