Prehistoric-Sized Dragonflies? It's in the Air

Oct. 18, 2006 — -- Before dinosaurs and birds came on the scene, dragonflies were king, with wingspans of about two and a half feet. That was 300 million years ago, during the late Paleozoic period.

Despite all the changes that have taken place since then, dragonflies are still around. Fortunately, they're much smaller.

For more than a decade now, scientists have been trying to figure out why the giant insects that roamed the planet so long ago are all gone, replaced by the smaller ones we see today. Predation by the first birds? Perhaps.

But new research suggests that the answer lies in the air, and if we could somehow tweak the atmosphere and pump up the oxygen, those big babies could be back.

The late Paleozoic was a time of huge trees, and giant ferns, many of which became buried in the soil, forming future hydrocarbon deposits, says Jon Harrison, an environmental physiologist at Arizona State University in Tempe and the principal investigator in an experiment described recently during a meeting of the American Physiological Society.

Through photosynthesis, the lush flora changed the chemical composition of the air, boosting the level of oxygen to 35 percent compared to 21 percent now.

Scientists have speculated for a while that more oxygen led to larger insects, and Harrison and colleagues at Argonne National Laboratory and Midwestern University, Glendale, Ariz., wanted to find out if that was indeed the case.

They centered their research on a fundamental difference between humans and insects. We have one trachea, the breathing tube that extends down into the lungs, which allows oxygen to be absorbed into the blood steam and carried to other parts of the body. Insects have many tracheal tubes that carry oxygen directly to all areas of their bodies.

If oxygen plays a role in limiting the size of insects, the scientists reasoned, then larger insects must dedicate a greater proportion of their bodies to the tracheal system.

"We picked a nice range of beetles so we could get a good idea of what the tracheal system looks like inside of them," says Alexander Kaiser of Midwestern University, lead author of the report. The four species of beetles are all members of the same family, and range in size from about 1.5 inches to a tenth of an inch.