The largest insects in Earth's history were griffinflies like Meganeuropsis permiana, which had wingspans exceeding 70 centimeters during the Early Permian, around 285 million years ago. These predators were not true dragonflies but distant relatives in the extinct order Meganisoptera. They thrived in an atmosphere with oxygen levels around 30–35%, much higher than today’s 21%, which may have supported their massive size by easing oxygen diffusion through their tracheal systems. For over 200 million years, maximum insect size closely tracked atmospheric oxygen levels, according to fossil analysis.

However, the story changed around 150 million years ago. Despite fluctuations in oxygen, insects did not regain their former size. Research by Matthew Clapham and Jered Karr suggests that the evolution of agile flying predators, especially birds, created new pressures. Large size became a liability in the face of aerial predation, effectively capping insect growth regardless of atmospheric conditions. Pterosaurs, which appeared earlier, showed a weaker effect on size limits, possibly due to less efficient flight or fossil data gaps.

Recent studies challenge the oxygen-diffusion hypothesis. A 2026 paper in Nature found that tracheole density in flight muscles scales only modestly across body sizes, suggesting oxygen delivery may not be the primary constraint. Other factors, such as air density aiding flight efficiency, may have played a role. The gigantism of griffinflies likely resulted from a combination of high oxygen, low competition, and favorable aerodynamics—conditions that no longer exist. Today’s largest odonate, a Central American damselfly, has a wingspan of just 19 centimeters, underscoring how profoundly ecosystems have changed.