Modern dragonflies are common insects, but they are dwarfed by their prehistoric relatives. The history of this group includes a period of dramatic gigantism, where species grew to sizes unimaginable today. This phenomenon was a direct consequence of Earth’s unique atmospheric conditions millions of years ago. Understanding the scale of these ancient fliers requires looking back into the deep past.
Defining the Prehistoric Giants
The largest prehistoric insects belonged to the extinct order Meganisoptera, informally called “griffinflies” or “giant dragonflies.” The most famous genus is Meganeuropsis, which soared through the skies during the late Carboniferous and early Permian periods (roughly 323 to 285 million years ago). These creatures were close predatory relatives, occupying a similar ecological niche to modern dragonflies.
The record holder for the largest insect wingspan ever discovered is Meganeuropsis permiana. Fossil wing impressions found in Kansas, USA, indicate a maximum wingspan reaching up to 75 centimeters (about 30 inches). This is roughly the wingspan of a modern crow or small hawk. Another giant, Meganeura monyi from the late Carboniferous, also boasted impressive wingspans of up to 70 centimeters.
The Mechanism Behind Maximum Size
The extraordinary size of these prehistoric insects was fundamentally linked to the atmosphere of the Paleozoic Era. Unlike vertebrates, insects do not have lungs or a closed circulatory system; they rely on the tracheal system, a network of internal tubes that delivers oxygen directly to tissues.
This system works primarily through passive diffusion, where oxygen moves from the air-filled tubes into the body. Diffusion efficiency places a strict upper limit on insect size. The larger the body, the longer the distance oxygen must travel, which quickly becomes insufficient to support a high metabolism.
During the Carboniferous period, however, atmospheric oxygen concentration was significantly higher, reaching up to 35% compared to the 21% we have today. This hyperoxic environment drastically increased the partial pressure of oxygen. The higher concentration allowed oxygen to penetrate much deeper into the tracheal system, effectively removing the size constraint that limits modern insects. This provided the physiological foundation that allowed the evolution of massive insects like Meganeuropsis.
Why the Gigantism Ended
The decline in insect size began with a shift in Earth’s atmospheric composition during the Permian period. Oxygen levels started to drop from their Carboniferous peak, moving closer to modern levels. This reduction in atmospheric oxygen made it metabolically unviable for the largest insects to sustain their enormous bodies. The tracheal system of a giant insect could no longer efficiently supply enough oxygen to the interior tissues, leading to size reduction over millions of years.
Adding to this environmental pressure was the emergence of new, more efficient flying predators. The evolution of early flying vertebrates, such as pterosaurs, introduced a new level of aerial competition and predation. Large, slow-moving insects became easy targets for these agile hunters. The combination of diminishing oxygen and increasing predatory pressure ultimately selected against gigantism, driving the evolutionary trend toward smaller, more maneuverable body plans.
Comparison to Modern Species
The scale of the prehistoric giants is best understood by contrasting them with the largest dragonflies alive today. The largest living species is the helicopter damselfly (Megaloprepus caerulatus), a close relative of the dragonfly. This impressive insect can achieve a wingspan of up to 19 centimeters (about 7.5 inches).
For true dragonflies, the largest species, such as the Giant Petaltail (Petalura ingentissima) or the Giant Hawker (Tetracanthagyna plagiata), have wingspans that typically reach 16 to 17 centimeters.
The ancient Meganeuropsis with its 75-centimeter wingspan was nearly four times larger than the biggest modern species. This difference illustrates the profound impact that atmospheric and ecological factors have had on insect evolution.