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 In the day of the dinosaur¹, insects had wingspans of nearly 2.5 feet. So why are today's bugs³ so puny⁴? According to researchers at U.C. Santa Cruz, we may have birds and bats to thank. Their concludes appear in the Proceedings⁵ of the National Academy of Sciences. If you ever sat through a high school biology course, you might remember hearing that  insects are limited in size by their ability to utilize⁶ oxygen. The bigger you get, the harder it is to get O2 to your tissues. And bugs don't have lungs to help. To test the oxygen collection, researchers turned to fossils. They charted the wingspan of more than ten thousands fossilized insects, and found that for the first 150 million years of bug² evolution, size tracked closely with atmospheric⁷ oxygen levels. The more O2, the bigger the bugs. But then insects started shrinking, even though oxygen continued to rise. This wave of reduction happens to coincide with the emergence⁸ of anatomical features that made birds more agile⁹  airborne predators¹⁰. And insects got even smaller about 6 million years ago when bats hit the scene. Being little makes you harder to catch---which may have given bugs with teeny wings an evolutionary¹¹ leg up.