The Collaborative International Dictionary of English v.0.48:

Hessian \Hes"sian\, a.
   Of or relating to Hesse, in Germany, or to the Hessians.
   [1913 Webster]

   Hessian boots, or Hessians, boot of a kind worn in
      England, in the early part of the nineteenth century,
      tasseled in front. --Thackeray.

   Hessian cloth, or Hessians, a coarse hempen cloth for
      sacking.

   Hessian crucible. See under Crucible.

   Hessian fly (Zool.), a small dipterous fly or midge
      (Cecidomyia destructor). Its larv[ae] live between the
      base of the lower leaves and the stalk of wheat, and are
      very destructive to young wheat; -- so called from the
      erroneous idea that it was brought into America by the
      Hessian troops, during the Revolution.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cecidomyia \Cec`i*do*my"i*a\
   (s[e^]s`[i^]*d[-o]*m[imac]"[i^]*[.a]), prop. n. [Nl., fr. Gr.
   khki`s, khki^dos, a gall nut + myi^a a fly.] (Zool.)
   A genus of small dipterous files, including several very
   injurious species, as the Hessian fly. See Hessian fly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Hessian fly
    n 1: small fly whose larvae damage wheat and other grains [syn:
         Hessian fly, Mayetiola destructor]