In the jumpy QuickTime Movie image on Jim Marden's computer screen, silvery waves precede the black figure of a stonefly as it pushes itself across the surface on the water, legs rigid, wings whirring.
Jim Marden, assistant professor of biology at Penn State, points to the intricate loops the fly's wings weave. It was this wing motion that first got Marden interested in studying the evolution of insect flight. "The problem isn't how insects developed larger wings for flight," Marden says, "but how they developed wing motion, period." The flight process requires not only wings, but specially developed muscles under the control on complex neuronal patterns.
The problem is not new to biology. Charles Darwin's opponents argued that insect flight could not be achieved through stepwise evolution. As Marden says, "The first evolutionary steps wouldn't get off the ground." Since then, the task of evolutionary biologists has been to explain how the intervening species could develop and use winglike structures for something other than flight.
Enter the stonefly.
Stoneflies spend much of their lives as underwater nymphs, but they must surface and travel to land to mate. To get from midstream to shore, an adult stonefly skims across the surface of the water, held up by its buoyant feet. The pads of the feet are covered with water-resistant hairs and are filled with gas—"like little life preservers," says Marden's undergraduate assistant, Melissa Kramer. Riding on these natural pontoons, the stonefly is pushed by its whirring wings, like a fan-boat cruising the bayou. Though the stoneflies never engage in true flight—they don't leave the water surface—they can reach speeds of 35 centimeters per second (by comparison, a wasp travels at over 100 centimeters per second).
The stonefly contradicts the theory that gliding insects evolved first, and eventually began flapping their primitive winglike structures to achieve true flight. "If they moved their wings, they'd be very poor gliders," Marden notes. "Gliding insects would need to hold their wings very rigid." Instead, Marden believes that primitive, flapping gill plates—still used by many insect species for underwater breathing—eventually became a means of locomotion above the water's surface. "After all," says Marden, "it's only a small step from circulating water to circulating air."
TO test this hypothesis, Marden and Kramer collected stoneflies from the banks of Bald Eagle Creek, near Julian, Pennsylvania. From January to March, they spent cold afternoons wading thigh-deep through freezing streams, scooping stoneflies into plastic jars. Once back in the lab, the flies were given various challenges to their flapping ability, including lower temperatures to decrease muscle efficiency and smaller wing areas (via scissors). "We couldn't change things like the neural pathways," Marden says, "so we worked with what we could change."
Marden and Kramer discovered that skimming speed increases directly with wing area, temperature, and muscle size. Yet even severely handicapped stoneflies, with wings trimmed to one-fifth their original size and operating at temperatures just above freezing, were able to skim. "They buzzed along quite well," says Marden.
The clipped wings approximated the size of external gills found on species of both living and fossilized aquatic insects. In an effort to find stronger correlations between wings and gills, Kramer has begun studying scanning electron micrographs of wingvein cross-sections, examining the thickness and shape of veins in various species of stonefly wings, which would strengthen his argument that the wings developed from gills.
They have also begun examining a second species of stonefly, one that is possibly more primitive than their previous subject. (Since wing size varies greatly within this species, no trimming will be necessary.)
Unlike its motorized cousin, this stonefly stands in the stream until it feels a breeze, and then raises its wings to let the wind blow it toward land. "On a sunny day," says Marden, "we'd sometimes see hundreds of these flies sailing into the bank of the stream, piling up on shore."
James Marden, Ph.D., is assistant professor of biology in the Eberly College of Science, 417 Mueller Laboratory, University Park, PA 16802; 814-863-1384. Melissa Kramer is an undergraduate student in biology. This project is funded by the National Science Foundation. It was reported in the 21 October 1994 issue of Science.