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Researchers at the US Department of Energy’s Argonne National Laboratory have discovered why giant insects that lived millions of years ago have disappeared.

In the late Paleozoic era, atmospheric oxygen levels reached record highs, which caused some insects to evolve into giants. When oxygen levels returned to lower levels, the insect giants went extinct.

Using detailed X-ray images produced at Argonne’s Advanced Photon Source (APS), the investigators examined the dimensions of respiratory systems in insects to find the reason for the gigantism.

Unlike vertebrates, where blood transports oxygen from the lung to the cell, insects deliver oxygen directly through a network of blind-ending tracheal tubes. As insects get bigger, this oxygen transport becomes far less effective, but an increase in atmospheric oxygen occurs, the longer trachael tubes can work, resulting in giant insects.

Recent research published in the Proceedings of the National Academy of Science journal confirmed the hypothesis that the tracheal system limits the size of an insect. It specifically explained how the constriction leading to the legs could limit the size of beetles.