THE LAST TIME I traipsed through unfamiliar woods after dark, I was 11 years old and hunting elusive snipes during a Camp Fire Girls initiation rite. Accompanied by a parent volunteer with no sense of direction, my fellow inductee and I became hopelessly lost for an hour or so, thoroughly enjoying our misadventure even as anxious troop leaders wondered whether to send out a search party.

I'm reminded of that fruitless but fun evening trek as I wait in the visitors center at Lake Accotink Park in Springfield. On this refreshingly unhumid Saturday in July, I'll once again search for hidden creatures of the night, but this time I'll be guided by professionals who know a thing or two about navigating murky trails. Frequent park visitors Mo Ghafori and his two daughters, 7-year-old Mahna and 8-year-old Nina, also are on hand for their first Nature Night Hike, an event conducted once or twice monthly year-round at the Fairfax County Park Authority site. Each 90-minute walk provides a unique after-hours experience for as many as a dozen visitors.

"Getting people out here to see a different side of the park" is what 22-year-old evening supervisor Axel Boy enjoys most about leading the hikes. Visitors never know exactly what they'll see.

Once, a group led by Boy came across a venomous copperhead "smack-dab in the middle of the road," he recalls. Much to the parents' dismay, "seven little kids rushed forward." Boy managed to intervene before anyone was bitten.

"We saw a pair of barred owls one outing, a male and female up in the trees," he recalls.

On this night, our party of six is waiting inside for the sky to get a little darker before we start out. Boy brings in a pillowcase-size muslin bag, knotted at the top.

"A little friend I caught earlier," he says as we gather around. Boy unties the knot, reaches in and pulls out a brown snake, not much bigger than a long worm. It's just one of the many types of animals we're most likely to see this time of year, when reptiles and amphibians abound at the park. During the colder months, hikers are more apt to run into mammals such as deer.

Boy returns the snake to the bag, then gives a brief introductory talk about the Accotink Stream Valley. He and his co-guide, staff member Charlotte Kluza, cover our flashlights with squares of orange cellophane. We'll still be able to see where we're walking, they tell us, but the dimmed beams won't startle the animals. We walk outside into the fading light -- most of us wearing bug repellent and hats -- where Boy tells us that although we can't help making noises with our feet, we should keep our voices low. At the edge of the trail, he talks a bit about nocturnal animals such as owls, points out the increasingly visible fireflies and then releases the little snake. Patches of gray-blue sky peek through the treetops as we descend a steep mulch path into the woods.

"Can you hear anything?" Boy asks, and we stop and listen. "I can hear the creek. I hear a cricket or other insect. I just heard a frog." The rest of us are still adjusting to the new sounds as Boy searches in vain for an owl; he's pretty sure he caught a glimpse of a bird's shadow. He stops to rescue a stray inchworm that's about to be stepped on in the dark.

With fireflies blinking like tiny beacons, we walk slowly, trying to sidestep puddles and splotches of mud. The gravel crunches beneath our sneakers no matter how lightly we attempt to step.

Boy tells us about the woods' vernal pools in which frogs and toads proliferate in the spring.

"I've come down here and I've counted as many as eight species of frogs and toads," he says. "If you come down here in the early springtime, they're so loud you can't hear yourself think."

Although we can't see them, we can hear trilling tree frogs and the occasional distinctive thrum of a bullfrog.

"If you cup your hands behind your ears, you can hear better, like an owl," Kluza whispers to the Ghafori children.

"It's getting darker and darker," their father observes.

We follow Boy as he ventures off the dim path toward what appears to be a pond or swamp. He asks us to close our eyes and wait for his instruction.

"Now open them," he says seconds later, and we immediately notice wings flapping rapidly in the long ray of light Boy shines above the water. Bats! How cool is this? One seems to miss us by mere inches, as Boy reassures us that the popular notion of bats flying into people's hair is just a myth. Kluza holds a bat detector, a gizmo that picks up the sounds the creatures emit -- which can't be heard by humans -- to orient themselves via echolocation and converts them into audible noises. The little box clicks like crazy as the animals swoop about, searching for prey.

"The brown bat eats 1,200 mosquitoes in an hour," Kluza says. She and Boy tell us more about the beneficial but misunderstood critters: They're the only mammals that fly, and they make up a quarter of all mammals on Earth.

Boy shuts off his light, and we reluctantly venture forth, crossing a grassy area and stepping onto a paved parking lot.

"Whoa-oh-oh! You almost put your foot on it!" Boy says excitedly as I stop in my tracks to see what I nearly squished: a millipede that has to be four or five inches long. The bug slows, its hard, brown plates illuminated by Boy's flashlight.

"Can you see his legs move there? Like little waves," our guide points out. "They will bite you, so it's best to leave 'em alone."

"See you, buddy!" Boy says as we walk away.

With the time already past 10, we begin heading back to the visitors center, taking a paved roadway instead of a wooded path. One of the children points to a small, dark object she finds in the flashlight's orange glow. It's a cockroach, very much alive. We move on. Boy tells us about flying squirrels, and we scan the trees expectantly to no avail. He shows us a humpbacked camel cricket but soon notices something a bit more spectacular.

"Oh, yeah, there we go!" he says, shining his light on a rather large wolf spider. The bug seems less menacing as Boy describes how the female spiders carry their babies on their backs. "They're kind of a ghost white, and the whole back will be covered with them!"

"Actually, I'm going to shoo him out of the road so my brother doesn't get him," Boy says, stamping his foot behind the spider to prod it to safety before Boy's sibling and co-worker, Asa, out patrolling the park by vehicle, reaches the scene.

Back at the center, Kluza shows me a bat house, and Boy disappears outside with his brother and a bucket. Our guide returns to show us one last animal, a rust-hued American toad.

"He's missing a foot!" Boy observes with surprise, but the handicap doesn't seem to bother the animal, one of the park's "door toads" that hang out by the visitors center. Naturally, Boy will take him back outside in short order.

Exiting the parking lot, I accidentally turn the wrong way and find myself traveling back down the road upon which we walked. I drive so slowly, scanning the headlights' beams for wayward creatures in need of a helping hand.

LAKE ACCOTINK PARK NATURE NIGHT HIKES -- 7500 Accotink Park Rd., Springfield. 703-569-0285. www.fairfaxcounty.gov/parks/accotink/index.htm. This Fairfax County Park Authority park hosts year-round guided Nature Night Hikes, usually on Fridays or Saturdays. The next scheduled hike is Aug. 19 at 8. $5 adults, $3 children ages 7 to 15. Children younger than 7 are not permitted. Reservations are required. Hikes also can be arranged for private groups, such as Scout troops. Bring flashlights. The park also holds monthly Full Moon Cruises featuring guided pontoon-boat tours of the lake. Upcoming cruises are Saturday, Aug. 20 and Sept. 17 at 8. $5. Required reservations fill up quickly.

CAPITAL HIKING CLUB -- Friday. Parking lot across from Old Angler's Inn, 10801 MacArthur Blvd., Potomac. 703-401-2719. www.capitalhikingclub.org.