Dragonflies are among the insect world's most powerful fliers.

"What good is half a wing?" asked Darwin's skeptical contemporaries. It is a question that cuts to the heart of evolution theory. If evolution proceeds in steps, how can something as complex as flight - and the necessary wings, nerves and muscles - ever arise?

In "Taking to the Air," Alan meets Pennsylvania State University biologist Jim Marden, who demonstrates exactly what evolutionary advantage half a wing might have had 350 million years ago before insect flight evolved.

Researcher clip stonefly wings to get to the roots of flight evolution.

Marden and his student Melissa Kramer looked to the rivers of eastern Pennsylvania, where insects called stoneflies hatch and rush to shore to find a mate. The winged but flightless stoneflies use their wings to propel themselves across the surface of the water.

Back in their lab, Marden and Kramer time the speed of the flies as they skim across the water. Then the scientists clip their wings and time them again. Even with their wings clipped down to little nubs, the stoneflies can still propel themselves across the water's surface. And that improves their chances of finding a mate. So that's what half a wing is good for.