Dismals Canyon - a famous habitat for fungus gnat larvae - as viewed during the day
Sometime around 1940, entomologist B.B. Fulton was in the midst of spending a long summer at his cabin in the mountainous western section of North Carolina. One hot evening, Fulton picked his way down a steep embankment to a nearby springhead behind the cabin to gather water. Stumbling in the dark, Fulton noticed something odd: the entire streambank was glowing with pinpoints of bright blue light.
The source of the strange blue glow – as Fulton would later discover – was the fungus gnat (Orfelia fultoni), a small insect almost invisible to the naked eye. Fungus gnats are fairly unremarkable as adults, appearing as a small flying insect not much bigger than a fruit fly. As immature larvae, however, fungus gnats have the unique ability of emitting what scientists have come to discover as the world’s brightest blue hue given off by an organism.
The fungus gnat’s incredible light show results from a "cocktail" of special proteins concentrated inside both ends of the larvae. These proteins are present in most bioluminescent (or glowing) organisms and are composed of two proteins, in particular: a luciferin protein that serves to produce the actual light, and an accompanying enzyme called luciferase that serves to catalyze - or facilitate - the light-producing reaction. Although the exact mechanism that triggers this reaction may differ across various species, this luciferin-luciferase complex produces light when activated, not all that unlike the reaction that occurs when a novelty glowstick is snapped to activate its light.
Interestingly, biologists studying O. fultoni's luciferin-luciferase complex have found that this species has a unique system for producing light; that is, the "trigger" that serves to cause a reaction between these two proteins is different than that known in any other light-producing organism. And beyond being a beautiful light show, the fungus gnat's blue lanterns serve a more practical purpose: capturing food. Biologists have specifically found this light show serves as a lure for other insect species. The gnat larvae's prey are attracted to the light, move toward it, and become trapped in a small web-like structure built by the larvae, where they become a meal. (For some amazing photos of the larvae at work, go here.)
Since their original discovery and formal scientific naming by Fulton in 1941, the fungus gnat has been documented from many sites across the southern Appalachians. Some of the most well-known sites include the western North Carolina mountains (near the site of Fulton's initial discovery), as well as Tennessee's Hazard Cave and northwest Alabama's Dismal's Canyon. One of the only other glow-worms known on the planet lives in cave systems within New Zealand - a whole world away from the streambanks and bluffs of the southern mountains. Even this New Zealand species, though, has been found to have an entirely different mechanism for creating its glow than O. fultoni.
A (small) photo showing the fungus gnat larvae at work in Dismal's Canyon, Alabama (photo in public domain).
So, why does the fungus gnat’s glow matter to evolutionary biology? The reason lies in the benefit that the larvae’s lure provides to the organism, specifically as an adaptation that may have arisen through natural selection. In his writings on evolution and natural selection in 1859, Charles Darwin produced several postulates that must be satisfied in order for natural selection to occur. This rough "checklist" for natural selection includes the following:
- Individuals within populations are variable
- Variation is heritable and can be passed to offspring
- In every generation, some individuals are more successful at surviving and reproducing than others
- The survival and reproduction of individuals is not random
In a very basic nutshell, natural selection occurs due to the development of heritable traits that allow some individuals to survive and reproduce better than others. When this occurs, the trait(s) that allow for this improved capability to reproduce get passed onto the next generation and become more common, while less-successful variations of those traits decline. In the fungus gnat's case, the larvae's blue lanterns would serve as one of these beneficial traits if and only if they somehow allow for improved survival and reproduction.
So how can we tell if natural selection may have influenced the development and persistence of the fungus gnat's glow? While we don't know for certain how or for what early purpose the genes coding for the gnat's luciferin and luciferase proteins originally arose (hypotheses suggest either directly for prey attraction or as an earlier signal of potential harm to predators), we can explicitly test to see if the lights produced by these proteins do, in fact, give the worm-like larvae an improved capability to survive and, by extension, reproduce - passing those light-producing genes onto the next generation.
Biologists tested this postulate indirectly in the 1980s using experiments in Highlands, North Carolina, in which some larvae had their lanterns hidden by opaque, dark-colored traps while others were covered with transparent traps which allowed the blue lanterns to remain visible. This setup then allowed biologists to observe the two groups of larvae directly, count the success of prey capture in each, and then use this information to measure how beneficial these glowing lanterns really were.
The results from this experiment were striking. Larvae with visible traps captured significantly more prey than those with their lures rendered invisible, thus showing the huge benefit that the lures provide. With this information, understanding how natural selection might have operated in O. fultoni becomes a simple numbers game: if some individuals acquired a luciferin-luciferase reaction through a mutation or other process, these lures would have allowed those individuals to capture more prey than those without. In a wild ecosystem where food and energy are at a premium, this ability would have allowed glowing individuals to better survive, ultimately long enough to reproduce and pass their lure system onto offspring. Over time, these lures would have increased in prevalence as less-successful, nonglowing (or faintly-glowing) individuals dwindled. This process - the increase in frequency of an inherited trait over generations due to a benefit to the organism - is a fundamental component of evolutionary biology and, in the fungus gnat's case, provides a beautiful example of an adaptation that almost went undiscovered....if it hadn't been for a thirsty biologist stumbling around in the dark.
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See it for yourself
- Dismal's Canyon (privately-owned preserve in northwest Alabama)
- Pickett State Park, Tennessee
- Highlands, North Carolina (general area)
Have you seen Fungus Gnat larvae glowing across the southern Appalachians? If so, snap a photo and upload it to iNaturalist via our project here.
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