300 million years ago, during the Carboniferous period of Earth’s history, there were giant insects flying around. These dragonfly-like creatures had wingspans of over 2.5 feet. It has long been suspected that the higher oxygen level at that time was the main factor allowing the insects to grow so large. John VandenBrooks of Arizona State University recently presented his experiments proving that this was so.
The oxygen content of Earth’s atmosphere has varied from 0% (before photosynthesis evolved) to a high of about 35% (during the Carboniferous period). Currently, our air is about 21% oxygen. VandenBrooks and his colleagues raised three groups of modern dragonflies in atmospheres containing 12%, 21% or 31% oxygen. As an aside, dragonflies are notoriously difficult to raise in the laboratory as they subsist by hunting live insect prey. The researchers were required to hand feed the insects, a task that has rarely been successful in the past.
In any case, they found that, as expected, the insects raised in a higher oxygen atmosphere grew faster and became larger adults. In addition, insects raised in high oxygen had much smaller tracheal tubes (the breathing apparatus that spans the insect body).
Interestingly, the same size increase was not true of some other kinds of insects raised in high oxygen, such as cockroaches. This corroborates the fact that no traces of giant cockroaches were found during the Carboniferous period. The authors speculate that although both groups of insects had smaller tracheal tubes when raised in high oxygen, some insects (dragonflies) used the high oxygen content to grow faster and larger, while others (cockroaches) invested the energy savings in eating or reproduction.