Source : Webster's Revised Unabridged Dictionary (1913)

Army worm \Ar"my worm`\ (Zo["o]l.)
      (a) A lepidopterous insect, which in the larval state
          often travels in great multitudes from field to field,
          destroying grass, grain, and other crops. The common
          army worm of the northern United States is {Leucania
          unipuncta}. The name is often applied to other related
          species, as the cotton worm.
      (b) The larva of a small two-winged fly ({Sciara}), which
          marches in large companies, in regular order. See
          {Cotton worm}, under {Cotton}.

Source : WordNet®

army worm
     n : noctuid moth larvae that travel in multitudes destroying
         especially grass and grain [syn: {armyworm}, {Pseudaletia
         unipuncta}]