NZ glowworms' secret fishing trick: Pee

Urine is the primary ingredient in Waitomo Caves' long threads, which are covered with sticky droplets made by the glowworm larvae to trap insects attracted by their glowing light. Photo / File

An international team of scientists have found an odd - and slightly gross - explanation for the sticky, glowworm-made "fishing lines" that hang in the famous Waitomo Caves.

In a study published today, the scientists report that urine is the primary ingredient in the long threads, which are covered with sticky droplets made by the glowworm larvae to trap insects that are attracted by their glowing light.

Using electron microscopy and chemical identification methods to analyse glowworms in New Zealand caves, the researchers found what that although the threads look very similar to spider webs, they are made of very different chemicals.

The droplets contain molecules that absorb water from the air, keeping the threads sticky, and are mostly made of urea - the main nitrogen-containing substance in urine.

The researchers think the difference between spiderwebs and glowworm threads exists because glowworms can only live in humid habitats such as caves and forests.

Spiders' webs may be the best-known sticky substances produced by animals, but other species use adhesive secretions in many and varied ways.

Larvae of the glowworm group, Arachnocampa, have long been known to secrete threads spaced with adhesive droplets - but the properties of these features had previously not been characterised in detail.

The authors hope that further research will examine how the habitats of different glowworm species shape the fishing lines and adhesive droplets that they produce.

"The adhesive threads of the world-renowned glowworm from New Zealand display a completely different prey capture system to those found in spiders or other glue-producing animals," said study lead author Janek von Byern, of Austria's Ludwig Boltzmann Institute for Experimental and Clinical Traumatology.

"These bioadhesives display a unique composition containing mainly water, hygroscopic salts, and to a very low extent also biomolecules as proteins and lipids."

The study was published in open-access journal Plos One.