Three hundred million years ago, insects ruled the skies like nothing we see today. We’re talking about Meganeuropsis permiana, a dragonfly the size of a hawk with a wingspan over 70 centimeters and a weight of 100 grams. When scientists first spotted the fossils, they asked the obvious question: why don’t bugs get that big anymore?
The answer seemed obvious. Thirty years ago, biologists developed what they called the “oxygen constraint hypothesis.” It was elegant, simple, and wrong.
The Theory Everyone Believed
The logic went like this: insects breathe differently than we do. No centralized lungs, no closed circulation system pumping oxygen through blood vessels. Instead, insects have this intricate network of tubes called tracheae that branch into impossibly thin capillaries called tracheoles, embedded directly in their muscle tissue.
As insects grow larger, oxygen has to travel further to reach the deepest tissues. And here’s where the diffusion problem kicks in. Oxygen doesn’t flow through these tiny tubes like water through a pipe. It creeps through via passive diffusion, which is notoriously slow.
The hypothesis argued that giant insects would need exponentially wider or far more numerous tracheoles to keep their muscles oxygenated. Eventually, you’d hit a structural limit where the breathing tubes would take up so much space that they’d crowd out the actual muscle fibers they were supposed to fuel. Flight performance would collapse.
This was supposed to be solved by the hyperoxia of the late Palaeozoic era, when atmospheric oxygen peaked around 30 percent compared to today’s 21 percent. More oxygen in the air meant giant insects could bypass their breathing system’s limitations and grow to absurd proportions.
Clean theory. Perfect answer. Completely wrong.
When 10,000-Fold Body Mass Changes Don’t Matter Much
Edward Snelling, a professor of veterinary science at the University of Pretoria, led a research team that decided to actually test this idea. What they found was surprising enough to make it into Nature.
The team gathered 44 insect species across ten distinct orders, representing nearly the entire range of modern flying bugs. On one end was Trioza erytreae, a tiny thing weighing 0.334 milligrams. On the other was Goliathus albosignatus, the famous Goliath beetle at 7.74 grams. That’s a 10,000-fold difference in body mass.
Using transmission electron microscopes, they took 1,320 high-resolution images of flight muscles and measured exactly how much space tracheoles occupied in each insect. They called this metric “tracheolar volume density.” If the oxygen constraint hypothesis was right, these breathing tubes should take up dramatically more space as insects got bigger, creeping toward a theoretical limit that would crush muscle power.
The results? In 0.5-milligram insects, tracheoles occupied 0.47 percent of flight muscle space. In 5-gram insects, that rose to just 0.83 percent. Over a 10,000-fold jump in body mass, the relative space occupied by these tubes increased by a factor of only 1.8.
For comparison, the blood capillaries serving the same function in bird and mammal muscles typically take up around 10 percent of tissue volume. Insect breathing tubes? Usually 1 percent or less.
There Was Always Room to Spare
When Snelling’s team extrapolated these findings to estimate tracheolar volume density in ancient giants like Meganeuropsis permiana, the numbers were striking. Even assuming a full 100-gram mass, their scaling equations predicted tracheoles would occupy only about 1 percent of the creature’s flight muscle volume, with an absolute upper statistical limit of 3 percent.
The giant insects had plenty of room to spare. Plenty.
And here’s the kicker. When the researchers ran sensitivity analyses using a standard 1-gram locust as a model, they found that tripling tracheolar volume density from 0.6 percent to 1.8 percent would increase oxygen-diffusing capacity by over four times. You could get a massive oxygen delivery boost without meaningfully sacrificing the muscle’s maximum mechanical work rate or peak metabolic rate.
If a giant insect needed more oxygen, it could simply evolve denser tracheoles. No anatomical roadblock. No flying power sacrifice. Just upgrade the breathing tubes and you’re set.
So Why Aren’t We Living with Pigeon-Sized Bugs?
If oxygen didn’t kill the giant insects, something else must have. And here’s where it gets interesting.
Snelling and his team suggest looking beyond molecular diffusion and considering the bigger ecological and physiological picture. There are actually several compelling hypotheses floating around.
One centers on the rise of aerial vertebrate predators. The fossil record shows maximum insect wing length started decoupling from atmospheric oxygen levels around 135 million years ago, right around when birds evolved and later when bats showed up. Giant insects were probably slow to accelerate, making them perfect high-calorie targets for more agile flyers. Being enormous stopped being an evolutionary win when the skies got competitive.
Then there’s thermodynamics. Flying generates massive heat. As animals get larger, their surface-area-to-volume ratio decreases, which means less efficient cooling. A hawk-sized insect might literally cook itself from the inside out with the heat of its own wing beats. The atmosphere’s higher density in the Palaeozoic era might have actually helped these creatures shed heat more effectively, not because of oxygen but through better heat dissipation.
Growing an XL-sized exoskeleton is another problem. Insects molt to grow, turning temporarily soft and vulnerable while their new shells harden. Surface tension and basic structural mechanics hold tiny beetles together fine, but scaling that up? Maybe not.
And don’t forget the insect cardiovascular system. Their open circulation is probably just too inefficient to power flapping flight in extremely large bodies.
The Mystery Isn’t Quite Solved Yet
Snelling mentioned one last frontier his team hasn’t fully explored. They focused on tracheoles, the tiny end-stage breathing tubes, but they haven’t thoroughly studied what’s “upstream”—the larger tracheae and especially the massive air sacs that act like bellows to ventilate the entire system.
“While we looked at tracheoles, we didn’t look upstream,” Snelling said. A comparative study on how air sac dimensions change with body size could reveal something unexpected.
He doesn’t expect this to resurrect the oxygen-constraint hypothesis. “Any limitation upstream can be compensated by investment in the tracheoles—there’s so much space down there,” he explained. But it would be intellectually satisfying to understand the complete picture of how these ancient giants were built.
This is the thing about science. Sometimes the simple, elegant answer that satisfied us for three decades turns out to be built on shaky ground. We had the framework backwards. The real question isn’t why giant insects don’t exist anymore. It’s why they ever needed to in the first place when technology and science show they could have evolved to stay small and thrived just fine.
The giant bugs are gone, and we still don’t fully know which of these competing pressures actually mattered most. Maybe it was all of them working together. Maybe it was something nobody’s thought of yet.
