Why do Fireflies light up and how?

Fireflies or lightning bugs make light within their bodies. This process is called bioluminescence and is shared by many other organisms, mostly sea-living or marine organisms. Fireflies light up to attract a mate. To do this, the fireflies contain specialized cells in their abdomen that make light.

Light in adult beetles was originally thought to be used for similar warning purposes, but now its primary purpose is thought to be used in mate selection. Fireflies are a classic example of an organism that uses bioluminescence for sexual selection. They have a variety of ways to communicate with mates in courtships: steady glows, flashing, and the use of chemical signals unrelated to photic systems.

The cells contain a chemical called luciferin and make an enzyme called luciferase. To make light, the luciferin combines with oxygen to form an inactive molecule called oxyluciferin. The luciferase speeds up the reaction.

The wavelength of light given off is between 510 and 670 nanometers (pale yellow to reddish green colour). The cells that make the light also have uric acid crystals in them that help to reflect the light away from the abdomen. Finally, the oxygen is supplied to the cells through a tube in the abdomen called the abdominal trachea. It is not known whether the on-off switching of the light is controlled by nerve cells or the oxygen supply.

 

About 2,000 species of firefly are found in temperate and tropical environments. Many are in marshes or in wet, wooded areas where their larvae have abundant sources of food. These larvae emit light and often are called "glowworms", in particular, in Eurasia.
Bioluminescence is used by many creatures.  It is used as a lure to attract prey by several deep sea fish such as the anglerfish below. A dangling appendage that extends from the head of the fish attracts small animals to within striking distance of the fish.

For more information visit:-
https://en.wikipedia.org/wiki/Bioluminescence
http://science.howstuffworks.com/zoology/question554.htm