The largest insects that ever lived were dragonflies with wingspans of more than two feet, thriving in an ancient atmosphere so rich in oxygen that nothing of that size could survive in the air we breathe today. This is a fascinating topic that invites us to explore the intricate relationship between insects, their environment, and the evolution of life on Earth. Let's delve into this captivating story and uncover the secrets of these ancient giants.
The largest insect known to science was a predator called Meganeuropsis permiana, with a wingspan of about 71 centimeters, a little over two feet. This creature, which lived in the Early Permian, roughly 285 million years ago, was not a true dragonfly but a griffinfly, a stem relative of modern dragonflies and damselflies. The skies it inhabited were devoid of birds, bats, and pterosaurs, and the air was significantly more oxygen-rich than what we breathe today. This abundance of oxygen is often cited as the primary reason for the massive size of these insects.
The Meganisoptera, often called griffinflies, resembled modern dragonflies and damselflies but were distant relatives. Two notable names in this group are Meganeura and Meganeuropsis. Meganeura, described in the 1880s from a French fossil, had a wingspan of around 65 to 70 centimeters, while Meganeuropsis, discovered by Frank Carpenter in 1939 from North American rock, had a body length of approximately 43 centimeters. In contrast, the largest living odonate, a Central American damselfly, has a wingspan of only about 19 centimeters, highlighting the immense size of these ancient insects.
Insects, however, do not breathe through lungs. Instead, they rely on a network of tubes called tracheae, which allow air to enter through openings along the body called spiracles. This system becomes less efficient as insects grow larger, as the distance oxygen must travel increases faster than the supply can keep up. This is where the atmosphere comes into play. Geochemical models suggest that the air in the Late Carboniferous and Early Permian periods had around 30 to 35 percent oxygen, which was denser than the air we breathe today. This higher oxygen content, combined with denser air, allowed insects to grow larger without the tracheal system becoming a limiting factor.
A 2007 study in the Proceedings of the National Academy of Sciences by Alexander Kaiser and colleagues provided experimental support for this hypothesis. They found that the tracheal system takes up a larger share of the body as beetles grow larger, which aligns with the prediction that an oxygen limit on size would occur. Additionally, rearing experiments have shown that some insects can grow larger over generations in oxygen-enriched air.
However, the story doesn't end there. The fossil record reveals a more complex picture. In 2012, Matthew Clapham and Jered Karr analyzed over 10,500 fossil insect wings spanning the Carboniferous to the present. They discovered that the relationship between maximum insect size and atmospheric oxygen held surprisingly well for about 200 million years before it broke down. Around the end of the Jurassic and the start of the Cretaceous, about 150 million years ago, insect size became uncoupled from oxygen levels.
The arrival of birds during this period is believed to have played a significant role. As fast and maneuverable flying predators filled the sky, being a large and less agile insect became a liability rather than an advantage. Predation, therefore, became a more significant factor in capping insect size, even as oxygen levels rose. The same analysis found only weak support for a comparable effect from pterosaurs, which had taken to the air earlier.
So, why don't we see two-foot insects today? It's not just because the air holds less oxygen. The aerial niche once occupied by griffinflies is now crowded with animals that would eat them. This shift in the ecosystem has contributed to the absence of such large insects in the present day.
The role of oxygen in insect size is well-grounded in physiology, but even the mechanism is contested. A 2026 study in Nature challenged the idea that oxygen delivery through the tracheolar-muscle system sets the ceiling on insect size. The authors argued that the relationship between oxygen delivery and insect size is more complex and may not be as straightforward as previously thought.
The fossil correlation is also debated. The link between size and oxygen weakens when temperature is considered, and other explanations have been proposed. One such explanation is purely mechanical: denser, oxygen-rich air reduces the power a large insect needs to stay aloft, which could favor bigger bodies for flight rather than respiration. The most defensible position is that the gigantism of these insects was likely a result of multiple factors, including a permissive atmosphere and an empty sky, with the relative weight of each factor still under debate.
In conclusion, the griffinflies represent a unique period in Earth's history where an insect body plan met an unusually oxygen-rich atmosphere and a sky free of aerial competitors. As these conditions changed, so did the size of insects. The fossil wings remain crucial evidence in understanding which factor was more significant in shaping the size of these ancient creatures.
This exploration of the largest insects ever and the factors influencing their size provides a fascinating glimpse into the past and the intricate interplay between biology, environment, and evolution. It reminds us of the dynamic nature of our planet and the ever-changing conditions that shape life as we know it.
