Source : Webster's Revised Unabridged Dictionary (1913)

Ichneumon \Ich*neu"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. ? track, footstep.]
   1. (Zo["o]l.) Any carnivorous mammal of the genus
      {Herpestes}, and family {Viverrid[ae]}. Numerous species
      are found in Asia and Africa. The Egyptian species({H.
      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 ({H. griseus}), known as the mongoose, has similar
      habits and is often domesticated. It is noted for killing
      the cobra.

   2. (Zo["o]l.) Any hymenopterous insect of the family
      {Ichneumonid[ae]}, of which several thousand species are
      known, belonging to numerous genera.

   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.

   {Ichneumon fly}. See {Ichneumon}, 2.