From WoWWiki

The Silithid Lights can be found in every hive or underground cave of the Silithids. They are able to produce light to iluminate the dark caves.

These Silithid Lights could be thought of as larvae of the Silithid considering these creatures are insect-like in their being. Just as a firefly produces a sort of glow, these youngling Silithid may be the primary source of light in their deep caverns.