"We think it's not just because oxygen affects the adults but because oxygen has a bigger effect on larvae," said study co-author Wilco Verberk of Plymouth University in the U.K.

"So a larval perspective might lead to a better understanding of why these animals existed in the first place, and maybe why they disappeared."

(Also see "Oxygen-Free Animals Discovered—A First.")

Fossils show that giant dragonflies and huge cockroaches were common during the Carboniferous period, which lasted from about 359 to 299 million years ago. (Explore a prehistoric time line.)

During this time, the rise of vast lowland swamp forests led to atmospheric oxygen levels of around 30 percent—close to 50 percent higher than current levels.

According to previous theories about insect