The Collaborative International Dictionary of English v.0.48:

armyworm \armyworm\, Army worm \Ar"my worm`\ (Zool.)
   (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 the noctuid moth
       (Pseudaletia unipuncta, formerly 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.

   Syn: army worm
        [1913 Webster]

   2. the larva of the fungus gnat; -- they march in large
      companies in regular order.
      [WordNet 1.5] Arna

The Collaborative International Dictionary of English v.0.48:

Caterpillar \Cat"er*pil`lar\, n. [OE. catyrpel, corrupted fr.
   OF. chatepelouse, or cate pelue, fr. chate, F. chatte,
   she-cat, fem. of chat, L. catus + L. pilosus hairy, or F.
   pelu hairy, fr. L. pilus hair. See Cat, and Pile hair.]
   1. (Zool.) The larval state of a butterfly or any
      lepidopterous insect; sometimes, but less commonly, the
      larval state of other insects, as the sawflies, which are
      also called false caterpillars. The true caterpillars have
      three pairs of true legs, and several pairs of abdominal
      fleshy legs (prolegs) armed with hooks. Some are hairy,
      others naked. They usually feed on leaves, fruit, and
      succulent vegetables, being often very destructive, Many
      of them are popularly called worms, as the cutworm,
      cankerworm, army worm, cotton worm, silkworm.
      [1913 Webster]

   2. (Bot.) A plant of the genus Scorpiurus, with pods
      resembling caterpillars.
      [1913 Webster]

   Caterpillar catcher, or Caterpillar eater (Zool.), a bird
      belonging to the family of Shrikes, which feeds on
      caterpillars. The name is also given to several other
      birds.

   Caterpillar hunter (Zool.), any species of beetles of the
      genus Callosoma and other allied genera of the family
      Carabid[ae] which feed habitually upon caterpillars.
      [1913 Webster]

WordNet (r) 3.0 (2006):

army worm
    n 1: noctuid moth larvae that travel in multitudes destroying
         especially grass and grain [syn: armyworm, army worm,
         Pseudaletia unipuncta]