Picture a flying predator with wings nearly seventy centimetres across, hunting over a vast wetland. That was Meganeura monyi, one of the largest flying insects ever discovered. It lived almost 300 million years ago and no living insect approaches its size.

So what allowed Meganeura to grow so large, and why is there nothing like it now? The familiar answer is oxygen. But a study published in 2026 argues that one important part of that explanation may be wrong.

What Meganeura actually was

Meganeura was not a true dragonfly. It belonged to the extinct group Meganisoptera, commonly known as griffinflies, whose members were probably close relatives of modern dragonflies.

It lived around 300 million years ago and was an aerial predator, probably hunting over open wetlands and waterways. Its enormous wingspan made it one of the most remarkable insects ever to take to the air.

Size like that is more than a curiosity. Because insect respiratory systems become less efficient as bodies grow larger, researchers have spent decades trying to explain how Meganeura reached such extraordinary proportions.

Why the ancient air was so strange

The world Meganeura lived in had a different atmosphere. Oxygen made up more than 30 percent of the air, compared with about 21 percent today. Vast wetland forests produced huge quantities of organic matter, and much of it was buried before fully decaying. That locked away carbon and allowed oxygen to accumulate in the atmosphere.

That oxygen-rich period is the setting for the classic explanation of giant insects, and the explanation depends on how insects breathe.

The oxygen idea, and how insects breathe

Insects do not have lungs. They draw air through openings in the body connected to a branching network of tubes called tracheae. These divide into microscopic tracheoles that deliver oxygen directly to tissues.

Oxygen must diffuse across the final gap into working cells, so researchers reasoned that a larger insect would face greater transport demands. Richer air could force more oxygen through the same system and raise the maximum size an insect could reach. The modern version of that idea was proposed in Nature in 1995.

There is evidence consistent with it. X-ray imaging of beetles found that larger species devoted a disproportionately greater share of their bodies to tracheal tubes, supporting the idea that respiratory plumbing becomes more costly as insects grow. Experiments have also found that some insects, including dragonflies and beetles, grow larger under high oxygen, although species respond differently.

Why some researchers now doubt it

A 2026 study in Nature led by Edward Snelling of the University of Pretoria examined the finest tubes feeding insect flight muscles. The team analysed 1,320 micrographs from 44 species across ten insect orders and compared the pattern with the estimated anatomy of giant fossil griffinflies.

They found that tracheoles typically occupied only about 1 percent or less of flight muscle. Their relative share rose just 1.8-fold even as body mass increased 10,000-fold across the species examined.

The argument is straightforward. If oxygen diffusion through flight-muscle tracheoles imposed a hard limit on insect size, larger insects should need to devote far more space to those tubes. Instead, the increase was small. “There is some compensation occurring in larger insects,” Snelling said, “but it is trivial in the grand scheme of things.”

Co-author Roger Seymour made the point by comparison. Capillaries occupy about ten times the relative space in the heart muscle of birds and mammals than tracheoles do in insect flight muscle. That suggests insects had considerable room to add more tracheoles if this part of oxygen transport were truly limiting their size.

The study does not prove that oxygen was irrelevant. It specifically argues that diffusion through the tracheoles of flight muscle did not set the maximum body size of insects. Oxygen delivery farther upstream, the needs of other tissues, the mechanics of the exoskeleton and ecological pressures could still matter.

A 2010 review also noted that giant insects appeared during periods of oxygen-rich air, but warned that the insect fossil record is too incomplete to test the relationship cleanly. The correlation is real, but the exact cause remains difficult to establish.

Another idea shifts attention to the aquatic young. Wilco Verberk suggested that high oxygen may have been harmful to larvae and that growing larger reduced surface area relative to volume, limiting oxygen exposure. “So a larval perspective might lead to a better understanding of why these animals existed in the first place,” he said, “and maybe why they disappeared.” It remains a hypothesis rather than a settled explanation.

There is also an ecological suspect: no vertebrate aerial predators existed when Meganeura flew. Later fossil patterns suggest that after birds evolved, insects became smaller despite rising oxygen levels, as the need for greater manoeuvrability favoured smaller flying bodies.