Three hundred million years ago, the sky of the late Paleozoic era was buzzing with giant insects. Meganeuropsis permianaThe wingspan of the predatory insect, similar to a modern dragonfly, was more than 70 centimeters, and its weight was 100 grams. Biologists looked at these ancient behemoths and asked why the bugs weren’t so big. Thirty years ago they made something the answer known as the “oxygen limitation hypothesis”.
For decades, we thought that any dragonfly the size of a hawk needed high-oxygen air to survive because insect respiratory systems are less efficient than those of mammals, birds, or reptiles. As oxygen levels in the atmosphere dropped, there wasn’t enough to support the giant bugs. “It’s a simple, elegant explanation,” said Edward Snelling, professor of veterinary medicine at the University of Pretoria. “But it’s wrong.”
Respiration of insects
Unlike mammals, insects lack a centralized pair of lungs and a closed circulatory system that delivers oxygen-rich blood to their tissues. “They breathe through internal tubes called a tracheal system,” Snelling said.
Air enters the insect’s body through special portholes called spiracles in their exoskeleton. From there, larger tubes, the tracheas, travel downward, which gradually branch into microscopically fine, blind-ended tubes known as tracheoles. These tracheoles are located deep in insect tissues, and mitochondria from neighboring cells are collected near them.
Insects can actively pump and expel air into the larger trachea by tilting their bodies, but this active pumping stops at the end of the line, in the smaller tracheoles. Here, oxygen delivery relies on passive diffusion to cross the final barrier into the tissue.
The problem with diffusion is that it is very slow. The oxygen limitation hypothesis argued that the larger the insect, the farther oxygen must travel to reach the deepest tissues.
“As the bugs get bigger, the diffusion problem gets bigger,” Snelling said.
To avoid muscle suffocation, a larger insect would need significantly wider or more tracheoles to maintain the oxygen supply, implying a structural tipping point. If an insect grows too large, the volume of trachea required to supply its muscles with oxygen will take up too much physical space. The tracheoles severely impaired the insect’s flight performance by compressing the muscle fibers they were trying to burn.
