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Science

Glowing Strong
by Carla Jansen

Travel to New Zealand, put on a wet-suit, and then go cave exploring. The caves are cool, wet, slippery, and dark—very dark. The air is still; there is little or no wind. This is the unique home for glow-worms.

Glow-worms really do glow! You can see them sparkle on the ceilings of caves as if they were stars. They range from 1/10 of an inch to 1 ½ inches long, and have a special organ on their tail that lights up to attract food.

How does the glow-worm eat? It is actually a vicious predator that sets traps for its food. Before it can lay its traps, the glow-worm spins a nest with a hammock for itself. It attaches the nest to the ceiling of a cave. Both the nest and the hammock are made from silk that comes out of the glow-worm's mouth. Finally, the glow-worm makes long threads of silk with sticky beads that hang down from its hammock. Then the glow-worm turns on its light. Insects see the light and fly towards it. The sticky silk threads trap flies and other cave bugs just like a spider's web catches insects. When a victim has been caught, the glow-worm pulls up the thread and kills the insect with a bite. The glow-worm may save part of the insect to eat later, or it may eat it all right away.

Why is the cave so important? The sticky silk lines can be 19 to 20 inches long and are usually very close to each other. If there is very much wind they would get tangled up. Fortunately, when there are small tangles, the glow-worm can cut the line and replace it. The darkness is also important. If it weren't so dark, flies would not head straight for the glow-worms' light, and the glow-worms would not have any food. They also like the temperature and moisture in a cave.

How long does a glow-worm live? A glow-worm is not really a worm at all. It changes by metamorphosis into a fly, just like a caterpillar changes into a butterfly. After the adult flies mate, the female lays about 100 eggs. A few weeks later, glow-worms crawl out of the eggs. The glow-worm eats all it can for at least six months. Then it covers itself with silk for a couple weeks. An adult fly emerges and lives for a few days—just long enough to mate and lay eggs. The grown adult flies do not eat anything, but they can glow anyways!

Glow-worms are common in many New Zealand and Australian caves and sometimes in other dark places where the air is still and the ground is moist. Now that you've traveled to their home, you're ready to return to yours and look for similar animals. A non-glowing larva in the United States (not yet named) also uses sticky strings to catch food, but has no light to lure the insects with. In addition, little beetles in the United States called fireflies use a different kind of light to find their mates. And lastly, some deep-sea fish use light to attract smaller fish for them to eat—much like the glow-worms use light to attract their food!

References:
Gibbs, George. Glow-worms. New Zealand: Reed Publishing, 2003.

Internet:
http://www.doc.govt.nz/upload/documents/parks-and-recreation/places-to-visit/southland/clifden-caves-brochure.pdf

http://www.ento.org.nz/nzentomologist/free_issues/NZEnto07_4_1983/Volume%207-4-419-424.pdf

http://soilbugs.massey.ac.nz/insecta.php

http://www.purdue.edu/UNS/html4ever/1998/980626.Turpin.fireflies.html

http://www.mbayaq.org/efc/living_species/default.asp?inhab=176

Photo:
Courtesy Wikipedia

Carla Mae Jansen has a BS in Biology Composite Teaching. She taught in Nebo School District, Utah for two years before having a baby. Prior to that she assisted teaching Biology at Brigham Young University and taught English in China. Carla enjoys her family, teaching, writing, exploring, and trying new things. She plans to resume teaching science this fall in Texas.