Form & Function

Photinus ignitus can either fly or crawl.  It is an arthropod so the legs are jointed appendages, which allows movement in one direction.  The part that makes fireflies so unique and helps them to stand out among other organisms is their bioluminescenct abdomen.  There is curiosity in how Photinus ignitus obtained its bioluminescence ability.  One idea is that it came from their common ancestor, the cantharoid beetle (Lewis & Cratsley, 2008).  The cantharoid beetle also displays bioluminescence, which Photinus ignitus may have inherited.  The bioluminescence trait began in firefly larvae as a warning to potential predators in the area.  It then evolved in the form of courtship in adult fireflies.  This type of shift in trait function is called exaptation.
                         

Photinus ignitus produce their bioluminescence light by a two step reaction that is sped up by the enzyme luciferase.  This process takes place in the light producing organs called lanterns.  The light producing organs are emitting light in all stages of development of the firefly.  The eggs, larvae, and pupae produce glows that are modulated, but when they are adults they control their bioluminescence to find mates (Lewis & Cratsley, 2008).  Courtship is based on an evolutionary process.  Females look at male signals to determine if they will provide a greater nutritional contribution at mating, which you can read more about in our reproduction page  Male courtship signals are used to determine which male will give more a nutritional gift.  The signals can also determine if the offspring will have desirable phenotypes.  The females determine all this by simply looking at the male light signals (Cratsley, 2004). 

Photinus ignitus has developed a defensive trait by making lucibufagins, a defensive steroid (Eisner et. al., 1978).  When predators eat fireflies the lucibufagins are also ingested, which gives the predator a bitter taste and can be lethal to them if too much is ingested.  Thus, the next time the predator spots a firefly, they will not be tempted to eat it since last time it left a bitter taste in their mouth.  The most accepted conclusion of how fireflies obtained this trait was from a common ancestor, which produced lucibufagins from ingested cholesterol. 

                                   

Photinus ignitus females used to emit pheromonal signals to attract males, but they evolved to use visual signals.  The advantages of using pheromonal signals is that females do not give there position away to predators, but a disadvantage is that it takes males a longer time to locate a female.  With males taking a longer time to find their mate, it gives predators a longer chance to eat the males.  The advantage of using visual signals is that it is easier to find each other; however, a disadvantage is that it is more identifiable in an open field where predators can seek the firefly more easily (Lewis & Cratsley, 2008). 

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