Rethinking giant Paleozoic insects: Oxygen wasn’t the whole story

Background

If you’ve ever seen a museum diorama with a dinner-plate-sized dragonfly perched on a Carboniferous fern, you’ve met one of Earth’s great paleobiological puzzles. During the late Carboniferous and early Permian (roughly 320–270 million years ago), dragonfly relatives such as Meganeura sported wingspans on the order of 60–70 centimeters—close to two feet. Then, somewhere along the path to the modern world, those outsized aerial hunters disappeared. Today’s largest dragonflies are spectacular but much smaller.

For decades, a tidy explanation reigned: ancient air held more oxygen—perhaps 30–35% of the atmosphere compared to today’s ~21%—and that extra oxygen allowed insects, which breathe through networks of air-filled tubes, to grow larger. When oxygen levels later declined, so did insect size. The idea was elegant because it linked a basic physiological constraint (moving oxygen across tissues without lungs) to a grand pattern in the fossil record.

That story has now hit turbulence. A new analysis reported this week revisits the mechanics of insect breathing and concludes that the oxygen-only account is too simple. By digging into how insect airways scale and how living insects actively pump air, the authors argue that even very large insects could likely have met their oxygen needs under much leaner atmospheres than the Paleozoic’s oxygen spike. If that’s right, the disappearance of giant fliers needs a different, more ecologically complex explanation.

How insects breathe—and why oxygen seemed like the master key

Insects don’t ferry oxygen with red blood cells. Instead, they inhale through valves (spiracles) that open to a branching tracheal system. These tubes subdivide until they end directly at tissues. Two things matter for getting enough oxygen to flight muscles:

• Diffusion: Oxygen molecules drift down a concentration gradient through fine tracheoles to mitochondria. Diffusion is fast but only over short distances.
• Convection (bulk flow): Larger airways and rhythmic body movements pump air, shifting oxygen-rich gas deep into the system and flushing out CO2.

The classic size limitation argument goes like this: as body size increases, distances within tissues grow faster than surface areas. Without lungs and a circulatory pump, an insect can’t increase long-distance oxygen transport enough to keep up—unless the external oxygen gradient is steeper. During the late Paleozoic, with far higher oxygen partial pressure, diffusion could reach deeper, overcoming the geometric penalty of big bodies. When oxygen dipped in the Permian and Triassic, diffusion slackened and giants became impractical.

Support for this view came from several lines of evidence:

• Paleo-oxygen proxies suggest atmospheric O2 peaks near 30–35% in the late Carboniferous, tracking a heyday of huge arthropods.
• Lab experiments show that insects reared in low-oxygen environments grow smaller; in oxygen-rich conditions, some species grow larger.
• Imaging studies of beetles and other insects suggested that tracheal tubes take up a bigger fraction of the body in larger species, hinting at a scaling limit where oxygen pipes would eventually crowd out other organs.

What happened: A closer look shows insects can breathe better than we thought

The new study challenges the lynchpin assumption: that lower ambient oxygen would have categorically strangled very large insects. It pulls together modern measurements of tracheal anatomy, metabolic demand, and ventilation, and then models how those features scale as bodies get bigger. Three insights emerge:

1. Airways scale hypermetrically in key regions.

• As insects grow, the diameters of major tracheae and the volume of compressible air sacs grow faster than simple geometric scaling predicts. By investing disproportionately in the parts of the system that move gas over longer distances, large insects mitigate the diffusion problem.

2. Active ventilation changes the math.

• Many insects rhythmically pump their abdomen, deform tracheal sacs, and even use wing-driven thoracic movements to drive air. Convection helps deliver oxygen deep into tissues and carries away CO2, allowing the fine tracheoles to handle only the final short diffusion step.
• The model indicates that with plausible pumping rates—comparable to those measured in modern robust fliers—oxygen delivery could keep pace with the energy demands of flight even when the atmosphere is closer to today’s composition.

3. Safety margins persist at realistic oxygen levels.

• When the researchers combined airway scaling and ventilation with measured flight metabolic rates, they found that the system retained headroom: the modeled oxygen supply met or exceeded demand across a range of oxygen fractions that includes modern Earth. In other words, oxygen alone doesn’t force giants off the stage.

Taken together, these points undermine the notion that a drop in atmospheric oxygen automatically spelled doom for two-foot “dragonflies.” Insects appear to possess multiple, scalable levers—bigger airways, more air-sac volume, and stronger ventilation—to keep muscles fueled.

Why the earlier logic faltered

• Correlation is not causation. Giant odonatopterans coexisted with high oxygen, but their disappearance overlaps with sweeping environmental upheavals near the end of the Paleozoic. Oxygen decline was one change among many.
• Scaling is flexible. The tracheal system is not a static plumbing network. Evolution can shift investment among tube diameters, branch architecture, and ventilatory mechanics, altering constraints.
• Performance depends on behavior. Many analyses focused on resting or average gas exchange. Flying insects are dynamic systems that recruit powerful, rhythmic flows when it matters: during flight and thermoregulation.

What this means for the rise and fall of ancient giants

If oxygen was not the sole gatekeeper, what else shaped the ceiling on insect size? Several interacting factors become more plausible protagonists:

• Aerodynamics and wing loading

  • Lift generation scales with wing area and airspeed, while body mass grows faster with size. Larger fliers need disproportionately large wings or higher speeds to stay aloft. Ancient odonatopterans had expansive, lattice-like wings that helped, but even slight changes to air density, turbulence regimes above ancient forests, or typical wind environments could have reset the feasible envelope for huge sizes.

• Habitat transformation

  • The lush, waterlogged coal swamps of the late Carboniferous gave way in many regions to drier, more seasonal landscapes by the Permian. Canopy structure, wind profiles, and the prevalence of open vs. cluttered airspace likely shifted. Large fliers are poorly suited to dense, tangled habitats where maneuverability trumps raw span.

• Life cycle bottlenecks

  • Odonatopterans have aquatic juveniles. Oxygen chemistry in lakes and swamps—affected by temperature, organic load, and circulation—can constrain nymph size and growth. Drying climates, altered nutrient cycles, and widespread ecological turnover could have squeezed larval stages even if adults could still fly.

• Predation and competition

  • Vertebrate aerial predators arrive later (pterosaurs in the Late Triassic, birds much later), so they can’t explain the initial disappearance. But as vertebrate fliers came to dominate the skies, any re-evolution of giant insects may have been suppressed by predation risk and competition for aerial niches.

• Thermal physiology

  • Insects are ectotherms with limited internal heat generation. Flight demands warm muscles. Giant insects would face tradeoffs between being large (which slows heating and cooling) and maintaining the temperatures needed for agile flight. Shifts in climate variability—colder nights, hotter days, or greater seasonality—may have made such tradeoffs tougher.

• Developmental and energetic tradeoffs

  • Even if oxygen supply keeps up, the cost of building and maintaining oversized airways and flight muscles may crowd out reproduction or immunity. Natural selection optimizes fitness, not maximum size per se.

None of these explanations needs to act alone; the end-Paleozoic world changed in many ways at once. The new respiratory analysis simply makes it much less likely that oxygen was the singular steering wheel.

What happened in the study: A bit more technical detail

Although the new work relies on living insects rather than fossil lungs (which rarely preserve), it uses tools that make strong inferences possible:

• High-resolution imaging (micro-CT and synchrotron scans) of modern insects to measure the geometry of tracheal trunks, branch networks, and air sacs across size ranges.
• Direct measurements of flight metabolic rates and ventilation patterns—abdominal pumping frequency, thoracic compression, and spiracle opening dynamics—in actively flying or exercising insects.
• Biophysical modeling that couples: (1) bulk flow in larger airways driven by abdominal and thoracic motions, (2) diffusion in terminal tracheoles, and (3) realistic oxygen partial pressures in different atmospheres.
• Sensitivity analyses that vary key parameters (e.g., ventilation amplitude, tracheal diameters) within measured biological ranges to see how robust the conclusions are.

The headline result is that with reasonable assumptions about how large insects could move air, calculated oxygen delivery surpasses the thresholds needed to power sustained flight at oxygen fractions substantially lower than the late Carboniferous high. This does not prove that all giant insects flew easily in today’s air, but it weakens the claim that oxygen decline alone forced them under a hard ceiling.

Key takeaways

• The long-standing “high oxygen made giant insects possible” story is incomplete. Insect respiratory systems scale and ventilate in ways that can offset lower atmospheric oxygen.
• Giant Paleozoic odonatopterans likely could have met oxygen demands for flight even under atmospheres closer to today’s 21% oxygen, given realistic airway architecture and pumping.
• The disappearance of giant fliers probably reflects ecological and environmental shifts—habitat structure, climate, life cycle constraints—rather than a single atmospheric knob.
• Future tests will rely on integrating biomechanics, paleoecology, and better reconstructions of ancient habitats, not just bulk atmospheric chemistry.

What to watch next

• Fossil-informed biomechanics

  • Detailed reconstructions of Paleozoic wings, thoraxes, and body masses—linked to aerodynamic simulations—could test how maneuverable and efficient meganeurans really were in different air densities and temperatures.

• Larval ecology under changing climates

  • Experiments and models on odonate larvae in variable oxygen and temperature regimes can probe how aquatic stages limit adult body size.

• Habitat-scale modeling

  • Coupling forest canopy reconstructions from the late Carboniferous and Permian with computational fluid dynamics could reveal how typical airflow and turbulence affected large-insect flight envelopes.

• Comparative respiratory scaling across lineages

  • Expanding tracheal scaling datasets to include the largest modern fliers (e.g., big moths, beetles, and dragonflies) will refine estimates of how much ventilatory headroom is available at different sizes.

• Multifactor experiments

  • Rearing modern insects under factorial combinations of oxygen, temperature, and habitat complexity can separate oxygen effects from other ecological pressures that influence maximum size and flight performance.

FAQ

• Were ancient “dragonflies” actually dragonflies?

  • The famous giants belonged to extinct relatives within Odonatoptera, not the crown-group dragonflies and damselflies (Odonata) we see today. They looked similar but were on a parallel branch.

• How big did they really get?

  • Wingspans around 60–70 centimeters (roughly two feet) are supported by fossils such as Meganeura. Body lengths were shorter than the wingspan; they were not two-foot-long bodies.

• Didn’t experiments prove oxygen controls insect size?

  • Oxygen affects growth: many insects grow smaller in low oxygen and sometimes larger in high oxygen. But those lab effects don’t uniquely explain Paleozoic gigantism. The new analysis suggests that, with scalable airways and active ventilation, very large insects could still function under lower oxygen than previously assumed.

• Does air density matter more than oxygen?

  • Air density influences lift. However, changing oxygen fraction alone only slightly alters total air density. Other gases and climate variables also shape density and viscosity. Oxygen strongly affects physiology via partial pressure; density enters the aerodynamic side of the ledger. Both matter, but the new study argues oxygen wasn’t the dominant size limiter.

• If oxygen wasn’t decisive, why don’t we see giant insects today?

  • Ecology likely sets the ceiling: habitat structure, predation and competition (especially once vertebrate fliers evolved), larval constraints, and climate variability. Natural selection may favor smaller, more maneuverable forms in today’s typical environments.

• Could giant insects evolve again?

  • It’s not impossible in principle, but modern ecosystems—with birds, bats, and complex, cluttered habitats—probably disfavor the extreme end of insect size. Evolution follows ecological opportunity, not just physiological possibility.

• How certain are ancient oxygen estimates?

  • They’re inferred from proxies like charcoal abundance (wildfire likelihood), sulfur and carbon isotopes, and biogeochemical models. While the broad outline—high late-Carboniferous oxygen, later decline—is robust, exact percentages and timings carry uncertainty.

Source & original reading

https://arstechnica.com/science/2026/03/leading-explanation-for-ancient-giant-flying-insects-gets-squashed/