INCREDIBLE INSECTS: A Celebration of Insect Biology

Research at Duke has revealed how the level of oxygen in the air can affect insect size. Insects breathe through a system of air-filled tubes called tracheae. As an insect grows it requires more oxygen, but the tracheae do not grow. Because of this, oxygen demand eventually outstrips supply and the insect stops growing. When insects are reared in a low oxygen atmosphere, they grow to a much smaller than normal size. It is thought that the very high oxygen levels during the Carboniferous and Permian eras enabled the evolution of giant insects. This chart shows estimated oxygen levels over geological time. The decline on oxygen to present levels coincided with the extinction of the giant insects.

Closeup images of the insect tracheal system. The tracheae are a finely-branching air-filled tubes that radiate from the spiracles on the sides of an insect. Early in the last larval stage, while the insect is still small, the tracheae are dense. Later, when the insect has grown, the distribution of trachea is much sparser and they are no longer able to keep up with the demand for oxygen of the growing body. This triggers the release of hormones (PTTH and ecdysone: see display on metamorphosis) that stop growth and start metamorphosis.

Caterpillars of the Cana leaf roller (Calpodes ethlius) have a transparent skin through which the tracheal respiratory system can be seen. The tracheae are the large white branching structures that radiate from the spiracles, which are the points at which air enters the system.

Meganeuropsis americana is a griffinfly, an extinct relative of the dragonfly, that lived during the Permian period, 290-280 million years ago. High oxygen levels during this period allowed insects to reach sizes that are now unattainable. This specimen had a two-foot wingspan. A modern dragonfly is shown for comparison.

This is a 3D printed replica of the largest griffinfly wing ever found. The original is at the Museum of Comparative Zoology at Harvard University.