Oct. 11, 2006 — The enormous dragonflies, millipedes and other insects that lived 300 million years ago grew to such sizes thanks to Earth's rich oxygen supply at the time, according to a new study.
"Why isn’t there a cockroach that ate Dallas from recent history?" posed co-author Jon Harrison. "We can answer that question without a time machine."
Harrison and his team analyzed how modern insects are designed for today’s 21-percent-oxygen air, focusing on the tracheal system — interconnected tubes that transport oxygen throughout bug bodies. By extension, the researchers envisioned what insects would have looked like millions of years ago when the air was composed of 35 percent oxygen.
Their findings help explain the remains of 4-foot-long millipedes, 4-inch cockroaches and dragonflies with 2 1/2-foot wingspans dating to the late Paleozoic period just before dinosaurs came on the scene.
Using X-ray imaging, the scientists compared the tracheal dimensions of four modern beetles ranging in size from 0.1 inches to 1.5 inches. The tracheal system took up 20 percent more of the insect’s body in larger beetles. In other words, the bigger an insect is, the more it must invest in air tubing.
And "more tracheae...mean less space for everything else in the body, such as the brain and reproductive system," Harrison explained.
In the oxygen-rich world of the Paleozoic era, insects needed less air to meet their oxygen needs. As a result, they could afford smaller trachea than modern insects.
The researchers concluded that although Paleozoic insects had the same basic body structure as modern insects, their maximum body size was larger because smaller trachea left more room for the other essential body parts necessary to sustain the heft.
The findings were presented at this week’s American Physiological Society "Integrating Diversity" conference in Virginia Beach.
Steve Heydon, curator and collections manager at the UC Davis Bohart Museum of Entomology, told Discovery News that the new explanation is plausible, but that there are other possible explanations for why insects grew smaller, such as competition pressure from birds.
"Remember that there weren’t any birds in the sky when the giant insects were out," said Heydon. "When flying dinosaurs and birds came around, the insects lost their exclusivity."
