What makes a glow-worm glow?

The female glow-worm is equipped with one of the most marvelous lighting systems in the world. A wingless beetle, she crawls about a night eating small insects. But on the lower side of her abdomen she possesses a “lantern” which she uses to signal to her winged mate flying above.

This “lantern” has a transparent layer of skin, like the lens of a lamp. Behind this is an oily layer of tissue which produces the light by a chemical process, and a second layer which acts as a reflector.

The glow-worm is able to control this remarkably bright light, using it only at certain times to attract a mate. In fact, the light is a sex-call, and the male has particularly large eyes to enable him to see the signal. An abundant supply of water and oxygen is needed by the glow-worm to maintain the chemical activity producing the light. For a time, even the insect’s eggs are luminous.

Glow-worms, which are about half an inch long, are natives of Europe. Other beetles with built-in lighting systems are called fireflies. Both male and female fireflies have wings and use lanterns to signal to each other and to warm off night birds who seem to find them unpalatable. The most famous are the large and brilliant cucujos of tropical America. On special occasions young women fasten them to their dresses where they shine like glowing gems.