For decades, natural history textbooks have offered a straightforward explanation for the existence of prehistoric monsters: the air was simply richer. The prevailing theory suggested that the towering “griffinflies” of the Carboniferous period—dragonfly-like insects with wingspans reaching up to 70 centimeters—could only survive because atmospheric oxygen levels were significantly higher than they are today.
However, a latest study published in the journal Nature is challenging this long-held biological assumption. Researchers from the University of Pretoria in South Africa and Adelaide University in Australia have found that the relationship between prehistoric insect size and oxygen levels may not be as direct as previously believed, suggesting that the giants of the ancient skies were not merely products of a high-oxygen environment.
The findings suggest that the physiological mechanisms insects use to breathe were not the primary bottleneck restricting their growth. This discovery prompts a fundamental reassessment of what actually limits the body size and energy demands of the most diverse group of animals on Earth.
The myth of the oxygen limit
Around 300 million years ago, the Earth was a vastly different world. The supercontinent Pangaea was home to dense, humid coal-swamp forests, and the atmosphere is believed to have contained roughly 45% more oxygen than the current 21% average. This oxygen-rich air not only fueled the growth of massive flora but also contributed to frequent, intense wildfires.
Because insects rely on a passive system of tubes called tracheae to deliver oxygen directly to their tissues—rather than using a heart to pump oxygenated blood like mammals—scientists long assumed they were limited by the efficiency of this diffusion. The logic was simple: more oxygen in the air meant oxygen could travel further into a larger body, allowing insects to evolve to gargantuan proportions.
Prof Edward (Ned) Snelling, an experimental physiologist in the University of Pretoria’s Faculty of Veterinary Science who led the research, argues that this explanation may be too simplistic. “Our findings suggest a need to reassess textbook explanations of what limits the body size and energy demand of the most diverse and abundant animals on the planet, insects,” Snelling said.
Uncovering the cellular evidence
To test the oxygen theory, the international team spent more than five years employing high-resolution electron microscopy to examine the flight muscles of insects. They specifically looked at tracheoles—the microscopic, fluid-filled ends of the tracheal system that facilitate gas exchange in the muscles.
If atmospheric oxygen were the primary driver of size, the researchers expected to see a significant “compensation” in the tracheal system—essentially, a much larger investment in the “plumbing” of the insect’s body to support larger muscles.
The results were surprising. The team found that tracheoles occupy only 1% or less of the flight muscle space, a proportion that remained remarkably consistent even when applying the data to ancient species. This indicates that insects possess a high degree of flexibility in how they handle varying oxygen levels, and that the physical space dedicated to oxygen transport does not scale up dramatically with body size.
“If atmospheric oxygen really sets a limit on the maximum body size of insects, then there ought to be evidence of compensation at the level of the tracheoles,” Snelling said. “There’s some compensation occurring in larger insects, but it’s trivial in the grand scheme of things.”
Comparing insect and vertebrate physiology
The discrepancy becomes even clearer when compared to the respiratory systems of mammals and birds. Dr Roger Seymour of Adelaide University noted that the cardiac muscles of vertebrates are far more “expensive” in terms of space. He observed that capillaries in the cardiac muscle of birds and mammals occupy about 10 times the relative space that tracheoles occupy in the flight muscle of insects.
According to Seymour, this gap suggests that insects have a massive amount of “evolutionary potential” to increase their oxygen-transport infrastructure if it were truly the factor limiting their size. The fact that they didn’t do so suggests the limit lies elsewhere.
| Factor | Traditional Theory | New Research Findings |
|---|---|---|
| Primary Driver | High atmospheric oxygen ($approx$ 30-35%) | Physiological flexibility/Other constraints |
| Limiting Mechanism | Tracheal diffusion efficiency | Tracheole space ($le 1%$ of muscle) |
| Proposed Constraint | Oxygen availability | Predation and exoskeleton load |
What actually stopped the giants?
If oxygen wasn’t the primary factor allowing “griffinflies” to reach 70-centimeter wingspans—and isn’t the primary factor keeping modern dragonflies little—what is? The researchers suggest that the decline of giant insects may have been driven by ecological and structural pressures rather than chemical ones.
Prof Snelling noted that modern insect size may instead be limited by “predation from small vertebrates” or “load limits on the exoskeleton.” As vertebrates like early reptiles and birds evolved and became more efficient predators, the risk of being a large, slow-moving target in the sky may have outweighed the benefits of size.
the exoskeleton—the hard outer shell of an insect—must support the animal’s weight. As an insect grows, its volume (and weight) increases cubically, while the strength of its exoskeleton only increases quadratically. Eventually, the shell simply cannot support the mass of the animal, regardless of how much oxygen is in the air.
The study, which drew from Africa’s vast insect diversity, was a technically grueling process. Dr Antoinette Lensink of the University of Pretoria described the insect material as “technically challenging to work with,” but noted that the reward was uncovering insights that challenge fundamental biological assumptions.
This research opens new avenues for understanding the evolution of the animal kingdom, suggesting that the prehistoric skies were shaped by a complex interplay of predation and physics rather than a simple atmospheric fluke. The team plans to continue exploring how these physiological traits influenced the survival and extinction of various insect lineages over millions of years.
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