What exactly, is Bernie Kraenzle up to night after night, running through meadows and pastures, waving a net? Why, for that matter, are his wife, Nancy, and their three children, Lori, 9, Shelly, 12, and Keith, 14, doing the same? A little cottage industry, it turns out. The energetic Kraenzles are harvesting an unusual crop on their 330-acre farm—the thousands of fireflies that appear each summer evening. The bugs are used in medical and scientific research.

The Kraenzles' property lies in the western part of Ste. Genevieve County, Mo. Nights are warm with plenty of moisture in the air, ideal conditions for twinkling members of the family Lampyridae. At dusk, starting in early June, the Kraenzles grab their bell-shaped nets, jump in the pickup and head for the creek. Catching fireflies may seem like child's play, but on the scale practiced by the Kraenzles—59,000 of them over the last four years, 1,100 in one record evening alone—it is a carefully executed business. This year has been their third best, with a catch of 12,803 so far—not bad, considering Bernie was out of action for two weeks in July with pneumonia.

"The key," he says, "is to be there at the right time and keep moving." Lightning bugs nestle in the grass during the day and start their flights as darkness gathers. They keep low at first, then fly high and scatter.

The result is a fairly frantic hour of chasing through the wet grass, an impromptu twilight ballet as each Kraenzle employs a vigorous—and different—technique. Shelly, for example, coaxes them into her net with a soothing chant: "Come here, you should be over here by me," or, "Hey, you're going the wrong way." Her mother is a wanderer, searching deep within the trees. "This," she says, "is the only good exercise Bernie and I get." Back home, their legs soaked to the knees, the Kraenzles pop the bugs into the freezer for 20 minutes to make them sluggish, then count them—each one is worth a penny.

The live bugs are packed in a Styrofoam box and sent by bus to the Sigma Chemical Co., a St. Louis firm which buys millions of fireflies every year. From them, Sigma extracts the two chemicals—luciferin and luciferase—which interact with a third, adenosine triphosphate (ATP), to create the flickering light. Because all living cells contain ATP, scientists use luciferin and luciferase as diagnostic agents to detect cancer, multiple sclerosis, cystic fibrosis and heart trouble. They are also used in tests for life-threatening air and water pollution.

The Kraenzles donate a nickel of every dollar earned to the kids' Silver Bells 4-H club. The rest will go toward rec room furniture to complement the stereo they bought earlier with their lightning bug bounty. Along with the lingering glow they get from knowing their work helps mankind, the net isn't bad either.