Three-hundred million years ago, the skies of the late Palaeozoic era were buzzing with giant insects. Meganeuropsis permiana, a predatory insect resembling a modern-day dragonfly, had a wingspan of over 70 centimeters and weighed 100 grams. Biologists looked at these ancient behemoths and asked why bugs aren’t this big anymore. Thirty years ago, they came up with an answer known as the “oxygen constrain hypothesis.”

For decades, we thought that any dragonflies the size of hawks needed highly oxygenated air to survive because insect breathing systems are less efficient than those of mammals, birds, or reptiles. As atmospheric oxygen levels dropped, there wasn’t enough to support giant bugs anymore. “It’s a simple, elegant explanation,” said Edward Snelling, a professor of veterinary science at the University of Pretoria. “But it’s wrong.”

Insect breathing

Unlike mammals, insects don’t have a centralized pair of lungs and a closed circulatory system that delivers oxygen-rich blood to their tissues. “They breathe through internalized tubing called the tracheal system,” Snelling explained.

Air enters the insect’s body through specialized portholes on their exoskeleton called spiracles. From there, it travels down larger tubes, the tracheae, which gradually branch into microscopically thin, blind-ending tubes known as tracheoles. These tracheoles are embedded deep within the insect’s tissues, and mitochondria in neighboring cells cluster next to them.

Insects can actively pump air in and out of the larger tracheae by flexing their bodies, but this active pumping stops at the very end of the line, in the tiny tracheoles. Here, oxygen delivery relies on passive diffusion to cross the final barrier into the tissue.

The problem with diffusion is that it’s notoriously slow. The oxygen constraint hypothesis argued that the larger the insect grows, the further the oxygen must travel to reach the deepest tissues.

“As the insects get bigger and bigger, the challenge of diffusion becomes greater,” Snelling said.

To prevent the muscles from suffocating, a bigger insect would need significantly wider or far more numerous tracheoles to maintain the supply of oxygen, which implied there had to be a structural tipping point. If an insect gets too big, the volume of breathing tubes required to supply its muscles with oxygen would take up too much physical space. The tracheoles would crowd the very muscle fibers they were trying to fuel, leaving the insect with severely impaired flight performance.