| Fossil Mayflies Weigh the Air | |
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John Cisne is a Cornell University paleobiologist who among other things has been studying fossils of mayflies. Every flyfisher knows about mayflies for their effect on trout, but they may not know that the mayfly order Ephemeroptera is 300 million years old. They are a very stable and successful order of insects, and their ancient fossils are not much different from something you'd see at your local stream.
Mayflies spend most of their lives as larvae in freshwater streams. Then they shed their skins and emerge into the air as adults with wings. At mating season, the adults do a peculiar courtship thing that involves fluttering up in the air then sinking slowly, wings spread wide, while they look for partners. Cisne was watching them in his yard one night, he says, "when it hit me: These remarkable little insects are experimentally measuring the density of the atmosphere. And they have been doing it for millions of years."
But a scientist needs more than a hunch—Cisne needed a hypothesis that could be tested. That was pretty simple: the muscles in the insect's pterothorax, the middle segment of the body, propel the wings. A large species of mayfly would have larger wings and more muscles than a smaller one, but the ratio of muscle to wing size is the same across all of the hundreds of species of modern mayflies. That's because they're all flying in the same atmosphere (there's an altitude effect too, but that can be subtracted).
Cisne used the length of the pterothorax L to stand in for the amount of wing muscle, and the length of the forewing R to stand in for the size (that is, the area) of the wing. This approximation works—modern mayflies all have roughly the same ratio R/L.
What about fossil mayflies? Insects are very rarely preserved as fossils, but there are a few mayflies among them. He examined specimens of Cretaceous age (65–200 million years) and Early Permian age (almost 300 million years) and found that the ratio was the same for them too.
Adult mayfly image courtesy Lower Colorado River Authority.
Mayfly body dimensions over the last 300 million years. Figure by John Cisne,
Cornell University.
Thus a handful of luckily preserved insect remains reveals something about the state of the Earth's atmosphere. Cisne told a San Francisco audience of scientists in December 1999, "As fossils reveal, relative wing length has remained practically the same since the Early Permian, suggesting that the atmosphere's mass likewise has remained much the same, and (from physiological considerations) that its oxygen content has remained at least as high."
Scientific knowledge is like a great wall, built slowly and methodically with sober attention to the foundation. Every paper in the journals, every talk presented at a meeting, is supposed to add to the wall, as a key stone or a grain of cement or perhaps just a coat of whitewash. But a few of the particles in that wall, like this elegant study of mayflies, have a special glitter to them.
It's not just flyfishers who appreciate mayflies. Take a look at the Mayfly Central site at Purdue University to see how deeply someone can get into the subject.
