The giant insects of the prehistoric world, such as the 71-centimetre wingspan predator Meganeuropsis permiana, thrived approximately 285 million years ago due to a unique combination of high atmospheric oxygen and an absence of aerial vertebrate predators. While historical theories pointed solely to oxygen levels, modern research suggests a complex interplay between physiology, mechanical flight requirements, and ecological pressure from birds.
Why did giant insects rule the skies?
During the Early Permian and Late Carboniferous periods, Earth’s atmosphere contained 30 to 35 per cent oxygen, compared to 21 per cent today, according to geochemical models like the GEOCARBSULF reconstruction. Because insects rely on a tracheal system—a network of tubes that deliver oxygen via diffusion—this dense, oxygen-rich air likely supported larger body sizes. Research published in Nature in 2026, however, challenges the idea that oxygen delivery was the sole limiting factor. The study found that tracheoles supplying flight muscles remain modest even in massive insects, suggesting that internal oxygen transport might not set a hard ceiling on insect growth.
The largest living odonate, a Central American damselfly, has a wingspan of about 19 centimetres. That is roughly one-third the size of the extinct griffinflies that roamed the Earth 300 million years ago.
How did the rise of birds change insect size?
A 2012 study by Matthew Clapham and Jered Karr of the University of California, Santa Cruz, published in PNAS, examined over 10,500 fossil insect wings. They discovered that while insect size tracked oxygen levels for 200 million years, the relationship decoupled around 150 million years ago. As birds evolved and filled the sky, they introduced a new selective pressure. Large, less agile insects became vulnerable to these fast, manoeuvrable predators, forcing a trend toward smaller body sizes even when oxygen levels later rose.
Are “griffinflies” actually dragonflies?
Calling these prehistoric giants “dragonflies” is a common shorthand that misrepresents the fossil record. These creatures belonged to the extinct order Meganisoptera, or griffinflies. While they were distant relatives of modern Odonata, the lineage containing today’s dragonflies and damselflies had not yet evolved when Meganeura lived 300 million years ago. Frank Carpenter, who described Meganeuropsis in 1939, helped clarify these distinctions, highlighting that the giants were a distinct evolutionary experiment in flight.
Pro Tip: Understanding the Mechanics of Flight
Some researchers suggest that beyond oxygen levels, the physical density of the air played a role. Denser air reduces the power required for a large insect to remain aloft, potentially favouring larger bodies for mechanical flight efficiency rather than just respiratory capacity.
Frequently Asked Questions
- Could giant insects survive today?
Some research, including a 2026 study in Nature, suggests there is no inherent reason why a griffinfly could not survive in today’s atmosphere, as the tracheal system’s efficiency is not strictly limited by current oxygen levels. - What is the largest insect known to science?
The predator Meganeuropsis permiana, which lived 285 million years ago, holds the record with a wingspan of approximately 71 centimetres. - Did pterosaurs impact insect size?
The 2012 PNAS study by Clapham and Karr found only weak support for a link between pterosaur emergence and insect size, likely due to gaps in the fossil record.
The evolution of insect size remains a subject of active scientific debate, balancing atmospheric chemistry against the harsh realities of predation. To stay updated on the latest findings in paleontology and evolutionary biology, subscribe to our newsletter or explore our archive of in-depth research articles.
