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… Why scientists blamed oxygen in the first placeInsects don’t breathe as we do. No lungs are involved. Instead, they have a system of tiny branching tubes, called tracheoles, which carry oxygen directly to the muscles by diffusion. The efficiency of diffusion decreases over longer distances, so scientists thought there was a hard limit to how big an insect could get, and that limit increased with more oxygen in the air.In 2010, a study Atmospheric oxygen level and the evolution of insect body size, identified several plausible mechanisms linking tracheal oxygen delivery to insect body size, and the case for high oxygen levels enabling prehistoric gigantism seemed well supported. It was a clean story: more oxygen, bigger bugs. Everybody moved on. However, the new research published in Nature suggests the clean story may have been wrong.

What researchers found when they looked closer, actuallyA team led by Edward Snelling of the University of Pretoria used high-powered electron microscopy to examine exactly how much space tracheoles actually occupy within insect flight muscles. The answer? Hardly any. Tracheoles make up about 1 per cent or less of the flight muscle in most insect species, and that pattern holds true for the ancient griffinflies.

That’s a surprisingly low footprint. In contrast, the capillaries in the heart muscle of birds and mammals occupy about ten times more relative space than the tracheoles in insect flight muscle. If oxygen transport really was the bottleneck limiting insect size, you’d expect evolution to have packed in far more tracheoles, especially during a period when giant insects were thriving. It didn’t.So, what actually made them so huge?Now here is where it gets really mysterious. No one knows for sure. The oxygen theory has been called into question, but nothing has come along to cleanly replace it.