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: Webster's Revised Unabridged Dictionary (1913)