There's something different about Karen and Lee's front yard this summer. It's covered with hundreds of bugs. There are bugs on the maple tree and bugs on the hedge. There are bugs on the sidewalk and bugs on the geraniums. There are bugs everywhere!

When the bugs first started appearing, Karen and Lee thought they were pretty awful. "They looked really disgusting," Karen says. "They crawled all over the place, and they shed their skins. I was scared of them. I didn't want to leave the house in case they bit me. But my mom told me that they're harmless. Now that I know they don't bite or sting, I think they're kind of neat to watch."

The cicadas that are blanketing the Washington area this year are harmless insects. They're making an appearance after 17 years of living underground. They crawl up from underground, coming to the surface to mate and lay eggs, not to feed. Unlike locusts, another kind of bug that many people confuse with them, cicadas do not eat leaves. They don't have teeth.

When they are underground, they live on juices that they get from roots, sucking the moisture in through a long tube-like mouth. If you look closely at a cicada, you'll see the tube. You'll also see its bright red eyes and long wings.

Cicadas are making an appearance all over the northeast part of the United States this year. They appear only around grass and trees, so if you live downtown you might have to go to the park to see some.

The females lay their eggs on tree twigs, usually making a slit in the bark. The eggs don't hurt the tree. During their short visit -- adult cicadas live only about three to four weeks above ground -- cicadas do no damage. Their shed skins make a mess -- but a few good rainstorms will wash all the castoff shells away.

"Cicadas are nothing to be scared of," says Rock Creek Park ranger Bill Yeaman. He says that the millions of cicadas now in our area will be around through June. The eggs they lay will hatch near the end of the summer, and the babies, called nymphs, will burrow back underground. The nymphs will live underground for the next 17 years, when they will -- you guessed it -- crawl back out and start the whole cycle over again. That means an "invasion" of cicadas won't happen again until the year 2004.

So you should take the opportunity to observe them while you can.

Are cicadas good for anything? Karen and Lee have noticed that the insects leave small, round holes in the earth when they crawl out. "The cicadas help aerate the soil much as earthworms do," explains Yeaman. "One of their other important functions is to be food for other animals," he adds. "Many kinds of birds, other insects and even box turtles feed on the cicadas. It's like a gourmet restaurant with its doors wide open."

In addition to providing an all-you-can-eat feast for other animals, the cicadas provide a rare chance to observe the behavior of insects close up. "It's a science lab right in front of you," says Yeaman. The cicadas come out at night to moult, or shed their skin. When they first come out, they're white. But soon their bodies harden, and the next day they're able to fly and sing. Only males sing. You have probably heard their buzzing song starting up in the last week or so.

"The cicadas provide a good lesson for kids to study and see the life cycle. It's a time to be fascinated," says Yeaman. "For naturalists, this is a very special event." The ranger says that cicadas are everywhere in Rock Creek Park. They're on the trails, on the trees, and -- unfortunately -- crawling across the roads. "When they try to cross the road at rush hour, they don't have a chance," he says. "But cicadas make up for their weaknesses by their sheer numbers. Many don't make it, but the ones that do manage to mate and keep the species going."

Even though there are many more cicadas than are needed to keep the species going, Bill Yeaman hopes that kids won't bring the insects indoors and put them in cages. Those who want souvenirs of the insects' visit can save some of the castoff shells, as Lee and Karen have done.

"You're fascinated, and that's natural," Yeaman says, "but watch them outdoors in their own environment. They're not here for very long, and they're nearing the end of their life cycle. Bringing a cicada inside to make a pet of it is depriving it of its mission." Tips for Parents

The cicadas' visit is a good chance to encourage your children to take an interest in nature. The Rock Creek Nature Center is open to the public from 9 to 5 on Tuesday through Sunday. For more information, call the center at 426-6829. In Chevy Chase, the Audubon Naturalist Society at 8940 Jones Mill Rd. has a 3/4-mile nature trail. The trail is open from dawn to dusk, and the society provides a brochure for visitors to refer to during their walk.Catherine O'Neill is a free-lance children's writer in Baltimore.