Skimmers or Fliers

The first phase began with a jar of specimens from the Smithsonian. Submerged in alcohol, the stoneflies were suspended eerily in cotton stoppered vials. The mottled brown insects ranged from puny little quarter inchers that hardly seemed to merit the lengthy scientific names recorded on the outside of their vials to whopping behemoths that threatened to push the alcohol soaked cotton wads clear out.

stone fly

Kathleen Walsh clipped off portions of the stoneflies' wings, treated them, and embedded them in individual blocks of resin. Then, using a scanning-electron microscope, or SEM, she brought their tiny tubelike wing vein structures into vivid view.

"That was really one of the coolest parts of the research," says Walsh, "looking into the SEM for the first time and seeing so much there."

Two glowing blue close-ups of stonefly wings, images captured under the microscope, show the thickness of the veins—a mere 5 microns for a "wimpy" stonefly that can only skim across water, a "whopping" 10 microns for a stonefly graced with flight.

Until recently scientists had no explanation for how swimming critters managed to leave behind their water bound lives and take to the air. Then in 1994, Penn State biologist James Marden and then-undergraduate Melissa Kramer published a paper in the journal Science about their work with stoneflies. "Within the stonefly order," explains Walsh, who learned of Marden and Kramer's research as a sophomore, "you have both fliers like Paragnetina media and skimmers like the Taeniopteryx." Skimmers come in three different types. "The first guy has wings that act like sails," says Walsh. "Gusts of wind push him along. The second one flaps his wings. He can fly when it's warm but he's a very clumsy flier. The third one goes up on his hind legs and skims across the water. All of them move above the surface of the water, but they rarely fly. That's why Marden thought it was a good order to look at for the evolution of flight."

Marden first set his sights on these America's Cup—evoking insects as they skimmed along the frigid surface of a river in upstate New York. This variety used their wings as sails, catching a breeze and gliding across the water's surface. Fascinated for years by the missing link of flight evolution, Marden wondered if these sailing stoneflies could offer any answers. After he came to Penn State in 1992, he began researching his hunch with Kramer. Creating a miniature movie set for the flies, complete with a wind machine and videocameras, Marden and Kramer simulated a breezy river setting and filmed insect travel. Even when their wings were chopped down to mere stubs, the flies could still sail. That wings could develop from gills had already been proposed by scientists, but no one had been able to explain what good these pathetic nubs would be. Marden and Kramer's experiment proved that even rudimentary wings would have been useful 300 million years ago. From wings suitable for sailing, larger, stronger wings necessary for flight could have gradually evolved until the insects could finally go airborne.

Now Walsh works to prove not only that flight could have evolved from gliding, but that it did. "I started out just looking at wings under the SEM," says Walsh. "I looked at the wing vein thickness first because you'd think a flier would have thicker wing veins. He'd need more support to be able to push his wings down to fly."

In the second phase of her research, Walsh and graduate student Melisande Wolf sequenced the ribosomal DNA of dozens of stonefly specimens from around the globe. A stack of blue three inch binders contains the results: Each sequence, resembling a crazed blue, black, green, and red seismograph, depicts a segment of DNA approximately 660 base pairs long. Now using special computer programs, Walsh is examining these DNA sequences for changes in the order of base pairs. The more changes in the DNA sequence, the more the species has evolved.

stone fly’s wings

"We made a tree based on the molecular data," says Walsh. From the DNA samples she has so far, Walsh helped to construct a branched line drawing called a phylogenetic tree—a lineage that shows the evolutionary relationships among present day stonefly species. On the tree each node, or intersection, is assigned a number. The shape of the wing and the species name are marked at the tip of each branch. "We thought that the more ancestral species would be at the bottom of the tree and the more recent at the top." Taeniopteryx, a weak winged skimmer, occupies one of the bottom branches.

"We made the tree, then plotted the vein thickness for some of the species," says Walsh. "So far the two have corresponded—the veins thicken as you move up the tree from skimmer to flier. But we need to increase the sample size to see if the trend holds."

The live skimmers Kramer and Marden videotaped could be "evolutionary remnants" from the past that never developed the ability to fly. Or, they could be insects that once could fly but have since adopted the surface skimming behavior as an adaptation to their surroundings. Walsh's phylogenetic tree will let her say for sure which came first, fliers or skimmers.

Kathleen Walsh graduated with a B.S. in biology from the Eberly College of Science in May 1998. She was an award winner at the Penn State Undergraduate Research Fair in March 1998. Her adviser is James Marden, Ph.D., assistant professor of biology in the Eberly College of Science, 416 Mueller Lab, University Park, PA 16802; 8148631384; Funding for this project comes from the National Science Foundation. Visit stoneflies on the web:

Last Updated September 01, 1998