Giant insects ruled the prehistoric skies during periods when Earth's atmosphere was rich in oxygen. Then came the birds. After the evolution of birds about 150 million years ago, insects got smaller despite rising oxygen levels, according to a study by scientists at the University of California, Santa Cruz. The study looks closely at the relationship between insect size and prehistoric oxygen levels. The leading theory attributes large insect size to high oxygen concentrations in the atmosphere, which allowed giant insects to get enough oxygen through the tiny breathing tubes that insects use instead of lungs. But, with predatory birds on the wing, the need for maneuverability became a driving force in the evolution of flying insects, favoring smaller insect body size.
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