[1] : The Collaborative International Dictionary of English v.0.48

ichneumon \ichneu"mon\, n. l., fr. gr. ?, lit., the tracker;
   so called because it hunts out the eggs of the crocodile, fr.
   ? to track or hunt after, fr. 'i`chnos track, footstep.
   1. zool. any carnivorous mammal of the genus herpestes,
      and family viverridae. numerous species are found in
      asia and africa. the egyptian species herpestes
      ichneumon, which ranges to spain and palestine, is noted
      for destroying the eggs and young of the crocodile as well
      as various snakes and lizards, and hence was considered
      sacred by the ancient egyptians. the common species of
      india herpestes griseus, known as the mongoose, has
      similar habits and is often domesticated. it is noted for
      killing the cobra.
      1913 webster

   2. zool. any hymenopterous insect of the family
      ichneumonidae, of which several thousand species are
      known, belonging to numerous genera.
      1913 webster

   note: the female deposits her eggs upon, or in, the bodies of
         other insects, such as caterpillars, plant lice, etc.
         the larva lives upon the internal tissues of the insect
         in which it is parasitic, and finally kills it. hence,
         many of the species are beneficial to agriculture by
         destroying noxious insects.
         1913 webster

   ichneumon fly. see ichneumon, 2.
      1913 webster
see also:
herpestes viverridae herpestes ichneumon herpestes griseus mongoose ichneumonidae 
ichneumon fly ichneumon 

[2] : WordNet (r) 2.0

ichneumon fly
     n : hymenopterous insect that resembles a wasp and whose larvae
         are parasitic on caterpillars and other insect larvae

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