Entomologists have found three species of 17-year cicadas within Brood X, each with a slightly different size, coloration, mating behavior and habitat preference. The males' songs differ, too, because they must attract females of their species.

One of the biggest cicada mysteries remains: How do bug brains keep track of the years?

Researchers suspect the cicadas can detect seasonal changes in the sugar or protein content of the tree sap they consume.

In experiments that artificially shortened seasonal cycles of light and temperature for plants and insects, the cicadas emerged after 17 cycles, even when each was just four months long. "How they're counting to 17, that's the totally unknown question," said Indiana University biologist Keith Clay.

However they figure it out, the Brood X nymphs have crawled to within a few inches of the surface, waiting for warmer weather. "Once it hits about 64 degrees 8 to 10 inches deep, that's the signal to come out," Clay said. When they do, "it's pretty synchronized, mostly all in one night."

Cicadas aren't the only ones counting: Officials at Boys' Latin School in North Baltimore tallied the years since 1987 and decided to move this year's June 5 commencement indoors, a reluctant break with the traditional celebration on the school's Lake Avenue lawn.

Cicadas also forced the school's 1987 commencement indoors.

"It wasn't just the fact that they were flying around," said public relations director Leslie Heubeck. "It was the noise they generated. Even though we have a PA system, they were loud enough that we were worried."

And with cicadas, there is always the "ick" factor.

"I don't speak for all women," Heubeck said, "but the cicadas scare me to death. I remember 17 years ago not wanting to be outside. ... I know they don't [bite], but they're really very ugly bugs. And they're big."

Change of plans

Johns Hopkins surgical resident Frank Lin and his fiancee were planning a May 30 wedding in an elm grove at the Audubon Society's Woodend Mansion in Chevy Chase. That is, until a fellow surgeon described a June 1987 wedding with cicadas that flew into guests' hair and landed in the food. "He said it was a day you would definitely never forget," Lin said.

Lin and his fiancee have postponed their nuptials to October.

For most of us, there is no way to avoid the ancient and inexorable spectacle - short of bugging out-of-state for a month.

The 1987 appearance of Brood X began about May 15. By the first week in June, the males were in full song. Noise levels in Milford, Rodgers Forge and Roland Park were measured at 80 decibels in the afternoon. That's nearly as loud as a heavy truck passing on the Beltway and well above the state's residential limit of 65 decibels.

Pedestrians complained that females flew like drunken sailors, bouncing off walls and windshields, and dropping onto car seats and hairdos. Homeowners covered lawn furniture. Some draped vulnerable saplings in cheesecloth.

This year, panicked gardeners have been calling Carrie Engel, greenhouse manager at Valley View Farms in Cockeysville.

"I think a lot of them are misinformed and think cicadas are much like locusts and chew everything up," she said.

In fact, they're not locusts, and they don't chew anything. Their feeding - sucking on tender plant parts for nutrients and moisture - causes no significant damage. Aside from the smelly mess when they die en masse, their only real impact will be some tree "flagging" - a dieback of leaves as the females slice the bark of small branches to lay their eggs. For the most part, it doesn't hurt the trees.

"It's just like a natural pruning," Engel said.