near Fort Meade as millions of cicadas whirred and droned in the trees
overhead. Then the weirdest thing happened: She was seized with the impulse to
flee.
It wasn’t the critters themselves – Williams owns a drawerful of cicada
T-shirts, a cicada key chain and a giant cicada doll – but their cries. It
sounded like a million madly whirling weed whackers. As the song swelled to
earsplitting intensity, she started to sweat.
“The sheer volume of the noise was painful,” said the 54-year-old state
Department of Agriculture entomologist. “I had to get out of the woods.”
It’s a feeling that many Marylanders can sympathize with these days, as
cicadas take over tree-lined neighborhoods across the state and create a
dawn-to-dusk ruckus that could reverberate into late September.
Cicadas come in two varieties. The first, “dog day” cicadas, appear each
July or August and provide the soundtrack to lazy summer afternoons. The
second are periodical cicadas, which emerge from the ground in plaguelike
proportions every 13 or 17 years, depending on their species.
Scientists have long been fascinated by the cicada’s deafening dirges.
These mournful love calls sung by the male can not only drive homeowners batty
but crank the needle of a decibel meter to near-rock concert levels. Brevisana
brevis, the African cicada, is the loudest recorded insect on Earth. Its
cries, notes The University of Florida Book of Insect Records, have hit 106.7
decibels, measured about 20 inches away.
17-year horde awaited
Even garden-variety cicadas can overpower the rumble of a lawn mower. And
if you think the insects are noisy now, wait until next year: In spring 2004,
a horde of 17-year cicadas are scheduled to emerge from the Maryland soil,
carpeting back yards and taking over trees across much of the state.
“It will be deafening,” says John Zyla, an amateur naturalist in Ridge, St.
Mary’s County, widely considered one of the foremost authorities on Maryland
cicadas. “If you’re standing underneath a tree, it will be hard to talk.”
One of the long-standing scientific mysteries surrounding cicadas is how a
finger-sized insect manages to belt out such earsplitting songs. It’s only
been in the past decade that a small group of biologists have begun to piece
together the puzzle.
The two most tenacious cicada detectives have been zoologist Henry
Bennet-Clark of Oxford University in England and David Young of the University
of Melbourne. Over the years the scientists have done everything from pouring
salt into dried carcasses to precisely determine their volume to recording the
sound-producing organs with specially modified microphones. In the process,
the scientists discovered that the cicada’s vocal system is far more elaborate
than previously thought.
Cicada songs begin inside a pair of drumlike organs – tymbals – on either
side of the insect’s abdomen. When the cicada wants to sing, it tenses a
muscle attached to each tymbal, which distorts the stiff structure much the
way a soda can dents when its lid is poked with a finger.
This buckling produces a pulse of sound – heard as several loud clicks. The
pulse travels into an large air sac in the insect’s abdomen. The pressure
generated by the tymbal pulse is intense: 158 decibels, roughly the force
created by an exploding grenade 3 feet away, Bennet-Clark notes.
But the real secret to the cicada’s sonic punch, Bennet-Clark and his
colleagues have found, stems from an even more unusual source: the insect’s
ears.
A pair of thin eardrums on the abdomen are not only designed for hearing
but for amplifying sound waves reverberating in the insect’s air sac. When
cicada song leaves the eardrums, it’s roughly 20 times louder than the clicks
produced by the tymbals, scientists have found.
“Strangely, this 158-decibel sound does not blast the male’s ears to bits,”
Bennet-Clark noted in a 1998 Scientific American article on his research.
Although scientists have yet to understand exactly how the cicada saves
itself from going deaf, Bennet-Clark has found one clue. The sensory part of
the ear is isolated from the rest of the structure by a small canal.
But it’s still unclear why, exactly, the insects evolved to be so noisy.
David Yager, a researcher at the University of Maryland who studies insect
communication, says one theory is that a tree full of buzzing cicadas might
confuse or overwhelm the ears of birds and other predators, driving them away.
Other scientists say the reason is more clear-cut.
“The sound has one purpose and one purpose only: To get a mate,” says
University of Connecticut biologist John Cooley. The ratio of males to females
is generally high, says Cooley. So sex-starved males must literally shout to
be heard over all the other cicadas advertising their amorous intentions.
Cooley and his colleagues have even discovered that the insects can jam
crucial sections of their rivals’ love calls. “This is not a friendly little
cooperative chorus,” he says. “These are males fighting tooth and claw to
drown out the other males.”
If the cicadas’ songs are weird, their lifestyles are even stranger,
fascinating scientists for centuries.
Maryland is home to 15 kinds of cicada – four periodical and 11 annual
species. Periodicals attract the most attention – and create the most chaos
when they emerge from the soil every 13 or 17 years.
Leaving their roots
Often mistakenly called “locusts” (which is technically a type of
grasshopper), periodical species spend all but a few weeks of their lives
underground, sucking nutrients from tree roots.
When the time comes to emerge – scientists still don’t know how the
creatures keep track – they dig neat, round escape shafts to the surface.
Their arrival is usually dramatic, since they typically all show up within a
few days.
Finding a vertical perch, the young cicada nymphs split their dry skins,
and pale, winged adults climb out to begin an urgent search for mates. That’s
when the singing starts.
After they mate, the female cicadas lay up to 600 eggs into cuts in the
tips of tree branches. In six to eight weeks, the eggs hatch. The nymphs drop
to the ground and burrow into the soil to feed for another 13 or 17 years,
leaving their parents to die.
While cicadas are harmless, periodic cicadas emerge in such numbers that
they can create havoc. Their intense noise can cause outdoor cafes to close,
while their intense numbers can drive school graduation ceremonies and
weddings indoors.
Some Marylanders are already bracing for the imminent arrival of a group of
17-year cicadas known as brood X. The largest of the periodical cicada
groupings, it last invaded Maryland in May 1987.
Gaye Williams at the Department of Agriculture says she has begun taking
calls from frantic brides preparing for spring weddings. Her advice: Forget
shrimp dip. And if you do serve it, she says, “Don’t eat the lumps.”
Information: Cicada Mania, www.dancentury.com/cicada
