Source : Webster's Revised Unabridged Dictionary (1913)

Glowworm \Glow"worm`\, n. (Zo["o]l.)
   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.

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

   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}.

Source : WordNet®

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