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.
"We took the beetles to the Argonne National Lab and put the live animals under the x-ray beam," he says. "We took video sequences and x-ray images of the live beetles, then we went back to Tempe and sat down at a computer and measured the tracheal volume, the dimensions of the structures."
They found that the larger beetles had to devote a greater proportion of their body mass -- up to 20 percent more -- to the tracheal system than the smaller beetles. It's the proportion of tracheal system to body mass that matters because as the insect grows larger, it must devote more and more of its resources to maintain a tracheal system that is robust enough to deliver enough oxygen to the body's tissues.
What that means, the scientists say, is the size of the tracheal system makes it possible for some insects to get bigger, but it also limits the maximum size.
"The finding that larger insects have to invest more in their tracheal system as they get bigger is sort of a method to limit the maximum size of insects," Harrison says.
More time at the computer led to this conclusion. A beetle can get no bigger than about six inches. And as it turns out, that's about the size of the largest known beetle, the Titanic longhorn beetle, Titanus giganteus, of South America.
If oxygen is the key, does it determine the size of insects over succeeding generations?
To find out, the researchers studied about 10 generations of fruit flies, varying the level of oxygen over multiple generations.
"They get smaller in low oxygen and bigger in high oxygen," adds Harrison, noting that while that may not be a smoking gun, at least it's a "smoking fly."
It's rather unlikely that the oxygen in our air is likely to change dramatically anytime soon, so for now at least we're safe from giant dragonflies that, incidentally, dine on mosquitoes and mate in mid-air.