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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.



In collaboration with researchers from MidwesternUniversityand ArizonaStateUniversity, the scientists used the X-ray images to examine the tracheal tubes in four beetle species that ranged in body mass by a factor of 1,000. By analysing the passageways that lead from the body core to the head and to the legs, they found that larger beetle species devoted a disproportionately greater fraction of their body to tracheal tubes than smaller species. The scientists believed that the orifices at the head and the legs were bottlenecks for tracheal tubes that limited the amount of oxygen that could be delivered to the extremities.



‘We were surprised to find that the effect is most pronounced in the orifices leading to the legs, where more and more of the space is taken up by tracheal tubes in larger species,’ said Alex Kaiser, biologist at Midwestern.



They then examined the tracheal measurements of the four species to try and predict the largest size of existing beetles. Although the head data predicted an unrealistic, foot-long beetle, in contrast, the leg data predicted a beetle that matched the size of the largest living beetle, Titaneus giganteus.



‘This study is a first step toward understanding what controls body size in insects. It’s the legs that count in the beetles studied here, but what matters for the other hundreds of thousands of beetle species and millions of insect species overall is still an open question,’ said Jake Socha, Argonne biologist.