The Largest Insects that Ever Lived Were Dragonflies with Wingspans of More than Two Feet, Grown in an Ancient Atmosphere so Much Richer in Oxygen that Nothing that Size Could Survive in the Air We Breathe Today

Why It Matters

Understanding why insects once grew so large clarifies how atmospheric chemistry and predator dynamics shape evolutionary limits, informing both paleobiology and modern climate‑impact studies.

Key Takeaways

• Meganeuropsis permiana had a 71 cm (2 ft) wingspan in the Early Permian
• Atmospheric O₂ reached 30‑35% then, versus 21% today
• High oxygen eased tracheal diffusion, allowing larger insect bodies
• Bird and bat evolution later limited insect gigantism despite oxygen
• Recent studies suggest size limits are multi‑causal, not just oxygen

Pulse Analysis

The Early Permian sky was dominated by griffinfly giants such as Meganeuropsis permiana, whose 71 cm wingspan dwarfed today’s largest odonates. Their unprecedented size coincided with a carbon‑rich world where atmospheric oxygen hovered around 30‑35%, a level that dramatically improves diffusion through the insect tracheal network. This physiological advantage meant that the primary constraint on body size—oxygen delivery to flight muscles—was relaxed, allowing insects to push beyond the limits observed in modern ecosystems.

Scientists have long pointed to the oxygen hypothesis, citing experiments where beetles reared in oxygen‑enriched chambers grow larger, and fossil analyses that correlate peak insect size with high‑oxygen intervals. However, a 2026 Nature study revealed that tracheolar investment in flight muscle scales only modestly across a vast mass range, challenging the notion that respiration alone caps size. Instead, researchers now view gigantism as a product of several interacting factors: atmospheric composition, air density, and the mechanical demands of flight in a denser medium.

The story complicates further when the rise of avian predators is considered. By the Jurassic, birds filled the aerial niche, turning large, less agile insects into easy prey. This predation pressure, combined with shifting climates and oxygen fluctuations, likely forced insects toward smaller, more maneuverable forms. The multi‑causal explanation underscores how climate, physiology, and ecosystem dynamics intertwine, offering valuable insights for predicting how modern insects might respond to today’s rapidly changing atmospheric conditions.