A new study published in Nature challenges the idea that high atmospheric oxygen levels enabled giant insects 300 million years ago. Researchers led by Edward Snelling of the University of Pretoria found that tracheoles in insect flight muscles occupy minimal space, leaving room for expansion. The discovery prompts scientists to seek alternative explanations for the insects' massive sizes.
Three hundred million years ago, during the time of the supercontinent Pangaea, Earth hosted enormous flying insects. Griffinflies reached wingspans of 70 cm, while mayfly-like species spanned 45 cm. Scientists had long attributed their size to atmospheric oxygen levels around 45% higher than today, which supposedly overcame limits in the insects' tracheal breathing system reliant on diffusion to flight muscles. A 1995 Nature paper supported this view, arguing diffusion inefficiency prevented such giants under modern oxygen conditions. Led by Edward Snelling, an associate professor at the University of Pretoria's Faculty of Veterinary Science, a team used high-power electron microscopy to analyze tracheoles in insect flight muscles. They discovered these tiny structures occupy just 1% or less of the muscle volume across species, including estimates for ancient griffinflies. Snelling stated, '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. There is some compensation occurring in larger insects, but it is trivial in the grand scheme of things.' Professor Roger Seymour of the University of Adelaide added, 'By comparison, capillaries in the cardiac muscle of birds and mammals occupy about ten-times the relative space than tracheoles occupy in the flight muscle of insects, so there must be great evolutionary potential to ramp up investment of tracheoles if oxygen transport were really limiting body size.' While some researchers note oxygen might still constrain other body parts, the study rules out flight muscle tracheoles as the bottleneck. Possible alternatives include predation pressures or exoskeleton limits, leaving the rise and fall of giant insects an ongoing mystery.