system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there no big insects?

I'm not talking about stag beetles or giant millipedes.  I'm talking horse size at least to qualify for "big".

> There are big mammals like whales and small mammals like voles.
> There are big birds like ostriches and small birds like kiwis.
> There are big reptiles like crocodiles and small reptiles like skinks.

Why are there no big insects?
gpt
Because insects don't "breathe" the way other animals do. They absorb oxygen through their "skin" and it sort of trickles through to the rest of their body's cells. Earth doesn't have high enough oxygen concentration to support insects beyond a certain size.