Three hundred million years ago, the skies at the end of the Paleozoic Era were buzzing with giant insects. Meganeuropsis permianaA predatory insect resembling a modern dragonfly, which had a wingspan of more than 70 centimeters and weighed 100 grams. Biologists looked at these ancient monsters and asked why insects weren’t so big anymore. Thirty years ago, they came up with an answer known as the “oxygen barrier hypothesis.”
For decades, we thought that dragonflies of any hawk-sized size needed highly oxygenated air to survive because the insects’ respiratory systems are less efficient than those of mammals, birds or reptiles. As atmospheric oxygen levels dropped, there was no longer enough oxygen to support the giant insects. “It’s a simple, beautiful explanation,” said Edward Snelling, professor of veterinary science at the University of Pretoria. “But this is wrong.”
insect breathing
Unlike mammals, insects do not have a centralized pair of lungs and a closed circulatory system that delivers oxygenated blood to their tissues. “They breathe through internal tubing called the tracheal system,” Snelling explained.
Air enters the bodies of insects through special pores on their exoskeleton called spiracles. From there, it travels to larger tubes, the trachea, which gradually divide into microscopic, thin, blind-ended tubes known as tracheoles. These tracheae are embedded deep within the insect’s tissues, and mitochondria in neighboring cells are stored next to them.
Insects can actively pump air in and out of the large trachea by bending their bodies, but this active pumping stops at the very end of the line, in the small trachea. Here, oxygen delivery depends on passive diffusion to cross the final barrier in the tissue.
The problem with propagation is that it is extremely slow. The oxygen barrier hypothesis argued that the larger an insect grows, the further oxygen must travel to reach deeper tissues.
“As insects get bigger, the challenge of proliferation gets bigger,” Snelling said.
To avoid muscle suffocation, a large insect would need a much wider or far more numerous trachea to maintain its oxygen supply, meaning there must be an anatomical tipping point. If an insect grows too large, the volume of breathing tubes needed to supply oxygen to its muscles will take up too much physical space. The trachea constricts the very muscle fibers they were trying to fuel, severely impairing the insect’s flight ability.
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