In the day of the dinosaur, insects had wingspans of nearly two-and-a-half feet. So why are today's bugs so puny? According to researchers at U.C. Santa Cruz, we may have birds and bats to thank. Their conclusions appear in the Proceedings 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 utilize oxygen. The bigger you get, the harder it is to get 02 to your tissues. And bugs don't have lungs to help. To test the oxygen connection, researchers turned to fossils. They charted the wingspan of more than 10,000 fossilized insects and found that for the first 1 50 million years of bug evolution size tracked closely with atmospheric oxygen levels: the more 02, the bigger the bugs. But then insects started shrinking, even though oxygen continued to rise. This wave of reduction happens to coincide with the emergence of anatomical features that made birds more agile airborne predators. And insects got even smaller about 60 million years ago, when bats hit the scene. Being little makes you harder to catch—which may have given bugs with teeny wings an evolutionary leg up.