For many years, scientists believed that the size of prehistoric insects, such as the griffin fly, was linked to elevated oxygen levels in the atmosphere. However, a groundbreaking study led by paleontologist Edward Snelling from the University of Pretoria suggests that the impressive size of these Carboniferous insects may have other underlying causes.

In the 1990s, researchers suggested that the high oxygen concentrations of the Carboniferous period, around 300 million years ago, coincided with the emergence of these giant insects.

They theorized that the increased oxygen demand due to the larger body sizes of these insects necessitated higher atmospheric oxygen levels.

This hypothesis is logical, given that insects rely on a specialized tracheal system for oxygen intake, a network of branching airways that culminate in trachea.

Insects require oxygen to travel by diffusion through this system, fueling their flight muscle cells.

Prior researchers posited that such giant flying insects could not exist today due to current atmospheric oxygen levels being insufficient to meet the high oxygen needs of their flight muscles.

In the latest study, Dr. Snelling and his team employed high-magnification electron microscopy to examine the relationship between body size and the number of tracheae within flight muscles.

They discovered that trachea typically occupy only about 1% or less of the flight muscles’ space in most species, a trend that holds true for the griffin fly as well.

This finding indicates that insect flight muscles are not limited by atmospheric oxygen levels; they can easily accommodate additional trachea within the muscle itself.

“If atmospheric oxygen truly restricts the maximum body size in insects, we should observe compensatory adaptations at the tracheal level,” remarked Dr. Snelling.

“There is some cost associated with larger insects, but overall it is minor.”

Professor Roger Seymour from the University of Adelaide pointed out, “In contrast, capillaries in bird and mammal heart muscles occupy approximately ten times more relative space than trachea in insect flight muscles. Thus, if oxygen transport is indeed a limiting factor for body size, significant evolutionary potential exists to enhance tracheal investment.”

Some researchers argue that oxygen flow to trachea and other body parts may still impose limits on insect size, leaving the theory of maximum size due to oxygen constraints open for debate.

“Regardless, these new findings clearly demonstrate that diffusion within the flight muscle trachea does not pose a constraint. Scientists need to investigate other factors that may explain the existence of these giant insects,” Dr. Snelling concluded.

If oxygen is not the limiting factor for insect size, alternative explanations like predation from vertebrates or limitations in the biomechanical support of the exoskeleton may be at play.

A detailed paper outlining this research was published in the latest issue of Nature.

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EP Snelling et al. Oxygen supply via the tracheal musculature does not suppress insect gigantism. Nature published online March 25, 2026. doi: 10.1038/s41586-026-10291-3