Biolumination, the creation of light by living things, is an evolutionary strategy that has formed multiple times for different reasons. But in the case of the fireflies, or lightning bugs if you prefer (despite them not being members of the true bugs, but that was last post), the cause for all the ephemeral lights generally comes down to one thing, sex.
Members of the family Lampyridea, or as they are more commonly known, fireflies, are a group of beetles that have developed the ability to use light for communication. Being beetles, a good portion of their lives are spent underground as larvae feeding off of soft bodied invertebrates such as slugs, snails and earthworms. Even at the larval stage, the glowing abdomen can be found in some species if flipped on their back. In this instance, the firefly is taking use of a secondary purpose behind glowing known as an aposematic signal. This is a warning to all those who find it that, due to chemicals unrelated to illumination, the creature possesses a terrible taste. In this case, the use of light is quite similar to the bright colors of certain poisonous frogs.
The production of light, in both the larvae and the adults, occurs in the abdomen. Here, on the underside of the posterior abdominal segments, within specialized cells known as photocyes, a chemical reaction occurs that produces the light. In the case of fireflies (as there are many ways to produce light), they begin with a florescent substrate known as Luciferin. While Luciferin can produce some light when oxidized, fireflies use an enzyme known as Luciferase to speed up the reaction. While Luciferin on its own only produces one wavelength of light, changes to the shape of the Luciferase enzyme in different species allows for the production of the wavelengths of yellow, green and even red light.
The reactions use of oxygen is key to the fireflies control over its light. While the nervous system does not directly contact the photocytes, it does connect to nearby cells. When the firefly wants to light up, it sends a signal to these neighboring cells to start producing nitrous oxide which is then absorbed by the adjacent photocytes. The photocytes have arranged their mitochondria (the source of energy for a cell) along the outside so that oxygen is used up by them before it can react with the Luciferin produced within the cell. But once nitrous oxide starts to be absorbed, the mitochondria start to metabolize using this gas instead, allowing for oxygen to diffuse deeper into the cell where it can finally be used in the reaction to produce light. As long as the supportive cells produce nitrous oxide, enough oxygen will diffuse throughout the cells for the firefly to glow. But once the production of nitrous oxide stops, the mitochondria go back to using oxygen which interrupts the oxygen flow, preventing the light producing reaction. In this round about manner, the firefly can appear to flick on and off.
All of these complex pathways have evolved for the sake of sex. For when it comes to reproduction, evolution will take any and all possible paths it can so that the individual’s genes may be passed on. The flashing is used as a sign of fitness, the longer and brighter the flash, the better suited the mate. Many females, who often do not develop wings and may even stay in a permanent larval form, use their own light to signal to the males whether they are interested or not.
The color, frequency and duration of the flashes are unique to each species. This prevents an individual from trying to attract a mate only to find out that it is the wrong species. However, one genus has taken advantage of this distinction. Females of the genus Photuris will mimic the flashes of the females of members of a different genus, Photinus, to lure in incompatible males. Once lured in, the female Photuris will eat the unsuspecting male so that it might gain the toxins it produces. It seems Photuris has found a more efficient way of becoming inedible than just producing the chemicals herself.
Thank you JuneBug for the request!