For a considerable duration, the immense size attained by griffinflies during the Carboniferous epoch, approximately 300 million years ago, was attributed to heightened atmospheric oxygen concentrations. However, novel research spearheaded by paleontologist Edward Snelling from the University of Pretoria suggests alternative explanations for these colossal creatures’ impressive dimensions.
Gigantic griffinfly in a Carboniferous forest.
During the 1990s, scientific consensus posited that a period of elevated atmospheric oxygen levels around 300 million years ago corresponded with the prevalence of extraordinarily large insect species.
The prevailing hypothesis was that the increased metabolic demands associated with the larger physiques of these giant insects necessitated a correspondingly higher concentration of oxygen in the atmosphere.
This proposition seemed logically sound, given that insects rely on a distinctive tracheal system for respiration. This system comprises a network of branching, tree-like airways that extend to the extremities of the organism, terminating in structures known as tracheoles.
For flight muscle cells to be adequately fueled, oxygen must traverse these tracheoles via diffusion, propelled by concentration gradients.
Consequently, researchers inferred that an insect of such gargantuan proportions would be incapable of existing in the contemporary environment, as current atmospheric oxygen levels are deemed insufficient to meet the substantial oxygen requirements of their flight muscles.
In a recent investigation, Dr. Snelling and his research associates employed high-power electron microscopy to meticulously evaluate the correlation between body size and the density of tracheoles within flight musculature.
Their findings indicated that the proportion of flight muscle occupied by tracheoles typically constitutes a mere 1% or less across the majority of insect species. This observation remained consistent even when extrapolating to the formidable griffinflies.
This suggests that the flight muscles of insects are not inherently constrained by prevailing atmospheric oxygen levels, as they possess the latent capacity to augment their tracheole network with considerable ease, given the minimal space these structures occupy.
“Were atmospheric oxygen genuinely dictating the maximum attainable body size for insects, one would anticipate observable compensatory mechanisms within the tracheole system,” stated Dr. Snelling.
“While some degree of adaptation is evident in larger insect species, its impact is negligible in the broader context.”
Professor Roger Seymour of the University of Adelaide commented, “In comparison, the capillaries within the cardiac muscle of avian and mammalian species occupy approximately tenfold the relative volume compared to tracheoles within insect flight muscle. This implies substantial evolutionary latitude to enhance tracheole investment should oxygen transport indeed be a limiting factor for body size.”
“Nevertheless, some scientific perspectives maintain that oxygen flow upstream of the tracheoles, or in other bodily regions, could still impose limitations on insect size, thus the theory of oxygen-constrained maximal insect size may still hold some validity.”
“Irrespective of ongoing debate, the current data unequivocally demonstrate that diffusion within flight muscle tracheoles cannot constitute such a definitive limit. Scientists will need to explore alternative factors to elucidate the reasons behind the existence of these colossal insects.”
“If oxygen is not the primary determinant of maximal insect size, then perhaps other pressures, such as predation by vertebrates or the structural integrity limitations of the exoskeleton, are responsible for the comparatively smaller size of modern insects.”
A scientific publication detailing this research was released this month in the esteemed journal Nature.
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E.P. Snelling et al. Oxygen supply through the tracheolar-muscle system does not constrain insect gigantism. Nature, published online March 25, 2026; doi: 10.1038/s41586-026-10291-3
