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

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "glowworm":
   blink, blinking, candle fly, coruscation, fire beetle, firefly,
   fireworm, glimmer, glimmering, glisten, glister, glitter,
   glittering, lampyrid, lantern fly, lightning bug, luciferase,
   luciferin, luminophor, phosphor, scintilla, scintillation, shimmer,
   shimmering, spangle, spark, sparkle, stroboscopic light, tinsel,
   twinkle, twinkling