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 fireflies, or lightning bugs if you prefer, the ephemeral light is generally caused by 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, they spend a good portion of their lives underground as larvae feeding on soft bodied invertebrates, such as slugs, snails and earthworms. Even at the larval stage, the glowing abdomen can be found in some species if they are flipped over. In this instance, the firefly is making use of a secondary purpose of glowing, known as an aposematic signal. This is a warning to predators 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 colours 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 specialised cells known as photocytes, a chemical reaction occurs that produces light. In the case of fireflies (as there are many ways to produce light), they begin with a fluorescent substrate known as luciferin. While luciferin produces some light when oxidised, fireflies use an enzyme known as luciferase to speed up the reaction. Pure luciferin only produces one wavelength of light, but variations in the shape of the luciferase enzyme in different species allows for the production of different wavelengths, such as those responsible for yellow, green and even red light.
The reaction’s use of oxygen is key to the firefly’s control of 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 neighbouring cells to start producing nitrous oxide, which is then absorbed by the adjacent photocytes. The photocytes’ mitochondria (the cells’ energy production centres) are arranged along the outsides, so that oxygen is used up by them before it can react with the luciferin produced within the cell. Once nitrous oxide starts being absorbed, the mitochondria metabolise this gas instead, allowing 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, interrupting 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 possible path it can to ensure that an individual’s genes will be passed on. The flashing is used as a sign of fitness: the longer and brighter the flash, the better 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 colour, 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 of 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 incompatible males. Once this has been achieved, the female Photuris will eat the unsuspecting male so that it can gain the toxins it produces. It seems Photuris has found a more efficient way of becoming inedible than just producing the chemicals herself.
Image by NEUROtiker