About 300 million years ago, the skies of the late Palaeozoic teemed with massive insects. Meganeuropsis permiana, a predatory species similar to a modern dragonfly, had a wingspan topping 70 centimetres and weighed roughly 100 grams. Scientists studying these prehistoric giants wondered why insects no longer reach such sizes. Three decades ago they proposed an answer, the so-called “oxygen constraint hypothesis.”
For many years the prevailing view was that dragonfly-sized insects required an atmosphere richer in oxygen to survive because insect respiratory systems are less efficient than those of mammals, birds, or reptiles. As atmospheric oxygen fell, the theory went, the conditions could no longer sustain gigantic bugs. “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 lack a central pair of lungs and a closed circulatory network that transports oxygenated blood to tissues. “They respire through an internal network of tubes called the tracheal system,” Snelling explained.
Air gets into an insect’s body through small openings in the exoskeleton called spiracles. From there it moves into larger tubes, the tracheae, which branch repeatedly into very fine, blind-ending tracheoles. Those tracheoles extend into the tissues, and mitochondria in adjacent cells often sit close to them.
Insects can actively move air through the larger tracheae by flexing their bodies, but this active ventilation does not reach the tiny tracheoles. At that final stage, oxygen must cross into tissue by passive diffusion.
Diffusion is slow, which is the central problem. The oxygen constraint hypothesis argued that as an insect grows, oxygen must travel farther to reach inner tissues.
“As the insects get bigger and bigger, the challenge of diffusion becomes greater,” Snelling said.
To keep muscles from suffocating, a larger insect would need much wider or many more tracheoles to sustain oxygen delivery, implying a structural limit. If an insect became too large, the volume of air tubes required to oxygenate its muscles would occupy too much space, crowding the very muscle fibers they were meant to feed and greatly reducing flight capability.
