system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why have bugs gotten smaller over time?
gpt
Because today's oxygen levels in the atmosphere are lower than they have been 70 million years ago. Insects don't have lungs, they can't force air in and out of their body, but they still need the oxygen to reach their cells. So if they have a large body, a lot of cells are further away from their "breathing holes", so they have to steal oxygen from neighboring cells, who steal it from their neighbors etc. until some cell with direct connection to the hole. This works if there is plenty of oxygen around, but nowadays there's so little that the innermost cells would die.