Femme Fatale

Don’t let her lovely luminescence fool you, the firefly is a tough predator.

Illustration by Robert Meganck

An important thing you should know for summer: A firefly is not a fly.

Neither, for that matter, is a lightning bug a bug.

Either or both, one and the other, what you have is a beetle. Also known variously as a moon bug, glow fly, glowworm, fire devil, big dipper—and of course, as everyone knows, “szentjánosbogár” in Hungarian—fireflies are found around the world on every continent except Antarctica, favoring warmish, wettish environments—like summer in Virginia.  

Sometime in early summer you’ll see it, that first brief burst of yellow-green flashing in the deepening twilight. Fireflies are among the Earth’s bioluminescent creatures, organisms that have marshaled chemistry to create light. Though not lending itself to easy explanation in layperson’s terms, the process by which the firefly generates its glow (in the part of its abdomen known, aptly, as its “lantern”) is a complex and not fully understood marvel of efficiency, involving the interplay of oxygen and two biochemical components, luciferin and luciferase (from the Latin for “light bringer” or “light bearing”). Called “cold light,” bioluminescence is almost all light and almost no heat; not surprisingly, it is, therefore, the subject of avid scientific inquiry as researchers try to understand and emulate the process and the structure of firefly bioluminescence to create more energy efficient lighting for human needs.

Among the estimated 2,000 firefly species worldwide, not all produce light, and of those that do, not all emit the familiar yellow-green. The ghost firefly (or “blue ghost”), which can be found in undisturbed forest areas along the Appalachians from Virginia south, produces a bluish-green, continuous glow as it drifts just above the forest floor. What you are likely to see in your own backyard or in a roadside meadow, however, is the brief on-and-off, yellow-green incandescence of the more common species found in Virginia; perhaps you chased those lights as a child, until out of the darkness you cupped a firefly gently into your hands.

Each species of firefly has a unique flash pattern, which serves to attract mates of its own kind. Lovely to contemplate on a summer eve, all those delicate glimmers, though, might tell a more unsettling tale at the level of the firefly. In particular, the female Photuris is a devious thing. Sometimes referred to as the femme fatale of fireflies, she engages in a behavior known as “aggressive mimicry,” in which she feigns the signal of another species in order to lure in a hapless male. Unfortunately for him, he’s not her date—he’s her dinner.

It is theorized that fireflies also glow in order to warn predators that they don’t taste good. As messaging points go, this seems rather cryptic and, obviously, fails to sway the opinion of the female Photuris. One likely reason Photuris females have a particular taste for Photinus males is because Photinus come with a healthy serving of a defensive compound (“lucibufagins”—which sounds like a character out of Tolkien) that the female Photuris cannot produce herself.

Photinus carolinus is a particular Photinus found commonly in Virginia. In one part of the Great Smoky Mountains National Park, these fireflies regularly, year after year, engage in a rare behavior of synchronous flashing—which, as the name suggests, involves large numbers of the fireflies all flashing on and off together, at the same time. So many tourists are drawn to the park to witness this annual phenomenon that the park runs a “firefly shuttle” to transport visitors to the site.

A reader contributor to Firefly.org reported witnessing another incident of synchronous flashing at a fireworks display in the Midwest, noting that each time one particular kind of firework exploded in the sky, the pyrotechnic light would be mirrored by a burst of firefly light nearby.

Even firefly eggs sometimes glow, as do the larvae, which is what earned them the colloquial name “glowworm,” though firefly larvae don’t look a thing like worms. What they resemble is something like a sturgeon with legs. In their lives, fireflies go from egg to larva to pupa to adult, a process known in the insect world as “complete metamorphosis,” but they spend most of their time in larval form, during which interlude they are not the gentle winged creatures of summer evenings, but rather the marauding scourge of earthworms, snails, slugs and other soft-bodied ground-dwellers. Firefly larvae seize upon their prey, injecting a toxin that paralyzes and then liquefies (or, to put it euphemistically, “pre-digests”) the innards of the unfortunate victim.

But wait—things go even more horror-movie at the next stage of metamorphosis, in which the pupal firefly more or less digests itself. It dissolves its own larval body, and then, from this goo, assembles the adult that will emerge a few weeks later. Imagine lying down for a nap as yourself and waking up as—well, you see where Kafka got his inspiration.

Transformed, a firefly passes only a few bright weeks as an adult, seeking love above your moonlit garden. Then the brief, magical glimmer of firefly light in velvety darkness, like summer itself, is too soon gone.

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