Glowing caves in New Zealand are a popular tourist attraction. The glowing in the caves is caused by bio-luminescent worms hanging from the cave ceiling. (via)
Glow-worms are in fact not actually worms, but rather the term refers to various forms of bioluminescent insect larvae (and adult females in some cases). This includes fireflies (Lampyridae), beetles (Phengodidae and Rhagophthalmidae) and fungus gnats (Mycetophilidae). The reason for glowing varies between these families. Adult female fireflies and beetles tend to glow to attract a mate. On the other hand, larvae may glow to warn predators of toxicity - thereby lessening their chances of being eaten - as seen in firefly larvae, or they may glow to attract small insects like midges into a sticky web so that they can be consumed, as seen in fungus gnat larvae. The likely source of the glowing in these New Zealand caves are fungus gnat larvae of the Arachnocampa genus, comprising 4 species that are endemic to New Zealand and Australia. For these gnats, the larval period is the longest life stage and involves the spinning of a silk ‘nest’ on which the larva sits, from which is hung up to 70 vertical threads each coated in sticky mucus - which is also poisonous in some species.
Through bioluminescence, the larvae draw insect prey including midges, mayflies, mosquitoes and moths towards their nest - hungry larvae glow brighter than those full from a recent meal! Once prey has become trapped in the sticky threads, it is ‘reeled in’ by the larva sitting at the top of the nest and consumed. I didn’t know anything about these guys until I saw a similar fungus gnat larvae, thought to be Macrocera nobilis, in a cave in Belize last month; though not bioluminescent, their nests were of identical structure. Very cool!