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nanoatzin t1_iwfvtkt wrote

Is it possible that air quality could be a significant factor?

Carbon dioxide has increased from around 250ppm to over 400ppm during the industrial revolution, with most of that increase occurring since 1940.

Carbon dioxide becomes toxic for humans above around 750ppm, however we have a fairly efficient respiratory system with a great deal of reserve capacity.

Insects breathe through tracheae and spiracles that run directly through the body seem less efficient. For example, insects grew to the size of birds during the Carboniferous period when oxygen was 35% of air and not the current 21%. Air quality seems to drive natural selection for insect size.

It seems that an increase in carbon dioxide would induce evolutionary pressure selecting for insects with smaller size because of respiratory efficiency limits.