Photinus pyralis
Insect · Beetle · BioluminescentOn a warm evening in late June, the first cold spark lifts out of the lawn just as the light goes blue. Then another, and another, until the whole field is blinking in slow, silent code. Each of those sparks is a beetle, not a fly, climbing the dark air to advertise itself.
The light is the firefly's whole language. In the last two segments of its abdomen, an enzyme called luciferase burns a fuel called luciferin with oxygen, and almost all of that energy leaves as light rather than heat. It is one of the most efficient lamps in nature, and the beetle switches it on and off by feeding the reaction air.
What looks like random twinkling is really a conversation. A male of this species flies low over the grass and gives one half-second flash that swoops upward at the end, a small hook of light, every five to seven seconds. A female watching from a grass stem waits a second or two, then answers with a single flash of her own. He turns toward her reply, and the two trade signals until he lands.
Follow one male's light, not the whole field. This species gives a single, unhurried flash of pale yellow-green that lifts upward as it fades, tracing a small J. That rising hook, repeated every five to seven seconds, is the surest way to name it, and the reason people call it the Big Dipper firefly.
At rest it is an unremarkable soft-bodied beetle about half an inch long. The giveaway is the pronotum, the shield behind the head: pinkish-red, rimmed in pale yellow, with a dark spot in the center. The wing covers are brown with thin pale edges.
Turn one over gently and the last two segments of the underside are waxy pale yellow. That is the lantern. In daylight it looks like nothing; at dusk those same segments are the part that lights.
Males do the flying, cruising knee-high in long, level passes over open grass. Females rarely fly during courtship; look for their answering flashes coming from low on a stem or blade rather than from the air.
Common Eastern Fireflies peak from mid-June through July across eastern and central North America, in the half hour after sunset when the air is warm and still. Look over unmown grass, meadow edges, and the borders of woods and water, places that stay damp enough for the snail-hunting larvae that live there for a year or two before this brief adult summer. Warm, humid, windless nights bring out the most lights; cold or breezy evenings shut them down. Turn off porch and yard lights to see the show, since artificial light drowns out the very flashes the beetles use to find each other.
Females of another firefly, the genus Photuris, have learned to crack this code. A Photuris female watches a flashing Photinus male, then imitates the answering flash of a Photinus female. When the hopeful male lands expecting a mate, she eats him, and in the bargain steals defensive chemicals his body carries that she cannot make herself. Among the gentle blinks of a summer field, a few of those lights are lures.
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