Giant Dragonflies With 2-Foot Wingspans Ruled the Skies 300 Million Years Ago — But Oxygen May Not Explain Why
Before birds or bats took flight, the sky was filled with insects, and some of them were huge. About 300 million years ago, dragonfly-like creatures with wingspans of up to 70 centimeters – almost the width of a modern hawk – flew over swampy forests. These “griffins” lived in a world shaped by coal swamps, frequent wildfires, and oxygen levels much higher than today.
These high oxygen levels have long been thought to explain how insects reached such massive sizes. But new research, published in Naturechallenges this idea, suggesting that these giants may not have relied on oxygen as much as researchers once believed.
“If atmospheric oxygen truly sets a limit on the maximum size of insects, then there should be evidence of compensation at the level of tracheoles,” lead author Edward Snelling said in a press release.
Learn more: Fossilized resin reveals rainforest full of insects and spiders 112 million years ago
Oxygen levels were thought to influence the size of giant insects
Comparison of insect flight muscle (left) with mammalian heart tissue (right).
(Image credit: Antoinette Lensink and Edward Snelling)
The oxygen theory made intuitive sense. Insects do not breathe like mammals. Instead of lungs, they rely on a branching network of air-filled tubes called tracheas, which deliver oxygen directly to their tissues. On the smallest scale, oxygen diffuses through even thinner tubes – tracheoles – into muscle cells, including those that power flight.
Since this system relies on diffusion, the researchers hypothesized that it would struggle to provide enough oxygen to support very large bodies, especially during fuel-intensive flights.
This idea began to take shape in the 1980s, when new techniques allowed geochemists to reconstruct the composition of ancient atmospheres. Their results showed that oxygen levels were significantly higher around 300 million years ago, about 45% higher than today.
In the 1990s, researchers linked these high oxygen levels to fossil evidence of giant insects, proposing that larger bodies would need more oxygen to fuel their flight. The timing aligned, and the idea quickly became the leading explanation for why insects once reached such massive sizes.
In this framework, griffin-sized insects should not be able to exist today.
A Closer Look at the Muscles of Flight
The new study revisits this hypothesis by examining how oxygen is actually delivered into insect flight muscles.
Using high-power electron microscopy, the researchers measured the space occupied by tracheoles inside muscle tissue across a range of insect sizes and extended these findings to griffins of the past.
What they found is that tracheoles take up about 1 percent or less of the space inside flight muscles, even in large insects.
This means there is considerable unused space that could, in theory, be filled with additional structures providing oxygen if needed. In comparison, the heart muscle capillaries of birds and mammals take up about 10 times more space.
“There is some compensation in larger insects, but it’s insignificant in the grand scheme of things,” Snelling said.
In other words, insects don’t seem to run into a hard limit on oxygen, at least not in their flight muscles.
If not oxygen, then what?
The results do not completely rule out oxygen as a factor. Some scientists suggest that limitations elsewhere in the body, such as how oxygen flows through the larger airways, could still play a role.
But the new data show that diffusion within flight muscle tracheoles is unlikely to be the bottleneck hypothesized by the researchers.
This opens the door to other explanations for why ancient insects grew so large and why modern insects did not.
One possibility is structural. As insects grow, their exoskeletons must support greater weight, which can impose physical limitations on size. Another reason is ecological: the rise of vertebrate predators may have made large size a liability rather than an advantage.
For now, Paleozoic insects remain a mystery. But the long-held explanation that they simply needed more oxygen is starting to look less certain.
Learn more: These amber fossils offer a ‘snapshot of life on Earth millions of years ago’
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