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.