Three hundred million years ago, the Earth presented a dramatically different landscape and atmosphere than what we witness today. During this era, known as the late Carboniferous period, the continents had consolidated into the supercontinent Pangaea. This vast landmass was characterized by extensive equatorial coal-swamp forests, which thrived under unusually high atmospheric oxygen levels. These elevated oxygen concentrations—estimated to be near 45%, compared to today’s 21%—contributed to environmental conditions that promoted rampant wildfires and sustained an unprecedented array of biodiversity, including colossal insects dominating the skies.
The Carboniferous period’s ecosystems were teeming with life: aquatic habitats flourished with various fish species, terrestrial environments were largely inhabited by amphibians, early reptiles, arthropods, and notably, giant cockroach-like insects. Among the most extraordinary denizens of the Paleozoic skies were enormous flying insects such as massive mayfly-like creatures boasting wingspans up to 17 inches (45 cm), and dragonfly-like “griffinflies” with wingspans soaring as large as 27 inches (70 cm) or more. Discovered nearly a century ago in fossilized impressions from fine-grained sedimentary rock in what is now Kansas, these griffinflies have long fascinated scientists seeking to understand how such gigantism was physiologically possible.
For decades, the prevailing scientific consensus attributed the existence of these giant insects largely to the high atmospheric oxygen levels of the time. The rationale was intimately tied to insect respiratory physiology: insects rely on a tracheal system—a complex, branching network of air-filled tubes culminating in tiny terminal branches called tracheoles—to deliver oxygen directly to their tissues, including the metabolically demanding flight muscles. Because oxygen diffusion through tracheoles is a passive process, it was hypothesized that today’s lower oxygen levels restrict insects from achieving similar gigantism, as their tracheal systems would inadequately supply the oxygen required for muscle function and flight.
The link between oxygen levels and insect size gained traction with geochemical reconstructions in the 1980s, which revealed evidence for this period of elevated atmospheric oxygen around 300 million years ago. These findings culminated in a 1995 landmark study published in Nature, which directly correlated giant insect fossils with peak oxygen concentrations during the Carboniferous and early Permian periods. This “oxygen constraint hypothesis” became the accepted paradigm, influencing subsequent research into arthropod physiology and evolutionary biology.
However, a recent groundbreaking study, published as of March 2026 in Nature, challenges this long-held assumption with compelling new physiological data. Led by Edward (Ned) Snelling from the University of Pretoria, the research team employed advanced high-power electron microscopy to scrutinize the spatial relationship between insect body size and the volume of tracheoles embedded within flight muscle tissue. The central question was whether the proportion of muscle volume occupied by these oxygen-delivering tracheoles scales up in giant insects, thereby reflecting a compensatory mechanism linked to oxygen availability.
Contrary to expectations, the study revealed that tracheoles occupy only about 1% or less of the flight muscle volume across a diverse range of modern insects. Strikingly, this ratio appears consistent even when extrapolated to the gigantic griffinflies of the Paleozoic Era with wingspans exceeding two feet (over 60 cm). This consistency suggests that the flight muscle’s oxygen supply system was not fundamentally constrained by atmospheric oxygen concentrations, as insects had sufficient physiological capacity for tracheole expansion within muscle tissue but seemingly did not employ it even at such colossal sizes.
Dr. Snelling explains, “If oxygen availability were indeed the limiting factor in maximum insect body size, we would expect to find evolutionary adaptations reflected as significant increases in tracheolar investment within the flight muscles, but our findings indicate that any such compensatory response is trivial at best.” This insight calls into question the oxygen constraint hypothesis as a definitive explanation for ancient insect gigantism and demands a reevaluation of factors influencing insect size evolution.
Moreover, Roger Seymour of Adelaide University, co-author of the study, highlights a striking comparison: “Capillaries in the cardiac muscles of vertebrates such as birds and mammals occupy roughly ten times the volume relative to their muscles compared to tracheoles in insect flight muscles. This disparity underscores the evolutionary potential insects have for ramping up oxygen delivery, implying that oxygen diffusion through tracheoles does not impose a physiological ceiling on body size.” This reasoning further disrupts the standing theory that past atmospheric oxygen richness was a prerequisite for supporting giant Paleozoic insects.
Although the new findings decisively refute tracheole-mediated oxygen limitation at the muscle level, some scientists caution that oxygen transport constraints could still exist in other parts of the insect respiratory system. For instance, airflow through larger tracheal tubes or oxygen diffusion in tissues upstream of the tracheoles could theoretically impose limits on maximum body size. Hence, while the muscle tracheole data dispel a major hypothesized bottleneck, the complete physiological picture remains complex and not fully resolved.
The implications of this research are considerable. If atmospheric oxygen concentration does not set a cap on insect size, scientists must seek alternate explanations for both the presence of giant Paleozoic insects and the markedly smaller maximal insect sizes observed today. Possible drivers include increased predation pressures from evolving vertebrates, biomechanical limitations on exoskeleton strength and weight-bearing capacity, or ecological and developmental constraints that modulate growth at the organismal level.
These alternatives invite fresh interdisciplinary research incorporating paleontology, biomechanics, evolutionary ecology, and comparative physiology. Understanding the precise factors governing insect body size throughout Earth’s history will enrich knowledge of evolutionary strategies as well as ecological interactions shaping terrestrial ecosystems.
In summary, the revelation that diffusion through tracheoles in flight muscles does not limit insect gigantism recalibrates our perspective of Paleozoic life and insect physiology. This paradigm shift underscores the dynamic interplay of environmental, anatomical, and ecological factors in the evolution of form and function. As scientists continue to unravel the mysteries of ancient ecosystems, such findings remind us that longstanding scientific beliefs remain open to challenge and refinement in the quest to comprehend life’s extraordinary diversity.
Subject of Research: Animals
Article Title: Oxygen supply through the tracheolar–muscle system does not constrain insect gigantism
News Publication Date: 25-Mar-2026
Web References: DOI: 10.1038/s41586-026-10291-3
Image Credits: griffinfly credit: Estelle Mayhew, adapted from image by Aldrich Hezekiah; giant petaltail credit: Estelle Mayhew
Keywords: insect gigantism, tracheal system, griffinflies, atmospheric oxygen, flight muscle physiology, Carboniferous period, Paleozoic insects, evolutionary biology, respiratory diffusion, insect flight, biomechanics, paleontology
