The Collaborative International Dictionary of English v.0.48:

Glowworm \Glow"worm`\, n. (Zool.)
   A coleopterous insect of the genus Lampyris; esp., the
   wingless females and larv[ae] of the two European species
   (L. noctiluca, and L. splendidula), which emit light from
   some of the abdominal segments.
   [1913 Webster]

         Like a glowworm in the night,
         The which hath fire in darkness, none in light. --Shak.
   [1913 Webster]

   Note: The male is winged, and is supposed to be attracted by
         the light of the female. In America, the luminous
         larv[ae] of several species of fireflies and fire
         beetles are called glowworms. Both sexes of these are
         winged when mature. See Firefly.
         [1913 Webster]

WordNet (r) 3.0 (2006):

glowworm
    n 1: the luminous larva or wingless grub-like female of a
         firefly