Oxygen is one of two factors allowing for large insects. Ecology is the other. As https://news.ucsc.edu/2012/06/giant-insects.html https://news.ucsc.edu/2012/

Oxygen is one of two factors allowing for large insects. Ecology is the other. As https://news.ucsc.edu/2012/06/giant-insects.html https://news.ucsc.edu/2012/06/giant-insects.html says, before birds you see a clear correlation between insect size and oxygen level. After birds, the correlation disappears.