Hundreds of millions of years ago, winged insects as big as today's hawks ruled the skies. But at some point in their evolutionary history, something changed, and the giant bugs began shrinking, eventually becoming the diminutive insects we're familiar with today. Now, researchers from the University of California Santa Cruz think they know why, and it may have something to do with dinosaurs' slow evolution into birds. Here's what you should know:

What were these giant bugs like?
About 300 million years ago, a dragonfly-like insect called the griffinfly buzzed about with a fluttery wingspan of about 28 inches. Scientists believe that extra oxygen in the sky (9 percent more than exists in today's atmosphere) gave the massive bugs more energy per breath, allowing them to power larger bodies. "When oxygen went up, insects got bigger," study author Matthew Clapham tells National Geographic. But around 150 million years ago, that pattern began to change.

What happened?
During the Jurassic period, the first birds — like the famous Archaeopteryx — began taking flight. Strangely, animals like the griffinfly stopped growing bigger, even though oxygen levels continued to climb. The new study, published in the Proceedings of the National Academy of Sciences, suggests that big bugs may have been supplanted by these newly airborne predators.

Why did big birds replace big bugs?
The birds, much more nimble than the flying reptiles that preceded them, found it easier than previous predators to lunch on the bugs. Or it could simply be that the birds bested large insects in the battle for resources. In any case, insects like the griffinfly began to shrink, evolving features that made them harder to catch. "The maneuverability of any sort of flying thing really scales with size," says Clapham. "Small things are much more maneuverable than large things."

How big would insects be today if not for birds?
Clapham thinks that, given current oxygen levels, the largest bugs today could be roughly three times bigger than they are.

Sources: Mercury News, Mother Nature Network, National Geographic, Red Orbit