Air's oxygen content constrains insect growth.

The size to which insects grow is limited by how much oxygen they can route to tissues in their legs, new airway measurements suggest.

The researcher knew that some insects grow particularly large when reared in high-oxygen laboratories and that massive insects that lived during the prehistoric Paleozoic period vanished.

Researchers have long suspected that the big bugs of the Paleozoic period could grow large because each milliliter of atmosphere then carried nearly twice as much oxygen as it does today (SN: 12/17/05, p. 395).

Unlike vertebrates, which move oxygen within the body by way of their bloodstreams, insects move air through their bodies via an internal network of hollow channels called trachea. To probe the relationship between body size and trachea volume, researchers compared related species of beetles that ranged from 3 millimeters to 3.5 centimeters in length.

The investigators, led by physiologist Jon Harrison of Arizona State University in Tempe, found that larger insects devote more of their interior space to trachea. At the point where each leg joins the body, the diameter of the trachea was so wide in the largest beetles that little space remained for other tissues, Harrison says.

Insects "hyperinvest in their respiratory system as they get bigger," he says.