for National Geographic News
UPDATE (NOVEMBER 21, 2007): Giant Sea Scorpion Discovered
For the giant insects that roamed Earth 300 million years ago, there was something special in the air.
A higher concentration of oxygen in the atmosphere let dragonflies sometimes grow to the size of hawks, and some millipede-like bugs reached some six feet (two meters) in length, a new study suggests. (Related: "Dragonflies Migrate Like Birds, Study Says" [May 10, 2006].)
Now that the proportion of oxygen has decreased, however, bugs can't grow much larger than they do now, the authors write.
The reason: The bigger an insect, the bigger the proportion of its body devoted to its tracheal system, which functions like a lung but is far less efficient at large sizes.
"[The tracheal system] explains why they are small," said study co-author Jon Harrison, a professor of environmental physiology at Arizona State University. "It takes up all the room."
The study appears this week in the journal Proceedings of the National Academy of Sciences.
Bigger Bodies, Bigger Lungs
Scientists have long puzzled over why bugs once grew to gigantic proportions but are now among Earth's more diminutive creatures.
"There were hundreds of ideas to explain the small size, but none of them could be proven," said lead study author Alexander Kaiser, of Midwestern University's Department of Basic Sciences.
So Kaiser and colleagues decided to test the idea that it was it was an insect's respiratory system that limited its size by studying beetles and fruit flies.
The team looked at beetles by peering through their exoskeletons with new x-ray beam technology at Argonne National Laboratory in Illinois.
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