One summer several years ago, Gaye Williams was walking in a wooded spot 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