BEIJING, Aug. 14 (Xinhuanet) -- Bugs were a lot bigger more than 250 million years ago -- millipedes longer than a human leg, dragonflies with wing spans like hawks -- and now a question that has long puzzled scientists has been explained.
The reason there are no humongous insects now is because of a bottleneck that occurs in insects' air pipes as they grow larger. They were able to surmount the problem in the Paleozoic Era thanks to a high-oxygen atmosphere.
Insects aren't like animals with backbones and deliver oxygen to their tissues directly and bloodlessly through a network of dead-end tracheal tubes. In bigger insects, this mode of oxygen transport becomes less efficient, but no one has been exactly sure why.
Alex Kaiser of Midwestern University and his colleagues at Argonne National Laboratory and Arizona State University delved deeper by shining X-rays on four living beetle species, ranging in body mass by a factor of 1,000. This allowed the team to measure the exact dimensions of the beetles' tracheal tubes.
Kaiser and collegues discovered the air passageways that lead from the body core to the legs turn out to be bottlenecks that limit how much oxygen can be delivered to the extremities, Kaiser said. The team also examined the passageways that lead from the body core to the head.
"We were surprised to find that the effect is most pronounced in the orifices leading to the legs, where more and more of the space is taken up by tracheal tubes in larger species," he said.
Kaiser and Argonne biologist Jake Socha also used the results to predict the largest size of currently living beetles. If data on the air passageways to the head were used as a limiting factor, they predicted a crazy-large, foot-long beetle, while the leg data predicted a beetle that matches the size of today's largest living beetle, Titaneus giganteus. The research is detailed in the Aug. 7 issue of the journal Proceedings of the National Academy of Sciences.
"This study is the first step toward understanding what controls body size in insects," Socha said. "It's the legs that count in the beetles studied here, but what matters for the hundreds of thousands of beetle species and millions of insect species overall is still an open question.