The flies grow to three centimeters long. The glow is a chemical reaction between Luciferin, a chemical substance, oxygen and ATP (an energy carrying molecule found in all living creatures). This glow helps the bug to attract food. The brighter they shine depends on how hungry they are.
If you visit the Glowworm Grotto cavern, you have to be very quiet. The bugs are very sensitive to sounds and will stop glowing if they think they will get hurt.