Tracheal tubes —which deliver oxygen to insects’ cells — are the key to understanding why giant insects of the late Paleozoic era became extinct. In the late Paleozoic era, with atmospheric oxygen levels reaching record highs, some insects evolved into giants.

When oxygen levels returned to lower levels, the insect giants went extinct.

The basis of this gigantism is thought to lie in the insect respiratory system.

In contrast to vertebrates, where blood transports oxygen from the lung to the cell, in insects oxygen is delivered directly through a network of blind-ending tracheal tubes.

Oxygen transport

As insects get bigger, this type of oxygen transport becomes far less effective. But if the atmospheric oxygen levels increase, as they did in the late Paleozoic, then longer tracheal tubes can work. This would allow larger-sized insects — even giants — to evolve.

A collaborative team of researchers from Argonne’s Advanced photon Source (APS), Midwestern University and Arizona State University wanted to study how beetles’ tracheal systems change as their body sizes increase.

X-ray images

The team took advantage of richly detailed X-ray images they produced at the APS to examine the dimensions of tracheal tubes in four beetle species, ranging in body mass by a factor of 1,000, according to an Argonne National Laboratory press release.

Overall, they found that larger beetle species devote a disproportionately greater fraction of their body to tracheal tubes than do smaller species.

The team focused in particular on the passageways that lead from the body core to the head and to the legs. They reasoned that these orifices may be bottlenecks for tracheal tubes, limiting how much oxygen can 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 University.

They then examined the tracheal measurements of the four species to see if they could predict the largest size of currently living beetles.

The head data predicted an unrealistically large, foot-long beetle.

In contrast, the leg data predicted a beetle that nicely matches the size of the largest living beetle, Titanus giganteus.

The research provides a specific explanation for what limits size in beetles: the constriction leading to the legs.