SAN FRANCISCO -- 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 int
Contact: Roger Segelken
Cornell University News Service