Some 300 million years super-sized insects – some as large as hawks – swarmed the Earth. The largest of these mega-insects was a predatory dragonfly-like creature with a wingspan of up to 75 centimeters.
That was during the late Carboniferous– early Permian periods, when the atmosphere was rich with oxygen. But then, 150 million years ago, birds showed up and the downsizing of insects began.
Scientists at the University of California, Santa Cruz believe high concentrations of oxygen in the atmosphere – over 30 percent of the air was 02 versus the 21 percent we have in today’s atmosphere – were responsible for the insects’ large size.
Insects have small breathing tubes instead of lungs, so the higher oxygen levels allowed them to take in and use more of the life-sustaining gas which encouraged their super sizes.
To reach their findings, the scientists examined data from more than 10,500 fossil wing lengths taken from various published records. They checked the size of the insects versus oxygen levels as they evolved over a period of hundreds of millions of years.
“Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years,” said Matthew Clapham, an assistant professor at UC Santa Cruz, who c0-authored a study published online in Proceedings of the Academy of Science. “Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds.”
With all over those hungry birds around, insects needed to become more maneuverable. Survival was a driving force in the evolution of flying insects. As result, the insects became smaller, which allowed them to survive and thrive, while their giant relatives died off.
Another transition in insect size took place more recently, at the end of the Cretaceous period some 90 and 65 million years ago, according to Clapman and Jered Karr, a UCSC graduate student who co-authored the study. They think several factors, such as the continued specialization of birds, along with the evolution of bats, and a mass extinction at the end of the Cretaceous period, may be behind this evolutionary transition. A shortage of fossils from that period has made it difficult for scientists to track insect sizes.
“I suspect it’s from the continuing specialization of birds,” Clapham said. “The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds.”
Clapham emphasizes their study wasn’t about determining average insect size during this time period, because the fossil records tended to favor the larger sized insects over the smaller ones, but instead concentrated on changes in the maximum size of insects over time.
“There have always been small insects,” he said. “Even in the Permian when you had these giant insects, there were lots with wings a couple of millimeters long. It’s always a combination of ecological and environmental factors that determines body size, and there are plenty of ecological reasons why insects are small,” said Clapman.