Glow-worm
Lore
Glow-worms are a breed of Crawlers. These slithering invertebrates have bright, luminescent skin and the ability to spew poison at their enemies. They will often congregate around a massive Glow-worm Matriarch that serves as the Queen-Mother of the colony. Elvish lore speaks of great Glow-worms that lived along Olórë Mallë, the Path of Dreams that were said to connect the mortal world with the Blessed Realm of Valinor. Whether the Glow-worms of Middle-earth have any kinship with those creatures is unknown. However, they have a far more aggressive temperament than the Glow-worms of legend.
Location
Glow-worms are found slithering around The Water-works and the Silvertine Lodes in Moria and in the Hall of Lights in Mirkwood. They have also been spotted in Gundabad, particularly the areas of Máttugard and Gloomingtarn. The Pit Worms in the Pit of Iron in the Gap of Rohan are certainly related.
Background
The great glow-worms of Olórë Mallë are mentioned in The Book of Lost Tales, Tolkien's earliest writings about Middle-earth. Their status in Tolkien canon is a matter of debate since while they do not physically appear in his other works, they are indirectly referenced in chapter VIII of The Two Towers wherein Frodo Baggins and Samwise Gamgee describe the Morgul-road as glimmering "like a glow-worm thread" in the Morgul Vale. Along with the real-life glowworms of our world, they are possibly an inspiration for the Glow-worms of The Lord of the Rings Online.