The largest insect known to science was a predator called Meganeuropsis permiana, with a wingspan of about 71 centimetres, a little over two feet. It flew in the Early Permian, roughly 285 million years ago, and despite the way it is usually described, it was not a true dragonfly. It was a griffinfly, a stem relative of modern dragonflies and damselflies.

The skies it hunted in had no birds, no bats and no pterosaurs. Nothing with a backbone had yet learned to fly. The air also held considerably more oxygen than ours, and that fact has carried most of the explanation for why insects then grew to sizes no insect reaches now.

What they actually were

The giants belonged to an extinct order, the Meganisoptera, often called griffinflies. They resembled modern dragonflies and damselflies and were distant relatives, but crown-group Odonata, the lineage containing today’s dragonflies and damselflies, had not yet evolved. Calling them dragonflies is a convenient shorthand that quietly misstates the family tree.

Two names anchor the group. Meganeura, described from a French fossil in the 1880s, lived in the Late Carboniferous around 300 million years ago and had a wingspan near 65 to 70 centimetres. Meganeuropsis, described by Frank Carpenter in 1939 from North American rock, was larger still, with a body around 43 centimetres long. For comparison, the largest living odonate, a Central American damselfly, has a wingspan of about 19 centimetres, roughly a third of the griffinfly’s reach.

The oxygen explanation

Insects do not breathe with lungs. Air enters through openings along the body called spiracles and travels to the tissues through a branching network of tubes, the tracheae, largely by diffusion. That design works well at small sizes and becomes harder to sustain as a body grows, because the distance oxygen must travel increases faster than the supply can keep up.

This is where the atmosphere comes in. Geochemical models of the deep past, such as the GEOCARBSULF reconstruction, put oxygen in the Late Carboniferous and Early Permian at around 30 to 35 per cent, against about 21 per cent today, in air that was also denser. More oxygen, more easily delivered, relaxes the size limit that the tracheal system imposes. The hypothesis has experimental support. A 2007 study in the Proceedings of the National Academy of Sciences by Alexander Kaiser and colleagues found that the tracheal system takes up a disproportionately larger share of the body as beetles get bigger, which is what an oxygen limit on size would predict, and rearing experiments have shown some insects growing larger over generations in oxygen-enriched air.

Where the simple story breaks down

The familiar version ends there: high oxygen built the giants, lower oxygen rules them out, so nothing that size could survive in the air we breathe. The fossil record complicates that ending.

In 2012, Matthew Clapham and Jered Karr of the University of California, Santa Cruz analysed more than 10,500 fossil insect wings spanning the Carboniferous to the present, published in PNAS. They found that maximum insect size tracked atmospheric oxygen surprisingly well for roughly 200 million years, before the relationship broke down. Around the end of the Jurassic and the start of the Cretaceous, about 150 million years ago, size came uncoupled from oxygen. Insects kept getting smaller even through later periods when oxygen rose again.

What changed around that time was the arrival of birds. Their reading is that once fast, manoeuvrable flying predators filled the sky, being a large and less agile insect became a liability rather than an advantage, and predation capped insect size in a way the atmosphere no longer governed. The same analysis found only weak support for a comparable effect from pterosaurs, which had taken to the air earlier, a result complicated by gaps in the fossil record.

So the reason there are no two-foot insects today is not only that the air holds less oxygen. It is also that the aerial niche the griffinflies once owned is now crowded with animals that would eat them.

What is settled and what is argued

Oxygen’s role is grounded in the physiology, but even the mechanism is now contested. The strongest support came from that beetle tracheal-investment result. A 2026 study in Nature pushed back, reporting that the tracheoles supplying insect flight muscle grow only about 1.8-fold across a 10,000-fold range in body mass, sit at roughly one per cent of muscle volume in most species, and stay modest even when the relationship is extended to Meganeuropsis itself. Its authors argue that oxygen delivery through the tracheolar-muscle system does not set the ceiling on insect size after all.

The fossil correlation is debated too. The link between size and oxygen weakens once temperature is accounted for, and other explanations have been put forward. One is purely mechanical: denser, oxygen-rich air reduces the power a large insect needs to stay aloft, which would favour bigger bodies for reasons of flight rather than respiration. The most defensible position is that the gigantism was probably multi-causal, with a permissive atmosphere and an empty sky both mattering, and the relative weight of each, along with the mechanism itself, still under argument.

The griffinflies mark a particular window in which an insect body plan met an unusually oxygen-rich atmosphere and a sky with no aerial competitors. Both of those conditions later closed. The fossil wings remain the main evidence for working out which one mattered more.