As daylight drains from the summer sky, a cool breeze rustles
through a hay-scented New England meadow. Relaxing in the
long grass, even the keenest observer might miss the miniature
army that’s now awakening from its daytime slumber. One by
one, tiny male fireflies are creeping upward along grass highways. They pause at each apex, ready to lift off like silent Black
Hawks. But as they prepare for their nightly search missions,
these firefly males aren’t motivated by military conquest. Their
quest? Genetic immortality. They’re hell-bent on procreation,
driven by an urgent need to propel their genes into the next
firefly generation. These resolute males are destined to spend
every night of their short adult lives valiantly broadcasting their
luminous signals. Unfortunately, the odds are stacked against
them as they head off into the night looking for love.
These are Photinus fireflies, the most common lightning bugs
in North America. We happen to know about Photinus sex lives
in intimate detail – more than we know about any other firefly
in the world. This deep knowledge is mainly due to Jim Lloyd,
a professor emeritus at the University of Florida who grew up
near the Mohawk Valley in New York State and went to school at
Cornell University in the mid-1960s. Like most college students,
Lloyd was fond of road trips, spending summers crisscrossing
the United States in his pickup. Unlike other students, he did
much of his traveling at night, driving backroads with his headlights off, head stuck out the window looking for firefly flashes.
As a doctoral student, Lloyd discovered that males of each
Photinus species emit a different pattern of flashes. Depending on
what species he belongs to, a Photinus male will court by emitting
a flash pattern with one, two, or several pulses of light. After a
short pause, he repeats the whole pattern again. Each flash pattern
is distinctive in how many light pulses it contains, how long each
pulse lasts, and the dark interval between pulses.
So it’s the timing of a male’s flash, rather than its color or
shape, which conveys the crucial information about his species
identity and sex. Like mariners who use the distinctive light
pattern to tell which lighthouse they’re approaching, female
fireflies use such differences in flash timing to distinguish the
males of their own species.
Heading Out Into the Night
The fireflies in our New England meadow are a species known
as Photinus greeni, whose males give a distinctive pair of quick
light pulses separated by 1. 2 seconds. Poised atop grass blades,
our army of firefly males has waited patiently for darkness.
Now each male lifts off into the air to start his nightly patrol.
As he flies, he earnestly advertises his availability about every
four seconds by flashing out to anyone who’s watching: “I’m a
greeni male, here I am! I’m a greeni male, here I am!” Each time
he shines his light, the male pauses for an instant in hopes of
spotting a female. So tonight across the meadow, it’s wink, wink,
hover and hope . . . wink, wink, hover and hope . . . wink, wink,
hover and hope. Males concentrate their efforts in places where
females are likely to gather and then fly along to the next likely
pickup spot. Hundreds of flying males now fill the meadow,
their flashes scintillating like sunlight glinting off the sea.
What’s a firefly’s worst nightmare? Sometimes I think it must
be the sudden rainstorm that hits just when they’re starting their
courtship flights. One night I watched as raindrops slammed
into their tiny bodies, driving them down to earth like sodden
shooting stars. Wet wings meant no more flying that night.
These soggy males were forced to continue their courtship on
foot, flashing sporadically as they plodded tediously through the
grass in their search for females.
But where are those obscure objects of their desire, the
females? Even though female Photinus greeni can fly, they
rarely waste their precious energy doing so. Instead, they perch
quietly on grass blades, like women perching on barstools at a
singles bar. If a female spots an especially attractive male, she
might deign to acknowledge his advances: she’ll flash him a
come-hither reply, aiming her lantern in his direction. Photinus
females usually give a single prolonged flash that rises in a cre-
scendo and then slowly fades out. Here again, timing is every-
thing. Across different Photinus species, females differ in how
long they wait before flashing out their response. And males use
these different response delays to identify females belonging to
their own species. In Photinus greeni, females have a very short
response delay: after the male flashes, they wait less than a sec-
ond before offering their crescendo response.
A hapless male whose courting days ended abruptly when he got trapped and
wrapped by an orb-weaving spider.