Research results have been presented that overturn the established theory that ancient insects were huge because of high oxygen levels.
Fossils discovered so far suggest that hundreds of millions of years ago, insects larger than today's pigeons roamed the Earth. The prevailing theory is that these giant species could thrive because the oxygen concentration in the atmosphere was higher than it is today, but new evidence has emerged that may overturn this theory.
Oxygen supply through the tracheolar–muscle system does not constrain insect gigantism | Nature
Massive insect body size 300 million years ago may not have been due to high atmospheric oxygen
The oxygen concentration in the atmosphere today is about 21%, but 300 million years ago it was over 30%. Insects consume a lot of energy during flight, so a higher concentration of oxygen would allow them to secure more energy, and it is believed that insects 300 million years ago were much larger than modern insects. In other words, 'insects cannot grow to such large sizes without a high oxygen concentration.'
However, research by Edward P. Snelling and colleagues at the University of Pretoria has presented evidence suggesting that the correlation between insect gigantism and oxygen concentration may be weak.
Insects breathe through openings called spiracles and tubes called tracheae. When Snelling and his colleagues used an electron microscope to study 44 species of modern insects, they found that in most cases, regardless of body size, the tracheal twigs, which are the terminal parts of the trachea, occupied less than 1% of the muscular space. This ratio is thought to have been the same even in species that lived 300 million years ago and grew to over 60 cm in length.
In comparison, the proportion of capillaries in the heart muscle of birds and mammals is much larger. Snelling et al. point out that there should be evolutionary room to increase the number of tracheae if the body is larger, and therefore the claim that oxygen concentration influences body size, and by extension the claim that modern insects are limited in size due to oxygen deficiency, is weak. If oxygen concentration were truly the reason for gigantism, modern insects should be able to grow larger by increasing the number of tracheal twigs, and the fact that this is not the case suggests that oxygen concentration is not the direct cause, according to Snelling et al.
One possible reason why insects grew to such enormous sizes is the lack of predators such as birds. As time progressed and vertebrate predators emerged, their size became a disadvantage, potentially leading to their extinction.
Some scientists argue that the flow of oxygen upstream of the tracheal twigs, or other parts of the body, may still be limiting body size, so the theory that oxygen limits the maximum size of insects has not yet been completely dismissed.
Snelling stated, 'If atmospheric oxygen levels really do determine the size of insects, then there should be evidence in the trachea that another organ is taking over that function. There is some compensation in larger insects, but overall it is insignificant.'
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