X-ray analysis of a beetle. Credit: Alex Kaiser, Jaco Klok, Wah-Keat Lee

Thursday, August 2nd, 2007--

Beetles might be bigger if the air they breathed had more oxygen in it, according to a new study published in the Proceedings of the National Academy of Sciences.

About 300 million years ago (during the late Carboniferous period and early Permian), some insects were much bigger than they are today. Some mayflies had a wingspan of nearly a foot and the ancestors of dragonflies were about three times that size. It is not clear why these insects were so big, but some research suggests that insect gigantism is linked to an abundance of oxygen in the atmosphere during that time.

Alexander Kaiser, the lead author on the study and an insect physiologist at Midwestern University in Glendale, Arizona, and his colleagues looked inside beetles to understand why more atmospheric oxygen could lead to super-sized bugs.

Instead of arteries and veins, insects rely on tracheal tubes to transport oxygen to the cells in their bodies. Kaiser and his colleagues used a type of x-ray to measure the size of the tracheal tubes in several species of darkling beetles (family: Tenebrionidae). They discovered that the tubes take up disproportionately more space in bigger beetles than they do in smaller ones. In other words, as the size of the beetle's body increases, the size of the tracheal tubes increases even faster. This space needed for tracheal tubes in bigger beetles could limit body size, the researchers report.

For example, Kaiser and his team extrapolated that if a darkling beetle grew to 17 centimeters long, the tracheal tube that connects the body to the leg would be so big that there would be no room for any other tissues. "We suggest that there is a limit of body size simply based on this scaling relationship of the tracheal system," Kaiser says.

Disproportionate scaling is unusual, Kaiser says: "The lungs of mammals scale proportionally with body size. The relationship between lung volume and body volume doesn't change no matter how big the animals are."

The reason that insects, unlike other organisms, need disproportionately larger tubes might be related to diffusion. Beetles rely on diffusion to move the air through the tubes. “And diffusion is heavily dependent on distance. So the longer the distance, the less effective is diffusion," says Kaiser. It may be that for oxygen to reach a big beetle's extremities, the beetle has to carry more air, the researchers say. But, if the air had more oxygen in it—like it did 300 million years ago—perhaps the tubes could be skinnier, and the beetle could grow bigger.

It's not yet clear whether beetles will become gargantuan if grown in oxygen-rich air, but those experiments are underway, Kaiser says.

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--Flora Lichtman

Sources

Alexander Kaiser
Department of Basic Sciences Midwestern University Glendale, Arizona

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