At the tail end of the Paleozoic Era, dragonflies with two-foot wingspans cruised through swamps, and five-foot-long millipedes crawled along the forest floors. Today the largest insect is a beetle no larger than a computer mouse.
Bugs used to be bigger, in part due to higher concentrations of oxygen. Unlike vertebrates, insects absorb oxygen directly through networks of air tubes. To find out how this mode of oxygen transport affected the size of insects, researchers at Argonne National Laboratory X-rayed beetles large and small. They found that the bigger the bug, the greater the proportion of the body dedicated to air tubes. Getting oxygen to the beetles’ legs was particularly constraining on size, however. In order to absorb enough to survive in today’s atmosphere, a beetle rivaling the size of its prehistoric ancestors would need tubes bigger than its leg openings could hold.