|MadSci Network: Zoology|
The short answer to your question is: chemicals. Of course, that could be the short answer to just about any question in biology, if you want to get philosophical about it.
Lightning bugs are a special family of beetles; their taxonomic position is reached through the hierarchy like this:
Kingdom: Animalia Phylum: Arthropoda (all the crunchy, jointed-leged invertebrates: insects, crustaceans, spiders, etc.) Class: Insecta Order: Coleoptera (beetles) Family: Lampyridae (lightning bugs)Many insects have the ability to glow or produce light. Insect physiologists believe that light production was probably an accidental side-effect of some regular metabolic process (a chemical reaction in the body) which then acquired significance of its own.
Some insects glow all over, and some only in certain places. Some glow all the time, and some only in bursts or flashes. In any event, the glowing is caused by the oxidation of luciferin by luciferase. In English, a specific chemical (called "luciferin" by early physiologists because it could "ignite," so to speak, to make light... "lucifer," get it?) is chemically changed ("oxidized;" electrons are removed from it resulting in its acquiring a positive charge) by a special enzyme ("luciferase;" an enzyme is a special kind of protein that speeds up chemical reactions). Chemical reactions can be of two basic types, one kind requires energy, and the other kind gives off energy. This luciferase-luciferin reaction is of the "energy-giving-off" variety. By "energy given off" scientists mean to say that some form of energy can be detected. The are more than a few forms of energy: heat is probably the most common kind of energy given off by a chemical reaction, but light (radiant energy) is also seen in many reactions. Incidentally, the luciferase-luciferin reaction also gives of heat energy, but it is negligible (98% of the energy is given off as light).
In lightning bugs, this reaction occurs in special cells on the ventral side (belly) of the posterior (rear end) abdomen (last segment of the insect). The special cells are probably derived from fat cells, and are sandwiched between a reflective layer of cells inside and a transparent cuticle ("skin," sort of) outside. These cells are under the control of the nervous system, which will supply the additional materials necessary to the cells to effect the chemical reaction, stuff like the enzyme itself, oxygen, blood, adrenaline and other compounds. Without some kind of control, the organs would flash all the time or simply stay lit, but you've probably noticed that fireflies only flash at night, dusk being the most popular time.
The cool thing about lightning bugs, of course, is what the flashing is used for; mating. The flashing patterns are different for each species of lightning bug, and through this radiant code, lampyrid love prevails. You can investigate all kinds of "flashing phenomomena" in these critters; some Asian species (only the males, I think) will flash in synchrony en masse, millions of them, creating essentially solid clouds of light that wink on and off in the middle of the night - an amazing thing to see. Some females (they always get a bad rap) use stealth flashing techniques to lure in males of different species - to eat, of course. The ethology (behavior) of these beetles is a lot more interesting than the actual chemistry that makes the flash, but the behavior would never have been there had it not been for the original chemical "accident."
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