In the Air That Once Held Giants: A Question That Refuses to Settle

There is a way we imagine the past—scaled slightly beyond the present, as if time itself once allowed for excess. Larger trees, heavier air, creatures that moved with a kind of amplified presence. Among these images, few are as striking as the idea of giant dragonflies, their wings stretching nearly two feet across, moving through skies that feel both familiar and distant.

For years, their existence seemed to carry a simple explanation. The atmosphere, it was said, once held more oxygen. With higher concentrations available, insects—whose breathing relies on passive diffusion through their bodies—could grow larger, their systems sustained by an abundance no longer present.

It was a tidy answer, one that linked environment directly to form. And for a time, it held.

But recent research within paleontology and evolutionary biology suggests that this explanation, while not without merit, may not be sufficient on its own. The story of why such large insects no longer exist appears to be more layered, less easily contained within a single cause.

Fossil evidence shows that giant insects, including ancient relatives of dragonflies like Meganeura, did indeed inhabit the skies during the Carboniferous and early Permian periods. Oxygen levels at the time were higher than they are today, supporting the idea that atmospheric conditions played a role. Yet new analyses indicate that size variation does not align perfectly with oxygen fluctuations alone.

In some periods, oxygen levels declined while large insects persisted. In others, oxygen remained relatively elevated, yet maximum insect sizes began to decrease. This inconsistency suggests that additional forces were at work, shaping the limits of insect growth in ways that go beyond atmospheric composition.

Among the factors now being considered is the role of predators—particularly the emergence and diversification of birds and other aerial vertebrates. As more agile and efficient flyers entered the skies, the advantage of large size may have shifted. What once aided survival could have become a liability, making insects more visible, less maneuverable, and more easily targeted.

There is also the matter of flight mechanics. Larger wings require different balances of strength, energy, and control. Environmental changes, including shifts in climate and vegetation, may have altered the conditions under which such large forms could thrive. The air itself, though still present, may have become a less accommodating medium for such scale.

In this view, size becomes not a simple function of oxygen, but a response to a network of pressures—biological, environmental, and ecological. Growth is shaped not only by what is available, but by what is required to survive within a changing system.

The image of the giant dragonfly remains, but its explanation grows more complex. It is no longer anchored to a single cause, but distributed across a range of influences that intersect over time.

Scientists report that while higher oxygen levels in Earth’s past contributed to larger insect sizes, this factor alone does not fully explain the existence or disappearance of giant insects. New research points to additional influences, including predation and environmental changes, as key factors in shaping insect size through evolutionary history.

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Sources

Nature Science National Geographic Smithsonian Magazine New Scientist