John L. Cisne/Cornell University

Size measurements of mayfly wings and flight muscles indicate similar atmospheric densities in the Permian and Cretaceous periods and in today's atmosphere.

Millions of years before humans invented the barometer to measure atmospheric pressure, a primitive winged insect was experimentally measuring air's density and leaving barometer readings in the fossil record, according to a Cornell University geologist.

Because that insect -- the common mayfly -- has persisted with little change since its appearance some 300 million years ago, scientists can use it to estimate the mass and composition of ancient atmospheres, John L. Cisne told the annual meeting of the American Geophysical Union (AGU) here today (Dec. 14, 1999).

"Mayflies work like little helicopters as they dance up and down in a mating swarm," Cisne said, explaining how measurements of a fossilized insect's body can reveal information about the air in which it once flew:

• A mayfly's main flight muscle fills most of the pterothorax, the body's two wing-bearing segments.
• The force that the muscle delivers to the wings is registered in the length of the pterothorax ("L" in the accompanying diagram).
• The corresponding force that the wings exert on the air is registered in the length of the forewing (the longer of a mayfly's two paired wings, "R" in the diagram).
• The air's density determines how large the forewing must be in relation to the pterothorax for a mayfly to be able to dance.

Citing evidence (plotted in the diagram) that the wing's relative size is practically the same in fossils from the Permian and Cretaceous periods and in modern forms, Cisne concludes: "The atmosphere's mass must have been practically the same for the last quarter-billion years, at least so far as mayflies can tell us."

Members of the insect order Ephemeroptera, with some 2,000 species, mayflies are found around the world in freshwater habitats. They are much beloved by fly fishermen, who know that a fresh hatch of mayflies means trout will be biting and deploy their artificial lures (called flies) accordingly. Maturing into adults after a year or more as water-dwelling larvae, mayflies perform a mating dance by beating their wings 20 to 30 times a second to rise in the air. They rest their muscles and "parachute" downward, then fly upward again and again in hopes of finding a reproductive partner.

Most mayfly species live only a few days as adults, but the occasional insect that isn't eaten by predators or otherwise recycled sometimes lands in soft sediment with a chance of becoming a well-preserved fossil.

Geologists have known from qualitative indications in rocks that Earth's atmosphere was, at certain times, quite different from today's. During the first half of the planet's history, for instance, the atmosphere contained almost no oxygen. Until now, however, the only way geologists could quantitatively gauge atmospheric pressure in the distant past was to measure the size of gas bubbles frozen into lava as it solidified.

Without more exacting evidence, such as "barometer readings" the mayflies promise to provide, many questions have gone unanswered:

• Has the mass of the atmosphere always been what it is today?
• Earth continually leaks hydrogen and helium gas into space (which is why the planet hasn't evolved into a "gas giant" like Jupiter or Saturn). How much of the atmosphere has been lost since the first mayfly flew?
• In particular, how much gas was blown off into space by the impact and explosion of asteroids like the one that hit what is now Mexico 65 million years ago? Though relatively small by astronomical standards, could catastrophic degassing events have contributed to mass extinctions, such as those 65 million years ago among flying reptiles (pterosaurs) and birds' flightless dinosaur relatives?

Cisne said he hopes that further study of mayflies and other fossil insects will provide more definitive answers to geologists' questions.

"Mayflies are particularly good subjects because their flight mechanism is relatively simple and easy to characterize and because they are a very ancient group," he said.

A scientist who studied arthropod anatomy before concentrating on geology, Cisne has long been interested in indicators of primeval conditions.

"I was looking out in the backyard one evening, watching the jitterbug dance of the mayflies," 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."

The idea didn't come to him while fly-fishing. Cisne doesn't fish.

Purdue University's Mayfly Central: http://www.entm.purdue.edu/entomology/research/mayfly/mayfly.html.